PHOSPHATE SORPTION BEHAVIOR IN BAJOA AND GOPALPUR SOIL SERIES
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Transcript of PHOSPHATE SORPTION BEHAVIOR IN BAJOA AND GOPALPUR SOIL SERIES
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PHOSPHATE SORPTION BEHAVIOR IN BAJOA AND GOPALPUR SOIL SERIES
COURSE TITLE: PROJECT THESIS COURSE NO: SS-4106
SOIL SCIENCE DISCIPLINE KHULNA UNIVERSITY
KHULNA
DECEMBER, 2014
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PHOSPHATE SORPTION BEHAVIOR IN BAJOA AND GOPALPUR SOIL SERIES
COURSE TITLE: PROJECT THESIS COURSE NO: SS-4106
This project thesis paper has been prepared and submitted to Soil Science Discipline, Khulna University, for the partial fulfillment of the four years professional B. Sc. (Hons) degree in Soil Science.
Submitted By
Md. Ali Reza
Student ID: 101309 Session: 2012-2013
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PHOSPHATE SORPTION BEHAVIOR IN BAJOA AND GOPALPUR SOIL SERIES
APPROVED AS TO STYLE AND CONTENT BY
.
KHANDOKER QUDRATA KIBRIA
Associate Professor Chairman of Examination Committee
Soil Science Discipline Khulna University, Khulna
Bangladesh
Soil Science Discipline Khulna University
Khulna December, 2014
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DECLARATION
This project thesis paper has been prepared and submitted to Soil Science Discipline, Khulna University, for the partial fulfillment of the four years
professional B. Sc. (Hons) degree in Soil Science.
Supervisor
..
Md. Sadiqul Amin [email protected] Associate Professor
Soil Science Discipline Khulna University, Khulna
Candidate
Md. Ali Reza
Student ID: 101309
Soil Science Discipline
Khulna University, Khulna
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Dedicated To
My Beloved Parents
Whose owes can never be fulfilled
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Acknowledgement First of all, I would like to express gratefulness to Almighty God who has enabled
me to accomplish this thesis work and to complete this project thesis paper
successfully in due time.
I would be privileged and proud to express unfettered gratification, sincere
appreciation and profound respect to my honorable teacher and supervisor Md
Sadiqul Amin, Associate Professor, Soil Science Discipline, Khulna University,
for his sincere supervision, valuable instruction, enthusiastic guidance,
constructive criticism and constant encouragement during the thesis work and the
preparation and compilation of this thesis paper.
I express my heartiest gratitude and sincerest appreciation to Md. Sanaul Islam
Associate Professor, Soil Science Discipline, Khulna University, for his advice,
encouragement and for all possible help during this thesis work.
I am really grateful to Afroza Begum, Head, Soil Science Discipline, Khulna
University, for his advice, encouragement andenthusiastic guidance to doing this
thesis work.
I am also thankful to Mr. Abdullah and Mr. Mahfuz, laboratory attendants for their
cooperation in the laboratory.
Special appreciation goes to my friends Anindita Das, Arpita Jotder and Farhana
Naznin Ema for their excellent co-operation.
Finally, I would like to express my gratitude to my parents for their moral,
inspiration and encouragement throughout the work.
December, 2014 Author
Md. Ali Reza
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Table of Contents Title Page
No.
Table of content I
List of Table IV
List of Figures V
Chapter One: Introduction
1. Introduction 1
1.2. Objective of the research 3
Chapter Two: Literature Review
2. Literature Review
2.1. Phosphorus status of soils in Bangladesh 4
2.2. Forms of phosphorus in soil 4
2.3. General characteristics of soil phosphorus 4
2.4. Role of phosphorus in soil 6
2.5. Soil phosphorus status and its availability 7
2.6. Phosphorus dynamics in soil 8
2.7. Requirement of phosphorus in plant 10
2.8. Soil factors affecting P uptake 10
2.8.1. Phosphate Buffering Capacity 11
2.8.2. Phosphate Diffusion in Soil 11
2.9. Phosphorus sorption and desorption 12
2.10. Adsorption isotherm 14
2.11.Types of adsorption isotherm 15
2.12. Factors affecting P sorption from soils 18
Chapter Three: Materials and Methods
3. Materials and Method
3.1. Sample Collection 22
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3.1.1. Bajoa Series 22
3.1.2. Gopalpur Series 22
3.2. Processing of soil sample 23
3.3. Physical analysis of soil 23
3.4. Chemical analysis of soil 23
3.4.1. Soil pH 24
3.4.2. Electrical Conductivity (EC) 24
3.4.3. Determination of organic carbon 24
3.4.4. Cation Exchange Capacity (CEC) 24
3.4.5. Available Phosphorus (P) 24
3.5. Phosphate sorption experiment 24
3.6. Fitting of phosphorus adsorption data in three equations 25
Chapter Four: Results and Discussion 27
4. Results and discussion 27
4.1. Physical Characteristics 27
4.1.1. Particle size analysis 27
4.1.2. Texture 27
4.2. Chemical Characteristics 27
4.2.1. Soil reaction or (pH) 27
4.2.2. Electrical conductivity (EC) 27
4.2.3. Organic Carbon 28
4.2.4. Available P 28
4.3. Phosphate sorption behavior 28
4.4. Freundlich adsorption isotherm 29
4.5. Langmuir adsorption isotherm 30
4.6. Temkin adsorption isotherm 31
4.7. Multi-point adsorption equations 32
4.8. Correlation between soil chemical parameters with phosphateadsorption 33
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5. Summary and Conclusion 36
6. Reference 37
Appendice 48
List of Tables Table No. Title Page No. Table 3.1 Table 4.2
Location and environmental conditions of the soils Phosphate adsorption parameters calculated from the isotherm
23 33
List of Figures Figure No. Title Page No. Fig 4.1 Fig 4.2 Fig 4.3 Fig 4.4 Fig 4.5 Fig 4.6 Fig 4.7
Phosphate sorption capacity of soils with different rates of phosphate application Freundlich adsorption isotherm for phosphorus in Gopalpur soil series Freundlich adsorption isotherm for phosphorus in Bajoa soil series Langmuir adsorption isotherm for phosphorus in Gopalpur soil series Langmuir adsorption isotherm for phosphorus in Bajoa soil series Temkin adsorption isotherm for phosphorus in Gopalpur soil series Temkin adsorption isotherm for phosphorus in Bajoa soil series
29 29 30 30 31 31 32
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Chapter I Introduction
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1. Introduction
Phosphorus (P) is an important naturally occurring element in the environment that can
be found in all living organisms as well as in water and soils. It is an essential component
for many physiological processes related to proper energy utilization in both plants and
animals. It is a component of key molecules such as nucleic acids, phospholipids, and
adenosine triphosphate (ATP). Phosphorus is also a critical element in natural and
agricultural ecosystems throughout the world (Onweremdu, 2007), as its limited
availability is often the main constraint for plant growth in highly weathered soils of the
tropics (Bunemannet al., 2004). Consequently, plants and animals cannot grow without a
steady supply of this nutrient (Theodorou and Plaxton, 1993). The efficiency of applied P
fertilizers to soils is low (He et al., 1994), because fast adsorption and/or precipitation
reactions occur due to the existence of clay minerals and to the presence of soil
components such as Ca, Al and Fe (Reddy et al., 1980). However, long-term continuous
application of P in the form of inorganic fertilizer and organic manures results in build-up
of soil P which ultimately increases the likelihood of elevated P in the soil solution and
surface runoff. Excess surface runoff containing P from soil eventually gives rise to
freshwater eutrophication. Therefore, frequent small applications of P have been
proposed as more economical in the long term (Linquistet al., 1996). Soils vary greatly in
the amount of P required to provide an adequate supply of available phosphorous to
plants. Plants also vary in their P requirements for optimal growth (Vander Zaaget al.,
1979).
Most of the phosphorus used by plant is inorganic phosphorus (Pionke and Kunishi,
1992). Different forms of phosphorus account for different portions in different soils, and
can be transformed under certain conditions (Fixen and Grove, 1990). The phosphorus
absorbed by plant directly comes from the soil solution, and there is a dynamic
equilibrium between phosphorus in the soil solution and on the surface of clay particles
(Barrow, 1983). Such equilibrium is governed by P sorption and release from the solid
phase and plant P uptake (Sharma et al., 1995).
