Basic properties of water
- Density: 1000 kg per cubic metre. So 1 litre = 1 kg.
- Pressure: water pushes on all sides. Pressure p = ρ × g × h, which is about 10 kPa per metre of depth (ρ = 1000 kg/m³, g = 9.8 m/s²).
- Flows easily and takes the shape of its container, but cannot be squeezed into a smaller volume easily.
- Surface tension and sticking let water climb thin gaps (capillary rise).
Basic properties of soil
Soil is a mix of solid grains, water and air. The empty space between grains is the pore space.
- Texture is the size of the grains: sand (largest, 0.05 to 2 mm), silt (middle), clay (smaller than 0.002 mm).
- Porosity = pore volume ÷ total volume × 100. Clay often has more pore space than sand, but its pores are tiny.
- Moisture content = water weight ÷ dry soil weight × 100.
Water in soil
- Gravity water drains down through big pores.
- Capillary water is held in thin pores and climbs up. Thinner pores lift water higher but slowly.
- Permeability is how easily water passes. Sand is high, clay is very low.
- A saturated soil has all pores full of water. Roots need some air too, so too much water harms crops.
This is why engineers test soil before they build a canal, a pond or a road.
Try it
In the 3D: in the last step press Pour water for sand, then silt, then clay. Which finishes first? Predict first.
At home: fill two cups with sand and with clay soil, each with holes at the bottom. Pour 100 ml of water in each and time how long the first drop takes to fall.
Key formulas and definitions
- Water: 1 L = 1 kg = 1000 cm³; density = 1000 kg/m³
- Pressure p = ρ × g × h ≈ 10 kPa per metre
- Porosity (%) = pore volume ÷ total volume × 100
- Moisture content (%) = water weight ÷ dry soil weight × 100
- Key terms: texture, pores, capillary rise, permeability, saturation
Worked examples
1. What is the weight of 25 litres of water?
1 L = 1 kg, so 25 kg.
2. Find the water pressure at 3 m depth (about 10 kPa per metre).
3 × 10 = 30 kPa. With g = 9.8 it is 29.4 kPa.
3. A wet soil sample weighs 130 g. After oven drying it weighs 100 g. Moisture content?
Water = 30 g. 30 ÷ 100 × 100 = 30%.
Common mistakes
- Thinking sand holds more water because its grains are bigger. Its big gaps drain fast.
- Thinking pressure depends on the width of the tank. It depends on depth.
- Dividing water weight by wet soil weight. Moisture content uses dry soil weight.
- Saying clay has no pores. It has many pores but very tiny ones.