Bresson 1990

Geoderma, 47 (1990) 301-325 301 Elsevier Science Publishers B.V., Amsterdam

Morphological characterization of soil crust development stages on an experimental field

L.-M. Bresson a and J. Boiffin b aLaboratoire de Science des Sols et Hydrologic, LN.A.P.-G., 78850 Thiverval Grignon, France

blNRA, Station d'Agronomie de Laon-P~ronne, B.P. 101, rue F. Christ, 02004 Laon-Cedex, France

(Received April 5, 1989; accepted after revision November 23, 1989)

ABSTRACT

Bresson, L.-M. and Boiffin, J., 1990. Morphological characterization of soil crust development stages on an experimental field. Geoderma, 47: 301-325.

Crust development on a loamy Aquic Hapludalf near Paris was studied in an experimental field to which various fertilizers and amendments had been applied. These treatments over a period of 57 years had induced a range of organic matter contents and exchangeable cation percentages. The macroscopic aspects of the soil surface was closely monitored during crusting, and microscopic characterization was carried out at every main crust development stage. Crusting followed a general pattern: ( l ) sealing of the soil surface by a structural crust; (2) development ofa depositional crust. Even in the sodic plot where the depositional crust appeared especially early, the structural crust developed first. The change from the first stage to the second one mainly depended on a decrease in infiltrahility due to the structural crust properties that induced micro-runoff and puddling under rainfall. Struc- tural crust development seemed to involve a coalescing process due to raindrop impact when moistening decreased soil cohesion. This process developed much faster in the sodic than in the limed plot. Morphology of the depositional crust, in particular microbedding and sorting, could be related to the hydrodynamic behaviour of the soil surface (abundance, size and duration of puddles).

INTRODUCTION

Soil crust formation under rainfall frequently induces failure of seedlings to emerge on unstable loamy soils. Moreover, crusting increases runoff and erosion hazards. Preventing these problems requires an understanding arid prediction of how soil degradation develops in the field. Crusting is governed by a complex sequence of soil particle detachment and transport processes at the soil surface (Farres, 1978; Boiffin, 1984).

These elementary processes, and their responses to soil and/or rainfall characteristics have usually been studied in the laboratory. First, particle detachment is achieved by various mechanisms: (1) disaggregation by entrapped air compression when moistening; (2) disaggregation by raindrop impact and/or flow turbulence; ( 3 ) microcracking by shrinking and swelling; (4) physico-chemical dispersion. These mechanisms occur with various in-

0016-7061/90/$03.50 1990 - - Elsevier Science Publishers B.V.

302 L.-M. BRESSON AND J. BOIFF1N

tensities depending on the soil composition and moisture status. As a result, the detached particle size distribution greatly varies (Le Bissonnais, 1988), which may induce significant differences in crust morphology and properties. Second, crusting also depends on the particle transport and sedimentation modes (Mticher and De Ploey, 1977; Chen et al., 1980; M/icher et al., 1981 ), which determine two main morphological types, "structural" crust and "depositional" crust (Chen et al., 1980). Third, compaction from raindrop impact may play a great part in soil crusting: McIntyre ( 1958 ) attributed "skin seal" formation under simulated rainfall to this process.

Field studies also are necessary, because the above-mentioned elementary processes greatly depend on specific conditions which are not easily repro- duced in the laboratory: ( 1 ) duration and intensity of rainfall events are er- ratic and often quite small; (2) the soil surface moisture is variable; (3) a suction gradient often occurs in the top few centimetres; (4) surface microrelief is more or less marked; and (5) water flow and particle movement can occur over several metres. As a result, the crusts described in situ show a great spatial and chronological variability in morphology and behaviour, which is especially clear in subtropical and arid environments (Valentin, 1981; Koois- tra and Siderius, 1986; Valentin, 1986; Escadafal and Fedoroff, 1987).

To understand and predict soil crust development in the field it is necessary to determine whether this phenomenon follows a unique general pattern in spite of its variability. Crust development on two loamy soils ( 10% and 20% clay) under natural rainfall and with various initial structural states and pre- ceding weather conditions was studied in a previous work (Boiffin, 1984, 1986; Boiffin and Bresson, 1987). Two successive stages were consistently observed irrespective of the conditions: ( 1 ) sealing of the soil surface by a structural crust; (2) development of a depositional crust. They were related to differentiations in the soil hydrodynamic regime induced by changes in infiltrability (Boiffin and Monnier, 1986 ).

The aim of the following study was ( 1 ) to test for the validity of this crusting pattern, which is required for reliable comparison between crusts differing in soil composition, and (2) to gain a better understanding of the genesis of crust microhorizons. For this purpose, a wider range of soil properties, such as organic matter content and exchangeable cation percentages, was needed. Such a composition range was found in a long-term experimental field which was established 57 years ago to show the effects of various fertilizers on soil physical properties. Crusts were described at all development stages, using the macroscopic and microscopic observation methods previously reported (Boiffin, 1984; Boiffin and Bresson, 1987), for monitoring the crusting process. The relationships between macroscopic aspect, microstructure and hydrodynamic behaviour of the soil surface were then studied.