Better management of phosphate fertilization can be achieved by studying the P sorption-
desorption behavior of the soil that reflects the partitioning of P between soil solid phase
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and soil solution. Understanding sorption-desorption of P on soil gives insight into the
mechanisms of soil P retention and release. From an agricultural viewpoint, maintaining
soil P concentrations in an optimum range for plant growth is essential to sustain soil
fertility and ensure the production of plants. Environmental concerns associated with P
focus on its stimulation of biological productivity in aquatic ecosystems, being
transported from agricultural fields to surface waters.
Sorption or fixation is the process by which phosphorus binds to the soil, thereby
becoming unavailable for leaching or run off. The adsorption of phosphate is the process
in which phosphate ions in solution react with atoms on the surface of soil particles
(Abedin and Salaque, 1998). This is an important property affecting both the fate of
phosphate fertilizer and the availability of phosphate to plants. When the soil P
equilibrium is disturbed by adding fertilizer, reaction between fertilizer and soil takes
place in two steps: a rapid step leads to adsorption of P and a slow reaction converts P to
a more firmly held form
An effective soil test can help to predict the fertilizer requirement of crops. However,
conventional soil tests provide information only about plant available P
(Barrow, 1978).
(Fixen and
Grove, 1990) and do not estimate the amount of P fertilizers needed unless calibrated for
the particular soil under test (Fox and Kamprath, 1970). Furthermore these tests do not
correctly predict the fertilizer-P requirement for a particular soil - crop system (Rashid
and Hussain, 1998). Consequently, P sorption isotherms, which relate concentration of P
in soil solution with P sorbed by the soil, have been used to correctly predict the P
fertilizer requirement of crops (Shah et al., 2003; Okunolaet al., 2010).
The soil P buffering capacity may be the limiting factor in P uptake (Holford, 1976; Nair
and Mengel, 1984). Phosphorus buffering capacities are derived from adsorption
isotherm plots (Holford 1976, Parfitt, 1978). Such relationships for P in the soil system
can be obtained by fitting data to suitable isotherm equations, such as the Langmuir,
Freundlich equation and the Temkin equation. The adsorption capacities of soils have
been important criteria in soil classification (Rajan, 1973; Breeuwsmaet al., 1986). The
adsorption curves provide an adequate basis for estimation of P requirements across a
diversity of soils and environment (Van Der Zee et al; 1979). Adsorption isotherms have
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an advantage over conventional methods of soil testing, because the isotherms consider
both intensity and capacity factors (Rajan, 1973; Tiarks, 1982). The relationships were
found tobe highly correlated. Use of P sorption isotherms for determining P requirements
can increase theefficiency of P fertilization in crop production.
1.2. Objectives of the research
The objectives of this study are to:
To study P sorption of selected soils.
To obtain sorption isotherms to evaluate the phosphate requirement of the selected soils.
To determine the best isotherm for the data.
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Chapter II Review of Literature
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2. Literature Review
2.1. Phosphorus status of soils of Bangladesh
Phosphorus status of Bangladesh soils covering the major soil types was assessed in
terms of parent materials and physiography. Total P concentration ranged from 172 to
604 mg kg-1 in the topsoil and from 126 to 688 mg kg-1 in the subsoil, and varied with
the physiography to which the soils belonged. In most soils, the available P concentration
was much higher for the topsoil than for the subsoil. The inorganic P concentration was
higher than the organic P concentration, except for one soil series from the Old
Himalayan Piedmont Plain, and was significantly and positively correlated with the total
P concentration. The high content of total phosphorus in these soils may be due to the
presence of phosphorus containing minerals in the soil materials. Islam and Mandal
reported that the contents of total phosphorus in soils of Bangladesh varied with the
variation of organic matter, pH and clay contents. In general, the P status was critically
low in paddy soils of the terrace area. Normal growth in this area is expected to be
difficult without application of P fertilizer. The available phosphorus contents in the soils
varried widely 2 to 15 mg kg-1
Phosphorus can be considered as one of the key elements in soil development because of
its great ecological significance and because practically all the P in natural systems must
be derived from the soil parent material. Soil phosphate can be broadly divided in three
categories: P in soil solution, P in equilibrium with the soil solution and fixed P
.Only 0.5 percent of the total P in the soil occur as
available P.As the soils are alkaline in reaction and contain calcareous materials, the
contents of available phosphorus are expected to be low because of its fixation.
However, the available P ratio in the soils indicated a poor releasing capacity of available
P in soils.Thus application of phosphatic fertilizers is recommended for maintaining the
nutrient balance for sustainable productivity.
2.2. Forms of phosphorus in soil
. Solution
P is immediately available for plant uptake and its concentration depends on the root
uptake and on the sorption and/or desorption reactions occurring between soil solution
and the solid phase. Data on an adequate optimum concentration of phosphorus in the soil
solution are scarce and inconsistent; in fertile arable soils they range from 0.03 to 0.3
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ppm.An adequate concentration of phosphorus in soil solution and its refilling from the
solid phase of soil are necessary for the required production of agricultural crops.The
equilibrium P concentration in the soil solution is also a good parameter for improving P-
soil testing. It agrees with the opinion that only an unconditionally necessary level of
soluble forms of phosphorus should be maintained in agriculturally productive soils to
ensure the necessary yield of agricultural crops. Recently adsorbed P at soil surfaces, can
be readi1y available due to P desorption into the soil solution. With increasing time, P
becomes more strongly retained by the soil matrix and therefore less available to plants
(Barrow and Shaw, 1975).The P fixation in soils depends upon many factors, namely the
pH of the soil, organic matter content, type of clay and sesquioxides. Morel et al.;
(1989) evaluated the phosphate fixing capacity of soils by the isotopic
exchange techniques in north-east France and reported that there was a significant
correlation between amount of phosphorus fixed, pH, exchangeable cations, clay content
and soluble phosphate. Soils around Beemanique near Rose-Belle, were found to have a
maximum P-fixation of 95.7%. Clay content is important when considering P sorption
(Reddy et al., 1980) due to substitution reactions between phosphate anions with OH-
anions or H2O molecules which are fixed to Al and/or Fe hydrous oxides present on the
surface of clay particles, resulting in the formation of a single bond (chemisorption)
which makes P less readily- available. Precipitation reactions between P with Al and/or
Fe have also been proposed as an alternative mechanisms that reduce soil P availability to
plants (Reddy et al., 1980; Iramuremyeet al., 1996), in most cases, chemisorption and
precipitation reactions are not easy to differentiate (Sample et al., 1980). In calcareous,
neutral and slightly acidic soils where CaCO3 is present, it reacts with available P.
Dissolution of Ca takes place forming Ca2+. P precipitates that undergo subsequent
reactions producing less Ca-P soluble forms .
2.3. General Characteristics of Soil Phosphorus
Phosphorus (P) is one of the most abundant elements and is essential for plant growth as
well as an important component in the developmental processes of agricultural crops
(Zhuo et al., 2009a) . Approximately two-thirds of inorganic P and one third of organic P
are not available in soil, especially in soils of variable charges. The rate of P use during
crop growth is very low. Phosphates fixed by Fe, Al, and Ca in soils is a major cause of
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low phyto availability (McBeath et al., 2005), because at least 70-90% of P that enters the
soil is fixed, making it difficult for plants to absorb and use. Therefore, research by
pedologists and plant nutritionists has primarily focused on the issue of increasing the P
use-rate of plants, both at local and international scales. Resolving this issue is
fundamental towards the continued development of agriculture. Organic supplements
have been reported to increase P availability in P-fixing soils (Iyamuremye and Dick,
1996; Agbenin and Igbokwe, 2006) and humic substances enhance the bioavailability of
P fertilizers in acidic soils (Hua et al., 2008). Decomposition products from manure such
as humic acids and citrate were reported to have greater affinity for Al oxides than for
PO4 (Violante and Huang, 1989). Various researchers have investigated the activity of P
in soil from the perspective of low-molecular-weight (LMW) organic acids, the
incorporation of different organic fertilizers, and soil types. The addition of LMW
organic acids activates Al-P and Fe-P in neutral and acidic soils and Ca-P in alkaline soil
(Zhuo et al., 2009b), causing an increase in the levels of readily-available P for use by
plants. The authors found that the P adsorption capacities of soils were dependent on the
type of organic fertilizer applied and the available soil type. Other studies have shown
that when poultry, cattle, and goat manure are applied to highly weathered tropical soil,
the P-sorption efficiency of the soil and P-buffering capacity decreased with an
increasing incubation period (Azeez and Averbeke, 2011). Variations in organic products
from supplements have also been reported to influence P sorption (Hue, 1991).
