(a) A smallholder farmer grows maize, beans, groundnuts and vegetables in a four-year crop rotation on the same piece of land. (i) State the meaning of the ...

(a)(i) Crop rotation is the practice of growing different crops in a planned sequence on the same piece of land over a period of years or seasons. [2] Each crop in the sequence is chosen to complement the others in terms of nutrient use, pest susceptibility, and soil structure effects.

(a)(ii) Including a legume in the rotation is important because legumes (such as beans and groundnuts) have root nodules containing nitrogen-fixing bacteria (Rhizobium). [1] These bacteria convert atmospheric nitrogen gas into ammonium/nitrates that remain in the soil after the legume crop is harvested or ploughed in, enriching the soil with nitrogen for the benefit of the next crop in the sequence. [1]

(a)(iii) Two other ways crop rotation helps maintain soil fertility (any two): [2]

1. Different crops have different nutrient requirements and root depths, so alternating them prevents the progressive depletion of any single nutrient and allows nutrients from different soil layers to be utilised. [1]
2. Rotation breaks pest and disease cycles that build up when the same crop is grown repeatedly on the same land. Pathogens and soil-borne pests specific to one crop cannot survive as easily when a different, non-host crop follows. [1]

(b)(i) Mass of nitrogen applied to the field:

\[ \text{Nitrogen} = \frac{20}{100} \times 200 \text{ kg} = 40 \text{ kg} \]

[1] for the correct method and [1] for the answer of 40 kg.

(b)(ii) Rate of nitrogen application per hectare:

\[ \text{Rate} = \frac{40 \text{ kg}}{0.5 \text{ ha}} = 80 \text{ kg per hectare} \]

[1]

(c) Two problems from excessive nitrogen fertiliser (any two): [2]

1. Excess nitrogen promotes excessive vegetative (leaf and stem) growth at the expense of grain or cob formation. The tall, lush plants may also be prone to lodging (falling over). [1]
2. Surplus nitrates that are not taken up by the crop leach through the soil into groundwater, contaminating drinking water sources and contributing to eutrophication of rivers and lakes. [1]

Other valid problems: delayed crop maturity; increased susceptibility to fungal diseases and pest attack due to soft, sappy growth.

(d) The role of phosphorus in plant growth: [2]

1. Phosphorus promotes strong root development and establishment, enabling the plant to access water and nutrients from a larger soil volume. [1]
2. It is essential for energy transfer within the plant (as a component of ATP) and is critically needed for flowering, fruiting, and seed formation. [1]