7 Fig. 7.1 shows a section of farmland where sheet erosion has removed the topsoil, leaving a lighter-coloured subsoil exposed. (a) Describe what is meant b...

Labelled answer diagram:

(a) Definition of sheet erosion [2]:

Sheet erosion is the uniform removal of a thin layer of topsoil over a wide area by shallow overland flow. [1] It is caused by rainwater that flows as a broad, shallow sheet rather than concentrating into defined channels or rills. [1]

(b) Why sheet erosion is more difficult to detect than gully erosion [2]:

1. The soil surface remains relatively smooth with no visible channels or gullies. There is no dramatic physical feature to draw the farmer's attention. [1]
2. The change happens gradually over many seasons. The farmer may not notice the slow thinning of topsoil until the lighter-coloured subsoil becomes visibly exposed or crop yields decline noticeably. [1]

(c) Three ways losing topsoil through sheet erosion reduces crop yields [3]:

1. Organic matter and humus are lost, reducing the soil's natural nutrient supply and its ability to support microbial life that cycles nutrients. [1]
2. Fine clay and silt particles are selectively removed (they are the lightest and first to move), reducing the soil's water-holding capacity. The remaining coarser particles drain too quickly. [1]
3. Soil structure deteriorates, becoming compacted or crusted, which makes root penetration harder and reduces seedling emergence. [1]

Also acceptable: applied fertilisers are washed away before crops absorb them; seed and young seedlings may be displaced or buried by moving soil.