Soil
Senior 1 · Agriculture
Soil
Lesson 3 of 4
22 min
Let's begin
Rub a pinch of moist soil between your thumb and finger. One soil feels gritty, another smooth like flour, another sticky enough to shape into a ribbon. Potters look for the sticky one; a vegetable grower hopes for something in between.
How a soil feels depends on the size of its particles. That same property decides how well it drains, how much water it stores, how easy it is to dig and which crops will do well in it.
Mineral particles are grouped by diameter. The limits below are those of the USDA system, the most widely used; other systems differ slightly.
| Particle | Diameter | Feel when moist |
|---|---|---|
| Gravel and stones | more than 2 mm | Not counted as fine soil |
| Sand | 0.05 to 2 mm | Gritty; you can see single grains |
| Silt | 0.002 to 0.05 mm | Smooth and silky, like flour |
| Clay | less than 0.002 mm | Sticky and plastic; can be moulded |
Soil texture is the relative proportion (percentage by mass) of sand, silt and clay in a soil. The soil types are named from their texture.
| Property | Sandy soil | Clay soil | Loam |
|---|---|---|---|
| Main particles | Mostly sand | Much clay (roughly 40% or more) | A balanced mix, roughly 40% sand, 40% silt, 20% clay |
| Pore spaces | Few, large | Many, very small | Mix of large and small |
| Drainage and aeration | Very good, fast | Poor, slow; waterlogs easily | Good |
| Water-holding capacity | Low; dries out quickly | High, but some water is held too tightly for roots | Good |
| Nutrient-holding | Low; nutrients are leached | High | Good |
| Workability | "Light": easy to dig at any time | "Heavy": sticky when wet, hard and cracked when dry | Easy to work |
| Capillarity (upward movement of water) | Fast but water rises only a short height | Slow but water rises highest | Moderate |
| Suitable crops | Cassava, groundnuts, carrots, sweet potatoes | Rice in valley marshlands, sugarcane, some vegetables after drainage | Most crops: beans, maize, vegetables, bananas |
| How to improve | Add plenty of organic matter to hold water and nutrients; mulch | Add organic matter so the particles form crumbs; drain it; avoid digging it when wet | Maintain organic matter |
Silty soils are fertile and hold water well, but their fine particles pack down and form a hard crust after rain, and they erode easily.
Organic matter improves every soil type: in sand it holds water and nutrients, and in clay it binds particles into crumbs so that air and water can move.
1. The feel (ribbon) test. Moisten a small ball of soil and squeeze it out between thumb and finger. Sand feels gritty and will not form a ribbon; loam forms a short, cracked ribbon; clay forms a long, smooth, shiny ribbon.
2. The sedimentation (jar) test. Half-fill a clear jar with soil, add water almost to the top with a pinch of salt or soap to separate the particles, shake hard for a few minutes and leave it to stand. The particles settle in order of size: sand first (within a minute), then silt (within an hour or so), then clay (can take a day or more). Organic matter floats on top.
Question: After settling, a jar shows 5 cm of sand, 3 cm of silt and 2 cm of clay. (a) Estimate the percentage of each. (b) Name the likely texture.
(a) Total depth = 5 + 3 + 2 = 10 cm.
Sand = 5 ÷ 10 × 100 = 50%; silt = 3 ÷ 10 × 100 = 30%; clay = 2 ÷ 10 × 100 = 20%.
(b) On the textural triangle, 50% sand, 30% silt and 20% clay lies in the loam class, close to its borders with sandy loam and sandy clay loam. The jar measures volume, not mass, so the answer is only an estimate; a laboratory gives exact figures.
Tip
Memory hook: "Sand sieves, clay keeps, loam is the happy middle": sand lets water through, clay holds on to it, loam balances the two.
Exam technique: in the jar test the ORDER of settling (sand, silt, clay) is the key point, and the reason is particle size: the heaviest particles sink first.
Define soil texture.
Solution: Soil texture is the relative proportion of sand, silt and clay particles in a soil.
Give three reasons why a sandy soil is less productive than a loam.
Solution: Any three: it holds little water, so crops wilt in dry spells; nutrients are leached quickly; it holds few nutrients on its particles; it often has little organic matter.
Explain why clay soils are called "heavy" soils.
Solution: Clay is sticky and hard to dig when wet and sets hard and cracks when dry, so it needs much more effort to cultivate. "Heavy" refers to the work, not the weight.
In a jar test the settled layers are 2 cm sand, 5 cm silt and 3 cm clay. Estimate the percentage of silt.
Solution: Total = 2 + 5 + 3 = 10 cm. Silt = 5 ÷ 10 × 100 = 50%.
How does adding compost improve (a) a sandy soil, (b) a clay soil?
Solution: (a) Humus holds water and nutrients that would otherwise drain away. (b) Humus binds the fine clay particles into crumbs, making larger pores that let air and water move and roots grow.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
