The timing and amount of irrigation on vines is typically determined by monitoring volumetric soil water content (VWC). This approach has proved useful over many years with the assumption that plant health, growth and yield can be directly correlated with VWC. While a good intuition will conclude that a healthy vine will grow on soil of adequate moisture, and not flooded or bone dry soil, intuition is not as clever at determining the optimal value of VWC. Therefore we often rely on hazy definitions such as field capacity and refill points to let us know when we should turn the pumps on and off. Measuring VWC has become so ingrained for many growers and scientists it is often forgotten exactly what is being measured. Increasingly, growers and scientists have come back to the realisation that VWC has vast limitations and measuring the more meaningful parameter of soil water potential is far more important. This article outlines what VWC actually is, how it is measured, why it is meaningless, and why soil water potential is important.
One of the surprising misconceptions I regularly come across in conversations about soil moisture sensors is what they are actually measuring. Sleek technology, sophisticated graphical software, 21st century telemetry communications, and even clever marketing, give the impression that soil moisture sensors precisely and accurately measure the water in the soil. In reality, there is no sensor currently available, nor has there ever been a sensor, which actually measures water in the soil directly.
All soil moisture sensors infer the amount of water in the soil indirectly. There are two classes of sensors which use different physical principles: neutron measurements as measured by the neutron probe; and dielectric permittivity as measured by capacitance (or frequency domain), time domain reflectometry (TDR), standing wave, or a combination of these (Figure 1).
Figure 1. Examples of sensors which measure soil volumetric water content: a neutron probe and a capacitance sensor.
The neutron probe measures the amount of hydrogen atoms in the soils. Although just about every substance contains hydrogen we are mainly interested in the change in hydrogen in the soil. Over hours, days and months, the only elements that really change in relative proportion is the amount of air and water. Therefore, as the count of hydrogen atoms from neutron probe measurements increases and decreases, this is proportionally related to the amount of water, or the volumetric water content, of the soil. To go from a raw count provided by the neutron probe to a VWC value we either calibrate the count to our soil type or use a standard calibration curve.
The dielectric permittivity is essentially how much electrical charge is stored in the soil. And the element in soil which stores electrical charge, and varies the most, is water. Therefore, when you download your data and you read values in VWC you are not actually reading values of water content, but water content derived from a measure of electrical charge which is related to the dielectric permittivity. Depending on how accurate your sensors have been calibrated, your measured value of VWC could be close, or way off, the actual VWC.
So our soil moisture sensors and instruments are not actually measuring moisture but something else. Another surprising misconception I come across is that plants are reliant on VWC for health and vigour. Plants have no idea, nor have any care for, the VWC of the soil. Plants only care about the amount of work they need to do to absorb water from the soil. So it’s not water content, but water potential, which is the key parameter for plants.
A water potential value of 0 kPa means the soil is saturated with water, field capacity is defined as -33 kPa and permanent wilting point is -1500 kPa. At field capacity plants find it easy to work and absorb water from the soil whereas at wilting point the work is so hard that plants can no longer absorb water and face mortality. These values, unlike VWC, are absolute and do not vary whether you are measuring on sandy soil or clay soil. It is similar to measuring temperature – 20°C is 20°C anywhere as is -33 kPa is -33 kPa anywhere.
Fortunately, there are numerous sensors which can measure soil water potential including G-Block gypsum blocks, MPS-6 Water Potential Sensor, and Jetfill Tensiometers (Figure 2). However, like soil moisture sensors, these sensors (apart from tensiometers) often infer soil water potential through some form of electrical measurement. Gypsum blocks measure the electrical resistance between two electrodes and this resistance is related back to soil water potential values. The MPS-6 sensor consists of ceramic plates which come into equilibrium with the soil water environment. A calibrated capacitance sensor then measures the water potential of the ceramics. Tensiometers are perhaps the most accurate sensor as they are the most direct measurement of water potential. Soil water comes into equilibrium with a water column in a tensiometer tube through a porous ceramic cup. As the soil dries the tension with the water column increases and a measurement of this tension is made and this is the water potential.
Figure 2. Example of sensors which measure soil water potential. From left to right: G-Block gypsum blocks, MPS-6 Sensor, Jet Fill Tensiometers.
Soil water potential sensors are not a silver bullet for vine water management. They certainly provide a more accurate indication of the true soil water condition which plants really need to know. However, in the next article I will argue that for vine water management it is far better to directly measure plant water potential rather than measuring the soil.