(a) Explain what is meant by green manuring. [2] (b) A farmer in a tropical region grows cowpeas as a green manure crop before planting cassava. (i) Describ...

(a) Green manuring is the practice of growing a crop (usually a legume) specifically for the purpose of ploughing it back into the soil while the plants are still green and actively growing, [1] in order to add organic matter and nutrients to the soil and improve its fertility. [1]

(b)(i) Two benefits the cowpea crop provides to the soil before it is ploughed in (any two): [2]

1. The cowpea roots bear nodules containing Rhizobium bacteria that fix atmospheric nitrogen, adding nitrogen to the soil even before the crop is ploughed in. [1]
2. The plant canopy covers the soil surface, reducing erosion from rain splash and wind, and suppressing weeds by shading them out and competing for light and space. [1]

(b)(ii) Why ploughing in at the flowering stage is more beneficial than waiting until seed maturity: [2]

1. At the flowering stage, the plant tissue has its highest nitrogen content and is soft and succulent, so it decomposes rapidly once ploughed in. [1]
2. After seed maturity, the stems become woody and lignified, with a high carbon-to-nitrogen (C:N) ratio. Material with a high C:N ratio decomposes slowly, and decomposing bacteria actually draw nitrogen from the surrounding soil to break down the carbon, temporarily reducing the nitrogen available to the next crop (nitrogen immobilisation). [1]

(c) Two other methods of adding organic matter to the soil, apart from green manuring (any two): [2]

1. Applying animal manure (farmyard manure / cattle or poultry manure). [1]
2. Adding compost made from decomposed crop residues and organic waste. [1]

Other acceptable answers: mulching with crop residues; applying biosolids / sewage sludge.