MORPHOLOGICAL CHARACTERIZATION OF SOIL CRUST DEVELOPMENT STAGES

MATERIALS AND METHODS

303

Experimental site

This study was carried out in the "42 plots" long-term experimental field ( 15 20 m) at INRA in Versailles ( 15 km W from Paris), which was set up in 1928 to test the effect of various fertilizers on the composition and properties of a loamy Aquic Hapludalf (Burgevin and H6nin, 1939).

We studied 5 of the 17 experimental treatments. "Reference", without any fertilizer; "Sodic"; 15 g m -2 per year o fNa (NaNO3); "Limed": 100 g m -2 per year of CaO (limestone); "Farmyard manure"; 10 kg m -2 per year of farmyard manure; "Ammoniated"; 15 g m -2 per year of N (NHaC1). The fertilizers were applied twice a year. The main analytical data for the Ap horizons (0-25 cm) are given in Table I. After 57 years of experimentation, these plots differed in the constituents directly brought by the fertilizers and also, to a lesser extent, in texture.

Plots, 22.5 m, were dug each year at fertilization, and kept free of any vegetation. On March 15th, 1985, digging was followed by raking to obtain a

TABLE 1

Main analytical data for the Ap horizons of the experimental plots

Ap (0-25 cm) Plots characteristics

reference limed farmyard sodic ammoniated manure

Particlesize (%): 0- 2 ~m 18 18 19 15 17 2- 20#m 22 22 22 22 22

20- 50# 37 37 36 38 38 50- 200#m 20 20 20 22 20

200-2000#m 3 3 3 3 3

Organic carbon (%o) 6.5 7.3 29.4 7.1 8.6 pH (H20) 5.5 8.2 6.4 5.7 3.7 CEC Metson 10.4 9.7 15.0 8.1 8.9

(10-2mmolg - I )

Exchangeable cations (10 2 mmol g- l) :

Ca 4.2 14.8 5.8 3.4 0.2 Mg 0.4 0.2 1 0.3 0 K 0.2 0.2 2.6 0.2 0.1 Na 0 0 0.2 1.1 0

304 L.-M. BRESSON AND J. BOIFFIN

structure and a microrelief similar to those of a quite coarse seedbed. Then plots were exposed to natural rainfall.

Observation procedures

Rainfall recording and observation Rainfall was measured using a raingauge recorder whose resolution was 0.1

mm rainfall and 60 s time. Two periods could be distinguished (Fig. 1 ); from March 15th to April 15th rainfall was almost uninterrupted so that the soil surface remained moist, and from April 15th to June 1 st the rainy sequences were shorter and interrupted by short periods of soil surface desiccation. The hydrodynamic behaviour of the soil surface under rainfall, especially the abundance, size and duration of puddles, was observed as often as possible.

Macroscopic observations Immediately after digging and raking, two rectangular microplots (50 35

cm) were delimited in each plot studied, where photographs and visual observations were repeatedly taken as crusting developed. Observations were made after every 5 to I0 mm of rainfall. The visual monitoring was based on the morphological features which are typical of the soil surface structural evolution (Boiffin, 1984): (1) area of "continuous patches" where inter-clod

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TABLE II

Typical morphological features of the two main surface macroscopic facies

305

Morphological features Surface macroscopic facies

F1 F2

Continuous patches area (%) 0 to 60 Size of the finest clods which are still unincorporated to crust 0.1 to 15 (mm) Disjunction evidence:

development No or very w. distribution Diffuse

Alteration of clods ( > 1 cm ): edges shape Sharp contact angles with underlying surface Acute

>50

10 to 50

Moderate to strong Mainly in microdepressions

Blunted

Obtuse

interstices disappeared and thus induced a continuous surface; (2) size of the finest clods which are still unincorporated to crust; ( 3 ) evidence of disjunction between plasma and skeleton grains; and (4) alteration of clod edges and contact angles between clods and underlying surface. The combination of these specific features defines two main "surface macroscopic facies", named F1 and F2, which are typical of the two crust development stages (Table II).

Microscopic observations 25 samples were taken from the reference (7), farmyard manure ( 5 ), limed

(4) and sodic ( 7 ) plots at each main crusting stage and in typical microsites. Two additional samples were taken from the depositional crust of the ammoniated plot. The sampling dates and microsites were chosen in such a way as to provide a range of surface macroscopic facies as large as possible. Air- dried samples were impregnated with a polyester resin in which a blue flu- orescent lighted dye (UV-Tex) was incorporated, and 5 X 8 cm thin sections were prepared according to the method described by Guillor6 (1983). The pedic, matrix, textural and plasmic organizations were described according to Bresson's scheme (1987) and using the terminology of Bullock et al. ( 1985 ). Photograms of thin sections were made according to the method described by Guillor6 ( 1987 ).

