4 A farmer in a semi-arid region has been growing sorghum on the same field for five consecutive seasons. Crop yields have declined each year despite regula...

(a) Three reasons why growing the same crop (sorghum) on the same land every season can reduce soil fertility: [6]

1. Nutrient depletion: The same nutrients are removed from the soil each season [1] because sorghum takes up the same minerals (particularly nitrogen and phosphorus) in similar quantities every crop cycle, and without replacement the soil becomes progressively depleted. [1]
2. Pest and disease build-up: Pests and diseases specific to sorghum accumulate in the soil over successive seasons [1], which reduces plant health and the plant's ability to take up nutrients efficiently. [1]
3. Soil structure deterioration: The soil structure degrades and compaction increases [1] because the same root pattern of sorghum does not break up different soil layers the way a rotation of crops with varying root depths would. [1]

(b) How the farmer could use green manuring to restore fertility: [3]

1. A fast-growing, leafy crop (such as cowpea, Crotalaria or sun hemp) is planted on the field. [1]
2. The crop is ploughed into the soil while still green and actively growing, before it has set seed. [1]
3. As the green plant material decomposes in the soil, it releases nutrients (especially nitrogen, if a legume is used) and adds organic matter (humus), which improves soil structure, water-holding capacity and microbial activity. [1]

(c)(i) What the numbers 15-15-15 on the fertiliser label represent: [3]

1. The first 15 means the fertiliser contains 15% nitrogen (N). [1]
2. The second 15 means it contains 15% phosphorus pentoxide (P\(_2\)O\(_5\)), which is the standard way of expressing the phosphorus content. [1]
3. The third 15 means it contains 15% potassium oxide (K\(_2\)O), the standard expression for potassium content. [1]

These three numbers are always listed in the order N-P-K and represent the percentage by weight of each nutrient in the bag.

(c)(ii) Three advantages of using organic manure rather than inorganic fertiliser: [3]

1. Organic manure improves soil structure and water-holding capacity in addition to supplying nutrients, whereas inorganic fertiliser only provides nutrients. [1]
2. Organic manure releases nutrients slowly over a long period as it decomposes, reducing the risk of nutrients being washed away by leaching. Inorganic fertiliser dissolves quickly and can be lost. [1]
3. Organic manure is often cheaper and more readily available to small-scale farmers in rural areas, since it can be produced on the farm from animal waste and crop residues. [1]

Other acceptable advantages: adds humus that supports beneficial soil organisms; does not cause soil acidification or salt build-up over time.