Soil
Senior 2 · Agriculture
Soil
Lesson 1 of 5
18 min
Let's begin
Two neighbours in Nyamagabe plant the same bean variety on the same day. One harvests three full sacks; the other harvests less than one. They had the same seed and the same rain. What differed was the soil.
Most Rwandan farms are small, often well under one hectare, so a farmer cannot simply move to new land when a field stops producing. The only way to feed a growing family from the same plot year after year is to keep that soil fertile. This lesson explains what fertility means, how to recognise a fertile soil, and how the wild plants growing on a plot can give you a first clue.
Soil fertility is the ability of a soil to supply crops with enough nutrients and water, in the right amounts and proportions, so that they grow well and give good, steady yields year after year. A fertile soil must also be free of substances that poison roots.
Fertility is not the same as productivity. A soil can be rich in nutrients and still give a poor harvest if the farmer plants late, uses poor seed or lets weeds take over. Fertility is what the soil can offer; productivity is what the farmer actually harvests.
| Characteristic | Why it matters |
|---|---|
| Deep enough for roots | Roots can spread widely to reach water and nutrients, and the plant is firmly anchored. |
| Good drainage and aeration | Roots need oxygen to respire. Waterlogged soil suffocates most crops (rice is an exception). |
| Plenty of organic matter (humus) | Humus darkens the soil, holds water and nutrients, and binds particles into crumbs. |
| Good water-holding capacity | Crops keep growing between rains. |
| Suitable pH, about 5.5 to 7.0 | Most crop nutrients are easiest for roots to take up in this range. |
| All essential nutrients in available form | Nutrients locked in rock or bound tightly to particles are no use to a plant. |
| Many useful soil organisms | Earthworms, bacteria and fungi break down residues and release nutrients. |
| Free from toxic substances, pests and diseases | Toxic aluminium, salt or soil-borne diseases can ruin a crop even in rich soil. |
| Good (crumb) structure | Crumbs leave spaces for air, water and roots to move. |
An indicator plant is a wild plant whose presence and vigour tell you something about the soil where it grows. Farmers have used them for centuries, because some plants only grow well in rich soil while others survive where crops struggle.
| What the soil is like | Common indicator plants |
|---|---|
| Fertile soil, rich in nitrogen and organic matter | Amaranth (pigweed), blackjack (Bidens pilosa, known in Rwanda as inyabarasanya), wandering Jew (Commelina), black nightshade, Oxalis. When these grow tall, dark green and thick, the soil is usually rich. |
| Poor, exhausted soil | Spear grass (Imperata cylindrica), love grasses (Eragrostis), and in cereal fields the parasitic witchweed (Striga). |
| Acid soil | Bracken fern (Pteridium), common on some high, rainy hillsides. |
| Waterlogged soil | Sedges (Cyperus) and papyrus. |
An indicator plant is only a clue. A single blackjack plant proves nothing; a whole field of tall, deep-green blackjack is strong evidence. Only a soil test can confirm the pH and nutrient levels.
Question: Mugisha has two plots. Plot A has dark brown, crumbly soil with thick, deep-green amaranth and blackjack. Plot B, higher up the hill, has pale, hard soil covered with short spear grass and bracken fern. (a) Which plot is likely more fertile? (b) What might be wrong with Plot B? (c) How can Mugisha be sure?
(a) Plot A. Dark colour shows organic matter, crumbs show good structure, and vigorous amaranth and blackjack indicate plenty of nitrogen.
(b) Spear grass suggests an exhausted soil with few nutrients, and bracken suggests the soil is acidic. Its pale colour shows little organic matter, possibly because topsoil has been washed down the slope.
(c) He should take soil samples from both plots for a soil test, which measures the pH and the main nutrients. The result tells him whether Plot B needs lime, manure or fertiliser, and how much.
Tip
Memory hook: a fertile soil is DAMP-TOP: Deep, Aerated, Moist (holds water), PH about 5.5–7, Toxin-free, Organic matter and organisms, Plant nutrients available.
Exam technique: when asked for characteristics of a fertile soil, say why each one helps the crop. "Has organic matter" earns less than "has organic matter, which holds water and releases nutrients".
Try each question before reading the solution.
Define soil fertility.
Solution: Soil fertility is the ability of a soil to supply crops with enough nutrients and water, in the right proportions, for good and sustained yields, while being free of substances that harm roots.
Explain why a very fertile soil may still give a poor harvest.
Solution: Yield (productivity) also depends on management and weather: poor seed, late planting, weeds, pests or drought can all reduce the harvest even when the soil itself is rich.
Name two plants that indicate a fertile soil and one that indicates an acid soil.
Solution: Fertile: any two of amaranth, blackjack (inyabarasanya), wandering Jew (Commelina), black nightshade, Oxalis. Acid: bracken fern.
A field of maize is heavily attacked by witchweed (Striga). What does this suggest about the soil?
Solution: Witchweed is most serious on poor, exhausted soils that are low in nitrogen and organic matter, so the soil probably needs manure and better management, such as rotation with legumes.
Why is a soil test better than relying on indicator plants alone?
Solution: Indicator plants only give a rough clue, and many things affect how they grow. A soil test measures the actual pH and nutrient levels, so the farmer knows exactly what to add and how much.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
