About the soil: The soil is a storage room for the plants' nutrients, and the medium through which water and nutrients move. It is the anchor for plants and the reservoir of water for plants' growth. There are various types of soil with differing characteristics, which determine what types of plants can be grown. Each type of soil will require a different drip or micro-sprinkler layout and spacing. In sandy soil, the water will tend to move straight down, so micro sprinklers or closely spaced (12" apart) 1or 2 gallon per hour drippers can be used. In loamy soil, the water will move slowly and will spread evenly, so here .5 or 1 gph drippers with a 16" to 18" spacing can be used. In clay soil, the water will percolate very slowly, and a low flow such as .5 or 1 gph drippers at a wider spacing of 20" to 30" can be used.
Soil water relationships: A micro irrigation system is essentially a transportation system which delivers water to a point in or near the plant's root zone. The final link in this transportation system is the soil. The soil's physical and chemical properties determine its ability to transport as well store water and nutrients. In the next few paragraphs we will try to explain the soil and water relationship, and the mechanisms by which moisture is transported and stored within the soil.
Capillary moisture is the water held in pore spaces by the surface tension between the water and the soil particles. Capillary moisture is the primary force in spreading the water horizontally and it is a primary source of water to the plant.
Gravitational water is free water in the soil which will move downward under the influence of gravity. After the soil is saturated, the gravitational water will percolate downwards, leaving the soil at field capacity.
Field capacity is a measure of the water held by the soil against the influence of gravity. If soil is saturated by rainfall or irrigation and then allowed to drain freely for 24 hours, the soil is usually at field capacity. For most plants, soil moisture content near field capacity is the ideal moisture level for vegetative growth, because there is a good balance between soil moisture tension and aeration. The soil will lose very little water after it has drained to field capacity if there are no plants growing in it. Plants will remove water by transpiration and reduce the soil moisture. On hot days, the plants may use water faster than the soil can supply the roots, or faster than the roots can supply the rest of the plant and the plant will wilt (the time to use the irrigation system). Normally, given sufficient soil moisture the plant will recover during the night.
Permanent wilting point is the soil moisture content at which the plant wilts and remains in a wilted state, ceasing normal growth and transpiration.