Soil is the loose surface of the land that roots grow in. A useful soil holds water, air, mineral particles and organic matter at the same time.
The idea
Mineral particles come in three sizes used in class. Sand particles are the largest. Water drains through sand quickly, and plant nutrients wash away with it. Clay particles are the smallest. Clay holds water and nutrients, and it can become waterlogged and hard when it dries. Silt sits between sand and clay in size. Loam is a mixture with enough sand to drain and enough clay and organic matter to hold water and nutrients. Many food crops prefer a loam. Texture is the mix of those particle sizes. Structure is different: it is how the particles clump. A crumb structure leaves small gaps for air and for roots.
Organic matter, from decayed plants and animal remains, improves structure and feeds soil life. A soil that is only mineral powder is a weak medium for crops. Soil pH describes acidity. Many arable crops do well near neutral. A strongly acid soil can be improved with lime, which raises the pH, but lime is not a fertiliser and it is not the answer on every field. Erosion is the removal of topsoil by water or wind. On a slope, water cuts channels if the soil is bare. Cover crops, mulch, contour ploughing and strips of permanent vegetation slow the water down. The topsoil is the part a farmer cannot afford to lose, because that is where most of the organic matter and the feeding roots are.
The General Agriculture tips guide sets out the papers, how to answer the essay and the practical, and the mistakes that cost marks. WASSCE General Agriculture tips
What earns the marks
Texture and structure are different marks. Texture is particle size. Structure is the arrangement into crumbs or clods. Do not call loam a particle size. It is a mixture. Sand drains fast and holds little. Clay holds water and can drain poorly. An erosion answer needs the agent, water or wind, and a practice that slows it. "Plant trees" is weak unless you say the trees or the cover break the force of the rain or the wind. Lime corrects acidity. It does not replace nitrogen.
Practice questions
These are practice questions in the style of the paper, with a model answer and a mark scheme. They are not a promise of what your paper will ask.
1Question 1
5 marks
(a) Distinguish between soil texture and soil structure. (4 marks)
(b) State what loam is. (1 mark)
Model answer
(a) Texture is the proportion of sand, silt and clay. Structure is the way those particles clump into crumbs or clods.
(b) Loam is a mixture of sand, silt and clay, usually with organic matter, that both drains and holds water.
Mark scheme
Texture is particle size, or the mix of sand, silt and clay2 marks
Structure is how particles are arranged or clumped2 marks
Loam is a mixture, not a single particle size1 mark
2Question 2
2 marks
Give one property of a sandy soil and one property of a clay soil that matter to a farmer. (2 marks)
Model answer
A sandy soil drains quickly and does not hold nutrients well. A clay soil holds water, and it can become waterlogged.
Mark scheme
Sand: fast drainage, low water holding, or easy nutrient loss1 mark
Clay: high water holding, slow drainage, or hardness when dry1 mark
3Question 3
4 marks
State two practices that reduce erosion by water on a slope, and explain how one of them works. (4 marks)
Model answer
Contour ploughing and a cover crop. The cover crop keeps rain from striking bare soil, so less soil is detached and washed downhill.
Mark scheme
One practice, such as contour ploughing, cover crops, mulch or strips of vegetation1 mark
A second practice, different from the first1 mark
How the chosen practice slows water or protects the surface2 marks
Practise General Agriculture with Anna
More questions on this topic are inside glearn, marked as you answer, with the reason written out. The first ones need no account.
What is the difference between texture and structure?+
Texture is the mix of sand, silt and clay. Structure is how those particles clump. Loam is a mixture of particle sizes, which is why many crops are grown on it. A crumb structure leaves space for air and roots.
How does a farmer slow erosion on a slope?+
Keep the soil covered, plough across the slope rather than up and down it, and use mulch or strips of vegetation. The aim is to stop rain hitting bare soil and to slow the water that runs downhill. All WASSCE General Agriculture past questions