There are three forms of water occurring in soil. Hygroscopic water is unavailable to plants because it is physically bound to soil particles. Gravitational water is the water that drains from pore spaces after rain. Plant growth can be affected if drainage is poor. Capillary water is the main source of water for plant needs. This is the water held in soil pore spaces against the force of gravity. Both the structure and organic material composition of the soil enable the soil to hold water in this manner. Vegetation density and type and the location of underground water tables are the determinants of how much capillary water is available to the plant.
Following rain or irrigation, gravity drains water away from the soil. The field capacity of the soil is the amount of water remaining after such a draining. This characteristic is mainly determined by the texture of the soil, however, structure and organic material content also play a role. When soil is at or near field capacity plants will readily absorb water. In the process of soil drying, the film of water around each soil particle becomes thinner and more tightly bound to the particle. As the water film binds to the particle, the likelihood of the water entering the root is lessened. A point of no return is eventually reached if the water is not added to the plant. This point is called the permanent wilting point and refers to the soil moisture. It is at this point that the plant is unable to absorb water at a rate sufficient for its needs. The plant wilts and will die. The permanent wilting point in clay soil is reached when the water content drops below 15%. In sandy soils, this point can be as low as 4%. The soil water between field capacity and the permanent wilting point is called available water.
Earlier we mentioned that the breakdown of minerals lends to the acidity of the soil. It is important to understand that the pH, acidity, or alkalinity of soil affects both the soil and the plant in a variety of ways. Soil that is not balanced, that is either too acidic or too alkaline, may be toxic to roots. However, normally an unbalanced soil pH will affect nutrient availability before they will affect the plant directly. In general, when soil is acidic the nitrogen-fixing abilities of plants are affected. Alkalinity will affect the availability of minerals such as copper, iron, and manganese. In areas of high precipitation, the soil is usually acidic because alkaline components are leached from the topsoil. Many agricultural operations will counterbalance any soil imbalances by adding lime (compounds of calcium or magnesium) to balance acidic soil, or by adding sulfur to alkaline soils. Bacteria in the soil will convert the sulfur to sulphuric acid, which will lower the overall pH which will make the soil more acidic. Nitrogen-based fertilizers may have the same effect on alkaline soils.
It is a fine balance to have, but soil quality directly influences plant quality.