Soil: more than dirt (Dokuchaev)
Soil is made of tiny bits of rock, air, water, dead plant and animal matter called humus, and living things such as worms, bacteria and fungi. The dark top layer, rich in humus, is where most roots grow.
The Russian scientist Vasily Dokuchaev (about 1880) studied soil in the field and showed that soil is a natural body of its own, not just crushed rock. It forms slowly under five factors: the parent rock, climate, living things, the shape of the land and time. He also showed that soil has layers (horizons) and that different regions have different soil types. The dark, humus-rich black earth (chernozem) of the steppe is a famous example.
Healthy soil holds water like a sponge, lets air in, and stores minerals for roots.
The zones of a root
A young root has five zones. From the tip going up:
- Root cap: a small hat of cells that protects the tip as it pushes through soil.
- Division zone: cells here keep dividing to make new cells.
- Elongation zone: the new cells stretch and make the root longer.
- Root hair zone (absorption zone): thousands of thin hairs take in water and minerals.
- Conduction zone: tubes here carry the water up to the stem. Side roots start here.
The hairs live only a few days. New ones keep forming near the tip as the root grows.
How roots take in water: root hairs and root pressure
Each root hair is one long cell. It has a very large surface and slips between the soil grains. The soil water holds few minerals. The sap inside the hair cell is stronger. So water moves into the hair by itself, like water moving toward a sugar solution. Minerals are pulled in too, using the plant's energy.
From the hair, water and minerals go across the root into the tubes (xylem) and up. The root also pushes the water a little. This push is root pressure. You can see it: cut a young plant just above the soil and sap drips from the stump. At night, when leaves are not pulling water, drops may appear on leaf tips (guttation). For tall trees, the main upward force comes from the leaves losing water, but root pressure still helps.
Mineral nutrition: macro-elements and micro-elements
Plants make sugar from air and water, but they must take minerals from the soil. The elements fall into two groups.
- Macro-elements, needed in large amounts: nitrogen (N), phosphorus (P), potassium (K), also calcium, magnesium, sulphur.
- Micro-elements, needed in tiny amounts: iron (Fe), zinc (Zn), manganese (Mn), copper, boron and others.
| Element | Helps with | If missing |
|---|---|---|
| Nitrogen | Green leaves, growth | Pale yellow leaves, small plant |
| Phosphorus | Roots, flowers, seeds | Poor roots, purple leaves |
| Potassium | Strong stems, fruit | Weak stems, brown leaf edges |
| Iron | Making chlorophyll | Yellow new leaves |
Fertilisers: feeding the soil
When we harvest a crop, the minerals leave with it. Fertilisers put them back.
- Organic fertilisers (compost, manure, green manure) come from living things. They release minerals slowly and add humus.
- Mineral (chemical) fertilisers, like urea or NPK mixtures, are quick and strong.
Use the right amount at the right time. Too much fertiliser makes the soil water stronger than the root sap, so water leaves the roots instead of entering. The plant gets burnt. Extra fertiliser can also wash into rivers and harm water life.
Try it: a bean seed and a salt test
1. Put a bean seed on wet cotton in a glass. When the root is 3 cm long, mark it with ink dots 5 mm apart. After two days see which dots moved apart: that is the elongation zone. Look with a lens for the fuzzy hair zone.
2. In the 3D, go to the last step. Predict the leaf colour for 10, 50 and 90, then check.
Key formulas and definitions
- Root zones (tip → up): cap, division, elongation, root hair, conduction
- Macro-elements (large amounts): N, P, K, Ca, Mg, S
- Micro-elements (tiny amounts): Fe, Zn, Mn, Cu, B and others
- Water enters root hair: weak soil water → stronger cell sap
- Soil factors (Dokuchaev): rock, climate, living things, relief, time
Worked examples
1. A bean root is marked with dots 5 mm apart over 20 mm near the tip. Two days later the dots nearest to the tip are 12 mm apart, those higher up are still 5 mm. Which zone is growing?
The part where the dots moved apart is the elongation zone, just behind the division zone near the tip.
2. A farmer's wheat leaves are pale yellow and small. Which macro-element is most likely missing, and what could he add?
Nitrogen is the most likely missing element. He can add compost, manure or a nitrogen fertiliser such as urea.
3. Why does a plant wilt after a heavy dose of fertiliser, even though the soil is wet?
The soil water becomes stronger than the sap in the root hairs, so water leaves the roots instead of entering. The plant cannot get water and wilts.
4. A pot soil test shows enough N and K but the new leaves are yellow with green veins. Which group of elements could be missing?
A micro-element such as iron. Even small amounts matter.
Common mistakes
- Saying roots only hold the plant. They also take in water and minerals.
- Mixing the zone order. From the tip: cap, division, elongation, root hair, conduction.
- Thinking more fertiliser always means more growth. Too much burns the roots.
- Believing micro-elements do not matter because the amount is tiny. Without them plants fall sick.