2.4. Role of Phosphorus in plants
Phosphorus is one of the seventeen essential nutrients required for plant growth
(Ragothama, 1999). Plant dry weight may contain up to 0.5% phosphorus and this
nutrient is involved in an array of process in plants such as in photosynthesis, respiration,
in energy generation, in nucleic acid biosynthesis and as an integral component of several
plant structures such as phospholipids (Vance et al., 2003). Phosphorus plays a critical
role in energy reactions in the plant. Deficits can influence essentially all energy
requiring processes in plant metabolism. Phosphorus stress early in the growing season
can restrict crop growth, which can carry through to reduce final crop yield. Deficiencies
during early growth generally have a greater negative influence on crop productivity than
P restrictions imposed later in growth. Plants respond to P deficiencies by adaptations
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that increase the likelihood of producing some viable seed. The adaptations increase the
ability of the plant to access and accumulate P and include modification of rhizosphere
pH, diversion of resources to root production, increased root proliferation in high-P
regions, and formation of associations with vesicular arbuscularmycorrhizae. Plants differ
in strategies adopted and in efficiency of P absorption. The low availability of
phosphorus is due to the fact that it readily forms insoluble complexes with cation such as
aluminum and iron under acidic soil condition and with calcium and magnesium under
alkaline soil conditions whereas the poor P fertilizer recovery is due to the fact that the P
applied in the form of fertilizers is mainly adsorbed by the soil, and is not available for
plants lacking specific adaptations. Moreover, global P reserves are being depleted at a
higher rate and according to some estimates there will be no soil P reserve by the year
2050 (Vance et al.,2003; Cordell et al., 2011).
2.5. Soil phosphorus status and its availability
Despite its importance for normal plant growth and metabolism, P is one of the least
accessible nutrients. Many soils are inherently poor in available phosphorus content
(Barber, 1995) although the total amount of P in soil may still be high (Vance et
al., 2003). This is evident from the extremely low soil solution P concentration (
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nutrients (Clarkson, 1981); consequently, plants do not deplete the total volume of the
rooted soil layer but only that part of the soil which is in the immediate vicinity of the
roots (Fohse and Jungk, 1983).
Phosphorus is commonly bound to iron and aluminium oxides and hydroxides through
chemical precipitation or physical adsorption (Kochian et al., 2004). As a result of
adsorption, precipitation and conversion to organic forms, only 10-30% of the applied
phosphate mineral fertilizer can be recovered by the crop grown after the fertilization
(Syers et al., 2008). The rest stays in the soil and may be used by crops in the following
years. Because of low P solubility and desorption, only a small proportion of phosphate
ions exist in the soil solution for plant uptake even under optimum P fertilization making
P fertilizer recovery to be lower compared to other nutrient containing fertilizers. This
suggests that chemical fertilizer application alone is not a cost effective way of increasing
crop production in many P-limiting soils (Tilman et al., 2002). Therefore, the use of
genotypes/cultivars with improved root traits able to unlock and absorb P from bound P
resources and/or effectively utilizing the absorbed P is of paramount importance for
enhancing the efficiency of P fertilization.
2.6. Phosphorus dynamics in soil
Soil P exists in various chemical forms including inorganic P (Pi) and organic P (Po).
These P forms differ in their behavior and fate in soils (Turner et al., 2007). Pi usually
accounts for 35% to 70% oftotal P in soil (calculation from Harrison, 1987). PrimaryP
minerals including apatites, strengite, and variscite arevery stable, and the release of
available P from theseminerals by weathering is generally too slow to meet thecrop
demand though direct application of phosphaterocks (i.e. apatites) has proved relatively
efficient for cropgrowth in acidic soils. In contrast, secondary P mineralsincluding
calcium (Ca), iron (Fe), and aluminum (Al)phosphates vary in their dissolution rates,
depending onsize of mineral particles and soil pH (Pierzynskiet al.,2005; Oelkers and
Valsami-Jones, 2008). With increasingsoil pH, solubility of Fe and Al phosphates
increases but solubility of Ca phosphate decreases, except forpH values above 8
(Hinsinger, 2001). The P adsorbedon various clays and Al/Fe oxides can be released
bydesorption reactions. All these P forms exist in complexequilibria with each other,
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representing from very stable, sparingly available, to plant-available P pools such aslabile
P and solution P .In acidic soils, P can be dominantly adsorbed by Al/Feoxides and
hydroxides, such as gibbsite, hematite, andgoethite (Parfitt, 1989). P can be first adsorbed
on thesurface of clay minerals and Fe/Al oxides by formingvarious complexes. The
nonprotonated and protonatedbidentate surface complexes may coexist at pH 4 to 9,while
protonated bidentateinnersphere complex is pre-dominant under acidic soil conditions
(Arai and Sparks, 2007). Clay minerals and Fe/Aloxides have large specific surface areas,
which providelarge number of adsorption sites. The adsorption of soilP can be enhanced
with increasing ionic strength. Withfurther reactions, P may be occluded in nanopores
thatfrequently occur in Fe/Al oxides, and thereby becomeunavailable to plants (Arai and
Sparks, 2007).In neutral-to-calcareous soils, P retention is domi-nated by precipitation
reactions (Lindsay et al., 1989),although P can also be adsorbed on the surface of
Cacarbonate (Larsen, 1967) and clay minerals (Devauet al., 2010). Phosphate can
precipitate with Ca, gen-eratingdicalcium phosphate (DCP) that is available toplants.
Ultimately, DCP can be transformed into morestable forms such as octocalcium
phosphate and hydroxyapatite (HAP), which are less available to plantsat alkaline pH
(Arai and Sparks, 2007). HAP dissolution increases with decreaseof soil pH (Wang and
Nancollas, 2008), suggesting thatrhizosphere acidification may be an efficient strategy to
mobilize soil P from calcareous soil. Po generally accounts for 30% to 65% of the total P
in soils (Harrison, 1987). Soil Po mainly exists in stabilized forms as inositol phosphates
and phosphonates, and active forms as orthophosphate diesters, labile ortho-phosphate
monoesters, and organic polyphosphates (Condronet al., 2005). The Po can be released
through mineralization processes mediated by soil organisms and plant roots in
association with phosphatase secretion. These processes are highly influenced by soil
moisture, temperature, surface physical-chemical properties, and soil pH and Eh (for
redox potential). Po transformation has a great influence on the overall bioavailability of
P in soil (Turner et al., 2007). Therefore, the availability of soil P is extremely complex
and needs to be systemically evaluated because it is highly associated with P dynamics
and transformation among various P pools .
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2.7. Requirement of Phosphorus in plant
P is an important plant macronutrient, making up about 0.2% of a plant's dry weight. It is
a component of key molecules such as nucleic acids, phospholipids, and ATP, and,
consequently, plants cannot grow without a reliable supply of this nutrient. Pi is also
involved in controlling key enzyme reactions and in the regulation of metabolic pathways
(Theodorou and Plaxton, 1993). After N, P is the second most frequently limiting
macronutrient for plant growth. The uptake of P poses a problem for plants, since the
concentration of this mineral in the soil solution is low but plant requirements are high.
The form of P most readily accessed by plants is Pi, the concentration of which rarely
exceeds 10 m in soil solutions (Bieleski, 1973). The form in which Pi exists in solution
changes according to pH. The pKs for the dissociation of H3PO4 into H2PO4 and then
into HPO4 2 are 2.1 and 7.2, respectively. Therefore, below pH 6.0, most Pi will be
present as the monovalent H2PO4 species, whereas H3PO4 and HPO4 2 will be present
only in minor proportions. Most studies on the pH dependence of Pi uptake in higher
plants have found that uptake rates are highest between pH 5.0 and 6.0, where
H2PO4dominates (Furihataet al., 1992), which suggests that Pi is taken up as the
monovalent form.
For efficient use of phosphorus fertilizer requirements should be assessed as accurately as
possible. Recommendations for phosphate fertilization must take into account of the
specific plant requirement,the available level of phosphate in the soil and soil P
adsorption characteristics. Bache and Williams (1971) have reported that phosphate
requirements of a soil can be estimated using quantity: intensity plots. Fox (1981/1982)
advocated that phosphate adsorption curves are the only method for precise fertilizer
recommendations, and especially if the properties of the soil are unknown.
2.8. Soil factors affecting P uptake
According to Kamprath and Watson (1980) the factors affecting the supply of P to plants
are the amount of soil P (quantity), the concentration of soil solution P (intensity), and
movement of P to roots (diffusion). In any assessment of the available P in soil by
chemical tests, one needs to consider the relationship between quantity, intensity and
diffusion and factors influencing availability of these components of P to plants.
