3 A farmer on a small vegetable farm decides to build a compost heap to produce organic matter for improving the fertility of the soil. Fig. 3.1 shows a cro...

Labelled answer diagram:

(a) Two materials the farmer could add to the compost heap:

1. Crop residues such as plant stems and leaves left after harvest. [1]
2. Animal manure from livestock housed on the farm. [1]

Other acceptable materials: kitchen vegetable waste, grass cuttings, straw, sawdust, or wood shavings. A good compost heap mixes carbon-rich (brown) materials like straw with nitrogen-rich (green) materials like fresh plant waste.

(b) The organisms mainly responsible for breaking down compost materials are bacteria (accept microorganisms, decomposers, or fungi). [1] These organisms secrete enzymes that digest organic matter externally, then absorb the soluble products. As they metabolise the material, they release heat, which is why an active compost heap becomes warm.

(c) The farmer should turn the compost heap regularly because:

1. Turning mixes the materials and introduces air (oxygen) into the centre of the heap. [1]
2. This allows aerobic decomposition to take place, which is faster and more efficient than anaerobic decay, and raises the temperature enough to kill weed seeds and pathogens. [1]

Without turning, the centre becomes anaerobic, decomposition slows dramatically, and the heap produces foul-smelling gases.

(d) Two ways adding mature compost improves a sandy soil:

1. Compost binds sand particles together to form larger crumbs (aggregates), which improves water-holding capacity. [1] Sandy soil on its own drains too freely, but organic matter acts like a sponge.
2. Compost increases the soil's ability to hold nutrients (cation exchange capacity), reducing leaching of fertiliser elements. [1]

Other acceptable answers: improves aeration or increases the population of beneficial soil organisms.

(e)(i) Green manure is a crop that is grown and then ploughed back into the soil while still green. [1] As the plant material decomposes, it adds organic matter and nutrients to the soil. [1] Leguminous green manures (such as clover or cowpea) are especially valuable because they fix atmospheric nitrogen through root nodule bacteria.

(e)(ii)

Advantage: Green manure adds organic matter that improves soil structure, unlike chemical fertiliser which supplies nutrients but does not improve the physical condition of the soil. [1] (Other acceptable answers: cheaper than purchasing chemical fertiliser; does not pollute waterways; leguminous types fix atmospheric nitrogen.)

Disadvantage: Green manure takes time to grow before it can be ploughed in, during which the field cannot be used for a cash crop. [1] (Other acceptable answers: nutrients are released slowly and may not be available when the next crop needs them.)

(f) Two reasons why crop rotation helps maintain soil fertility:

1. Different crops take up different nutrients, so rotating prevents the depletion of any single element from the soil. [1]
2. Legumes in the rotation fix atmospheric nitrogen through symbiotic bacteria in their root nodules, increasing the soil's nitrogen content for the following crop. [1]

Other acceptable reasons: rotation breaks pest and disease cycles, allowing soil biology to recover; different root depths access nutrients from different soil layers.