Soil
Senior 2 · Agriculture
Soil
Lesson 3 of 5
22 min
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A soil can contain plenty of phosphorus and still leave a bean crop starving for it. What matters to a plant is not how much of a nutrient is in the soil, but how much is available: dissolved in the soil water, in a form the roots can absorb.
When a crop cannot get enough of a nutrient, it shows signs on its leaves, stems and fruits. A farmer or extension officer who can read those signs can often act before the harvest is lost.
Plants need 17 essential elements. Carbon, hydrogen and oxygen come from air and water. The rest come from the soil:
The first clue is where the symptom appears. Some nutrients are mobile in the plant: when they run short, the plant moves them from old leaves to new growth, so the older, lower leaves show symptoms first. Other nutrients are immobile: once built into a leaf they stay there, so the young leaves and growing tips suffer first.
| Nutrient | Mobile? | Typical deficiency symptoms |
|---|---|---|
| Nitrogen (N) | Yes | Stunted, spindly plants; older leaves turn pale yellow (chlorosis). In maize the yellowing runs from the leaf tip down the midrib in a V shape. |
| Phosphorus (P) | Yes | Stunted plants with dark green or purplish leaves (young maize often turns purple), poor roots, late maturity. |
| Potassium (K) | Yes | Edges of older leaves turn yellow then brown and dry (scorching); weak stems that fall over (lodging); poor fruit. |
| Magnesium (Mg) | Yes | Yellowing between the veins of older leaves while the veins stay green (interveinal chlorosis). |
| Calcium (Ca) | No | Young leaves deformed, growing tips die; weak roots; blossom-end rot in tomatoes. |
| Sulphur (S) | No | Young leaves uniformly pale yellow (looks like N deficiency, but on new leaves). |
| Iron (Fe) | No | Interveinal chlorosis on the youngest leaves. |
| Zinc (Zn) | Partly | Small leaves bunched together because the stem stops lengthening (rosetting). |
| Boron (B) | No | Terminal buds die; hollow or cracked stems; poor flowering and fruit set. |
Visual symptoms are only a guide. Several deficiencies look alike, disease or drought can cause yellowing too, and a crop can lose yield before any symptom shows. Leaf and soil analysis give the certain answer.
Question: Umutoni's maize is knee-high. The lower leaves are yellow, starting at the tips and spreading down the middle of the leaf; the top leaves are green. The plants are shorter than her neighbour's. The field has had maize for four seasons with no manure. Identify the likely deficiency and suggest a remedy.
Step 1: where? Older, lower leaves, so a mobile nutrient (N, P, K or Mg).
Step 2: what pattern? Yellowing from the tip along the midrib (V-shape), with stunting. Potassium would scorch the leaf edges; magnesium would yellow between veins; phosphorus would give purplish leaves. The pattern fits nitrogen deficiency.
Step 3: why? Four seasons of maize with no manure have used up the soil's nitrogen and organic matter.
Remedy: top-dress with a nitrogen fertiliser such as urea now; in future seasons add compost or farmyard manure and rotate with beans or soybean.
Tip
Memory hook: "Mobile moves out of the old": N, P, K and Mg show on old leaves first; Ca, S, Fe and B show on the young ones.
Exam technique: in a diagnosis question, state the position of the symptom, then the pattern, then your conclusion. Examiners reward the reasoning, not only the name of the nutrient.
Explain why a soil with plenty of phosphorus may still produce phosphorus-deficient beans.
Solution: The phosphorus may be unavailable. In a strongly acid soil it is bound to iron and aluminium; roots damaged by aluminium also absorb less. Dry soil, poor aeration or cold can also reduce uptake.
Leaves at the top of a tomato plant are pale yellow between the veins, while the lower leaves are healthy. Is the missing nutrient mobile or immobile? Suggest which one.
Solution: Immobile, because the young leaves are affected. Interveinal chlorosis on young leaves points to iron.
A farmer digs in a large amount of dry maize straw and the next crop turns pale. Explain.
Solution: Straw has a lot of carbon and little nitrogen. Microorganisms decomposing it take nitrogen from the soil for their own growth (immobilisation), so less is left for the crop until decomposition is well advanced.
State the effect of liming on (a) aluminium toxicity, (b) phosphorus availability in an acid soil.
Solution: Lime raises the pH. (a) Less aluminium dissolves, so toxicity falls. (b) Less phosphorus is locked up by aluminium and iron, so more becomes available.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
