Walk the tightrope

Using soil moisture sensors correctly helps maintain plant health with optimal water use.
By Mike Mills

As soon as Archimedes built the first screw pump to bring water into the Egyptian irrigation canals, the burning question for the farmer was, “How much water should go on the field?” To this day, scientists and engineers continue to research the water needs of agriculture plants for optimum yield.

Maintaining the necessary water available in the soil is critical to success.

It’s like walking a delicate tightrope when irrigating, applying just enough water to fill the soil to field capacity without causing deep percolation or runoff. While knowing the “sweet spot” has traditionally been more art than science, modern moisture monitoring devices can find that optimum soil moisture and reach it without overwatering.

Understanding sensors

Soil moisture devices can be divided into three basic types: volumetric, tensiometers and solid state. Volumetric sensors directly measure the amount of moisture in the soil and are primarily used in research applications. These devices are generally the most accurate, but they can be very expensive and require a lot of calibration at installation. Tensiometers measure the amount of tension between the soil and water particles. The higher the tension, the more difficult it is for plants to extract water. These devices are relatively inexpensive and accurate but require a lot of maintenance throughout the season. Solid state devices work by measuring the electrical conductivity of a soil and calculating water tension to determine the amount of water available to the plant. These are typically the most affordable sensors and are generally easy to install, but they can lose accuracy in high salinity soils.

Installing a soil moisture sensor is the equivalent of adding a “water gauge” to a field indicating when that field is “full” (field capacity), “empty” (wilting point) and the current water level in between. Knowing this information allows the irrigation operator to accurately calculate exactly how much water is required, if any at all. With this powerful tool, operators can control pumping costs while reducing or eliminating nutrient leaching and also increasing both yield and quality.

The financial benefits from proper soil moisture monitor utilization can be significant. With the high cost of energy and maintenance for pumping plants, reducing water events throughout the season can return thousands of dollars to the operator in terms of saved expenses. In addition, by maintaining a proper soil moisture level, field crops often see an increase in both yield and quality. One study of an actual production operation measured “side-by-side” performance of two fields with similar soil composition, the same seed varieties and plant populations, and identical cultivation practices. One field was watered using the growers’ traditional irrigation scheduling methods, the other field followed the irrigation recommendations of the moisture sensor. At the end of the season, the field scheduled using soil moisture monitoring saw irrigation costs reduced by $19/acre while also increasing revenue by $33/acre for a total positive impact of $52/acre.

Experience and data

Today’s grower is a wealth of knowledge and experience earned after years of hard work, successes and failures. That firsthand experience has taught the grower exactly what practices achieve success. By adding a soil moisture sensor, many long-held beliefs about water application and timing are challenged. It is difficult to delay an irrigation event simply because the sensor says so when all the grower’s experience says it’s time to water. Likewise, reducing the amount of water applied during a necessary irrigation event also sometimes seems counterintuitive. The recent Farm and Ranch Survey of more than 229,000 growers showed that almost 40% still use the “feel” method of determining soil moisture for irrigation scheduling while less than 10% use a monitoring device to report soil moisture. Growers are reluctant to allow their soil to become “too dry” for fear of losing their crop. Add to that the belief that extra applied water will just go back to the water table without any adverse effects means that growers are often just too reluctant to change what they practice.

Water is the lifeblood of crops and withholding that is a big leap of faith. Common beliefs hold that the more water applied, the better a crop will perform. However, research continues to demonstrate that overwatering can have detrimental effects on both yield and quality. When the root zone is beyond field capacity, there isn’t enough pore space for air. As that excess water drains due to gravity, it also takes with it vital and expensive nutrients.

Soil moisture sensing technology has significantly improved in recent years. Modern probes are much easier to install and extract. They also have the capabilities to automatically detect the soil type where they’re installed and some even provide nutrient information along with soil moisture. They are usually connected to a service that imports current weather details then interprets all the data for the grower. Growers may also have the option of entering field-specific data such as crop type, planting date and other details. By uploading this information to a central database, the grower can receive information about their fields including current soil moisture in relation to field capacity and wilting point along with recommendations on whether an irrigation cycle is necessary and how much water should be applied.

The cost of these types of devices has become much more affordable as technology has improved. Some growers choose to purchase their own devices so they can do their own installation and extraction. This is often the lowest cost method, but it does involve some labor and education by the grower. More popular is a leasing model where a local service will install, maintain and extract the device each year. During the growing season those services can offer the grower highly detailed reports of their soil moisture, crop development and watering recommendations. In the off season, they perform device maintenance and storage.

Additionally, because of the positive environmental effects of utilizing these devices, there are many states that will provide financial incentives for the grower. Soil moisture sensing is an approved practice by the Natural Resources Conservation Service, and many locations will pay a fixed amount per acre for each field where a sensor is installed. The funding opportunities sometimes end up covering 100% of the device and services, leaving the grower with no out-of-pocket expenses.

The addition of soil moisture monitoring is an inexpensive and environmentally friendly way for a grower to increase revenue while also being responsible with natural resources. If a grower is willing to work a little outside their comfort zone and follow the recommendations, they can see rewards both financially and environmentally.

Mike Mills is the manager of dealer development and training/director of sustainability solutions at Reinke Manufacturing Company.
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