Soil Moisture Monitoring

Regular soil moisture monitoring during the irrigation season can identify the optimum water supply needed by a dairy farm to achieve maximum crop and pasture yields, which is important to generating long-term success with soils.

Soil moisture sensors

Soil moisture data is used to assist with scheduling the next irrigation to optimise plant growth and water use efficiency. For surface irrigations this generally relates to the timing of the watering. For pressurised systems, it is more likely to help with both irrigation timing and the amount to apply.

There are two main types of commercially available soil moisture sensors – suction-based and volumetric-based systems. Essentially, suction-based sensors measure how tight water is held in the soil. Usually shown in kilopascals (kPa), this type of sensor quantifies how hard the plant must work to extract water from the soil. Suction-based sensors are consistent across different soil types. Gypsum blocks and tensiometers are two commonly used suction-based tools. For suction-based sensors, research has shown ryegrass-white clover pasture growth is optimised between 10 and 30kPa at 20cm depth. At 35kPa and above, pasture experiences difficulties extracting water and should be irrigated before reaching this critical point.

Volumetric-based tools use a measurement of the soil ‘dielectric’ which reflects the capacity of a material to transmit electromagnetic waves or pulses. Capacitance probes and total domain reflectometry (TDR) capacitance spikes are examples of commonly used volumetric-based monitoring tools. Sensors can either be ‘manual-read’, which display readings on the device, or they can be ‘screen-read’ systems where data is automatically logged and transmitted wirelessly to a PC, mobile phone or tablet and can be viewed in a graphic format. It is recommended that farmers use a reputable irrigation agronomist to discuss their soil moisture monitoring equipment needs.

Readily available water

Irrigators use soil moisture sensors to indicate whether their soils are in the zone of 'Readily available water' (RAW). RAW is the amount of water that a plant can easily extract from the soil and is influenced by soil type and plant type.

The high-water level is called ‘field capacity’ – the point at which adding more water to the soil will cause losses as either runoff across the soil surface or infiltration below the effective root zone. The low water level is called the ‘refill point’. If soil moisture levels fall below this, the plant needs to use more energy to extract the remaining water in the soil.

Therefore, soil moisture levels for optimum plant production should be maintained in the RAW zone, between the field capacity and the refill point. Furthermore, understanding the RAW for soil and plant type helps define the amount of water that can be applied at any one time without water wastage.

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