RESULTS

Macroscopic observations

Surface macroscopic facies The soil surface evolution was found to be gradual in all the plots. The

succession of the two main facies F1 and F2 was always observed, even in the


Photos 1 to 6. Surface macroscopic facies (35 50 cm microplots, frame width 60 cm). 1. Ref- erence plot: initial fragmentary facies F0.2. Reference plot: F1 facies (structural crust), with continuous patches (arrow). 3. Reference plot: F2 facies (depositional crust), with soil loss areas (micromounds, in dark colour, m) and depositional areas (in light colour, d). 4. Farm- yard manure plot: F2 facies (depositional crust ). 5. Sodic plot: F 1 facies (structural crust ), with continuous patches (arrow). 6. Sodic plot: F2 facies (depositional crust ), with flattened surface and large and longlasting puddles ( in dark colour, p).

p lots where c rus t ing was rap id (Photos 1 -6 ) . First, cont inuous patches appeared in the areas wi th the f inest c lods, and expanded by incorporat ing more and more coarse c lods (Photos 2 and 5). The d i s junct ion features between

MORPHOLOGICAL CHARACTERIZATION OF SOIL CRUST DEVELOPMENT STAGES 307

granomass (skeleton) and micromass (plasma), i.e. bare silt and sand grains, remained scarce and diffuse, and no spatial segregation of basic particles could be observed. Second, clear features of the basic particle disjunction and segregation developed, and the surface continuity markedly increased. Soil loss areas (micromounds) and depositional areas (microdepressions where puddles formed during rainfall) could be distinguished at a decimetric scale (Photos 3, 4 and 6 ).

Time and rapidity of the change from the first facies to the second varied with the plots (Fig. 2). The sodic plot changed early and suddenly, and the reference and ammoniated plots slightly later and gradually. The farmyard manure and limed plots, where the F2 facies was confined to some places and was not well developed, changed much later and slower.

Puddling During rainfall, puddles appeared more or less early depending on the plots.

In the sodic plot, small (centimetre-size) puddles formed on March 22nd and lasted only some minutes after rainfall had stopped. Afterwards, they formed under most, even light (=2.8.10 -4 mm s -~), rainfalls. Heavier rainfalls (March 26th, April 5th, 7th and 11 th ) induced a several-day flooding of half the plot (Photo 6). In the reference and ammoniated plots, first puddles occurred on March 25th, then even under very light rainfalls, but never lasted longer than some ten minutes after rainfall had stopped. In contrast, within the farmyard manure and limed plots, small puddles (< 2 cm wide) could only be observed on April 7th under a 2.8.10 -3 mm s-l rainfall, and disap-

Cumulative rainfall amount from the last tillage operation

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LEGEND transitional

FI ~ (FI_F2) i i ["2

Fig. 2. Surface macroscopic facies in the various plots according to the cumulative rainfall amount.


peared before it ended. Afterwards, no puddling was observed for low rainfall intensities ( I< 1.4.10 -3 mm s -~ ).

Identification and characterization of typical microhorizons

The microhorizons observed in the reference plot are described in Table III, and the main difference between the treated plots are given in Tables IV- VII.

The three typical microhorizons m l, m2 and m3 previously defined by Boiffin and Bresson (1987) could be identified (Photos 7-10). The m3 and m2 microhorizons mainly differed from the initial one (ml ) at the pedic organization level by a continuous, instead of a fragmentary, microstructure. At the matrix level, the m3 was very compact, while the m2 and the m 1 were rather porous. At the textural level, the m3 was characterized by the disjunction between micromass (plasma) and granomass (skeleton), which was generally expressed by alternate submillimetric microbeds more or less con- trasted in texture and uncomformable with the underlying microhorizon.

A fourth typical microhorizon, named ml-2, allowed a more accurate description of the microstructural evolution during crust development. It showed intermediate micromorphological characteristics (Table III); most macropores were typical packing voids, with a polyconcave shape (as in m l ), but in thin sections the solid phase was 2D (two-dimensional) continuous (as in m2). Nevertheless, the 3D void interconnection was indicated by both the small relative spacing of the packing voids and their rather elongated shape (Photo 7 ).

Chronosequences of microprofiles

Crust development could be described in each plot as a chronosequence of microprofiles consisting of superposed typical microhorizons: m 1, m2/m 1- 2 /ml and m3/m2/ml -2 /ml (Fig. 3).

Typical microprofiles of the three macroscopic facies Microhorizon arrangement was closely related to the soil surface macro-

scopic facies, regardless of the plot, as reported for other soils by Boiffin and Bresson (1987). In the initial fragmentary facies, F0, only the m l microhorizon was present. The soil surface was microscopically rough. The FI facies was associated with m2/ml -2 /ml microprofiles which occurred when and where patches of structural crust began to develop (Photo 7 ). The m2 microhorizon was well developed only within the continuous patches, and very thin ( < 1 mm) and discontinuous in the remaining parts of the crust. In the F2 facies, the depositional crust appeared within microdepressions and was characterized by m3/m2/m 1-2/m 1 microprofiles (Photo 8). However, the


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TABLE IV

Comparison of the initial states (m 1 microhorizon) between the plots

Organization level

Differentiation criteria Plots

reference limed farmyard sodic manure

Pedic Abundance of aggregates > 10 mm (%)

Interaggregate porosity (%)