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2.8.1. Phosphate Buffering Capacity
The P equilibrium between solid phase and solution phase is characterized by phosphate
buffering capacity of a soil. This is the ability of a soil to maintain its P concentration in
solution as P immobilized by soil through various processes or as P is added by
fertilization. Thus the availability of soil phosphate is described by both intensive and
extensive parameters which are determined by the concentration of phosphorus in soil
solution and quantity of phosphate adsorbed on the soil solids respectively. These two
parameters determine the buffering capacity of soil (Barrow, 1967; Holford, 1976). The
buffering capacity of a soil is the slope of the adsorption isotherm at some arbitrary
concentration, which may reflect the actual P concentration found in the solution (Beckett
and White, 1964). It is an indication of the ability of the soil to replace a unit change in
soil solution P and maintain a productive solution concentration. The soil P buffering
capacity may be the limiting factor in P uptake
Soils with high phosphate adsorption maxima have higher phosphate buffering capacity
than those with low phosphate adsorption maxima
(Holford, 1976; Nair and Mengel, 1984).
(Rajan, 1973). According to Kamprath
and Watson (1980) the buffering capacity of acid and neutral soils is a function of the
amounts and crystallinity of hydrated oxides of Fe and Al, whereas in calcareous soils the
amounts of exchangeable Ca and CaCO3
Diffusion of phosphorus through the soil to the roots is the dominant mechanism
governing the P-uptake by roots growing in all soils, except those extremely high in
phosphorus.
determine the P buffering capacity.
2.8.2. Phosphate Diffusion in Soil
Barrow (1989) suggested that phosphate mostly moves to plant roots by
diffusion and it is only the phosphate in the soil solution that is free to move.
The concentration gradient in soil across the root surfaces is an important factor
influencing P diffusion (Kamprath and Watson, 1980).Soil texture is another factor
affecting diffusion of P (Olsen and Watanabe 1963).As the clay content increases, the
diffusion coefficients increase due to a decrease in tortuosity and an increase in buffering
capacity. The diffusion of phosphate persists until the equilibrium is established. Since
the diffusion of phosphorus occurs essentially in the liquid phase and an individual
phosphate ion spends a relatively short time in this phase, the diffusion coefficient of
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phosphorus in the soil solution will be different from that in free solution. Diffusion
coefficient of phosphate through soil is in the range of 10-8 to 10-11 cm2s-1. Fitter
(1992) stated that a phosphate ion normally moves less than a millimeter through the soil
in a day. The diffusion coefficient for phosphate ion in water is 0.89 X I0-5cm2s-1.
Phosphorus diffusion through soil is slower than in pure water for three reasons, (i) soil
water occupies only part of the soil so the cross-sectional area for diffusion is less; (ii) the
diffusion path is tortuous because the water is present as films around soil particle; and
(iii) most of the diffusible phosphorus is adsorbed on soil surfaces which equilibrate with
and buffers the small amount of phosphorus in soil solution. All the factors that govern
the rate phosphorus diffusion to the root and the extent of root growth are important in
determining the availability of phosphorus to growing plants in a soil.
2.9. Phosphorus sorption and desorption
Establishment of phosphate (P) retention and release capacity of soils is essential for
effective nutrient management and environmental protection. The main source of plant
available P is generally termed the labile pool. This provides fairly rapid exchange with
soil solution, maintaining the solution concentration. The remaining fraction is the non-
labile pool. This contains a large quantity of insoluble phosphate, which is very slowly
released into the labile pool. Various organic and inorganic phosphates constitute these
labile and non labile pools. In general the labile pool can be considered as orthophosphate
adsorbed onto surfaces of clay minerals, hydrous oxides and carbonates plus iron and
aluminium phosphates. The relationship between the quantity of phosphorus in the labile
pool and the soil solution concentration depends particularly on soil texture and pH
(Archer, 1988).
When soluble P compounds are added to the soil, they react rapidly with various soil
components and are quickly converted to slowly available forms thus creating one of the
main problems relevant to the maintenance and improvement of soil fertility. Phosphorus
fixation is a serious problem in alkaline and calcareous soils (Sharif et al., 2000). The soil
can rapidly and firmly adsorb large amounts of P from solution and once adsorbed, they
are difficult to release (Huang, 1998). In calcareous soils, the dynamics of P is controlled
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by many soil properties that strongly retain P and consequently maintain low P
concentration in soil solution (Bertrand et al., 1999).
In recent years plant available phosphate is dependent on the factors - the concentration
of phosphate in the soil solution, amount of exchangeable phosphate and the relative rate
of adsorption from the soil.
The first two factors are static and can be related to each other by adsorption isotherms.
The exchangeable P can be measured by using a suitable technique. Desorption by
Olsen's extraction method (0.5 M NaHC03 solution) has been well correlated with plant
yield in a wide range of soils (Rashid and Rowell, 1988).
The immediate source of phosphate-P to plant roots is the soil solution. Phosphate
deficiency in soil usually occurs from too low concentration of orthophosphate in the soil
solution rather than from an inadequate total P content. The concentration of P in the
solution is governed by a dynamic equilibrium between solid and solution phases where
phosphate is continually released from and re-adsorbed by the solid phase. Any change in
the P concentration of soil solution will initiate physico-chemical processes to re-
establish the equilibrium.
Phosphate adsorption is a process in which phosphate ions in solution react with atoms on
the surface of soil. When soluble phosphate compounds are added to the soil they
undergo a series of complex reactions. These compounds react rapidly with soil minerals,
by precipitation reactions and adsorption onto surfaces, and the availability of this added
P declines. A simple cation exchange may be essentially completed within minutes,
whereas the adsorption of orthophosphate can continue increasing for two days even after
these proceed at a very slow rate for some months (Wild, 1988).
The equilibrium between solid phase and solution phase P is usually expressed by the
buffering capacity of a soil in the shape of adsorption isotherm, which is a line showing
the relationship between quantity of P adsorbed by the soil and the changing
concentration of P in the surrounding solution. According to Holford (1989) the buffering
capacity or sorptivity of a soil is controlled by the two fundamental soil properties: one is
the extent or number of P-reactive sites, and the other is the affinity of these sites for P.
These processes may be on the soil colloidal surfaces (adsorption) or in the solution
-
(precipitation). Numerical estimates of these properties can be obtained by fitting a
suitable equation such as Langmuir or Freundlich equation to the adsorption isotherm.
Some parameters of adsorption can be obtained from these equations to compare the
behaviour of P in soil.
The phenomenon of phosphorus sorption and desorption in soils is widespread because of
its agricultural importance, and has been the subject of considerable study and
controversy (Barrow, 1978). Several factors determine the flux of P to plant roots. These
were stated as mainly intensity, quantity, capacity and mobility factors. Phosphate
concentration in soil solution represents the intensity factor and it has been shown that
plants vary in this requirement. The quantity factor is the amount of table P in soils. The
capacity factor is described as the quantity of P retained at specific conditions of
intensity. Predicting requirements for P fertilizers using P sorption isotherms takes into
account both intensity and capacity factors, and may be considered a valid concept
relevant in the P supply mechanism and fertilizer management.
Phosphorus sorption capacity is an important soil characteristics that affects the rates and
plant response to Phosphorus fertilizer application (Fox and kamprath, 1970). Several
factors apart from the nature of the soils used may affect adsorption of phosphate. Barrow
(1978) enumerated these as: the period and temperature of contact between soil and
phosphate solution, the method of shaking, the P solution: soil ratio; the identity and
concentration of the supporting electrolyte used; the moisture content of the soil prior to
treatment, and previous addition of phosphate or other specifically adsorbed anions.
2.10. Adsorption isotherm
The process of adsorption is usually studied through graphs know as adsorption
isotherm.An adsorption isotherm is an invaluable curve describing the phenomenon
governing the retention (or release) or mobility of a substance from the aqueous porous
media or aquatic environments to a solid-phase at a constant temperature and Ph.
Adsorption equilibrium is established when an adsorbate containing phase has been
contacted with the adsorbent for sufficient time, with its adsorbate concentration in the
bulk solution is in a dynamic balance with the interface concentration. In order to
optimize the design of an adsorption system for dye removal from solutions, it is
-
important to find the most appropriate correlation for the equilibrium curve (Crini and
Badot, 2008).
