SOIL CHEMISTRY- COMPETENCE BASED ESSAY QUESTIONS WITH ANSWERS

 

SOIL CHEMISTRY – FORM FOUR

Competence-Based Questions with Answers

Question 1

A farmer notices that crops growing in one part of the farm are not growing well because the soil is too acidic. Explain five ways in which soil acidity can be reduced.

Answer:
i. Apply lime such as calcium carbonate (CaCO₃) to neutralize excess acidity.
ii. Apply wood ash, which contains basic substances that can reduce soil acidity.
iii. Add organic manure to improve soil properties and help buffer soil pH.
iv. Avoid excessive use of fertilizers that increase soil acidity.
v. Test the soil regularly and apply suitable materials according to the soil-test results.

Question 2

A farmer wants to determine whether the soil in his farm is acidic, neutral or alkaline. Describe three methods that can be used to determine soil pH.

Answer:
i. Use a pH meter to measure the hydrogen ion concentration and obtain the pH value.
ii. Use universal indicator and compare the colour produced with a standard pH colour chart.
iii. Use pH indicator paper and compare its colour change with the given pH scale.

Question 3

Explain six importance of soil pH in agricultural production.

Answer:
i. It determines the availability of essential mineral nutrients to plants.
ii. It influences the activities of microorganisms responsible for decomposition and nutrient cycling.
iii. It affects the solubility of different minerals and chemical compounds in the soil.
iv. It influences the growth and development of crops.
v. It affects the efficiency and effectiveness of some fertilizers.
vi. It helps farmers to select crops that are suitable for particular soil conditions.

Question 4

A farmer applies ammonium fertilizer continuously and notices that the soil becomes more acidic. Explain how excessive use of some fertilizers can cause soil acidity.

Answer:
i. Some ammonium fertilizers undergo nitrification in the soil.
ii. During nitrification, hydrogen ions (H⁺) are released.
iii. The increase in hydrogen ions lowers the soil pH.
iv. Continuous application of such fertilizers can therefore make the soil increasingly acidic.

Question 5

Different soil samples have pH values of 4, 7 and 9. Classify each soil sample and explain the meaning of each value.

Answer:
i. pH 4: The soil is acidic because it has a relatively high concentration of hydrogen ions (H⁺).
ii. pH 7: The soil is neutral because the concentrations of H⁺ and OH⁻ ions are approximately equal.
iii. pH 9: The soil is alkaline because it has a relatively higher concentration of hydroxide ions (OH⁻).

Question 6

Explain five effects of highly acidic soils on crop production.

Answer:
i. Essential nutrients such as phosphorus may become less available to plants.
ii. Toxic concentrations of aluminium and manganese may develop in the soil.
iii. Root growth may be restricted.
iv. The activities of beneficial soil microorganisms may be reduced.
v. Poor nutrient availability and toxicity may lead to reduced crop growth and yield.

Question 7

A farmer adds calcium carbonate to acidic soil. Explain the chemical importance of this practice.

Answer:
i. Calcium carbonate reacts with hydrogen ions present in acidic soil.
ii. The reaction reduces the concentration of hydrogen ions.
iii. The soil pH therefore increases towards neutrality.
iv. A simplified reaction is CaCO₃ + 2H⁺ → Ca²⁺ + CO₂ + H₂O.
v. Liming can therefore create more favourable chemical conditions for crop growth.

Question 8

Explain five ways organic matter improves the chemical properties of soil.

Answer:
i. It supplies nutrients to plants when organic matter decomposes.
ii. It increases the soil's ability to retain nutrient ions.
iii. It improves the availability of some mineral nutrients.
iv. It helps to buffer sudden changes in soil pH.
v. It provides favourable conditions for microorganisms involved in nutrient cycling.

Question 9

A farmer uses large amounts of chemical fertilizers without following recommended application rates. Explain five possible chemical effects on the soil.

Answer:
i. The soil may become excessively acidic.
ii. Soluble salts may accumulate in the soil.
iii. An imbalance of nutrients may develop.
iv. Some nutrients may become chemically unavailable to plants.
v. Excess nutrients may be leached into groundwater and other water sources.

Question 10

Explain the importance of nitrogen, phosphorus and potassium (NPK) in soil fertility.

Answer:
i. Nitrogen: It promotes vegetative growth and is essential for the formation of proteins and chlorophyll.
ii. Phosphorus: It promotes root development and plays an important role in energy transfer in plants.
iii. Potassium: It helps regulate water balance, improves plant strength and supports resistance to some stresses.

Question 11

A soil sample contains very little nitrogen. Predict five effects this may have on plants.

Answer:
i. Plants may show stunted or poor growth.
ii. Older leaves may become pale or yellow.
iii. Protein formation may decrease.
iv. Plants may develop small leaves and weak stems.
v. Crop production and yield may decrease.

Question 12

Explain five causes of soil alkalinity in agricultural areas.

Answer:
i. Accumulation of basic salts in the soil can increase alkalinity.
ii. Irrigation with water containing high concentrations of dissolved salts can contribute to alkalinity.
iii. Poor drainage can cause salts to accumulate near the soil surface.
iv. Weathering of alkaline parent rocks can release basic minerals into the soil.
v. Excessive application of certain alkaline materials can increase soil pH.

Question 13

A student adds a small amount of soil to distilled water, shakes the mixture and filters it before testing the filtrate with universal indicator. Explain why these steps are performed.

Answer:
i. Distilled water is used to avoid adding ions that could interfere with the pH test.
ii. Shaking allows soluble substances in the soil to dissolve into the water.
iii. Filtration removes insoluble soil particles from the mixture.
iv. Testing the filtrate allows the pH of the soil solution to be determined.

Question 14

Explain five methods of maintaining soil fertility using knowledge of soil chemistry.

Answer:
i. Apply appropriate fertilizers to replace nutrients removed by crops.
ii. Use organic manure to add nutrients and organic matter to the soil.
iii. Apply lime to acidic soils when soil testing indicates that liming is necessary.
iv. Practice crop rotation to reduce continuous depletion of particular nutrients.
v. Conduct soil testing before applying fertilizers and soil amendments.

Question 15

A farmer discovers that phosphorus fertilizer applied to his acidic soil is not giving the expected results. Explain why soil pH can affect phosphorus availability.

Answer:
i. In strongly acidic soils, phosphorus can react with iron and aluminium compounds.
ii. These reactions may produce compounds that are less soluble.
iii. The phosphorus becomes less available for absorption by plant roots.
iv. Correcting excessive soil acidity can improve phosphorus availability.
v. Soil testing helps determine the appropriate treatment before applying fertilizers.