Making irrigation decisions based on soil & data

Irrigation involves many considerations, but soil scientist Dr. Andrew Bierer, Appalachian Fruit Research Center, Kearneysville, WV, believes soil is number one.

“Soil is made of three different size class particles,” said Bierer, who spoke recently at the Mid-Atlantic Fruit & Vegetable Convention. “The largest are sands, in the middle are silts and the smallest are clays. The proportions of these different particles determine how much water the soil can store, how it releases water and how available it is to the plant.”

Sands drain quickly, while clays have the highest water holding capacity but don’t give it up easily. This matters because pore space influences water use and compaction potential. This concept is also important when it comes to placing soil sensors to determine irrigation needs.

“Understanding pore space is critical for water management,” said Bierer. “Typical soil is approximately half solids and half pore space. We can affect that with compaction.”

Compaction compresses pore space that would otherwise be available for water and air, so when soil is compacted, there isn’t sufficient space to hold irrigation water.

Bierer explains soil water in terms of force, or “force of extraction.” “We like working in terms of force of extraction because it’s universal across soil types,” he said. “This pressure refers to the force required to extract water from the soil. Thinking of a scale in terms of soil water tension means looking at the force required to extract it – it’s a spectrum. Plants have to work that hard to extract moisture from the soil. They do this from increasing osmotic compounds in roots to draw water in.”

Field capacity is the amount of water soil can hold. Any amount above field capacity is gravitational water. Irrigation is based on field capacity and wilting point, which is the point at which there is irreversible harm to plants. Variation in soil layers, such as sand on top of clay, can complicate water management, which should be considered when placing sensors.

Regarding data-driven approaches to irrigation management, Bierer explained evapotranspiration and soil sensors. “Evapotranspiration estimates the evaporation occurring from soil plus the transpiration from plants,” he said. “It’s tracking rain and watering events and plant water use to adequately replace lost water.”

Soil sensors directly measure the soil water status beyond the “look and feel” method of checking soil moisture. These come at a higher price, depending on the technology. Real-time sensors can be wireless radio module devices or spot checking with a handheld tool should be reserved for the highest value crops. Soil sensors are well integrated into decision support platforms to make data easy to use. Sensors require maintenance and appropriate deployment for favorable results.

Some growers manage water through their own experience or a field manager who checks the ground and crops daily, looking for signs of stress and deciding irrigation priorities. This method relies on experience to determine how a clod of soil feels in the hand and varies among users. Sensors provide real- time data to go beyond that step.

Growers can improve profit margins with data-driven precision. “Yield increases have been shown in multiple crops by using sensor-based irrigation,” said Bierer. “In tomatoes, it’s anywhere from 20% to 30%. In blueberries, pulse irrigation is popular and that’s highly reliant on sensor-based irrigation with similar yield increases.”

Labor reduction is another bonus with a good irrigation system. If the farm owner or manager is taking time off and hired individuals are hand-watering or checking field moisture, the labor reduction gained with sensors over time contribute to positive ROI for the initial costs of equipment for soil moisture monitoring.

“Identify your goals and budget,” said Bierer. “Is spot checking something you are interested in for lower cost versus continual real-time logging? Are you looking to have peace of mind to see if you are on the right track or are you looking for valve control? A lot of decision support systems now offer valve control at the touch of a button on a phone app.”

Growers using automated irrigation should determine how data will be used and plan for data-driven action. Will data be incorporated into decisions, and if someone else on the farm will be reading data, will they respond and act through a standard protocol?

In some cases, general guidelines and recommendations won’t work for a specific cultivar. If a grower is trying a new variety or crop and the irrigation system isn’t working well, be willing to adjust it. For irrigating high-value crops, consider soil moisture sensor needs vs. its lower value.

“Having a lot of data at your fingertips can be powerful but it can also be paralyzing,” said Bierer. “Implement your checks and contingency plans. Check sensor calibration at least yearly or semi-annually. What happens if system fails? If someone else is taking over irrigation when you are away, if a sensor fails or it gives erroneous readings due to poor soil contact, can you switch back to business as usual?”

by Sally Colby