The adsorption capacity of soils have been important criteria in soil classification
(Breeuwsmaet al., 1986). The adsorption curves provide an adequate basis for estimation
of P requirements across a diversity of soils and environment (Van Der Zee et al.,
1979).Muljadiet al., (1966) and Olsen and Khasawneh (1980) revealed that the isotherm
from a plot of phosphate retained against different equilibrium concentrations could be
divided into three regions corresponding to three distinct stages in soil phosphate
interaction: (a) The first region corresponding to low 27 phosphate addition resulting in
practically complete adsorption or a negligible fraction of the added phosphate remaining
in the equilibrium solution. The adsorption isotherm rises steeply and remains close to the
Y-axis; (b) the second region is the strongly curved portion of the isotherm which is
convex to the Y-axis. Bache (1964) showed that adsorption in this region varies
logarithmically with the equilibrium phosphate concentration; and (c) The third portion
of the isotherm approaches linearity and occurs at medium to high phosphate
concentrations. Here the adsorption varies linearly with the amount of P in equilibrium in
solution. At high level of this region, the slope of line is small and the isotherm, for most
soils, tends to become more or less parallel to the X-axis.
2.11. Types of adsorption isotherm
The reaction between phosphate and soils in particular has been described
mathematically by several adsorption isotherm equations i.e. Langmuir equation (Bolster
and Hornberger, 2007; Jiao et al., 2008), Freundlich equation (Zhang and Selim, 2007;
Jiao et al., 2008), Temkin equation (Anghinoniet al., 1996 and Ahmed et al., 2008) and
Elovich equation (Olsen and Watanabe, 1957; Fox and Kamprath, 1970). Among these
equations the Langmuir,Freundlich and Temkin equations are the most frequently used to
describe the relationship between equilibrium P added and P sorbed by the soils.
2.11.1. Langmuir Isotherm
Langmuir equation, proposed by Langmuir in 1916 for adsorption of gases on clean solid
surfaces, was first used by Olsen and Watanabe (1957) to describe phosphate adsorption
-
in soils. It is based on the assumption that the energy of adsorption is independent of the
surface coverage. In its linear form, the Langmuir equation can be written as:
C/X = 1/Kb + C/b
Where C = equilibrium concentration of phosphate in solution (g P/ml),
X= mass of phosphate adsorbed (g)/ mass of soil (g)
K= adsorption maximum (mg P/g soil), b is related to the binding energy of soil.
A plot of C/X against C should give a straight line, from which the adsorption maximum
K, is the inverse of the slope and the constant b, (b = slope/intercept) related to energy of
adsorption or binding energy can be readily calculated.
The use of Langmuir equation appears satisfactory because its derivation is acceptable on
theoretical grounds and it contains parameters, which have physico-chemical significance
(Olsen and Watanabe, 1957; Holfordet al, 1974) representing the extensive (adsorption
capacity) and intensive (affinity) properties of the adsorbent for the adsorbate(Holford,
1982). However, deviations from the expected linearity have been reported at high
phosphate additions (Olsen and Watanabe, 1957).
Langmuir isotherm can often be used to give a measure of the energy by which
phosphorus is bonded to the solid and an adsorption maximum. Based on this maximum,
calculation of the degree of phosphate saturation can be made, which has been shown to
be related to plant uptake of soil phosphorus (Holford and Mattingly, 1976 b).
2.11.2. Frendluich Isotherm
Freundlich equation was the first model to be used in describing phosphate retention
(Russell and Prescott, 1916). Barrow (1978), advocated that the adsorption data from
dilute solution could be fitted to Freundlich equation in the following form:
X= KC1/n
Where K and n are empirical parameters, a is the sorption energy and n the sorption
constant.
C is the equilibrium concentration of adsorbate in mg/L and
-
X = mass of adsorbed P (g)/mass of soil (g)
The equation was originally empirical, without any theoretical physico-chemical
foundation, and no significance can be attached to the coefficients (Olsen and Watanabe,
1957). It implies that energy of adsorption decreases exponentially as the fraction of
covered surface increases (amount of adsorption). Freundlich equation can be derived
theoretically by assuming that the decrease in energy of the adsorption with the
increasing surface coverage is due to surface heterogeneity. Freundlich equation is
normally used in its logarithmic form;
log X = 1/n log C + log a.
A plot of log X against log C should give a straight line. Though it is the oldest
adsorption equation in the literature on phosphate adsorption, it has been shown to give
better fits to adsorption data than the Langmuir isotherm (Fitter and Sutton, 1975)
especially in many soils over limited concentration ranges
Freundlich relationship has been used only for the theoretical treatment of phosphate
adsorption. It has not been possible to compare quantitatively adsorption data for soils
obtained from plot of Freundlich equation because the equation was assumed to be
empirical
(Barrow and Shaw, 1975).
(Arshedet al., 2000). However, some workers suggested that the intercept and
slope of a linear Freundlich plot could be used to compare phosphate adsorption in soils
(Kuo and Lotse, 1974; Holford, 1982). Freundlich equation has also the limitation that it
does not predict a maximum adsorption capacity. Despite its limitation the equation was a
better fit to phosphate adsorption isotherms in most of the soils than the most widely used
Langmuir equation (Fitter and Sutton, 1975) and as good fit as the more complex two -
surface Langmuir equation (Barrow, 1978; Sibbesen, 1981).
2.11.3. Temkin Adsorption Equation
X/m= a + B lnC
Where X/m =, mass of adsorbed P (g)/mass of soil (g)
C is the equilibrium P concentration (ug/ml),
and a and B are parameters.
-
A plot of X/m against ln C gives a straight line if the adsorption process fits the model.
The values of a and B are obtained from the intercept (a) and the slope (B), respectively.
The B value of Temkin equation is considered as the P-buffering capacity (retention
capacity of adsorbed P) of soil (ug/g), (Anghinoniet al, 1996).
2.12. Factors affecting P sorption from soils
The soil characteristics that influence P fixation include the amount and type of clay-
fraction minerals, soil pH, soil organic matter content, time of reaction, exchangeable
Al3+, soil redox condition (Sanchez and Uehara, 1980), and root exudates.
2.12.1. Soil pH
Soil pH has profound effect on the amount and manner in which soluble phosphates
become adsorbed. When soil is acidic, the dominant P ion species present is H2PO4- and
when soil becomes alkaline (Higher pH), the dominant ion becomes PO43-(Gillian and
Sample, 1968). Adsorption of phosphorus by iron and aluminium oxides also declines
with increasing pH (White, 1980). Gibbsite (Al(OH)3 adsorbs greatest amount of
phosphate between pH 4 and 5. Phosphorus adsorption by goethite (an-FeOOH)
decreases steadily between pH 3 and 12
)
(Huang, 1975). Phosphate availability in most
soils is at a maximum in the pH range of 6.0 to 6.5 (Tisdale et al., 1985). At lower pH
values the retention results from the reaction with iron and aluminium and their hydrous
oxides. Above pH 7.0 the ions of calcium, and magnesium and their carbonates cause
precipitation of added phosphorus, which decreases its-availability.
2.12.2. Effect of Soil Carbonate
Lajtha and Bloomer (1988) regarded calcium carbonate as the primary geochemical agent
capable of retaining P in the soils of a desert ecosystem. The presence of calcium or
magnesium ions must accompany high pH values. At pH values above 7.5 the ions of
calcium and magnesium as well as their carbonates cause precipitation of the added P,
and their availability decreases. If the increase of these ions (calcium and magnesium)
continue, there will be a decrease in solubility of soil phosphorus. However, liming acid
soils increase the solubility of phosphorus. In calcareous soils calcium bound phosphate
Ca-P is the most important and dominant P fraction (Kuo and Lotse, 1972). The reaction
-
of added phosphate with calcareous soils as CaCO3 involves initial adsorption of small
amounts of phosphate followed by precipitation of high levels of Ca-P.
2.12.3. Effect of Clay Mineralogy
Several workers have reported a significant correlation between clay content and P
sorption parameters (Chauderyet al.,2003). The clay content of a soil has great impact on
phosphate adsorption. Soils containing large quantities of clay will adsorb more
phosphate than those with less clay content.Many studies have shown that there are close
relationships between clay content and P sorption by calcareous soils of Mediterranean
regions .
2.12.4. Effect of organic matter
Several authors noted correlation between organic C and the amount of P adsorbed by
soils (Woodruff and Kamprath, 1965). According to Tisdale et al, (1985) the availability
of phosphorus increased from decomposition of organic residues has been due to: (a) the
formation of phosphohumic complexes which are more easily assimilated by plants, (b)
anion replacement of the phosphate by the humate ions, and (c) the coating of
sesquioxide particles by humus to from a protective cover and thus reduces the phosphate
retention capacity of the soil. It was suggested that certain organic anions form stable
complexes with iron and aluminium, thus preventing their reaction with phosphorus by
blocking the adsorption sites (Leaver and Russell, 1957). It was further stated that these
complex ions release phosphate previously retained by the same mechanism. Harter
(1969) suggested that it is OH groups in organic matter which affects phosphate
adsorption through anion exchange, while the results of Appeltet al, (1975) showed that it
is the Al and to lesser extent the adsorbed by the organic colloids which are active in P
adsorption. Organic matter does lower the adsorption of P. it also provides a method of
increasing the P availability without the use of fertilizers. The evolution of carbon
dioxide after the decomposition of organic residues has a favorable effect on phosphate
availability. The gas is dissolved in water to form carbonic acid which is capable of
decomposing certain primary soil minerals. On the basis of available evidence, it is clear
that the addition of organic materials to mineral soils may increase the availability of soil
phosphate.