Matrix Matrix porosity (%)

Textural Micromass (


Photo 7. Structural crust in the reference plot (vertical section, plain light): m 1-2 numerous, polyconcave and probably 3D-continuous voids; m2 very thin and dense upper layer, and porous lower layer with convexo-concave to vesicular voids; arrow is small spot of early m3 (pocket- like distribution, poor sorting and microbedding, presence of microaggregates).

during the F 1 facies, especially within the cont inuous patches where it reached its max imum thickness at the init ial stage of the F2 facies. Outs ide these patches, the m2 deve loped slower but for a longer t ime so that in the F2 facies


Photo 8. Depositional crust in the reference plot (vertical section, plain light): ml-2 very thick, with convex and smooth matrix voids; m2 very dense and quite thick especially in the micromounds; matrix voids are less abundant, smaller and more smooth than in the structural crust; m3 strongly microbedded, with a microdepression facies (the clayey microbeds are convergent from the middle to the sides of the puddle).

it was slightly th icker in mic romounds than in microdepress ions. The m2 was quite thick (2 -6 mm) in the reference plot, very thin (0 .5 -2 mm) in the l imed (Photo 9 ) and fa rmyard manure plots, and very thick (4 -12 mm) in


Photo 9. Depositional crust in the limed plot (vertical section, plain light): ml undisturbed: m 1-2 very thin, discontinuous; m2 very thin; m3 poorly sorted and bedded, presence of microaggregates, pocket-like distribution.

the sodic plot (Photo 10). The m3 microhorizon was quite thick (up to 5 mm) in the reference plot, and thinner (up to 3 mm) in the limed (Photo 9) and farmyard manure plots. It was very thick (up to 8 mm) in the sodic plot (Photo 10), and reached up to 20 mm in thickness in the ammoniated plot.

Time-dependent variations of the typical microhorizons.

Perceivable changes in some microscopic characteristics of the typical microhorizons occurred during crust development. The main modifications observed in each typical microhorizon were the following in the reference plot.

In the m 1-2 microhorizon, the macropores remained as packing voids, but their abundance slightly decreased from the F1 facies (15%) to the F2 facies (10%). The size of the voids was constant, but their shape obviously changed in the continuous patches of the F1 facies: elongation and/or tortuosity decreased and polyconcavity (Photo 11 ) was altered by angle rounding (convexo-concavity) while microscopical smoothness increased (Photo 12 ).

Initially, the m2 was not porous at the matrix level, but during the first crusting stage (F1 facies), vesicles could be observed within the thickened m2 below a thin ( < 1 mm ) aporous layer. The matrix porosity did not change greatly during the second crusting stage (F2 facies): vesicles were less abundant and slightly smaller (50-250 ~tm instead of 100-500 ~tm), indepen- dently of the possible development of a m3 microhorizon.

314 L.-M. BRESSON AND J, BOIFFIN

Photo 10. Depositional crust in the sodic plot (vertical section, plain light): m2 very thick; m3 well sorted and bedded, with a deltaic facies (the clayey microbeds are divergent from the middle to the sides of the puddle.

The m3 features which developed in small pocket-like holes at the end of the Fl facies (Photo 7) were different from the typical m3 of the F2 facies (Photo 8 )" the bedding was less distinct and the sorting was poorer in that some microaggregates were often mixed with poorly sorted basic components.

Interplot differences of typical microhorizons

Depending on the plot, the microscopic characteristics of the initial microhorizon m 1 differed at all organization levels. The main differences can be seen in Table IV. Moreover, partial fabrics as in m2 (presence of vesicles) or as in m3 (bedding and sorting) could be observed, especially in the sodic plot. They did not show any preferential orientation and distribution.

The m 1-2 microhorizon always had the same plasmic, textural and matrix organizations as the underlying m 1, and so differed with plots. On the other hand, the m 1-2 pedic organization markedly differed from that of the m l, but remained quite different between plots (Table V). The obvious evolution of voids during the first crusting stage (F1 facies), which was previously described in the reference plot, could also be observed in the sodic plot. In contrast, the m 1-2 matrix voids did not change within the limed plot (Photo 9 ).

The m2 microhorizon was apedic in all the plots. At the matrix level, only


REFERENCE FARMYARD MANURE

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LEGEND

Fig. 3. Typical microprofiles in the various plots according to the surface macroscopic facies and the cumulative rainfall amount.

slight interplot differences related to void shape and size could be observed (Table VI). The m2 had the same plasmic and textural organizations as the underlying m 1-2 and m 1 ones, and therefore slightly differed from one plot to the other.

The m3 microhorizon was apedic (pedic level ) and aporous (matrix level ) in all the plots. Its textural and plasmic organisations markedly differed between plots especially with regard to the relative distribution of the microbeds, their thickness and textural porosity, and the sorting and orientation of their basic components (Table VII), and Photos 14, 15 and 16 ).


