Monitoring Soil Moisture

Introduction

As of 2018, US farmers applied 83.4 million acre feet of water for irrigation (USDA NASS 2018). That is equivalent to 27,176,009,142,857 gallons. Lake Erie holds around five trillion gallons. Thinking about that amount of water being utilized for crops, it brings up a few questions. How much energy is used for moving this much water? How much erosion can be caused by over irrigation? How much nutrient is moved below the root zone because of over irrigation? Is there a way to minimize these risks and improve energy efficiency? As weather patterns change and more intense rain events occur followed by lengthy periods of dry or drought conditions, monitoring soil moisture and having the potential to irrigate may help to alleviate some crop stress.

Methods of Monitoring

The first place to start with soil moisture is to know what soil types are in the field. Web Soil Survey (usda.gov) is a good place to start. Once an area of interest has been outlined and a soil map created, water holding capacity can be found within the data for that soil type, which gives a good estimate of how much moisture the soil can hold at various depths, and from there maximum allowable depletion can be determined. Maximum allowable depletion is determined by taking the water holding capacity at a specific depth to the crop, and multiplying by a percentage of moisture loss the farmer feels comfortable with, usually 40 to 60 percent.

There are many methods of soil moisture monitoring. One of the most basic is to observe the feel and appearance of the soil. Take a sample of soil near the rooting depth of plants, ball it up. If it falls apart, it's likely that the plants will need moisture soon. NRCS has a document to help estimate moisture by feel and appearance, found here: Estimating Soil Moisture (usda.gov) (PDF).

Beyond the feel and appearance method, there is the option to use soil tensiometers, which measure the water tension in the soil. Tensiometers need to be calibrated occasionally and are usually read in the field. In some soils, especially those with heavy clay, tensiometers can clog around the ceramic tip and need to be cleaned and recalibrated.

Another common option is to use a Time Doman Reflectometry unit, or TDR. TDR measures moisture by sending an electromagnetic pulse through the soil, and the time it takes to go from probe tip to probe tip is related to soil moisture content. This is a quick and relatively accurate measurement method if the probe is calibrated for the soil type being sampled.

Capacitance sensors are another option. Many probes are drill and drop, while a tensiometer requires making a soil slurry and backfilling the hole around the tensiometer. Many capacitance sensor readers have the option for Bluetooth or cellular capability, and companies marketing them have rental plans for the units. Information is sent to your phone or computer with the soil moisture, soil salinity, and often Evapotranspiration data for that area.

Evapotranspiration and Crop Coefficients

Evapotranspiration is the measurement of water evaporation from soil and transpiration from plant leaves. It is influenced by temperature, humidity, wind speed, and solar radiation. The higher the reference evapotranspiration values are, the faster the soil moisture is depleted, and the need for irrigation increases. Some mobile weather stations have evapotranspiration estimates that can help determine the need for irrigation. Crops have varying water requirements at different growth stages. This is known as the Crop Coefficient. Knowing what the coefficient of a crop at a specific growth stage can also help estimate the needs of that crop, based on what soil moisture readings are, and what Evapotranspiration rates are.

Climate Smart

Monitoring soil moisture for irrigation allows for providing the right amount of irrigation at the right time. It allows for more efficient use of water. Throughout the growing stages of plants, the crop coefficient changes, and there are times when plants need less water. The other side of the coin is when plants are stressed, knowing how much irrigation to put on is equally important to avoid over-irrigation. Over-irrigation can lead to increased potential for fungal and bacterial diseases and increased potential runoff or leaching of nutrients. Proper timing at rates of irrigation can increase energy efficiency and reduce unnecessary wear and tear on equipment. Well-pump usage can be decreased. If using a generator, there can be a reduction of fuel consumption.

Monitoring soil moisture for irrigation is an opportunity to adapt to climate variability. It allows an adjustment for extended drought conditions. It provides the information to conserve water when it is not necessary to irrigate. Knowing when and how much to irrigate can help to maintain yield potential in a dry year. There are many methods to monitor soil moisture when trying to determine the need for irrigation. Which method to use depends on how much one is willing to invest versus the return of investment potential.