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Irrigation made easy part 6: Scheduling of water usage I
t is crucial to irrigate your crops on a daily basis to keep the upper zone of the soil wet. This is the region where the roots will not have to work hard to suck up water to the plant, where it enables all the chemical growth processes to occur.
Productive plant growth will largely depend on how hard roots must suck to draw water from the soil (see Figure 1). 2m
A single bubble of air in the straw will break the suction. The plant will try to suck up water through a broken straw, but will only suck up air.
Hard enough to suck water up a straw to a height of 2 m: Water is freely available to the plant. It is easy for the plant to find and suck up water. The plant will grow well, with no stress.
4m
Hard enough to suck water up a straw to a height of 4 m: The plant must work harder to draw up water. Some energy may be diverted from leaf or fruit growth to sucking up water.
6m
Hard enough to suck water up a straw to a height of 6 m: The plant must now start to work even harder to get water. Growth will slow down and the plant could wilt in the heat of the day.
8m
Hard enough to suck water up a straw to a height of 8 m: It is very difficult for the plant to suck up water needed to survive. Growth will stop and the plant could die in hot, dry conditions.
ProAgri Zambia 55
Plant production depends largely on how hard the root system must work to abstract the water that the plant needs from the soil. Soil attracts and holds water molecules close to the surface of particles. The plant needs to work hard to draw off the water that is held close to the surface of a particle in the root zone. Irrigation systems are designed so that normally only 50% of the water that can be held in the soil profile is drawn off by the plant. The suction force needed to draw off this quantity of water is normally between 5 and 6 metres (or about -50 to -60 kPa). Much of the water held in the soil is not readily available to the plant. Scheduling describes the management of irrigation applications, supplying the correct quantity of water, at the right time, to ensure that enough water is freely available to allow the plant to grow and prosper. Scheduling involves 15