-12 1 mm

13 1 mm

Photos 11 to 13. Evolution of the matrix voids according to crust development in the reference plot (vertical section, circular polarized light, negative). 11. Polyconcave packing void, rough (ml) . 12. Convexo-concave matrix void, characterized by a development of convexities and a smoothness increase (ml-2). 13. Increase in smoothness and convexity of matrix voids, from polyconcave packing voids (lower part of the ml-2) to typical vesicles (upper part of the m2 ).


TABLE V

Comparison of the m 1-2 pedic organizations between the plots

317


reference limed farmyard sodic manure

ammoniated

Void size (mm) 1 to 5 0.5 to 2 1 to 5 2 to 5 0.2 to 0.5 *~ Microscopic roughness Moderate Strong Moderate Weak Strong *~ Polyconcavity distinctness Moderate Strong Moderate Weak Strong*

*t Related to fluffy microstructure (microaggregated c / f rdp).

TABLE VI

Comparison of the m2 matrix organizations between the plots


reference limed farmyard sodic ammoniated manure

Void size (mm) 0.2to 1 0.1 to0.3 0.1 to0.5 0.3to 1.5 0.1 to0.3 Microscopic roughness Weak Moderate Moderate Very weak Moderate Vesicle abundance Moderate Low Moderate High Moderate

DISCUSSION

Relationships between surface structural evolution and hydrodynamic regime

The development of the F2 facies, i.e. the depositional crust, closely coin- cided with puddle appearance in all the plots. This shows that puddling induced a thorough change in the conditions under which translocation and deposition of the solid particles detached by rainfall occurred. This hypothesis is supported by the contrast between the m 1, m 1-2 and m2 microhorizons and the m3 microhorizon: bedding and sorting in m3 and unconformability between m3 and underlying m2 evidenced the sedimentary origin of the m3 microhorizon (Chen et al., 1980; Miicher et al., 1981; Valentin, 1981; Boiffin, 1984; Arshad and Mermut, 1988 ).

The early stage of surface crusting, which did not show textural disjunction nor elementary particle segregation, also occurred in the sodic plot in spite of its high dispersability (Fig. 2). Therefore, development of the depositional crust seemed to be mainly determined by changes in surface hydrodynamic regime and not directly by the soil composition.

Conversely, for a given rainfall sequence, the surface hydrodynamic regime

318 L-M. BRESSON AND J. BOIFFIN

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Photos 14 to 16. Sorting of the m3 microbeds of the depositional crust in different plots (vertical section, plain light). 14. Reference plot: the clayey microbed includes coarse grains (g) and is capped by a thin film of pure clay (arrow). 15. Sodic plot: the clayey microbed shows a differentiation of grain size distribution according to the depth, which is typical of a non turbulent sedimentation regime. 16. Ammoniated plot: the clayey microbed is constituted by 50 pm microaggregates (arrow).


could be related to structural crust development. Indeed, puddles appeared more rapidly on the plots where the m 1-2 and m2 microhorizons had rapidly thickened. In fact, pore abundance and continuity of these microhorizons decreased in such a way (Tables IV-VI) that spreading and thickening of the structural crust could induce a strong decrease in surface infiltrability. Pre- vious work (Boiffin, 1984; Boiffin and Monnier, 1986) showed that, in similar soils, infiltrability ranged from 1.1-1.4- 10 -2 mm s- 1 for the initial state ( F0 facies ) to less than 1.4- 10- 3 mm s- ~ for well-developed structural crusts (F1 facies) and to 3-6- 10 -4 mm s -1 for the depositional crust (F2 facies).

Basic processes of crust genesis

Structural crust The main characteristics of the m 1-2 were intermediate between those of

the m l and the m2 (3D-continuous void space and 2D-continuous solid phase) and its boundaries with m 1 and m2 were diffuse, which suggests that it was a transitional microhorizon.

The m 1-2 and most of the m2 (except the upper part) seemed to result from the gradual packing of the m 1. This process is mainly suggested by the pedic and matrix void changes from m 1 to m 1-2 and then to m2 (Tables IV- VI ) as observed in the reference plot: decrease in porosity and void number, elongation, roughness and polyconcavity with development of convexities (Photos 11, 12 and 13 ). This hypothesis is supported by the comparison of the treated plots: the ml-2 and m2 thickening (Fig. 3) and the decrease in void roughness and polyconcavity (Tables V and VI) were more rapid and complete in the sodic plot than in the limed one (Photos 9 and 10).

On the other hand, the interstitial clogging process related to splash, which was suggested as a mechanism of structural crust formation (McIntyre, 1958; Boiffin, 1984; Onofiok and Singer, 1984; Boiffin and Bresson, 1987), should not play an important part in the m 1-2 genesis. Firstly, no disaggregation could occur within this microhorizon. Indeed, the overlying m2 formed a protec- tion against raindrop impact, and on the other hand, slaking from entrapped air compression could not be very efficient under such moist conditions (Boiffin, 1984; Norton, 1987; Le Bissonnais, 1988) whereas microcracking from shrinking and swelling could not occur due to too low clay content (Valentin, 1981; Le Bissonnais, 1988). Second, the overlying m2 which is 3D-continuous prevented the particles released from the soil surface by the splash, from migrating within the m 1-2.

