Question: How much water would be good for soil or growing crops

Asaf Shnel answered on 19 Nov 2020:

Every soil is different. It depends of the amounts of clay, sand, and organic matter in the soil.

There is a term called field capacity. That is when a soil is wetted and all the water has drained from the bigger pores in the soil by the action of gravity. At that point all the smaller pores are still filled with water and there is also plenty of air in the bigger pores that have drained. So there is plenty of water for roots to easily take up and also air that roots need for respiration.

A second term is wilting point. That is when a soil dries to the point where is very difficult for plant roots to take up water. This happens because of something called capillary forces, in other words water tends to bind strongly to soil particles, when there is plenty of water in the soil there are a lot of free water molecules that are not binding to soil particles and it is easy for roots to take up that water, but when the soil reaches the wilting point it is very difficult for roots to take up water.

For plants to grow best, the soil has to be above the wilting point and up to the field capacity of that soil.
Soils with more clay can take up more water before reaching the field capacity compared to sandy soils. But because the clay particles can hold on tightly to water molecules this soils will reach the wilting point at higher water contents then a sandy soil, because sand holds water molecules more loosely than clay. Sandy soil also drain faster because the bigger sand particles produce bigger pores that drain faster, compared to smaller clay particles that produce more pores, but smaller pores that drain slowly.

So a soil with more clay can be watered with more water then a sandy soil, because it takes more water to reach field capacity, and less often then a sandy soil because it drains slowly.
A sandier soil will need less water when irrigated but will need to be irrigated more often.