Fig. 1 shows a cross-section through a soil profile taken from a piece of agricultural land. (a) State the name given to each of the following layers in the...

Labelled answer diagram:

(a) Names of the soil profile layers [3]

• (i) Topsoil / A horizon [1]. This is the uppermost mineral layer, dark in colour because it is rich in humus (decomposed plant and animal material). Most biological activity and root growth occur here.
• (ii) Subsoil / B horizon [1]. This layer lies below the topsoil and is where minerals leached from above (such as iron oxides and clay particles) accumulate. It is typically lighter in colour and denser than topsoil.
• (iii) Parent rock / C horizon / bedrock layer [1]. This is the lowest part of the profile, consisting of partly broken and weathered rock fragments. It is the geological material from which the soil above is gradually forming.

(b) Two visible differences between topsoil and subsoil [2]

Any two differences that could be observed in the diagram (1 mark each):

• The topsoil is darker in colour than the subsoil because it contains more humus [1].
• The topsoil shows more root penetration than the subsoil, where roots are sparse or absent [1].
• The topsoil contains more organic matter and living organisms than the subsoil [1].
• The subsoil may show more mineral deposits or clay accumulation (sometimes visible as a different colour band or mottling) compared to the topsoil [1].

(c) How weathering of parent rock contributes to soil formation [3]

Soil formation from rock is a slow, multi-process transformation:

1. Physical weathering breaks the solid rock into progressively smaller fragments through forces such as freeze-thaw action (water expanding as it freezes in cracks) and temperature changes that cause repeated expansion and contraction [1].
2. Chemical weathering dissolves minerals within the rock fragments through reactions with water and weak acids (such as carbonic acid from dissolved CO₂), releasing mineral ions into the developing soil [1].
3. Biological weathering contributes as plant roots grow into cracks and physically prise rock apart, and organisms such as lichens and bacteria chemically attack rock surfaces [1]. Over long periods, the weathered rock fragments mix with organic matter from dead plants and animals to produce the mineral and organic components of soil.

(d) Why plant roots concentrate in the upper soil layers [2]

Any two valid reasons (1 mark each):

• The upper layers contain the most nutrients and organic matter, which roots need to absorb for plant growth [1].
• The upper layers have better aeration - more air-filled pore spaces that supply the oxygen roots require for aerobic respiration [1].
• The upper layers are less compacted and have a looser structure, making it physically easier for roots to grow through them [1].
• The lower layers are progressively denser, rockier and harder, creating physical resistance that roots cannot easily penetrate [1].