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Irrigation made easy – part 6: Scheduling of water usage

Photo: Pixabay

It 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).

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.

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.

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.

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. 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 the planned replacement of water that has been drawn off by the crop in the soil profile. Various methods can be used to schedule irrigation.

These include:

Using a fixed irrigation cycle

Early in the season, enough water is applied to fill the soil profile with water. It is accepted that much of the water used by the plant will be provided by rainfall.

The seasonal estimated irrigation requirement (crop usage, minus effective rainfall) is divided into daily or weekly increments. Fixed quantities of water (about 20 to 30 mm/week) are applied on a fixed cycle. It is assumed that temporary supply shortfalls during periods of peak demand will be met by water stored in the soil profile at the beginning of the season.

A fixed irrigation cycle can work well for several crops. Specific crops do, however, have critical growth stages. For example, any water stress in maize during the critical tasselling period will result in a significant reduction in crop production.

A common mistake is to over-design the fixed cycle application, rather than to make provision for an additional irrigation at critical growth stages. In most cases where a fixed irrigation cycle is used, the crop is over-irrigated. Water, energy, labour, and fertiliser are then wasted, and crop production can also be lower.

Determining soil moisture content by drying soils

Example

From laboratory tests:

Dry density of soil = 1 600 kg/m³

Field capacity soil water = 133 mm/m

The function of the first water application is to fill the soil profile with water. Photo: Pixabay.

From field test:

Sample tin weighs = 138,5 g

Wet sample plus tin = 236,7 g

Dry sample plus tin = 229,9 g

Dry sample = 229,9 – 138,5 = 91,4 g

Water lost = 236,7 – 229,9 = 6,8 g

Moisture content = (6,8 / 91,4) × 100 = 7,44% = (6,7 / 91) × 1 600 = 119 mm/m

  • Crop = Cabbage with 400 mm rooting depth.

Field capacity = 133 mm/m (given)

Readily available water = 66,5 mm/m (50%) sample moisture = 119 mm/m

RAW in sample = 119 – 66,5 = 52,5 mm/m

RAW in root zone = 52,5 × 0,4 = 21 mm

If current plant water use is in the order of 7 mm per day, then irrigation is not needed for a further 21/7 = 3 days. The drying of soil samples can be a good and accurate method of scheduling.

It does, however, require a good deal of measurement and calculation, together with a good understanding of the soil being tested and its properties.

Next month we shall look at scheduling according to weather conditions. This series is published with acknowledgement to the ARC Agricultural Engineering for the use of their manuals. Visit www.arc.agric.za for more information.

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