Finally, the main process involved in the genesis of the m 1-2 and the most part of the overlying m2 seemed to be a gradual coalescence of the initial aggregates by compaction under plastic conditions. Since these microhorizons developed from the surface, the compacting agent was rainfall rather than the weight of the overlying soil material. In such a process, the part of


rainfall was double: dynamic compaction in depth by raindrop impact (Sor and Bertrand, 1962; Boiffin, 1984) and plasticity increase by moistening. This is consistent with the correlation between soil crusting susceptibility and soil consistency showed by De Ploey ( 1978 ). The m2 thickening in the F2 facies which occurred within the micromounds, and not within the sedimentary depressions, confirmed that raindrop impact played a major role.

These results also suggest that the genesis of vesicles might be ascribed to compaction of initial packing voids due to plastic strain under semi-liquid conditions (Photos 11, 12 and 13).

However, the upper part of the m2 ( < 1 mm) which constituted the soil- atmosphere interface, could result from a different process. Indeed, right at the start of the m2 development, this upper part showed a very low textural porosity and a vesicular matrix porosity (Table III and Photo 7 ). Moreover, this thin interface did not thicken (0.5 to 1 mm) during crust development, unlike the whole m2. Last of all, the differences between plots were quite small. Therefore, the interstitial clogging processes of splash or muddy flow and sub- sequent compaction by raindrop impact (Mclntyre, 1958; Onofiok and Singer, 1984; Valentin and Ruiz Figueroa, 1987) probably took place in the upper part of the m2 microhorizon.

Finally, in the structural crusts studied, the dispersion process which has been advanced to explain the structural crust formation (Mclntyre, 1958) especially in sodic soils with low electrolyte concentration water (Agassi et al., 1981; Tarchitzky et al., 1984; Shainberg and Singer, 1985; Levy et al., 1966) did not seem to affect directly the ml-2 and m2 genesis. Indeed, the plasmic and textural organizations between the m2 and the underlying m 1-2 and m 1 did not differ, as confirmed by the similar m2 microhorizons of the sodic and reference plots.

Depositional crust Both time-dependent and interplot variations of the m3 microstructure

suggest four main stages in the genesis of the depositional crust. At the first stage, the microbeds, filling in small pocket-like holes, were

poorly sorted and generally included some microaggregates (Tables III and Photo 7 ). This could be related to a muddy flow process which induced short- distance translocation of solid particles.

The second stage of the m3 development was clearly related to a runoff- sedimentation regime. Indeed, clayey and silty or sandy microbeds (Table III) alternated within small microdepressions. Each pair of microbeds might be related to a rainfall event: the coarser particles would deposit when the running water sheet suddenly stopped in the puddles, whereas the finer particles suspended in the puddles would begin to deposit slowly only after the rainfall had stopped. The first coarse microbeds were generally silty, even though the following ones were sandy, which might indicate that runoff was


still quite low when the F2 facies began to develop. The clayey microbeds which often included sand and silt grains probably developed under turbulent conditions during rainfall, but they were generally capped by a thin film of pure clay which probably slowly deposited after the rainfall had stopped (Photo 14). This stage was hardly reached after a month in the limed plot where puddles were very transient.

The third stage was characterized by the unconformability between the coarse and fine microbeds. This deltaic aspect occurred within the areas where puddles notably spread ("basins"), which suggest that during rainfall the coarser particles mainly deposited in the external part of the basin, where the running water sheet suddenly stopped, while the clay particles settled on the whole surface of the basin (Photo 10). The clayey beds were all the thicker, better sorted and more birefringent as they were located in the middle of the basin (Photo 15 ), probably because, after rainfall, puddle subsiding concen- trated the finest clay particles in the central part of those basins where they slowly deposited. The "deltaic" features, i.e. nature and relative distribution of the clayey beds, were more marked in the sodic plot where puddles were very large (Table VII ).

The fourth stage was related to the general flattening of the plot surface. Indeed, the deposited microbeds were thinner and more uniformly distrib- uted. After a month, this stage was reached only in the sodic plot and, to a lesser extent, in the reference plot (Table VII).

The direct influence of soil dispersability on the m3 microhorizon genesis could not be clearly stated from this experiment, even though a high soil dispersability is supposed to increase basic particle disjunction during detachment, transport or deposition. On one hand, the good sorting and the strong birefringence of the clayey microbeds within the sodic plot (Table VII ) could be ascribed to high dispersability (Southard et al., 1988 ). Moreover, the textural distribution (c/ f rdp: coarse/fine related distribution pattern) of the coarse microbeds within the limed plot was still quite chitonic, which shows that the disjunction between micromass and granomass was poor: such close binding could be related to the low dispersability induced by the high calcium carbonate supply. On the other hand, the well sorted clayey microbeds in the ammoniated plot were formed by stable 50/~m microaggregates (Photo 16 ). In the same way, the sorting of the m3 was very poor in the farmyard manure plot, although dispersability seemed to be rather high as indicated by disjunction between granomass and micromass (enaulic of rdp).

On the contrary, puddling duration, which was very short in the limed and farmyard-manure plots and long in the sodic one, seems sufficient to explain the m3 variability described above.

