Agricultural Science Paper 2, Nov/Dec 2010  
Questions: 1 2 3 4 5 6 7 8 9 10 Main
General Comments

Question 3

(a) List two sources of carbon dioxide in the atmosphere.                   [2 marks]

(b) State (i) four causes of soil acidity;                                               [4 marks]
             (ii) two ways by which soil pH influences crop growth.          [ 2 marks]

(c) List three ways by which each of the following biotic factors affects agricultural production:
                   (i) parasites;
                  (ii) soil organisms.                                                          [ 6 marks]

(d) State two advantages of zero tillage.                                             [2 marks ]


This question was not popular with the candidates and their performance was below average. In question 3 (a), many candidates were able to list sources of carbon dioxide in the atmosphere.

However, in question l(b)(i & Ii), most of the candidates could not state the causes of soil acidity and ways by which soil pH influences crop growth. In question 3(c) (i & ii), most of the candidates were able to list the ways by which biotic factors such as parasites and soil organisms affect agricultural production. While in question
3(d), many candidates could not state the advantages of zero tillage.

The expected answers include:

Causes of Soil acidity

- Leaching
- Respiration of soil organisms
- Prolong application of nitrogenous fertilizers
- Accumulation of salt in arid areas from irrigated water
- Crop removal
- Presence of sulphur
- Acid rain
- Nutrients uptake by plants
- Waterlogging
- Presence of humus
- Biological activities in the soil.

Ways by which biotic factors affect Agricultural Production:


- cause damage to the host e.g. tapeworms.
- transmit certain diseases to the host e.g. ticks and nematodes.
- reduce the quality of farm produce like fruits, seeds, meat skin etc.
- cause death of plants and animals.
- reduce the quantity of farm produce.

Soil organisms: They

- add manure and humus to the soil by putrefaction.
- help to fix atmospheric nitrogen into the soil e.g rhizobium leguminosarium.
- help to aerate the soil.
- help in soil formation.
- reduce the quantity of crops.
- reduce the quality of crops.
- cause plant and animal diseases e.g. nematode.

Advantages of zero tillage

- Lessens the direct impact of raindrops on bare soil
- Minimizes degradation of soil structure
- Slows down the rate of mineralization, leading to more sustainable use of nutrients in the organic matter
- Requires less labour than maximum tillage
- Can be practiced on marginal soils that might not otherwise be suitable to cultivate


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