-
Soils with a high content of organic matter (colloidal) had very low capacities to adsorb P
(Fox and Kamprath, 1971).Soluble P fertilizer was readily leached from such soils; the
addition of large amounts of exchangeable Al almost completely.
2.12.5. Ionic Strength of Soil Solution
Both organic and inorganic anions compete with phosphate for adsorption sites to varying
extent. In some cases it may result in a decrease in the adsorption of added phosphate or
desorption of retained phosphate. Weakly held inorganic anions such as nitrate and
chloride are of little significance, whereas specifically adsorbed anions like hydroxyl,
sulphate and molybdate are competitive.
The strength of bonding of the anions with the adsorption surface determines the
competitive ability of that anion. For example, sulphate, even though considered to be
specifically adsorbed anion, is unable to desorb much phosphate (Zhang et al., 1987).
Species and concentration of cations in the soil solution also influence the adsorption of P
by soils.
Divalent cations enhance P sorption more than monovalent cations(White, 1981). For
example clays saturated with Ca2+ have the capacity to retain greater amounts of P than
those saturated with Na+ or other monovalent cations. The explanation for this effect of
Ca2+ involves the making of positive charges edge sites of crystalline clay minerals more
accessible to P anions for sorption. On the other hand, both organic and inorganic anions
compete to varying degrees for P sorption sites, resulting in some cases, a decrease in the
sorption of added P (Moshi et al., 1974).
2.12.6. Soil Mineral Type
Numerous studies show that aluminosilicate clay minerals play an important role in P
sorption by soils. The surface charge of clay minerals (and oxides) is partly pH dependent
so that anion exchange capacity increases as pH decreases. Soils with significant contents
of iron and aluminium oxides have large phosphorus fixation capacities because of their
high surface areas. The higher the Al and Fe oxide contents of soil clay and the less
crystalline (more amorphous) the soil minerals, the greater an acid soils P fixation
capacity. This is largely attributed to the greater surface area which these conditions
-
represent (Quintero et al., 1999). Among the layer silicate clays, 1:1 type clays have a
greater phosphate retention capacity than 2:1 type clays. Soils containing large amounts
of kaolinite group clay minerals will retain larger quantities of added phosphate than
those containing the 2:1 type clay minerals.
2.12.7. Effect of temperature
Temperature affects most physical processes and the speed of chemical reactions
generally increases with a rise in temperature. If temperature at which phosphate reacts
with soil is increased, the rate of reaction is considerably increased (Barrow, 1989). It
also has an important theoretical application. As the temperature increases, the kinetic
energy of the molecules increase, which enables them to jump over the energy barrier
into a new reaction state High temperatures are expected to slightly increase the molar
solubility of compounds such as apatite, hydroxyapatite, octacalcium phosphate, variscite
and strengite. Increase in temperature also stimulates biological activity which enables
phosphate to be released from organic residues. Wild, in 1950 estimated that an increase
in temperature from 298 K to 308 K increased P adsorption in soils. The soils of the
warm regions of the world generally adsorb more phosphates than the soils of temperate
regions. These warmer climates also give rise to soils with higher contents of the hydrous
oxides of iron and aluminum. Many workers agreed that phosphorus retention increases
at higher temperatures (Muljadiet al., 1966; Kuo and Lotse, 1974).
2.12.8. Plant Root Geometry
Phosphate uptake is more dependent on plant root activity than is the case for other major
nutrients. Plant root geometry and morphology are important for maximizing P uptake,
because root systems that have higher ratios of surface area to volume will more
effectively explore a larger volume of soil (Lynch, 1995). For this reason mycorrhizae are
also important for plant P acquisition, since fungal hyphae greatly increase the volume of
soil that plant roots explore.In certain plant species, root clusters (proteoid roots) are
formed in response to P limitations. These specialized roots exude high amounts of
organic acids (up to 23% of net photosynthesis), which acidify the soil and chelate metal
ions around the roots, resulting in the mobilization of P and some micronutrients
(Marschner, 1995).
-
Chapter III Materials and
Method
-
3. Materials and Methods
The study was conducted in laboratory with two soil series of Ganges Floodplain in
Bangladesh to evaluate the sorption behavior of Phosphate.
3.1.Sample Collection
Soil samples were collected from the top soil (0-15 cm) by using composite soil sampling
method as suggested by the Soil Survey Staff of the USDA (USDA, 1951) from different
location as follow-
3.1.1. Bajoa Series
Bajoa soils are developed in tidal floodplain basins. They are seasonally shallowly to
moderately deeply flooded, poorly drained soils developed in tidal deposits. It contains a
grey to olive grey silty clay loam subsoil with moderate to strong blocky structure in the
B horizon. They have five phases: Highland; medium high land, non-saline; medium
lowland and medium lowland, flood hazard.
3.1.2. Gopalpur Series
Gopalpur series includes poorly drained, seasonally flooded soils developed in
moderately fine textured Gangetic Alluvium. They are developed on the summits to
upper slopes of gently undulating ridges. Gopalpur soils consist of olive-brown, friable,
calcareous clay loams and silty clay loams usually overlying a moderately fine textured
substratum. They show a weak coarse blocky structure in the sub soil with patchy
coatings on ped faces. They are calcareous and moderately alkaline in reaction
throughout the profile. They have five phases: Highland, smooth relief; highland,
irregular relief; medium highland, irregular relief and medium lowland, flood hazard.
-
Table 3.1 General information of studied soil
Characteristics
Soil Series
Bajoa Gopalpur
Location
Village: Milemara
Union: Batiaghata
Upazila: Batiaghata
District: Khulna
Village: Dokkhinpara,
Chaderdanga
Union: chaderdanga
Upazila: Batiaghata
District: Khulna
Latitude &
longitude
N: 22043.219
E: 089028.256
N: 22040.727
E: 089027.151
Land type Medium high land High land
Land use Seasame-Boro-Falow Seasame-Boro-Falow
Effervicence Slightly Calcarious Calcarious
3.2. Processing of soil sample
The collected soil samples were dried in air spreading on separate sheet of papers. After
drying in air, the larger aggregates were broken gently by hammering with a wooden
hammer. Then the crushed soils was passed through a 2.0 mm sieve. The sieved soils
were then preserved in plastic container and labeled properly. These were later used for
various physical and chemical analyses.
3.3.Physical analysis of soil
The particle size analysis of the soils was carried out by hydrometer method as described
by Gee and Bauder (1986). Textural classes were determined using Marshalls Triangular
Coordinator system.
3.4. Chemical analysis of soil
3.4.1. Soil pH
Soil pH was determined electrochemically using a 1:2.5 (w/v) soil: distilled water ratio
using a pH meter.
-
3.4.2. Electrical Conductivity (EC)
The electrical conductivity of the soil was measured at a soil: water ratio of 1:5 by the
help of EC meter.
3.4.3. Determination of organic carbon
Organic carbon of samples was determined by Tyrins method as outlined by Jackson
(1962).
3.4.4. Cation Exchange Capacity (CEC)
The CEC of the soils was determined by extracting the soil with neutral 1 N NH4OAc at
pH 7, followed by the replacing the ammonium in the exchange complex by 1 N KCl at
pH 7 (Black, 1965). The displaced ammonium was determined by colorimetric method at
685 nm wavelength as suggested by Baethgen and Alley (1989).
3.4.5. Available Phosphorus (P)
Available Phosphorus was extracted from the soil with0.5 M NaHCO3 (Olsens Method)
at pH 8.5 and Molybdophosphoric blue colour of analysis was employed for
determination (Jackson, 1967) and P was determined by ascorbic acid blue colour method
(Murphy and Riley, 1962).
