Soil consists of solid particles, with pore spaces which can contain water and/or air. The particles vary enormously in size, from clay particles less than 0.002 mm in diameter to sand greater than 1 mm diameter. Soil texture depends on the proportion of sand, silt and clay particles. In addition, soils will have a structure of ‘clumps’, called aggregates or peds, with large pores between the peds.
The amount of water that a soil profile can hold, and the rate that water can enter the soil, depends on the soil structure, texture, and water content. Soils with the same texture can have an entirely different structure and water holding capacity.
The pressure in the pore water when soil is saturated is atmospheric - tension = 0. As it drains, because of capillary action, it will become negative - the soil moisture tension increases.
Clay can also have water inside the clay particles, held under tension. In addition, the pore spaces will be smaller than sand, so that as clay dries high tensions can form in the remaining pockets of water.
Idealised structure of soil matrix, with different amounts of volumetric soil water...
| Saturated | Full Point | Refill Point | Permanent Wilting |
| (Free water at surface) | (Maximum water holding capacity) | (Crop starts losing yield) | (like a parrot...) |
| 0 kPa | 6 kPa | 60 kPa | 200 kPa |
A sandy soil can retain water in the pore spaces between particles, but virtually no water is held in sand particles. Some of the pore water can freely drain out under gravity, and most of the rest can be extracted by plant roots.
The rate at which water enters the soil profile is controlled by gravity and the water conductivity of the soil. The conductivity is not constant, but changes as the soil goes from dry to wet. Water will flow vertically, under gravity.
A clay soil will behave entirely differently. When clay is wet it has an extremely low conductivity - almost no water can pass through it - but when dry the high soil tension sucks water into the soil matrix. Clay will therefore absorb water when dry, but will not allow water to pass when wet. The force moving the water is tension, and water will flow in all directions, not just vertically.
In practice, agricultural soils will be a mixture between sand and clay, and will have a structure of peds. The pores between the peds will allow water to infiltrate.
A sandy soil will allow water to move down through the profile. A clay soil may appear to show similar movements, but the water has percolated down between the peds, and then been sucked horizontally into the peds by soil tension.
The different strength behaviour of clay and sand is due not to any inherent difference in the material, but to the different rate that water can move through the soil.
The 'strength' of soil is proportional to water suction in the pores
Clay is held together in a ‘clumps’, with a high unconfined strength, because the water pressure in the pores is under suction, holding the particles together. This suction force can only be reduced if water can flow in to the soil structure. If water eventually gets in to the structure, under prolonged wet conditions, the negative pore pressure increases and the strength falls.
Sand has a very low unconfined strength - water can easily flow into the structure, and
the pore water pressure is zero. It is only when the sand is confined, at depth, does it
have strength.