Delphinidin

Water in the soil occurs in three forms. Hygroscopic water is physically bound to the soil particles and is unavailable to plants. Gravitational water drains out of the pore spaces after a rain. If drainage is poor, it is this water that interferes with normal plant growth. Plants are mainly dependent on the third type, capillary water, which is water held against the force of gravity, in pores of the soil. The structure and organic matter of the soil—which enable the soil to hold water against the force of gravity—the density and type of vegetational cover, and the location of underground water tables largely determine the amount of capillary water available to the plant.

The ancient Incas of Peru knew that water would rise just so far in some areas. Where the water table was close to the surface, they removed the upper 0.6 meter (2 feet) of soil and planted their crops down in the hollowed-out areas so that the roots would be able to reach the capillary water.

They knew that in some areas having sandy soils and low annual precipitation, soils could be compacted by a heavy roller to create finer capillaries to raise water from below. This technique is effective only if the available water is within 1.5 to 3.0 meters (5 to 10 feet) of the surface and if the soils do not contain much silt or clay.

After rain or irrigation, water in the soil drains away by gravity. The water remaining after such draining is referred to as the field capacity of the soil. Field capacity is mainly governed by the texture of the soil, but the structure and organic content also influence it to a certain extent. Plants readily absorb water from the soil when it is at, or near, field capacity. As the soil dries, the film of water around each soil particle becomes thinner and more tightly bound to the soil particle and less likely to enter the root. If water is not added to the soil, eventually a point is reached at which the rate of absorption of water by the plant is insufficient for its needs, and the plant wilts permanently. The soil is then said to be at the permanent wilting point. In clay soils, the permanent wilting point is reached when the water content drops below 15%, while in sandy soils, the permanent wilting point may be as low as 4%. Available water is soil water between field capacity and the permanent wilting point.