Soil
Senior 1 · Agriculture
Soil
Lesson 2 of 4
22 min
Let's begin
When a road is cut through a Rwandan hill, the fresh bank shows the soil in layers: a dark band at the top, a redder or paler band below it, then broken, crumbly rock. A farmer building a radical (bench) terrace digs into those same layers, and if the dark top layer is buried, the new terrace bench may grow poor crops for several seasons.
This lesson looks at what a soil is made of and at the layers you see when you cut down through it.
A typical soil is about half solids and half pore space (the spaces between particles). By volume, a good agricultural soil (a loam) is roughly:
| Component | Share by volume (typical) | Role for the crop |
|---|---|---|
| Mineral matter (sand, silt, clay, stones) | about 45% | Forms the soil's skeleton, anchors roots and releases mineral nutrients as it weathers |
| Organic matter (humus, dead plants and animals) | about 5% | Holds water and nutrients, releases nitrogen, phosphorus and sulphur as it decays, binds particles into crumbs, feeds soil organisms |
| Soil water (with dissolved nutrients, the "soil solution") | about 25% | Dissolves nutrients so roots can absorb them; keeps plants turgid; needed for photosynthesis |
| Soil air | about 25% | Supplies oxygen for root respiration and for useful soil organisms |
Water and air share the pore space, so when one increases the other decreases. After heavy rain the pores fill with water and air is pushed out; that is why a waterlogged soil suffocates roots.
Soil also contains living organisms: earthworms, termites, insects, fungi, bacteria and plant roots. They make up only a tiny share of the volume but do most of the work of decomposing organic matter and mixing the soil.
(a) A fresh soil sample weighs 50 g. After drying it weighs 42 g. What percentage of the fresh soil was water?
Water = 50 − 42 = 8 g. Percentage = 8 ÷ 50 × 100 = 16%.
(b) A 40 g sample of the dry soil is heated strongly and then weighs 37.6 g. What percentage of the dry soil was organic matter?
Organic matter = 40 − 37.6 = 2.4 g. Percentage = 2.4 ÷ 40 × 100 = 6%.
(c) 50 cm³ of dry soil is added to 50 cm³ of water. When the bubbles stop, the level reads 88 cm³. What percentage of the soil's volume was air?
Expected volume = 50 + 50 = 100 cm³. Air = 100 − 88 = 12 cm³. Percentage = 12 ÷ 50 × 100 = 24%.
A soil profile is a vertical section through the soil from the surface down to the parent rock. It shows layers called horizons, which differ in colour, texture, structure and organic matter.
| Horizon | Name | Features |
|---|---|---|
| O | Organic layer (litter) | Fallen leaves and plant remains, fresh at the top and partly decomposed below. Thin or absent on cultivated land. |
| A | Topsoil | Dark (humus), loose and crumbly; most roots and soil organisms; most fertile. Minerals are washed down out of it (leaching). |
| B | Subsoil | Lighter or redder, firmer; collects clay, iron and other materials washed down from the A horizon; few roots and little humus. |
| C | Weathered parent material | Broken, partly weathered rock; no humus; very few roots. |
| R | Bedrock | Solid, unweathered parent rock. |
Tip
Memory hook: read the profile from the top: "Old Apples Become Compost Remains" gives O, A, B, C, R.
Exam technique: in percentage calculations, write the formula first: percentage = part ÷ whole × 100, and say what the "whole" is. The water percentage uses the FRESH mass; the organic-matter percentage uses the DRY mass.
Name the four main components of soil and give the typical share by volume of each in a loam.
Solution: Mineral matter about 45%, organic matter about 5%, water about 25%, air about 25%.
Explain why crops suffer in a waterlogged soil.
Solution: Water and air share the same pore spaces. In waterlogged soil water fills the pores and pushes out the air, so roots and useful organisms get no oxygen for respiration.
A fresh sample weighs 80 g and 68 g after drying. Find the percentage of water in the fresh soil.
Solution: Water = 80 − 68 = 12 g. 12 ÷ 80 × 100 = 15%.
Give two differences between topsoil and subsoil.
Solution: Any two: topsoil is darker because it contains humus, subsoil is lighter or redder; topsoil holds most roots and organisms, subsoil few; topsoil is loose and crumbly, subsoil firmer; topsoil loses minerals by leaching, subsoil collects them.
Why should a farmer digging a terrace keep the topsoil aside and spread it back on the new bench?
Solution: The topsoil holds most of the humus, nutrients and organisms. If it is buried under subsoil, the bench surface will be infertile and crops will grow poorly until organic matter is built up again.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
