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New Opportunities for Soil Moisture Monitoring

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Whether it’s having too much or too little, water is a constant struggle in agriculture. The adage that you can’t manage what you can’t measure is just as true for water as it is for finance and business.

Given how important water is for growing crops, it’s surprising that technologies providing soil moisture and weather data aren’t more widely used. However, with the new year comes new goals, and what better goal is there than more accurately monitoring crop water needs?

The soil moisture monitoring market is saturated (forgive the pun) with hundreds of companies and systems. Many have seen the opportunity in the marketplace and are trying to enter the space.

Soil Characterization
When evaluating a site for installing a soil moisture monitoring system, it’s important to find an area of the field that is representative of normal or average field conditions. Installing a soil probe in a problematic area, such as a low elevation spot where water pools or soil texture is markedly different from the rest of the field, can cause issues when it comes to interpreting data and making recommendations.

The first thing to do is to understand the soil at the identified site. A great place to start is using digital Web Soil Survey maps from the Natural Resources Conservation Service (NRCS). These maps cover most of the agriculture land in the United States and provide data on which soil types to expect in an area. While these maps aren’t a perfect representation of field variability and soil types, they are helpful in understanding what conditions might be present in a field. 

Next, it’s important to do a physical characterization of the soil. This involves taking a soil sample and looking at the layers of soil and textural composition. Specific things to look for are the textural class, such as whether the soil is a sand, silt, or clay. These characteristics affect how well water will infiltrate into the soil, the water holding capacity, and things such as irrigation timing and amount. Another characteristic to look for is any layers of compaction, such as a plow pan, that might hinder water movement through the soil.

After a suitable site is found that represents the field, the soil moisture monitoring system is ready for installation.

Most systems have a main gateway that collects, stores, and transmits the data. A soil moisture probe connects to this gateway, primarily through a cable, although some are wirelessly enabled.

It’s important that the selected site will allow the gateway and probe to communicate, whether they’re situated right next to each other or some distance apart. Consideration should be given to whether equipment, such as tractors, sprayers, or pivot irrigation systems, will need to operate in the field. The systems should be in a place so as not to be damaged in-season or marked so that they are visible as the crop grows taller over the course of the season.

There are two primary methods for installing soil moisture probes. One is using an auger to make a hole deep enough for the probe and packing a mud slurry around it. Drawbacks to this process are that it’s time consuming and messy. There may also be issues with data accuracy, as many soil moisture probes have sensors at different depths that provide soil moisture, temperature, and electrical conductivity (EC) readings at different depths. If a mud slurry is made and packed around the probe, the natural variations in the soil as depth changes are effectively erased and the probe may not provide accurate readings.

The second method is known as “drill and drop,” as used with the probes developed by Sentek. This requires an auger that is the exact diameter of the soil moisture probe. With careful installation, this results in a tight seal around the soil moisture probe with good soil contact. This method is much faster, cleaner, and in theory will provide more representative data than the auger method.

Checking the Data
Once the soil moisture monitoring system is in place, it’s important to check the data. First, the system should be checked to ensure that the probe and gateway are communicating, and that data is either being collected in a data logger or being sent to the cloud.

These values should be checked to make sure that the system was installed correctly and that none of the values are off. If the values look incorrect, this could be an indication that the system wasn’t installed properly and that there may be a pocket of air along the soil probe.

The system should be carefully monitored during the first precipitation or irrigation event. Typically, it will take water some time to infiltrate the soil and percolate down through the soil profile. A user should be able to watch the varying depths of soil increase in moisture sequentially, with the 2-inch depth increasing soil moisture content minutes or even hours before 6 inches, 10 inches, and so on.

If all the depths being monitored have a soil moisture spike right around the same time during a precipitation or irrigation event, it may indicate that there is not adequate soil contact. In this case, water may be running down the sides of the probe and into the soil profile, throwing off soil moisture readings.

Interpreting the Data
Interpreting soil moisture data, while a science, is also a bit of an art. There are potentially hundreds of factors to consider when interpreting the data, and users need to be wary of correlation vs. causation. This essentially means that while some data may look like it has some type of relationship, it doesn’t necessarily mean that one causes the other.
As an example, it’s not uncommon to see soil salinity values increase as soil temperatures rise. However, high soil temperatures do not necessarily mean that soil salinity will increase. Soil salinity is more a factor of precipitation and irrigation events and saline water evaporating from the soil and leaving salts behind. As such, the two values may be correlated, but soil temperature does not necessarily cause soil salinity values to change.

However, there are a few areas where soil moisture data can clearly be interpreted. One of these is plant available water.

With some observations, it is possible to establish the thresholds for saturation and field capacity of the soil and the permanent wilting point of the crop that is being grown. The space between the permanent wilting point and field capacity is the plant available water. This can be monitored over the course of the season and used to schedule irrigation events. 3PlantAvailWater_png_AnhGl2K2

By collecting data on precipitation events and irrigation, it is also possible to determine how many inches of water are needed to raise soil moisture by a given amount.

An interesting possibility of crop management made possible by collecting soil moisture data at different depths is determining the rooting depth of the crop. A goal for in-season crop production is to grow plants that are resilient and able to adjust to changing environmental conditions. Key to a plant’s ability to do this is how deep their roots grow. Plants that reach deeper into the soil are able to scavenge for water and nutrients in a greater area and are typically better able to tolerate weather extremes.

Through clever irrigation timing, plant roots can be encouraged to grow deeper into the soil profile. This can be monitored through soil moisture data, as the soil profile at the different depths will take on a more stair-stepped pattern as the plants take up water for respiration during the day and slow down water uptake at night.

Making Decisions
Ultimately, when adopting any new precision agriculture technology on the farm, the goal should be to use the data and information to make more informed decisions. These decisions should have a clear economic benefit, such as making accurate irrigation recommendations, knowing when and how much fertilizer to apply, and knowing when conditions are ripe for certain diseases or pests.

There are many hardware options on the market, but few have taken the next step of providing automation or decision-making tools. One such company is Pessl Instruments, an agriculture sensor manufacturer based in Austria.

Pessl recently announced a partnership with John Deere to provide solutions to customers, which includes having weather and soil moisture data link to the John Deere Operations Center software. With its sensors, Pessl Instruments is able to model possible disease pressure on 38 crops, with a total of 82 different diseases. This includes diseases such as Cercospora on sugarbeet, late blight of tomato, Sclerotinia stem rot of soybean, and many others.

As with any new technology, the only way to learn is to try it. Don’t expect to be able to use the soil moisture data right away, as it can take time to understand.

The best way to start is by installing a system and collecting data for a full season. With this data, patterns will start to emerge, and the data will hopefully be useful in making more informed water and crop management decisions.

Another best practice would be to work with an agronomist or irrigation specialist to understand the data and find ways to utilize it. It’s a new year, and this would be a great technology to adopt and make the goal to understand.


Interested in more about precision agriculture? Read Nate’s latest article about automation powering lettuce thinners.

Learn more about precision agriculture offerings from RDO Equipment Co. Browse listings of available used agriculture equipment for sale or find options by visiting your local RDO Equipment Co. store.