Medium- and long-term effects of soil crusting

Interplot comparison of the initial states (F0 facies, m l microhorizon) shows (Table IV) that two main differences could be related to feed-back

MORPHOLOGICAL CHARACTERIZATION OF SOIL CRUST DEVELOPMENT STAGES 3 2 3

effects of soil crusting. First, some fragments of the m2 and/or m3 microhorizon which were inherited from previous crusts could be observed in the m 1 microhorizon. Such features occurred more frequently in the plots which were most crusted and least crumbled, such as the sodic one. Second, experimental treatments produced a differentiation at the textural organization level which was much more significant than the enrichment of the groundmass in organic matter or calcium carbonate alone (Table III): (1) decrease in micromass content in the sodic plot (see also analytical data Table I ); (2) more chitonic nature of the c/ f rdp in the limed plot, showing a closer association between micromass and granomass; ( 3 ) enaulic c/f rdp of the farmyard-manure plot, showing that the micromass, despite of its greater amount, was not closely bound to the granomass; and (4) microaggregated c/ frdp (fluffy microstructure) of the ammoniated plot, which was consistent with the strong acidifi- cation of this plot. Therefore, the addition of direct and induced effects of the fertilizers for over 50 years resulted in a real pedogenetic differentiation.

CONCLUSIONS

The general pattern of soil crust development was confirmed by this study of a long-term experimental field where various fertilizers induced a wide soil composition range: ( 1 ) first, sealing of the surface by a structural crust; (2) then, formation of a structural crust; ( 3 ) the change from the first to the second stage depended on the hydrodynamic behaviour of the soil surface, which was partly controlled by the structural crust development. This pattern should be taken into account for reliable comparisons of crusting in soils of different composition, climatic environment and land use and management.

In the sodic soil, the earlier and more rapid depositional crust development was mainly due to the more rapid spreading and thickening of the structural crust. Moreover, even in such a dispersable soil, the structural crust formation could not be directly attributed to dispersion processes. Although dispersion is thought by some authors to be a major process in soil crusting, its part should be reconsidered.

The genesis of structural crust (m 1-2 microhorizon and the most part of m2 microhorizon) could mainly be attributed to a coalescing process under raindrop impact. Such a process probably played a significant part in the evolution of infiltrability together with the already known process of interstitial filling due to soil particle detachment and transfer by splash or mass micro- movements. This coalescing process which could be related to the decrease in soil cohesion induced by moistening, greatly depended on soil composition, and was most developed in the sodic soil.

In the depositional crust, the morphology of the typical m3 microhorizon could not be explained without reference to the puddling process which was


partly controlled by the previous development of the underlying ml-2 and m2 microhorizons.

From a methodological point of view, the various elementary processes in soil crusting occurred in various ways according to space and time. Therefore, spatial relationships between the various functional sites (for example eroded micromounds and sedimentary microdepressions) as well as the chronological relationships between the successive development stages should be taken into account. So, the space and time experimental scale appears to be especially important for reliable estimating of soil crusting susceptibility.

REFERENCES

Agassi, M., Shainberg, I. and Morin, J . , 1981. Effect of electrolyte concentration and soil sod- icity on infiltration rate and crust formation. Soil Sci. Soc. Am. J., 45:848-851.

Arshad, M.A. and Mermut, A.R., 1988. Micromorphological and physico-chemical characteristics of soil crust types in northwestern Alberta, Canada. Soil Sci. Soc. Am. J., 52: 724-729.

Boiffin, J., 1984. La d6gradation structurale des couches superficie!les du sol sous Faction des pluies. Th~se Doct. Ing., I.N.A.P.-G., Paris, 320 pp.

Boiffin, J., 1986. Stages and time-dependency of soil crusting in situ. In: F. Callebaut, D. Ga- briels and M. De Boodt (Editors), Assessment of Soil Surface Sealing and Crusting. Flanders Research Center for Soil Erosion and Soil Conservation, Ghent, pp. 91-98.

Boiffin, J. and Bresson, L.M., 1987. Dynamique de formation des crofites superficielles: apporl de l'analyse microscopique. In: N. Fedoroff, L.M. Bresson and M.A. Courty (Editors), Mi- cromorphologie des Sols/Soil Micromorphology A.F.E.S., Plaisir, pp. 393-399.

Boiffin, J. and Monnier, G., 1986. Infiltration rate as affected by soil surface crusting caused by rainfall. In: F. Callebaut, D. Gabriels and M. De Boodt (Editors), Assessment of Soil Surface Sealing and Crusting. Flanders Research Center for Soil Erosion and Soil Conservation, Ghent, pp. 210-217.

Bresson, L.M., 1987. Comportement hydrique d'un sol argileux acide, 6tude aux diff6rents ni- veaux d'organisation. In: N. Fedoroff, L.M. Bresson et M.A. Courty (Editors), Micromor- phologie des Sols. A.F.E.S., Plaisir, pp. 513-520.

Bullock, P., Fedoroff, N., Jongerius, A., Stoops, G. and Tursina, T., 1985. Handbook for Soil Thin Section Description. Waine Research Publications, Wolverhampton, 152 pp.

