Plant-available Water In The Soil
By Professor Nils Nykvist
In a mineral soil, about half the volume consists of soil particles, and the other half of pores that are filled with water when the soil is water-saturated and with air when it is completely dry.
In a gravelly or sandy soil, the pores are large and mostly filled with air, unless the soil is saturated because of high groundwater. Access to water is for these soils the most limiting factor for plant production, unless the roots reach down to the groundwater.
The fine-grained soils, however, have small pores and can therefore suck up much water. For these soils the availability of oxygen is the most limiting factor for many plants. In agriculture, there is a rule of thumb that at least 10% of the pores should be filled with air for good plant growth.
The content of water in the soil when the pores are completely filled with water is called maximum water capacity. In nature, maximum water capacity is rarely achieved, because air is often enclosed in the pores and has to be sucked out from the soil.
When the soil drains, the larger pores are emptied first, because the gravity is greater than the water-binding force in the soil. The so-called gravitational water runs off. The water content of the soil when the downward water movement has ceased is called field capacity. It varies with the depth to groundwater when the soil water through its pore system is in contact with the groundwater.
In order to characterize a soil with respect to its ability to bind water, there is a need to use a measure of field capacity that is independent of the depth to groundwater. The most commonly used value is that of the soil drained to a depth of one metre.
Plant roots can, through their suction power, use water until it binds in the soil with a tension greater than what the plants are able to create. The water content of the soil when plants start to wilt and cannot recover when water is supplied is called the permanent wilting point.
For many agricultural plants this corresponds to a negative pressure of a 150-metre water column. This value is often used as a guide for the permanent wilting point, but it must be emphasized that there are great differences between different plants in their ability to utilize soil water. These differences are important in natural plant communities, where certain plant species better can survive dry spells, in contrast to others.
The difference in water content between field capacity and permanent wilting point is the amount of water normally available to the plants. One cubic meter of clay loam can, after draining, bind about 420 liters of water, of which 180 liters is available to plants.
The corresponding figures are, for a sandy soil, about 40 and 30 liters of water per cubic meter. Converted to millimeters rainfall, these water volumes correspond to 420 and 180 millimeters for the clay loam, and 40 and 30 millimeters for the sand.
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