Drying of a wet soil

Soil water is lost from the soil surface by evaporation and from the rooting zone by plant transpiration.

Evaporation
The rate of water loss from the soil by evaporation depends on the drying capacity of the atmosphere just above the ground and the water content in the surface layers. The evaporation rate is directly related to the net radiation from the sun which can be measured with a solarimeter. Evaporation rates increase with higher air temperatures and wind speed or lower humidity levels. As water evaporates from the surface, the water films on the soil particles become thinner. The surface tension forces in the film surface become proportionally greater as the water volume of the film decreases. This leaves water films on the particles at the surface with a high surface tension compared with those in the films on particles lower down in the soil. The increased suction gradient causes water to move slowly upwards to restore the equilibrium. Whilst the surface layers are kept moist by water moving slowly up from below, the losses by evaporation, in contrast, are quite rapid. Consequently the surface layers can become dry and the evaporation rate drops significantly after 5 to 10 ‘mm of water’ is lost. Evaporation virtually ceases after the removal of 20 ‘mm of water’ from the soil. Maintaining a dry layer on the soil surface helps conserve moisture in the soil below. Evaporation from the soil surface is almost eliminated by a leaf canopy that shades the surface, thus reducing air fl ow and maintaining a humid atmosphere over the soil. Mulches can also reduce water loss from the soil surface.

Evapotranspiration
As a leaf canopy covers a soil the rate of water loss becomes more closely related to transpiration rates. The potential transpiration rate represents the estimated loss of water from plants grown in moist soil with a full leaf canopy. It can be calculated from weather data (see Table 19.2 ).

As roots remove water it is slowly replaced by the water film equilibrium, but rapid water uptake by plants necessitates root growth towards a water supply in order to maintain uptake rates. At any point when water loss exceeds uptake, the plant loses turgor and may wilt. This tends to happen in very drying conditions, even when the growing medium is moist. Wilting is accompanied by a reduction in carbon dioxide movement into the leaf, which in turn reduces the plant’s growth rate (see photosynthesis). The plant recovers from this temporary wilt as the rate of water loss falls below that of the uptake, which usually occurs in the cool of the evening onwards. Continued loss of water causes the soil to reach the permanent wilting point because roots can extract no more water within the rooting zone.

When the soil has reached its permanent wilting point (PWP) there is still water in the smallest of the soil pores, within clay particles and in combination with other soil constituents, but it is too tightly held to be removed by roots. Typical water contents of different types of soil at their permanent wilting point (PWP) are given in Table 19.1.


Available water
Roots are able to remove water held at tensions up to 15 atmospheres within the rooting zone, and gravitational water drains away. Consequently the available water for plants is the moisture in the rooting depth between field capacity and the permanent wilting point. The available water content (AWC) of different soil textures is given in Table 19.1 . Fine sands have very high available water reserves because they hold large quantities of water at FC and there is very little water left in the soil at PWP (see sands). Clays have lower available water reserves because a large proportion of the water they hold is held too tightly for roots to extract (see clay).

Roots remove the water from films at field capacity very easily. Even so, plants can wilt temporarily and any restriction of rooting makes wilting more likely. Water uptake is also reduced by high soluble salt concentrations (see osmosis) and by the effect of some pests and diseases (see vascular wilt diseases). As the soil dries out, the water films become thinner and the water is more difficult for the roots to extract. After about half the available water content has been removed, temporary wilting becomes significantly more frequent. Irrigating before available water falls to this point helps maintain growth rates. Plants grown under glass are often irrigated more frequently to keep the growing medium near to field capacity. This ensures maximum growth rates since the roots have access to ‘ easy ’ water, i.e. water removed by low suction force.

Soil consistency
The number of days each year that are available for soil cultivation depends on the weather, but more specifically on soil consistency (sometimes referred to as the workability of the soil). It also infl uences the timing and effect of cultivations on the soil.

It is assessed in the field by prodding and handling the soil. A very wet soil can lose its structure and flow like a thick fluid. In this state it has no load-bearing strength to support machinery. As the soil dries out it becomes sticky, then plastic. When plastic the soil is readily moulded. In general, the soil is difficult to work in this condition because it still tends to stick to surfaces, has insufficient load-bearing strength, is readily compacted and is easily smeared by cultivating equipment. As the soil dries further it becomes friable. At this stage it is in the ideal state for cultivation because it has adequate load-bearing strength, but the soil aggregates readily crumble. If the soil dries out further to a harsh (or hard) consistency the load-bearing strength improves considerably, but whilst coarse sands and loams still readily crumble in this condition, soils with high clay, silt or fine sand content form hard resistant clods. The friable range can be extended by adding organic matter (see humus). At a time when bulky organic matter is more difficult to obtain it is important to note that a fall in soil humus content narrows the friable range. This allows less latitude in the timing of cultivations and increases the chances of cultivations being undertaken when they damage the soil structure.

Whereas many sands and silts can be cultivated at field capacity, clays and clay loams do not become friable until they have dried out to well below field capacity, i.e. heavier soils need more time for evaporation to remove water through the soil surface.