Monitoring
Water is an essential, perhaps the most essential, nutrient for plant growth. It is needed in much greater quantities than any other nutrient. But while other nutrients are retained by the plant, water is on a one-way continuous flow from the soil through the plant roots to the leaf surface to be transpired to cool the leaf.
Water is the single most important factor influencing yield and quality of any crop — grains, seeds, grapes fruit. The response of crops when water is limited is to:
> Decrease the rate of evapotranspiration and photosynthesis,
> Decrease the rate of leaf growth, and
> Decrease grain or seed growth.
The extreme outcome of limiting water supply to crops is plant death.
Rainfall is the most variable aspect of weather, thus sufficient moisture for plant growth poses a major risk for growers. Irrigation is necessary to ensure crops always have sufficient readily available soil moisture so that production is optimised. Not only is irrigation essential, so is the proper management of the irrigation. Rarely during any growing season on any soil type will irrigation not be required.
The Irrigation Season
Irrigation can begin in August and not stop until May, i.e. a season of 7–8 months. You must be prepared to start early, finish late and in some seasons not stop in between. Irrigation is a 7-day a week task that simply cannot be missed if production is to be maintained.
How Much Irrigation
This will depend on soil type, daily crop water use and the climatic environment. These three factors are inextricably connected to the need to supply water to the root zone before it becomes so dry the plants cannot readily remove water for growth. There are two extremes in soil types and local environment:
1. Light stony soils (usually less than 300mm top soil) have only 30–35mm of readily available moisture in the top 400mm of soil — NOT very much! Regions with these soils are also prone to the effect of the Northwesterly winds. These winds can have dramatic effects and increase the daily water usage significantly.
2. Heavy soils are usually the deepest soils and have the most available soil moisture. They require the least irrigating.
The Effect of Getting Behind
The most likely time for an irrigating farmer to get behind is during a westerly weather cycle. Daily water use by the some crops can be as high as 7–8mm per day during these times. Once soil moisture drops below the critical deficit (the soil moisture content below which plants struggle for water) plant growth is affected. Production will decrease by 0.2–0.3% per mm below critical deficit. For example, pasture production could fall from 70–80kg dry matter per day to 50–55kg dry matter per day if irrigation is 7-10 days late.
Measuring Soil Moisture
HydroServices measures soil moisture through the soil to accurately determine irrigation requirements. Soil moisture is measured with a neutron probe. Direct measurement removes any assumptions and doubts associated with evapotranspiration models. At key locations aluminium access tubes are set into the soil. Soil moisture is measured on a weekly basis at several depths. The readings from the tubes are averaged to provide an accurate assessment of the soil moisture content. This allows an irrigation management programme to be developed in consultation with the grower.
How the measurements are used
For each field or crop monitored, the amount of moisture in the critical root zone layer of the soil and subsoil layers is plotted on an irrigation management report. This report is provided for the grower, is in the form of an easy-to-read graph and is updated weekly or whenever measurements are made.
Irrigation management aims to keep the moisture between the full and stress points so that there is sufficient moisture for optimum growth. If the moisture falls below the stress point, yield will be decreased and quantity diminished. If irrigation raises soil moisture above the full point, the excess drains through the soil, wasting water and leaching valuable nutrients.
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