Physical Science Applications in Agriculture

Unit

Physical Science Systems

Problem Area

Agricultural Production Systems

Testing Common Substances for pH

Lesson

Ever see limestone being applied on a grower’s field?

• What is so important about this powdery substance?

• What does it do for the soil?

• Does the composition or type of limestone (liquid, dry, palletized) make a difference in its effectiveness?

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Which plant looks healthier?

• Examine the pictures or actual plants that have received different levels of fertilizer.

• Which plant looks healthier?

• Which plant would produce better and more product?

• How much fertilizer do they apply to get optimum growth of our vegetable plants?

• Is there a way to determine this? How?

Learning Objectives

1. Define pH and discuss its role in plant nutrition.

2. Explain how soils become acidic.

3. Explain how soil pH is measured.

4. Explain why lime is applied to acid soils.

5. Discuss the effectiveness of lime on acidic soils.

Terms

• Acid• Calcium carbonate equivalent• Cation• Cation exchange capacity• Lime requirement• Percent base saturation• pH scale• Soil pH

What is pH?

• Soil pH is the measure of acidity or alkalinity of the soil.

What is the pH scale?• The pH scale is a fourteen point scale used to

measure pH.– A neutral pH is 7.0.– A solution with a pH between zero and 6.9 is

considered acid.– A solution with a pH between 7.1 and 14.0 is

considered alkaline or base.– The scale is expressed in logarithmic terms.

• Each unit change in pH corresponds to a tenfold change in acidity or alkalinity.

• A pH of 6.0 is 10 times more acidic than a pH or 7.0.

How does pH affect plant nutrition?

• The pH value of soil is important to agriculturalists because certain nutrients become unavailable to plants if the pH value is too high or too low.

• The amount of nutrients that are available is dependent upon soil pH.

How does pH affect plant nutrition?

How do soils become acidic?

• pH is determined by the concentration of hydrogen (H+) ions and hydroxyl ions (OH-) in the soil solution.

How do soils become acidic?

• A sample of pure water has an equal number of H+ and OH- and is neutral.

• An acid is a substance that releases hydrogen ions.

• When saturated with H+, a soil behaves as a weak acid.

• The more H+ held on the exchange complex, the greater the soil’s acidity.

Several factors influence soil pH.

• Soil organic matter is continuously being decomposed by micro organisms into organic acids, carbon dioxide, and water, forming carbonic acid.

• Carbonic acid reacts with Ca and Mg carbonates in the soil to form more soluble bicarbonates, which are leached away, leaving the soil more acid.

Several factors influence soil pH.

• As water from rainfall passes through the soil, basic nutrients such as calcium and magnesium are leached.

• They are replaced by acidic element including aluminum, hydrogen, and manganese.

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Several factors influence soil pH.

• Soils formed under forest vegetation tend to be more acidic than those developed under grasslands.

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Several factors influence soil pH.

• Soils often become more acidic when crops are harvested because bases are

• removed.

• Legumes generally contain higher levels of bases than grasses.

• Legumes also release H+ ions into their rhizosphere when actively fixing atmospheric N.

Several factors influence soil pH.

• Nitrogen from fertilizer, organic matter, manure, and legume N fixation produces acidity.

• Nitrogen fertilization speeds up the rate at which acidity develops.

• At lower N rates, acidification rate is slow, but is accelerated as N fertilizer rates increase.

ACID AND ALKALINE SOILS

How is soil pH measured?

• The two most commonly accepted methods of measuring soil pH are indicator dyes and the pH meter.

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How is soil pH measured?

• Indicators are frequently used in the field to make a rapid pH determination and must be used by a trained hand to avoid major error.

• The more accurate and widely used method is the pH meter used in soil testing laboratories.

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Why is lime applied to acidic soils?

• Since various plants require different pH levels for optimum growth, growers must attempt to adjust soil pH to suit the crop or plant being grown.

• This involves the use of limestone to raise pH or the use of alum to lower pH.– Lime requirement is the amount of

agricultural limestone needed to establish the desired pH range.

Why is lime applied to acidic soils?• Soil pH is an excellent single indicator of soil

acidity, it does not determine lime requirement.• Lime requirement of a soil is not only related to

the pH but also to its buffer capacity or cation exchange capacity.– Cation exchange capacity is the total

number of exchangeable cations, an ion with a positive charge, a soil can adsorb.

– The relative amount of the cation exchange capacity filled with basic cations is called percent base saturation.

– Soil pH is a measure of the percent base saturation.

Why is lime applied to acidic soils?

• Lime replaces hydrogen and aluminum on the cation exchange sites with calcium and changes hydrogen ions to water.

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Factors to figure how much lime is required

1. the present pH

2. the desired pH

3. the cation exchange capacity of the soil

4. the liming material to be used

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The effectiveness of lime on soil.

• The effectiveness of lime depends on:– Purity– Fineness– Rate it dissolves

• Measured as the calcium carbonate equivalent

What determines the effectiveness of lime on acid soils?

• The neutralizing power of lime depends upon its purity, measured as the calcium carbonate equivalent.– Neutralizing values of all liming materials are

determined by comparing them to the neutralizing value of pure claim carbonate.

– Setting the neutralizing value of calcium carbonate at 100, a value for other materials can be assigned.

What determines the effectiveness of lime on acid soils?

• When a given quantity of lime is mixed with the soil, its reaction rate and degree of reactivity are affected by particle size.1. Coarse live particles react more slowly and

less fully.2. Fine lime particles react more rapidly and

much more completely.3. Cost of lime increases with the fineness of

grind. 4. The goal is a material that requires a

minimum of grinding, yet contains enough fine material to cause a rapid pH change.

Other important factors determining the effectiveness of lime

1. Placement for maximum contact with the soil in the tilled layer is essential.

2. Even when properly mixed with the soil, lime will have little effect on pH if the soil is dry.• Moisture is essential for lime-soil reaction to

occur.

3. The full benefit of lime is not seen until the second or third year after application.• Lime does not react with acidic soil very far

from the lime particle.

Review/Summary

• What is pH and how does it affect plant nutrition?

• How do soils become acidic?

• How is soil pH measured?

• Why is lime applied to acidic soils?

• What determines the effectiveness of lime on acid soils?