Soil moisture meters allow better irrigation scheduling, preventing the drench-then-drought behaviour that sacrifices potential grass growth and wastes valuable resources.
The aim of irrigating pastures is to maximise growth – to apply water when the plant needs it. Over-watering leads to wastage and drainage events. Underwatering leads to decreased production as the plants are suffering from water stress.
Maximising pasture growth and limiting nutrient loss was the theme of a Wairarapa Moana Wetlands project field day held in South Wairarapa in late January. Dan Bloomer, an experienced irrigation and precision agriculture consultant and IrrigationNZ board member, outlined some soil moisture meter basics.
He said meters fit into three general categories: tensiometers, soil dielectrics and neutron probes.
Tensiometers are effectively a water-filled tube, often plastic, with a porous cup at the base and a vacuum sensor attached, sunk into the soil. As the soil dries it pulls water out through the porous base, creating a tension that is measured by the vacuum sensor. The drier the soil, the greater the tension (vacuum).
“It is a technology that measures how hard the plant has to suck to get water,” Bloomer said.
“It’s an interesting thing to know because there is a limit at which plants cannot suck any harder.”
Relatively low tech, tensiometers were the basis of most early soil monitoring systems.
Today, the most common form of soil moisture meters are dielectrics. Rather than physically measuring soil moisture, dielectrics measure soil electrical properties. As moisture levels vary in the soil, so does the dielectric constant; water has a dielectric constant of 80 while air is 1.
The soil dielectric category is itself divided into two sub-types – time-domain and frequency-domain.
Time-domain sensors bounce an electrical pulse along a known length of transmission line, measuring how long it takes for the signal to travel to the end and back.
It then calculates the moisture content based on an equation-type relationship with the dielectric constant.
Frequency-domain meters have a series of capacitor-based sensors at measured intervals along its length.
The sensor, which may be permanent or portable, is placed inside a PVC tube sunk vertically into the soil. Each of the sensors generates an electrical field. Once again, the soil moisture content is calculated based on a mathematical relationship with the dielectric constant.
Bloomer said both types of dielectric meters require careful installation. For time-domain sensors, the transmission line must be as close to horizontal as possible at the desired soil depth.
A tape-type sensor must be installed on its narrow edge to avoid water collecting on the sensor and giving false readings.
Air pockets around the sensors will lead to inaccurate readings. The soil around the sensors needs to be representative of the soil as a whole – slurry used to fill in any gaps around the sensors might not be widely representative.
Another factor to keep in mind is that some sensors will take several months to bed in.
In stony or gravelly soils, Bloomer recommended using the third type of soil moisture meter, the neutron probe.
With a neutron probe, a nuclear source is lowered into the soil through an aluminium access tube, and neutrons are emitted into the soil.
Neutrons that encounter hydrogen atoms in soil water molecules are bounced back and a count of those that bounce back can be used to indicate soil moisture content.
Because it involves the use of a nuclear source, special training and certification is required to operate a neutron probe. When combined with their high cost per unit, they are most commonly used by consultants servicing multiple clients.
Bloomer recommended measuring soil moisture at multiple points to get good management information. Having sensors at multiple sites means more variation can be captured.
“Soils are incredibly variable – more variable than we’d like to believe.”
He recommended having the sensors at three different sites, depending on soil type.
Having multiple sites can also make it easier to recognise strange or inconsistent readings, potentially indicating poor installation or technology failure.
Bloomer said deciding on the right locations for the sensor sites needs a bit of research.
“The biggest errors come when people don’t know the history of their paddock, don’t think about the dry spots and wet spots.”
He suggested checking out satellite images on Google Earth, using the timelapse feature. This could help to identify drains, filled hollows, headland areas – all factors that could impact on the identification of appropriate sensor sites.
Having sensors at two or more different soil depths allows operators to manage irrigation scheduling so that water can be applied at optimal times, minimising drainage events and preventing production declines due to water stress.
Keeping track of terminology
Field capacity (FC) – soil water content at its fullest point; any further water application (rainfall or irrigation) leads to drainage.
Permanent wilting point (PWP) – the lower limit for plant water extraction; at this point, plant growth completely halts; will vary with the nature of the plant, shallow-rooted versus deeper-rooted.
Available water capacity (AWC) – the amount of soil water available for a given root depth, the difference between FC and PWP; a shallower-rooted crop like pasture will have a lower AWC than a deeper rooted crop like lucerne.
Trigger point (TP) – the soil water content when water stress occurs and plant growth declines, a rough guide is 50% of AWC; also known as the refill point.
Refill amount (RA) – 80% of the trigger point (TP).
Readily available water (RAW) – amount of soil water between FC and TP.
Distribution uniformity (DU) describes how evenly an irrigator applies water to a paddock. A low DU means irrigation water is being distributed unevenly, potentially leading to over- or underwatering.
An irrigation system will never achieve 100% DU because of imperfect sprinkler patterns and the possible effects of wind.
The type of irrigator used also has an impact on DU. For example, centre pivots can achieve a DU of 80-90% while rotating booms (like Rotorainers) are commonly about 60-70%.
Onfarm, DU can be checked relatively easily. Guidelines and a calculator are available at www.pagebloomer.co.nz/resources/irrig8lite/.
