Water and farmland soil
Soil is made of grains (sand, silt, clay) and a little rotted plant matter. Between the grains are pores, which hold air and water. In good soil, about half of the volume can be pores.
- Sandy soil: big grains, big gaps. Water drains fast and the soil dries quickly.
- Clay soil: tiny grains, tiny gaps. It holds a lot of water and drains slowly, so it can get waterlogged.
- Loam: a mix of both. It holds enough water and still lets extra water drain. Most crops like it.
After rain, extra water drains down. The water that stays in the small gaps is what roots can use. When the soil is full of water, there is no air in the gaps.
Water and growing crops
A crop uses water in three ways. It is part of the plant's body, it carries food and minerals inside the plant, and it escapes from the leaves as vapour. This escape is called transpiration. It pulls more water up from the roots, like drinking through a straw.
Plants need water at the right amount and time.
- Too little: the plant wilts, grows slowly and gives a small harvest.
- Too much (waterlogging): water fills all the gaps. Roots cannot get air, so they weaken and leaves turn yellow. Some crops, like paddy rice, are made to cope with standing water.
A handy fact: 1 mm of water on 1 square metre of land is 1 litre. So a farmer can turn a water depth into litres for the whole field.
Water and forest soils
A forest floor is covered with fallen leaves and has many roots and tiny animals. This makes the soil loose and full of gaps. So forest soil works like a sponge.
- It soaks rain in. Less water runs off over the top.
- It holds the soil. Roots and leaves slow the rain drops, so mud is not washed away.
- It gives water out slowly. Stored water seeps into streams and springs, so they keep flowing between rains.
- It helps keep water clean. Soil and roots trap dirt before the water reaches the stream.
Trees also use some water and give out vapour. On bare, hard soil the water runs off fast, carries soil away (erosion) and floods the stream, and then the stream may run dry later. The numbers in the 3D (20% and 80%) are made up to show the idea; real values change with soil and slope.
Try it: the sponge test
Fill two cups, one with sand and one with garden soil, to the same height. Slowly pour the same amount of water over each and watch how fast it drains out of a hole at the bottom. Predict which holds more water, then check. Which one would you use for a plant that likes lots of water?
Key formulas and definitions
- 1 mm of water over 1 m² = 1 litre
- Water volume (L) = depth (mm) × area (m²)
- Water the soil can hold ≈ soil volume × pore fraction
- Days a soil store lasts = available water (mm) ÷ daily use (mm)
- Key terms: pore, loam, transpiration, waterlogging, erosion, runoff
Worked examples
1. How many litres of water fall on a 100 m² plot in 30 mm of rain?
Litres = 30 × 100 = 3,000 L.
2. A farmer gives a 1-hectare field 50 mm of irrigation (1 ha = 10,000 m²). How much water is that in litres and in m³?
Litres = 50 × 10,000 = 500,000 L. 1 m³ = 1,000 L, so that is 500 m³.
3. A 200 cm³ sample of soil has 40% pores. At most how much water can it hold?
40% of 200 = 80 cm³ (80 mL).
4. In 50 mm of rain, a bare slope soaks in 20% and a forest slope soaks in 80%. How many mm run off from each?
Bare: 50 × 0.8 = 40 mm runs off. Forest: 50 × 0.2 = 10 mm runs off.
5. A crop uses 5 mm of water a day. The soil has 60 mm the plant can reach. If no rain comes, after how many days is the store used up?
60 ÷ 5 = 12 days.
6. Growing 1 kg of rice needs roughly 2,500 L of water. A family eats 300 kg of rice a year. About how much water was used to grow it?
300 × 2,500 = 750,000 L, which is 750 m³ (a rough figure).
Common mistakes
- Thinking soil is solid all through. It always has gaps that hold air and water.
- Believing more water is always better. Waterlogged soil has no air for roots.
- Thinking plants only take water for food. Most water leaves through the leaves as vapour.
- Saying forests only drink water. They also soak up rain, protect soil and feed streams.