Question 2
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(a) With the aid of litmus paper and water provided, classify specimen G, H and I according to their pH (3 marks)
(b) Outline the steps taken in classifying specimens G, H and I. (5 marks)
(c) (i) If the pH value of one of the soil sample provided is 4.5, what condition does
this pH value indicate in the soil sample . (1 mark)
(ii) State three effects of the condition indicated in 2(c)(i) on crop production.(3 marks)
(iii) List three factors that could cause the condition indicated in 2(c)(i). (3 marks)
Observation
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Most candidates were able to classify the soil samples according to their pH.
(b) Candidates could not outline the steps taken to classify the soil samples. Some
candidates only recorded their observations.
(c) Most candidates stated that a soil sample with pH of 4.5 is acidic. However, they
found it difficult to stating the effects of soil acidity on crop production as well as
factors that could cause soil acidity.
The expected answers include:
2. (a) Classification of specimens G, H and I according to their pH
Specimen G - neutral soil
H - acidic soil
I - alkaline/basic soil
(b) Steps taken in classifying specimens G, H and I
- Add some quantity of water to each of specimens G, H and I
- Stir thoroughly
- Test each of the specimens with litmus papers
- Specimen G - no change in colour of litmus paper - neutral
- Specimen H – blue litmus paper turned red – acidic
- Specimen I – red litmus paper turned blue – alkaline/basic
(c) (i) Condition indicated by pH value of 4.5
Soil acidity
(ii) Effects of soil acidity on crop production
- Could damage the roots of plants
- Hinders the absorption of mineral ions
- Lowers the rate of decomposition of organic matter due to its adverse
effects on soil microorganisms
- Humus could easily be leached out of the soil
- Causes nutrient deficiency especially calcium and phosphorus
- Reduces microbial population
- Causes certain nutrients to be present in the soil in toxic levels e.g.
manganese and iron
- Depresses nitrification because of its adverse effect on soil microbes/low
pH reduces the activities of soil microorganisms
- Low pH reduces the formation of pods in legumes
- It causes reduced growth and yield of crops(iii) Factors that could cause soil acidity
- Absorption of basic cations/Nutrient uptake by plants
- Use of nitrogen/ sulphur based fertilizers
- Leaching
- Erosion
- Acidic parent material
- Acid rains in industrial areas
- Presence of sulphur in the soil which leads to formation of weak acid.