3.5. Phosphate sorption experiment
For determining phosphate adsorption characteristics of the soils, phosphorus sorption
was estimated according to the procedure outlined by (Nair et al,.1984).One gram of air-
dried sieved soil was taken into a 50 mL centrifuge tube. Seven initial P concentrations,
namely 0, 1, 2, 5, 10, 25 and 50 g P mL-1 in 0.01 M CaCl2 solution were added
separately to each centrifuge tube using a soil/solution ratio of 1:20 (w/v). The resultant P
contents were 0, 20, 40, 100, 200, 500 and 1000 g P g-1 soil. The centrifuge tubes were
then shaken and equilibrated for 16 h. The mixtures were centrifuged and the
supernatants were analyzed for phosphate following the ascorbic acid blue color method
(Murphy and Riley 1962). Sorbed P was inferred from the difference between the
concentration of soluble P added in the initial solution and the concentration of P in the
solution at equilibrium. Each treatment was replicated three times.
-
3.6. Fitting of phosphorus adsorption data in three equations
The sorption values of each soil were fitted according to the Langmuir, Freundlich and
Temkin equations.
Linear form of the Langmuir (1918) equation is:
CX-1 = (KL bL)-1 + CbL
-1
where, X is the amount of P sorbed (mg kg-1), C is the equilibrium P concentration (mg
L-1) in solution, bL is the adsorption maximum (mg P kg-1), KL is the bonding energy
constant (L mg-1 P). A plot of CX-1 (y-axis variable) against C (x-axis variable) will yield
a straight line with a slope of 1/bL and a y-intercept of 1/KLbL. The Maximum Buffer
Capacity (MBC) of the soil, which is the increase in sorbed P per unit increase in final
solution P concentration, was estimated from the product of Langmuir constants KL and
bL (Holford, 1979).
Freundlich (1926) equation is:
X = KfC
Logarithmic form of the
N
Freundlich (1926) equation is:
log X = log Kf
+ N log C
where, X is the amount of P sorbed (mg kg-1), C is the equilibrium P concentration (mg
L-1) in solution, Kf is the proportionality constant (mg kg-1), N is the empirical constant.
A plot of log X (y-axis variable) against log C (x-axis variable) will yield a straight line
with slope N and a y-intercept log Kf
Temkin equation (
.
Temkin and Pyzhev, 1940) is:
X = a log C + b
-
where, X is the amount of P sorbed (mg kg-1), C is the equilibrium P concentration (mg
L-1
) in solution, a and b are constants. A plot of X (y-axis variable) against log C (x-axis
variable) will yield a straight line with slope b and y-intercept.
-
Chapter IV Results and Discussion
-
4. Results and discussion
The investigation was conducted to study the phosphate sorption of two soils of Ganges
Floodplain. The results of the study and discussion are presented here.
4.1. Physical Characteristics
4.1.1. Particle size analysis
The sand percentage of Gopalpur and Bajoa series were 8% and 12% respectively. Sand
percentage of Bajoa were greater than Gopalpur. The silt percentage of Gopalpur and
Bajoa were 58% and 57% respectively. The silt percentage of Bajoa were greater than
Gopalpur series. The clay percentage of Gopalpur and Bajoa were 34% and 31%
respectively. The clay percentage of Bajoa were greater than Gopalpur.
4.1.2. Texture
The texture of Gopalpur and Bajoa were both silty clay loam. SRDI staff (1965-86) found
that on the Ganges Tidal Floodplain, soils of river banks and extensive basins are heavy
clay to silty clays.
4.2. Chemical Characteristics
4.2.1. Soil reaction or (pH)
The pH of the Bajoa and Gopalpur soil series were 7.9 and 6.9 respectively. It indicates
that the soils are neutral to moderately alkaline soils. This is a common feature of the
Ganges Tidal Floodplain soils of Bangladesh (Brammer, 1971). This may be due to the
presence of low exchangeable bases in the surface soil. The almost equal range of pH in
all the soil series is probably due to the existence of strong buffering systems
(McConchie, 1990).
4.2.2. Electrical conductivity (EC)
The EC value of the Gopalpur and Bajoa soil series were 1.8 and 5.52 dSm-1. These
results indicate that the soils of the study area were nonsaline to moderately saline.
-
4.2.3. Organic Carbon
The organic carbon percentage of the Gopalpur and Bajoa soil series were 0.464% and
0.696% indicating the lower amount of organic matter (BARC, 2005).
4.2.4. Available P
Phosphorus content of the Gopalpur and Bajoa soil series were 15 ppm and 9 ppm, which
indicates low level of Phosphorus (BARC, 2005).Bhuiyan (1988) observed that the
available phosphorus of different soil series of Bangladesh ranged from 2.2 to 14 ppm.
Bajoa was below the critical level of available P.
4.3. Phosphate sorption behavior
Soils were equilibrated with 0.01 M calcium chloride solution containing graded
concentrations (0 to 50 g mL-1) of phosphorus. The resulting change in the sorbed
phosphate was then calculated from analysis of the equilibrium solution. Phosphate was
sorbed at all other rates in different amounts and proportions by the soils included for the
present study (except 0 g P mL-1 application) (Fig. 4.1). At 0 g P mL-1, there were
some desorption in all the soil series. Desorption of P at control was also reported by
several other scientists (Vaananenet al., 2008; Afsaret al., 2012). In the present study,
phosphate sorption increased gradually with increasing phosphate application in all the
soil series. Increase in P sorption with increasing phosphate in equilibrium solution was
also reported by other scientists (Afsaret al., 2012). At phosphate application rates, the
Bajoa soil series, occupying non salinity, sorbed the greater amount of phosphate. On the
other hand, the Sara soil series of moderately salinity sorbed the lower amount of
phosphate. P sorption was higher in Bajoa soil series than Gopalpur.
-
Fig. 4.1. Phosphate sorption capacity of soils with different rates of phosphate application
4.4. Freundlich adsorption isotherm
The R2
Fig. 4.2.Freundlich adsorption isotherm for phosphorus in Gopalpur soil series
value of the Freundlich equation of Gopalpur and Bajoa soil series were 0.8008
and 0.7062 where greater value 0.8008 was observed in Gopalpur soil than Bajoa soil
(Fig. 4.2 and 4.3). The intercept of Gopalpur and Bajoa soil series were 0.8998 and
0.8234. The slopes of the sorption curves show that the amount of P sorbed by the soils
differed between the soil series. The greater slope of 1.11 was observed in Gopalpor soil
than 0.8234 in Bajoa soil series.
0
50
100
150
200
250
300
350
400
0 200 400 600 800 1000 1200
Ads
orbe
d P
(g
P p
er g
soil)
Applied P (g P per gram soil)
gopal SORBED g P per g soil
bajoa SORBED g P per g soil
y = 1.11x + 0.899R = 0.800
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
0 0.5 1 1.5 2 2.5 3
Log
X
Log C
-
Fig. 4.3.Freundlich adsorption isotherm for phosphorus in Bajoa soil series
4.5. Langmuir adsorption isotherm
The Langmuir adsorption isotherm explains the adsorption maxima and energy of
adsorption. The Langmuir equation was found to be a favorable method to explain the P
adsorption in most soils. The R2 value of the Langmuir equation of Gopalpur and Bajoa
soil series were 0.9834 and 0.9988 respectively (Fig. 4.4 and 4.5). The greater value of R2
was observed in Bajoa Soil and the lowest in Gopalpur soil series. The intercept of
Langmuir equation ofGopalpur and Bajoa soil series were 0.4698 and 0.0034. The greater
value of intercept was observed in Gopalpur Soil than Bajoa soil series. The slope of
Langmuir equation of Gopalpur and Bajoa soil series were 0.0078 and 0.0031. The
greater value of slope was observed in Gopalpur soil and the Bajoa soil series.
