How does water act in the soil?

Soil pores can be divided into 3 sizes depending on their role in the movement and storage of water:transmission (function is aeration and plant-root extension), storage or residual.

• The largest macropores or ‘transmission pores’ allow the rapid movement of water and gas through the soil. Water in these large pores usually drains within 48 hours.
• Mesopores or ‘storage pores’ hold water against the force of gravity; they are particularly important in storing water for plants.
• Finally, micropores or ‘residual pores’ hold water very tightly. Although this water can still play a part in chemical reactions, it is too tightly held for plant use.

The stability of macroaggregates can only be maintained if there is a continuous replenishment of organic matter to replace the temporary and transient binding agents that are constantly being degraded by soil microbial organisms. Reductions in macroaggregate stability and the subsequent loss in macropores can reduce the infiltration rate of water and the exchange of gases with the atmosphere. Aggregate instability is a particularly serious problem in soils that have a high proportion of sand and silt. These can slake even under the mildest rain. As aggregates break open, sand silt and clay particles are released and washed into soil pores, preventing further water infiltration; this is called ‘soil crusting’. Crusting effectively seals the soil surface so that instead of infiltrating the soil, rain collects in puddles where it is then evaporated.

The available water capacities of some soil texture types:-