Field Soil pH Organic matter / % Available nitrogen / mg per kg Previous crop A 6.2 3.1 28 Maize B 4.5 1.8 12 Cassava C 7.1 2.6 35 Groundnuts D 6.8 5.4 42 G...

(a) The field with the most acidic soil is Field B, with a pH of 4.5. [1] This is the lowest pH value in the table, and lower pH means higher acidity.

(b) Why Field D has a higher organic matter content than the other fields: [2]

• Field D has the highest organic matter percentage at 5.4%. [1]
• The previous crop was grass fallow, meaning the land was left under grass rather than being cultivated. During fallow, grass roots, dead plant material and organic residues accumulate in the soil without being disturbed by ploughing, allowing organic matter to build up over time. [1]

(c) Two steps to make Field B suitable for maize production (which requires pH 5.8-7.0): [4]

1. Apply lime (calcium carbonate, CaCO\(_3\)) to raise the soil pH from 4.5 into the required range of 5.8 to 7.0. [1] Liming neutralises soil acidity, reduces aluminium toxicity (which stunts root growth at low pH) and makes essential nutrients more available to the plant. [1]
2. Add organic manure or compost to increase the organic matter content from its current low level of 1.8%. [1] This improves the soil's nutrient supply, water-holding capacity and structure, all of which support healthy maize growth. [1]

(d) Two reasons why soil organic matter is important for maintaining soil fertility: [2]

1. As organic matter decomposes, it releases essential plant nutrients (nitrogen, phosphorus, sulphur) in forms that plant roots can absorb. [1]
2. It improves soil structure and aggregation, which increases the soil's ability to hold water and air, both of which are needed for root growth and nutrient uptake. [1]

Other acceptable reasons: supports soil organisms (earthworms, beneficial microbes); increases cation exchange capacity (the soil's ability to hold and release nutrient ions).