Measuring Soil Moisture
It is common landscape practice to supplement rainfall with the use of an irrigation system to keep plants looking their best. Many systems are automatic: the more complex units are connected to a climate-based electronic controller and run when weather and evapotranspiration data dictate; the simpler ones run a set schedule linked only to a time clock. Either of these systems may apply more water than is necessary to maintain a healthy landscape. For a clear picture of when and how much to water plantings, agricultural managers have long relied on soil moisture measurements; landscape professionals can do the same to maximize irrigation efficiency in landscape and turf plantings.
Soil Moisture Terminology
The following terms are commonly used to describe how soil moisture is quantified. More detailed information on all of these can be found in the agricultural extension publications listed under Resources.
- Soil water content is a measurement of the amount of water in a known amount of soil; it can be expressed as % water by weight or volume of soil, or inches of water per foot of soil.
- Soil water potential or soil moisture tension is a measurement of how tightly water clings to the soil and is expressed in units of pressure called bars (one bar is equal in strength to the pressure of one atmosphere). Generally the drier the soil, the greater the soil water potential and the harder a plant must work to draw water from the soil.
- Plant available water (PAW) is the amount of water in the soil between the soil's field capacity (soil water content after gravity has removed any freely draining, excess water) and its permanent wilting point (soil water content at which most plants can not recover from wilting). It is expressed as inches of available water per foot of soil.
This figure is important because it is within this range (between field capacity and wilting point) that irrigation should occur, based on the amount of PAW that can be depleted in the soil without harming plant growth and development. Plants with shallow roots and low root densities should be watered before the soil moisture level comes too close to the permanent wilting point since they will be less able to absorb all available water than plants with deeper roots and higher root densities.
A useful tool for estimating PAW in different soil types is a hydraulic properties calculator, which is readily available online. The calculator is straightforward, but requires the user to know the percentages of sand and clay in his or her soil. This kind of soil textural analysis can be requested in a soil test from the Soil and Plant Tissue Testing Laboratory at UMass (https://ag.umass.edu/services/soil-plant-nutrient-testing-laboratory).
Measuring Soil Moisture
A wide range of tools are available for determining soil moisture, and the devices mentioned here are typically used for irrigation management purposes. They are not much more expensive than simple soil probes (but are much more accurate), and are straightforward to operate.
- Tensiometers are devices that measure soil moisture tension. They are sealed, water-filled tubes with a porous ceramic tip at the bottom and a vacuum gauge at the top. They are inserted in the soil to plants' root zone depth. Water moves between the tensiometer tip and surrounding soil until equilibrium is reached, and moisture tension registers on the gauge at the top of the unit. Readings indicate water availability in the soil. Tensiometers operate best at soil moisture tensions near field capacity and need to be serviced before reuse if they dry out. Average cost for a tensiometer is $50-$100 (and generally more than one is installed at a location) (Cregg, 2003).
- Electrical resistance blocks, also known as gypsum blocks, measure soil water tension. They consist of two electrodes embedded in a block of porous material, usually gypsum; the electrodes are connected to lead wires that extend to the soil surface for reading by a portable meter. As water moves in or out of the porous block in equilibrium with the surrounding soil, changes in the electrical resistance between the two electrodes occur. Resistance meter readings are converted to water tension using a calibration curve. Gypsum blocks operate over a wider range of soil moisture tensions than tensiometers, but tend to deteriorate over time and may even need to be replaced yearly (Werner, 2002). Individual blocks can cost as little as $1.25 each and the meter is around $300 (Cregg, 2003). Granular matrix sensors are newer devices that are similar to gypsum blocks but are less susceptible to degradation. The sensors are more expensive than gypsum blocks, in the $30 range.
- Time Domain Reflectometry (TDR) is a newer tool that sends an electrical signal through steel rods placed in the soil and measures the signal return to estimate soil water content. Wet soil returns the signal more slowly than dry soil. This type of sensor gives fast, accurate readings of soil water content, and requires little to no maintenance. However, it does require more work in interpreting data, and may require special calibration depending on soil characteristics. The cost ranges from $100 to $500 (Ling, 2005).
For more information about the theory and practice of measuring soil moisture, refer to the following extension publications:
- Cregg, B. 2003. Soil moisture measurements in nurseries and landscapes. Crop Advisory Team Alert, Vol. 18, No. 12. Michigan State University Extension, East Lansing, MI.
- Evans, R., D. Cassel, R. Sneed. 1996. Measuring soil water for irrigation scheduling: monitoring methods and devices. Publication Number: AG 452-2. North Carolina Cooperative Extension Service. Raleigh, NC.
- Ling, P. 2005. A review of soil moisture sensors. Vol. 12, Issue 3. Ohio Floriculture Online. The Ohio State University Extension. Columbus, OH.
- Scherer, T., B. Seelig, D. Franzen. 1996. Soil, water and plant characteristics important to irrigation. EB-66. North Dakota State University Extension Service. Fargo, ND.
- Werner, H. 2002. Measuring soil moisture for irrigation water management. FS876. South Dakota State University Cooperative Extension Service. Brookings, SD.
Written by: Jennifer Kujawski