Measuring and controlling soil moisture is critical to growing and maintaining healthy plants. To a novice, many of the terms regarding soil moisture can be confusing. In this primer we attempt to define and relate the various technical terms related to soil moisture, and to describe state of the art soil moisture sensors.
The best way to think of soil is to use the analogy of a sponge. When you dip a dry sponge into water it will absorb water slowly until it is completely saturated. When you pull it out of the water, water will gush out quickly, because of the effect of gravity, and after a few minutes the water will drip out of it at an increasingly slower rate until it stops dripping. The point at which the sponge is full of water, yet gravity can no longer pull water out of it is analogous to the measurement we call field capacity. When the soil has been saturated, and any excess water has been removed by gravity, the soil is at field capacity. This is also referred to water holding capacity (WHC).
Now suppose you take a vacuum cleaner and place its hose on the sponge. If powerful enough, the suction of the vacuum cleaner will pull water out of the sponge, until most of the water is removed. Note that regardless of how strong the vacuum is, a little bit of water will remain in the sponge, and it will appear moist. To drive out all of the water from the sponge, you would have to heat it. We compare this to soil where the vacuum represents the roots of a plant. The roots suck water out of the soil with a pressure determined by capillary action. The plant will be able to suck excess water out of the soil until the capillary pressure can no longer overcome the soil's tension to retain the water. This point at which a plant's root can no longer extract water is called the "wilting point", which as you can imagine is a critical parameter.
One more important term is the "plant available water". This is the available amount of water in soil that can actually be used by the plant. Just because soil may have water in it doesn't mean that the plant has enough "suck" to pull it out. So the definition of plant available water is the holding capacity minus the wilting point. Good soils have large plant available water, meaning they have high holding capacity, and low wilting points, so that water is available, and easy for the plant to extract.
As soil varies in composition, so do these parameters. Soil types are defined by their particle size. Sand is coarse - of course, and clay is made up of very fine particles, while silt is a medium particle size. Because clay soil has very fine particles it tends to hold moisture well, but it also holds on to it so the wiling point of clay is quite high, making it difficult for plants to extract the moisture. Sandy soil is very porous and so water flows out easily, and a result it has low holding capacity. The perfect soil has high holding capacity, and a low wilting point. To achieve this perfect soil, soils of different particle size are mixed together with organic matter such as humus.
Now that we have discussed how soil holds water, we can discuss how to measure soil moisture. Since the purpose of measuring soil moisture is to know if plants are getting enough water, we would want to measure the water that is available to their roots. Ideally we would measure the water with an "artificial" root. One very accurate method of doing this is with a tensiometer, which measures the water as a function of pressure. Since it measures pressure or tension its units are also in terms of pressure. The tensiometer doesn't tell you what the absolute moisture content of the soil is, but hearkening back to our soil moisture analogy, tells you how much pressure it takes to suck water out of the soil. Many technical articles describe results from tensiometers and give units in pressure such as bars, etc. Now if you happen to know what sort of soil the tensiometer is measuring, then you can compute the absolute soil moisture or at least get an estimate of it. A clay soil may have high moisture content, and at the same time have a high pressure, rendering the moisture useless to the plant. While tensiometers are accurate, and provide useful information they are delicate and expensive scientific instruments that require specialized knowledge to operate and interpret. They are also slow in the sense that they have to come into equilibrium with the surrounding soil before a measurement can be made, so they are not ideal for use in making quick measurements.
Another similar approach to the tensiometer is the gypsum block. This is essentially 2 stainless steel electrodes that are encased in plaster. As moisture absorbs into the gypsum resistivity decreases. The gypsum serves as a salt barrier. Many cheap soil moisture sensors consist of two stainless steel rods that insert into the soil. This approach is highly inaccurate due to salts in the soil which can wildly change the resistance of the soil, and thus give inaccurate readings of moisture content. The gypsum block sensor partially overcomes salinity issues with the gypsum barrier. The main disadvantages with gypsum blocks is that they are typically slow and bulky. After a block is placed in the soil, there is a lag before the gypsum comes to the same moisture level as the surrounding soil. Because they are large and obtrusive they can't be used in potted plants. The output of a gypsum block is an electrical resistance, this is in turn related to moisture in the units of pressure with the use of look up tables.
Modern soil moisture sensors use electronics to measure the dielectric constant of the surrounding material which happens to be related to moisture content. These sensors are also known as capacitive soil moisture sensors, or TDR soil moisture sensors. These sensors are small and unobtrusive so they can be used with potted plants, provide instant readings, are simple to use, are very affordable, and many are low power. For example, Vegetronix (www.vegetronix.com) produces several types of electronic soil moisture sensors. The Vegetronix VH400 consumes only a few miliamps of power, and can be purchased for $36.95 in small quantities. Because of their low cost and low power requirements, these sorts of sensors are being massively deployed in irrigation systems in wireless mesh networks such as Zig bee networks.
These sorts of electronic probes measure the soil moisture in absolute terms, namely the volume of water to the volume of soil, also know as VWC. Another related soil moisture measurement unit is GWC or gravimetric water content, which is defined as the mass of water, to the mass of soil. VWC and GWC are related by the bulk density of the soil, so if you know the density of the soil you can convert from one to the other. VWC is more commonly used. VWC is also related to pressure, to convert from one to the other the type of soil must be known. As was mentioned, a clay soil may have a high VWC, but a plant may have a hard time extracting water from it.
Accurate measurement and interpretation of soil moisture data, can allow individuals or computerized systems make decisions about water usage, saving valuable water resources, and promoting healthy plants.