Soil moisture monitoring is an imperfect science with a lot of complexity to it. There is no replacement for good old know-how; the feel of the soil in your hand and how the plants respond, but soil moisture probes can provide additional information. Soil moisture monitoring technology gives you the ability to bury probes at root depth and/or to get moisture readings at multiple depths, without picking up a shovel. It’s just one more tool in the toolbox.
There are a lot of different styles of soil moisture probes that work in various ways, these include: Electrical Conductivity (EC), Volumetric Water Content (VWC%), and Soil-Water Tension/Soil Metric Potential.
Electrical Conductivity (EC)
This type of soil moisture sensor measures how well soil conducts electricity (as the name implies). While pure water is a very poor conductor, water with salts and other nutrients dissolved in it is a good conductor. So it is the solution of water and nutrients that allow for the moisture level to be read. But measuring soil moisture based on EC can have pitfalls. Depending on fertilization rates and soil salts, EC reading may not directly reflect soil moisture content. Many California growers deal with increasing salinity, which “fools” EC sensors into thinking the soil is wetter than it is. Many of your garden variety moisture probes work on EC and usually give you a 1-10 reading on the dial (1 being dry and 10 being very wet).
Volumetric Water Content (VWC%)
The VWC method of determining soil moisture content is based on the ability of a material to hold its charge — something called dielectric constant. Without going into too much detail, probes of this type distinguish between salt-driven conductivity and actual water content, giving you a bit more accuracy when compared to EC. These types of probes give you readings of 0% to ~60%. The limitation of this type of probe is that soil type (sandy versus clay) has a large impact in the VWC reading. In the case of hand-held moisture probes you really need to know your soils and find a good chart to guide you. But with the more advanced probes, it is possible to calibrate the probe to the soil type for more accurate readings. There are a growing number of the long-range radio wave (LoRaWAN) remote moisture sensing platforms utilizing this VWC type of probe (some of which test for both VWC and EC).
Soil-Water Tension
Soil-water tension is the amount of force or “effort” plant roots must exert to extract water from the soil. Driving soil tension dynamics are the forces of adhesion and cohesion. Water likes to electromagnetically stick to soil particles (adhesion) and other water particles (cohesion). An example of this is when you put your finger over the top of a straw, lift it out of your water glass, and the water stays in the straw — that is adhesion and cohesion in action. When soil is wet, it is very easy for plants to extract water. When soil dries, tension increases, making water extraction physically harder for plants.
We measure soil-water tension (also called metric potential) using a tool called a tensiometer. Tensiometers measure that level of suction the plants are feeling. Some tensiometers are digital and some are very low tech. Soil tension is measured in kilopascals (kPa) and measurements generally range from 0 kPa (VERY wet) to -100 kPa (dry). Most vegetable crop types like a kPa range of somewhere between -5 to -70. As an example, lettuce, which wilts very easily, will want to stay somewhere in the range of -7 to -10 kPa. In contrast a tomato plant can handle much dryer soils and will be comfortable in kPa ranges as low as -30.
The kilopascal (kPa) unit of measurement can be a tough one to wrap our heads around. I get a lot of blank stares trying to explain tensiometers. I try to make the analogy of temperature. You probably can’t tell me what a Fahrenheit degree actually signifies, but you know at 30 degrees you’re going to need a jacket, 70 degrees you’ll be comfortable and at 130 degrees Fahrenheit you might die. Tensiometer measurements are another way we can get an indicator of what’s going on in the sub-soil. It’s a way to manage for a moisture range where your plants will be comfortable.
You with me? Good! Because we have another problem to unbox.
Soil-water tension varies with soil type. Clay soils hold water much more tightly when compared to more loose sandy soils. Clay particles are very small, and because of that the forces of adhesion and cohesion are much stronger than in loamy and sandy soils. Since clay holds onto water tightly, the tensiometer will read “drier”/higher on the kPa scale, even when the soil still has usable moisture. With time and observation you can calibrate your reading of the soil-water tension to your particular soil composition, allowing for an accurate, at-a-glance reading of subsoil moisture conditions. Read more about tensiometers here: Soil Moisture Monitoring with Tensiometers
Each soil moisture monitor has its weaknesses and strengths but all can be of use on a small-scale farm setting. Take the time to consider what could be a good fit for your operation.
This article was compiled with funding from the Department of Water Resources Underrepresented Communities, California Tribes, and Small Farmers Groundwater Technical Assistance Program.