- FORMS OF SOIL WATER
All plants require water for growth, but they also require oxygen around their roots. If there is too much water, then oxygen is pushed out of the soil. A balance of‐air and water in the soil produces the best plant growth, and the best conditions for bacteria and other soil micro‐organisms which are important in the soil.
The three main states of soil water are gravitational water, water at field capacity and water at the permanent wilting point.
After very heavy rain a soil will become saturated. In this state, all the pore spaces in the soil will be full of water and pools will form on the soil surface. Once the rain stops and the soil begins to dry out, the surface water will be removed by evaporation. Some of the water in the soil will be drawn down through the soil by the force of gravity, and this water is known as Gravitational Water.
Figure 1: The situation in a saturated soil and the situation with a saturated soil and pore space.
Once the Gravitational Water has drained out of the soil, the water remaining in the soil is being held against the pull of gravity, and the soil is said to be at Field Capacity. When a land is irrigated, the object is to bring the soil to Field Capacity, with the soil full of water, but not saturated. At this stage there is plenty of both Air and Water in the soil for maximum plant growth.
PERMANENT WILTING POINT
If the soil is at Field Capacity and no further rain falls, the water in the soil is gradually used up by the plants until the point at which the plants wilt from lack of water. If it then rains, or the crop is irrigated, the plants will recover and continue to grow. If, however, no water is added to the soil, the plants will wilt further and die. This stage at which plants die through lack of water is called the permanent wilting point. There is still some water in the soil, but it is so tightly held by the soil particles that it is unavailable to the Plant.
Figure 2: Soil water at Permanent Wilting Point and the pore space
- MOVEMENT OF WATER IN SOILS
Water moves through the soil by the following methods:
- The run‐off of surface water. This should be controlled by using mechanical methods of soil conservation to prevent damage to the soil surface.
The percolation downwards of Gravitational Water. This depends on:
- The soil texture
- The soil structure
- The wetness of the soil
- The depth of the water table or drainage trenches
- Capillary water: The movement of both True Capillary Water and Apparent Capillary Water.
- Hygroscopic Water: There is no movement of this water in the soil.
This is water which the soil cannot retain against the force of gravity and which percolates down through the soil either to the Water Table or into the drainage system, if one exists.
This water moves through the large pore spaces and passages in the soil and moves rapidly. It moves more rapidly through sandy soils than through Clay soils.
This is the water held in the small pore spaces and the spaces inside the soil crumbs. When a soil is at Field Capacity, the water in the soil is known as the Available Soil Water because this is the water available to the plants for growth. This available soil water is capillary water and moves upwards, downwards, and sideways through the fine pore spaces of the soil by a process known as capillary action.
Sandy soils have few very fine capillary pores because the sand particles are large. These soils have a low water‐holding capacity, and there is very little lateral, i.e. sideways, movement of water. Clay soils have many very small particles and so have many fine capillary pores. These soils have a high water‐ holding capacity and there is much lateral movement of water.
Figure 3: Soil structure of sand and clay
The poor lateral movement of water in sandy soils is used in the water planting of tobacco seedlings. Water is added to the hole in the soil before the seedling is planted. This water does not move sideways but straight down through the soil to connect up with the water table. see diagram below:
Figure 4: Movement of water through a sandy soil and a clay soil
In a soil with a high water table, the capillary water links the soil surface to the water table. Water is drawn upwards through the soil by capillary action and this movement of water is fairly rapid.
Figure 5: Movement of water through capillary action
In a soil with a low water table, one that is 15 to 20 meters below the surface, there is an area above the water table of True Capillary Water. When such a dry soil is rained upon will fill up to field capacity to a certain depth, e.g. 2 meters. This water is known as Apparent Capillary Water. Between this Apparent Capillary Water and the True Capillary Water lower down there will be a Dry Zone. Movement of water into the Dry Zone will be very slow, 10 ‐ 12 centimeters per month.
Plants growing on this type of soil and whose roots are in the area of the Apparent Capillary Water will be well supplied with water but they will have to seek it out. This is because as they use up the water, the root area dries out as this area is not being replenished from the Water Table.
A dry spell, after planting tobacco or when maize plants have become established, helps to develop a good root system by forcing the roots to extend themselves in their search for water.
Figure 6: Shows where apparent and true capillary water exists
3. AVAILABILITY OF WATER TO THE PLANT
Water is added to soils in the following ways:
Water is lost from soils by:
- Evaporation from the soil surface and within the pore spaces in the top 12 centimetres of the soil.
- Transpiration: this occurs down to the root depth of the plant.
4. EFFECTS OF IRRIGATION
At Field Capacity, no water is being lost from the soil by drainage. Any water lost from the soil is by evaporation and transpiration. Potential transpiration is an estimate of this water lost. This is further discussed in the Irrigation Lectures.
SOIL MOISTURE DEFICIT
This is the amount of water required to bring a soil up to Field Capacity. Crops growing in clay soils can withstand higher moisture deficits than those growing in light soils. Crops growing in light, sandy soils require more frequent rainfall or irrigation than those growing in clay soils. The total amount over a season however, may not be greatly different.