Water is held within the soil pores with varying degrees of tenacity, depending on the amount of water present, and the size of the pores. Soil solution, which is very important as a medium for supplying nutrients to the plants, is made up of soil water and dissolved nutrients.
There are three basic forms of soil moisture:
1) Hygroscopic: A thin layer of moisture surrounds the soil particles; there is a very strong attraction between the soil particles and the water molecules, and if the film of moisture is thin enough, the attraction can be so strong that the water is unavailable for plant use.
2) Available or Capillary: When the soil’s water content increases, the layer of moisture surrounding the soil particle thickens, and the attraction between soil and water decreases. The outer layers will then flow to areas of thinner films, and the excess can be used by plants.
3) Gravitational: If the water content becomes so great that the attraction from the soil can not overcome the pull of gravity, the water will move downward through the pore spaces. This moisture is mostly not available to plants, as it will not remain in the root zone for any length of time.
Because of the irregular sizes and shapes of soil particles, spaces or pores develop between the mineral and organic matter particles. These pores are occupied by water, micro-organisms, or air. Optimally, the percentage of air should be 25% by volume. (C02) content is much higher, and Oxygen (02) much lower in soil then that of the atmosphere. In fact, CO2 in soil is often several hundred times higher that the .03% normally found in the atmosphere.
The volume and composition of soil air are determined by the water content of the soil. Air is found in those pores not occupied by water. After a rain or irrigation the large pores are the first to be emptied of soil water, (capillary action), followed by medium size pores as moisture evaporates or is used by plants.
This easily explains the tendency for soils with a high proportion of tiny pores to be poorly aerated. In this type of soil, water dominates, and the air content is low, as is the rate of exchange of air into and out of the soil from the atmosphere. The result is excessive levels of CO2 and deficient levels of O2; conditions unsatisfactory for plant, root, and microbial growth.