Burgevin, R. and H6nin, S., 1939. Dix ann6es d'exp6riences sur l'action des engrais sur la composition et les propri6t6s d'un sol de limon. Ann. Agron., 6: 771-779.

Chen, Y., Tarchltzky, J., Brouwer, J., Morin, J. and Banin, A., 1980. Scanning electron microscope observations on soil crusts and their formation. Soil Sci., 130: 49-55.

De Ploey, J., 1978. A consistency index and the prediction of surface crusting on Belgian loamy soils. In: H. Vogt and T. Vogt (Editors), L'Erosion Agricole des Sols en Milieu Temp6r6 non M6diterran6en. Univ. L. Pasteur, Strasbourg, pp. 133-137.

Escadafal, R. and Fedoroff, N., 1987. Apport de la micromorphologie ~ une 6tude multi-scalaire de la surface des sols en r6gion aride (Tunisie m6ridionale). In: N. Fedoroff, L.M. Bresson and M.A. Courty (Editors), Micromorphologie des Sols. A.F.E.S., Plaisir, pp. 409-413.

Farres, P., 1978. The role of time and aggregate size in the crusting process. Earth Surface Pro- cess., 3: 243-254.

Guillor6, P., 1983. M6thode de fabrication m6canique et en s6rie des lames minces. D6parte- ment des Sols, INA P-G, Thiverval-Grignon, 22 pp.

Guillor6, P., 1987. Analyse photographique ~ grand champ de lames minces. In: N. Fedoroff,


L.M. Bresson and M.-A. Courty (Editors), Micromorphologie des Sols/Soil Micromorphol- ogy. A.F.E.S., Plaisir, pp. 49-56.

Kooistra, M.J. and Siderius, W., 1986. Micromorphological aspects of crust formation in a sa- vanna climate under rainfed subsistence agriculture. In: F. Callebaut, D. Gabriels and M. De Boodt (Editors). Assessment of soil surface sealing and crusting. Flanders Research Center for Soil Erosion and Soil Conservation, Ghent, 374 p.

Le Bissonnais, Y., 1988. Analyse des m6canismes de d6sagr6gation et de la mobilisation des particules de terre sous l'action des pluies. Th6se, Universit6 d'Orl6ans, 225 pp.

Levy, G., Shainberg, I. and Morin, J., 1986. Factors affecting the stability of soil crusts in sub- sequent storms. Soil Sci. Soc. Am. J., 50: 196-201.

Mclntyre, D.S., 1958. Soil splash and the formation of surface crusts by raindrop impact. Soil Sci., 85:261-266.

Mticher, H.J. and De Ploey, J., 1977. Experimental and micromorphological investigation of erosion and redeposition of loess by water. Earth Surface Process., 2:117-124.

MiJcher, H.J., De Ploey, J. and Savat, J., 1981. Response of loess material to simulated translocation by water: micromorphological observation. Earth Surface Process. Landforms, 6: 331-336.

Norton, L.D., 1987. Micromorphological study of surface seals developed under simulated rainfall. Geoderma, 40: 127-140.

Onofiok, O. and Singer, M.J., 1984. Scanning electron microscope studies of surface crusts formed by simulated rainfall. Soil Sci. Soc. Am. J., 48:1137-1143.

Shainberg, I. and Singer, M.J., 1985. Effect of electrolyte concentration on the hydraulic properties ofdepositional crust. Soil Sci. Soc. Am. J., 49: 1260-1263.

Sor, K. and Bertrand, A.R., 1962. Effects of rainfall energy on the permeability of soils. Soil Sci. Soc. Am. Proc., 26: 293-297.

Southard, R.J., Shainberg, I. and Singer, M.J., 1988. Influence of electrolyte concentration on the micromorphology of artificial depositional crust. Soil S~i., 145: 278-288.

Tarchjtzky, J., Banin, A., Morin, J. and Chen, Y., 1984. Nature, formation and effects of soil crust formed by water drop impact. Geoderma, 33:135-155.

Valentin, C., 1981. Organisations Pelliculaires Superficielles de Quelques Sols de R6gion Sub- d6sertique (Agadez, R6p. du Niger). Dynamique de formation et cons6quences sur l'6con- omie en eau. Th6se Doct. 3Cycle, Univ. Paris VII, Paris, 213 pp.

Valentin, C., 1986. Surface crusting of arid sandy soils. In: F. Callebaut, D. Gabriels and M. De Boodt (Editors), Assessment of Soil Surface Sealing and Crusting. Flanders Research Center for Soil Erosion and Soil Conservation, Ghent, pp. 40-47.

Valentin, C. and Ruiz Figueroa, J.F., 1987. Effects of kinetic energy and water application rate on the development of crusts in a fine sandy loam soil using sprinkling irrigation and rainfall simulation. In: N. Fedoroff, L.M. Bresson and M.A. Courty (Editors), Micromorphologie des Sols/Soil Micromorphology. A.F.E.S., Plaisir, pp. 401-408.

Bresson 1990

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