1 Fig. 1.1 shows a cross-section through a soil profile in a tropical farming region. Fig. 1.1 (a) Name the soil horizon labelled A on Fig. 1.1. [1] (b) Sta...

Labelled answer diagram:

(a) The soil horizon labelled A is the topsoil (also called the A horizon). [1] This is the uppermost layer of the soil profile that plant roots primarily grow in.

(b) Two reasons why horizon A is darker in colour than the layers below: [2]

1. It contains more humus (decomposed organic matter), which is dark brown to black in colour. [1]
2. It contains more decomposed plant and animal remains because organic material accumulates at the surface where dead leaves, roots and organisms are deposited and broken down by soil organisms. [1]

(c) How leaching reduces soil fertility: [2]

1. Soluble mineral nutrients (such as nitrates, potassium and calcium) are dissolved and washed downward through the soil profile by percolating rainwater. [1]
2. These nutrients are carried beyond the reach of plant roots into deeper soil layers or the water table, so they are no longer available for plant uptake. [1]

Leaching is particularly severe in tropical regions with high rainfall and light, sandy soils that have poor nutrient-holding capacity.

(d) Two ways to improve fertility of a leached soil: [2]

1. Add organic manure or compost to replace lost nutrients and increase the soil's ability to hold nutrients (through improved cation exchange capacity). [1]
2. Apply inorganic (chemical) fertiliser containing nitrogen, phosphorus or potassium to directly replenish the specific nutrients that have been lost. [1]

Other acceptable answers: add lime to raise pH and reduce leaching of cations; use cover crops or mulching to slow water movement through soil.