Research Highlights

Investigating Variable Rate Irrigation and Its Impact on Water Use and Soil Nutrients

By Sarah Hill

Pivot irrigation is not new to soybean production. However, using variable rate irrigation (VRI) to apply different amounts of water to specific areas of soybean fields is a newer practice that hasn’t caught on yet in Kentucky. Ole Wendroth, professor in the Department of Plant and Soil Sciences at the University of Kentucky, has been researching using different irrigation rates on different soil zones within soybean fields, through research projects funded from the Kentucky Soybean Board.

Trevor Gilkey, who farms near Princeton, Ky., allows Wendroth to utilize his farm for research on a large scale and owns a pivot irrigation system. 

“Irrigation is different from natural rainfall,” according to Wendroth. “If a farm receives a rainfall of 1 inch in 10 hours, farmers call that a ‘million-dollar rain’ because it’s soft and easily infiltrates into the soil. If the same amount of water is applied with a pivot system that runs over a whole soybean field, the water application rate is much higher than natural rainfall.”

The 600-foot-radius center pivot takes only several minutes to pass over a given section in the soybean field, moving on its own, applying more water in a shorter amount of time than a natural rainfall would. In Gilkey’s field, a complete cycle with pivot irrigation takes about 48 hours, applying half an inch of water to cover the entire field during that time. 

Erosion Creates the Foundation

“Kentucky soils are extremely spatially variable because of karst topography and erosion events in the past before no-till was introduced,” Wendroth says. “There is a lot of dolomitic limestone in the southern U.S., so precipitation percolates through it and creates caves beneath the surface. When the ceiling becomes too thin due to weathering, the limestone breaks and the soil above it collapses, creating sinkholes and the typical landforms with shoulder, backslope and footslope.”

“Due to erosion, in footslopes, we find deep, dark, fertile soils with a high soil organic matter content that are formed by soil material eroded from adjacent slopes and accumulating in low landscape positions,” Wendroth adds. “On the other hand, at the slope positions, the second layer with higher clay content is now the primary soil layer, and this happens in nearly every field on every farm in this part of the southeastern U.S.”

Spatial soil variability means that there are different soil qualities across the same soybean field, making its management difficult. Some areas may allow infiltration and retain water more easily than others, so more water is available for soybean plants for a longer period of time. In sloped areas with more clay-rich material, water uptake is slower, and soybean plants can’t take up as much water, so those areas will have lower yields than footslopes and shoulders. 

Technology Controlling Irrigation

In VRI, the many nozzles on a center pivot are controlled with computer software. After the initial pressure is set, the site-specific irrigation rate is influenced by simply turning the nozzles on or off. The computer program allows the soybean farmer to set the base rate, then the pivot moves at the right speed to accomplish that base rate or less but cannot apply more than 550 gallons of water per minute, the standard base rate for center pivots. 

“In the first year of research, we did intensive study on identifying if the pivot does what we want it to do,” Wendroth notes. “The farmer gave us access to the software control system, so we programmed an app with the geographic information from the field and the different rates to apply for the different areas of the field. We remotely uploaded the application map to the pivot and ran it.”

Wendroth’s research had a layout of different experimental strips, each with different rates of irrigation and nitrogen (N) application. 

“When the pivot came to the border of a treatment plot, the nozzle would shut on or off as needed,” he says. “The pivot crosses a lot of topographic zones in the soybean field. We combined the scheme with different water application rates to also look at N dynamics, since N and water go hand in hand.”

VRI allowed the soybean farmer to not over-irrigate the soybean field, so nutrients wouldn’t leach out of the root zone more than in non-irrigated fields. Wendroth and his team collected agroecological information on nitrates in the soil and water supplies to determine how much N the plants were getting. They are still analyzing the data and will publish the results when analysis is complete.

Applying the Data

“Among others, USDA-ARS scientists developed agroecosystem process simulation models since the 1990s,” Wendroth says. “We give them computer program data on important inputs, soil properties of the different soil layers, daily weather information, planting date, crop variety, how sensitive the soybeans are to temperature, and other environmental conditions.”

These sophisticated models can be used to create decision tools for soybean farmers where they would only need to enter a few simple pieces of data related to soil type to find out the best variable rates for topographic zones on their farms.

“Farmers and researchers in Kentucky haven’t had much experience using VRI, and upgrading center pivots with the technology is expensive,” Wendroth adds. “While soybean farmers can judge extremely well when to turn on or off the irrigation system, these model calculations will support decisions for VRI and show how well the soybean farmer can do with VRI.”

Wendroth says that VRI has the most potential in years when the weather isn’t consistently so dry. 

“We want to demonstrate what the benefits of VRI are — if the result is worth the investment,” he says. “We need to figure out if we should let the soil dry down to a certain point and then apply water uniformly, or if non-uniform irrigation rate and scheduling has great advantages.”

Published: Feb 27, 2023