Fig. 4.4. Langmuir adsorption isotherm for phosphorus in Gopalpur soil series
y = 0.823x + 1.369R = 0.706
0
0.5
1
1.5
2
2.5
3
3.5
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5
Log
X
Log C
y = 0.007x + 0.469R = 0.983
0.001.002.003.004.005.006.007.00
0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00
CX
-1
C
-
Fig. 4.5. Langmuir adsorption isotherm for phosphorus in Bajoa soil series
4.6. Temkin adsorption isotherm
The R2
Fig. 4.6.Temkin adsorption isotherm for phosphorus in Gopalpur soil series
value of theTemkin equation of Gopalpur and Bajoa soil series were 0.8097 and
0.8077 where greater value 0.8097 was observed in Gopalpur soil than value 0.8077 in
Bajoa soil (Fig. 4.6 and 4.7 and ).The slope of Temkin equation of Gopalpur and Bajoa
soil series were 105.91 and 96.146. The intercept of Temkin equation was greater for
Gopalpur (63.109) thanBajoa soil series (99.484).
y = 0.0031x + 0.0034R = 0.9988
-0.50
0.00
0.50
1.00
1.50
2.00
2.50
-100.00 0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00
CX
-1
C
y = 105.9x - 63.10R = 0.809
-100.00-50.00
0.0050.00
100.00150.00200.00250.00300.00350.00
0.00 0.50 1.00 1.50 2.00 2.50 3.00
X
Log C
-
Fig. 4.7.Temkin adsorption isotherm for phosphorus in Bajoa soil series
4.7. Multi-point adsorption equations
The P sorption data of the two soil series were plotted according to the conventional
Langmuir, Freundlich and Temkin equations (Table 4.2). Among the three adsorption
equations, Langmuir equation was best fitted to the equilibrium P sorption data (R2 =
0.9834,0.9988). Different phosphate sorption parameters were calculated from the three
sorption equations (Table 4.2). The Langmuir equation is used to derive the maximum P
sorption capacity (bL) of the soils. The bL values of the Gopalpur and Bajoa soil series
were 128.20 and 322.58 mg P kg-1. An increasing trend in the bL values was observed
along the catena of the studied soil samples. It should be noted that the bLvalues are more
empirical curve-fitting parameters than true sorption maxima, since input concentration
were not sufficient to saturate the soil (D'Angeloet al., 2003). The energy of adsorption
expresses the binding energy required to adsorb phosphorus. The P binding energy KL of
the Gopalpur and Bajoa soil series were 0.016 and 0.911. Maximum P buffering capacity
(MPBC) is the product of P sorption capacity (or monolayer coverage in mol P kg-1of
soil) and phosphate affinity constant related to the binding strength (Dalal and
Hallsworth, 1976) and regulates the partition of P between solution and solid phase. The
Maximum P Buffer Capacity (MPBC) of the Gopalpur and Bajoa soil series were 2.0512
and 293.87 where the Bajoa soil series had the greater value than Bajoa soil series.
Unlike P buffering capacity, MBC does not vary with solution phosphate concentration
(Poteet al., 1999). Majumdaret al. (2004) suggested that management practices such as
y = 96.14x + 99.48R = 0.807
-100.00-50.00
0.0050.00
100.00150.00200.00250.00300.00350.00400.00450.00
-2.00 -1.00 0.00 1.00 2.00 3.00 4.00
X
Log C
-
soil conservation measure, application of manure, etc., influence MBC of P. The
phosphate sorption data were also adequately described by the Freundlichand the Temkin
equations. Among the two soil series, Bajoa soil series had the greater Kf
X: Total sorbed P; C: Equilibrium P concentration in solution; K
values (23.39)
than the Gopalpur (7.93) soil series determined from the Freundlich equation (Table 4.2).
The Buffering index (BI) of the Gopalpur and Bajoa soil series were 8.802, 19.17 where
the Bajoa soil series had the greater value than Gopalpur soil series.
Table 4.2. Phosphate adsorption parameters calculated from the isotherm
f and N are empirical constants; BI:
Buffering index; bL: Phosphate sorption maximum; KL
The phosphate sorption capacity increased along the catena of the soil. The higher P
sorption capacity of the Bajoa soil series might be attributed to its high organic matter
and clay content. Significant relationships between P sorption capacity and several soil
properties like organic matter and clay contents have been reported by several authors
(
: P binding strength; MBC: The maximum buffer
capacity of the soil; a and b of Temkin equations are constants.
4.8. Correlation between soil chemical parameters with phosphate adsorption
Adsorbed amount of phosphorus from the soil series were correlated with each other and
with other soil chemical properties.
Daly et al., 2001; Hossainet al., 2012).Soil containing high contents of clay adsorb more
P than those with small amounts (Pena and Torrent, 1990).
Freundlich equation Langmuir equation Temkin
equation
Soil series
log X =
log Kf N + N
logC
K BI f
CX-1 = (KL bL)-1 +
CbL
b-1
KL MBC L X = a logC
+ b
Gopalpur y = 1.11x
+ 0.8998 1.11 7.93 8.802
y = 0.0078x
+ 0.4698 128.20 0.016 2.0512
y = 105.91x
- 63.109
Bajoa
y =
0.8234x +
1.3692
0.82 23.39 19.17 y = 0.0031x
+ 0.0034 322.58 0.911 293.87
y = 96.146x
+ 99.484
-
There is a significant correlation between maximal value of surface-adsorbed phosphorus
and clay content of soil suggesting that clay content affects the ability of a soil to adsorb
P applied to soil. Therefore, as clay content increases, the P sorption capacity of a soil
increases. The sorbed P will increase the ability of a soil to replenish the soil solution as a
plant withdraws P. The role of clay content on P sorption has been reported by some
researchers (Pena and Torrent, 1990; Afifet al., 1993)
Organic matter is known to influence phosphorus sorption greatly (Quanget al., 1996).
Soils that are highly weathered and the presence of organic matter reduces P- sorption
capacity due to direct result of competition for sorption sites between phosphate and
organic ligands (Hakim, 2002). Khan (1999) reported that less phosphorus adsorption in
soils with high organic matter is due to occupation of adsorption spaces by organic
anions. In the present study, the amount of P sorption was positively correlated to organic
matter but the correlation coefficients were not significant. Organic matter was also
positively correlated with the maximum buffering capacity, the buffering capacity and the
sorption capacity obtained from the Freundlich plot.
A negative and significant relationship exists between P sorption maximum and pH(Fig.
4.5). pH has been considered to be a measure of exchange acidity which measured Al3+
in addition to H+. The negative relationship between P sorption maximum and pH shows
that sorption decreases with increasing soil pH. This can be partly explained by the
decrease in positive charge of hydroxy-surfaces with increasing pH, namely the
disappearance of -H2O
+ ligands that have very strong affinity for phosphate (Shang et al.,
1992). The negative relationship between P sorption and pH is consistent with the
findings of other workers (Ioannouet al., 1994; Zhang et al., 1996).
Among the sorption equations, Langmuir equation was best fitted to the sorption data.
Similar results have been reported for Langmuir equation over Freundlich and Temkin
equations by other scientists as well (Gichangiet al., 2008; Moazedet al., 2010). Based on
R2 values, the Freundlich equation was better in predicting the P sorption capacity of
calcareous soils than the other two equations (Zhou and Li 2001). The good fit of the
-
Langmuir adsorption equation indicates that the P sorption affinity of soils remained
constant with increasing surface saturation (Mead 1981).
The Bajoa soil series had the highest KL values. Mehadi and Taylor (1988) suggested that
high KL value indicates strong bonding of phosphate by soil particles. As a result, due to
the highest KL values, the Bajoa soil series will retain P better Gopalpur soils and
possibly be the better sink at similar P adding rates.
The Gopalpur soil series had the lowest KL
values. The soils that show lower P buffering
capacities may need more frequent application of p fertilizer than soil with relatively high
buffering capacities.
-
Chapter V Summary and
Conclusion
-
5. Summary and Conclusion
In this study, P adsorption isotherms were obtained for two soil samples. The isotherms
were similar indicating similarity in the nature of adsorption reaction, but differed in the
intrinsic characteristics such as the slopes of the isotherms and adsorption capacity.
Maximum P adsorbed by the soil ranged from 19.96 to 345.29 g P g-1 soil. Differences
in the amounts adsorbed may be attributed to differences in soil texture, clay mineralogy,
organic matter contents in the soil.
In the investigation it was noted that Langmuir equation provided the best fit to P
adsorption data in the soils compared to the Freundlich and Temkin equations. The r2
values calculated from the Langmuir plots of soils ranged with an average of 0.991,
followed by the Freundlich plots with an average r2 = 0.7535, and Temkin plot with
average r2 = 0.8087. Langmuir equations were able to explain the P behavior in the soils
with respect to buffering capacity and P supplying capacity. The Maximum P Buffering
Capacity (MPBC) of the soilsare 0.0512 and 293.87. MPBC were higher in the Bajoa soil
series and lower in Gopalpur soil series. Phosphate sorption maximum were higher in the
Bajoa soil series and lower in Gopalpur soil series. Phosphorus binding strength
werehigher in the Bajoa soil series and lower in Gopalpur soil series. In soils with high P
sorption capacities and strong P binding energy like ours, add to soils a less soluble P
source that releases P to soil solution in smaller concentrations and slow down the P
fixation reaction in soil and to maintain fertilizer P in plant available form for a long
period.
P sorption data enabled the prediction of the amount of fertilizer needed for a soil. This
will prevent the arbitrary addition of fertilizer to soil which can cause eutrophication.
From the physical and chemical properties of the soils studied, soils which contain larger
amounts of the soil properties (texture, clay mineralogy, organic matter, aluminum, iron
and calcium contents of the soil) and the sorption parameters derived from the three
equations contain more P.
-
Chapter VI References
-
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