Why measure soil moisture?
Monitoring soil moisture can help growers determine:
- How close a crop is to running out of available soil moisture.
- If stored soil moisture is low at seeding time, whether a crop with a lower CLL (better able to access moisture lower in the profile) should be considered, such as a cereal instead of canola, particularly in lower rainfall areas.
- Whether a mid-season nitrogen application is justified based on soil moisture levels and yield potential.
- Whether irrigation is required in irrigated cropping systems.
- Potential grain quality issues, such as screenings, and yield forecasting to help guide marketing decisions.
While it is possible to estimate soil moisture based on experience and weather data, measured soil moisture can give a more accurate assessment, which improves the quality of information on which decisions are based.
Soil moisture monitoring can provide key information for both irrigated and dryland growers to help with irrigation, yield prediction, crop and variety selection and fertiliser applications. While high-tech solutions with high capital outlay can provide a wealth of data, there are simpler and cheaper options for growers who want to understand more about soil moisture without major expense.
Rural Directions Pty Ltd consultant Tony Craddock says soil moisture monitoring can be used in conjunction with plant available water capacity (PAWC) to understand how much water is available to the plant.
“PAWC is the difference between the drained upper limit (DUL), also known as the field capacity, which is the highest moisture content the soil can store before water runs off or drains away, and the crop lower limit (CLL), which is the lowest moisture content that plant roots can extract from soil,” he said.
“If soil moisture is above the DUL, no more water can be held in the soil so additional rain or irrigation will not be of benefit and can lead to waterlogging and surface runoff. If soil moisture is below the CLL, while there is still moisture in the soil, it is not able to be extracted by the crop as it is too tightly bound to the soil. The volume of water between these two limits is available to be used by the plant.”
PAWC is sometimes referred to as the soil moisture ‘bucket’. Monitoring the soil moisture level can inform growers how full the ‘bucket’ is at any time. There is a range of equipment that can be used for measuring soil moisture.
A capacitance probe telemetry unit and automatic rain gauge during soil moisture monitoring trials in 2009 in SA.
Source: Tony Craddock
Gypsum blocks and tensiometers
The cheapest and simplest option for getting started with soil moisture monitoring is a gypsum block. This gives a reading of soil water tension which measures how difficult it is for plant roots to withdraw water from the soil.
Gypsum blocks have a limited life span of three to five years because they are susceptible to salinity or high levels of nitrogen. However they are essentially maintenance free and low cost, which can outweigh their limitations.
Tensiometers are a probe installed in the soil which read the same soil water tension as gypsum blocks. The advantage of a tensiometer compared to a gypsum block is that they do not have a fixed lifespan but require more maintenance. Both types of systems start at about $500.
A capacitance probe contains multiple sensors, often at 10-centimetre intervals, measuring soil moisture at different points in the root zone. The sensors directly measure moisture content.
Capacitance probes have higher initial costs than tension-based systems, and require soil-specific calibration. However they give a better understanding of moisture levels throughout the root zone, and can provide continuous data which can be remotely uploaded to a computer via a telemetry unit, compared to tensiometers or gypsum blocks for which measurements are obtained manually in the field.
A basic capacitance probe system with telemetry unit can be purchased and calibrated for about $4000, though there are many options to improve the outputs, including ‘add-ons’ such as temperature, automatic rainfall and wind gauges, which can be purchased with capacitance systems.
Neutron probes are the high end of the cost and accuracy spectrum for soil moisture monitoring. They emit neutrons which collide with the hydrogen atoms in water, which in turn are detected by the probe. Neutron probes are only recommended for researchers due to the high cost and complexity.
GRDC Booklet: Estimating Plant Available Water Capacity
Video: Using soil moisture probes for more informed decision making - a chat with SA grower Tom Robinson
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