Jarrod Miller, Extension Agronomist & James Adkins, Irrigation Engineer

One method to uncover soil variability and crop response is to use precision soil sampling, including either grid or zone methods. These maps can then be used to make variable rate applications, where application rates will change as you cross the field, and GPS helps guide the rates. For this study, soil samples were taken from the upper eight inches of soil on our irrigation research farm in Harbeson, DE. Samples were taken on a 90 by 90-foot grid in April 2022 (Figure 1).

Figure 1: Grid sampling points overlain onto a September 2024 image of the Warrington Irrigation Research Farm, including both center pivot and linear irrigation.

From these grids, maps of nutrients and soil properties were created, including soil pH and buffer pH (Figure 2). Soil pH is measured using water and is the pH your crop will react to. So you use the soil pH measurement to decide if you want to raise or lower your pH. The buffer pH helps figure out any acidity within the soil that will react with lime, causing the need for a higher lime rate (Figure 2b). These two pH measurements were combined using UD recommendations to create a variable rate lime map (target pH = 6.2), which was applied in 2023.

Figure 2: Grid sampling maps of 2022 soil water pH (a), soil Adams-Evans buffer pH (b), and the final lime rate based on University of Delaware recommendations (dark blue is greater lime recs). The overlain box shows an area with low pH (requires more lime), but less buffering acidity (requires less lime), resulting in a lower lime rate. The circle shows an area with lower soil pH (requires more lime), more buffering acidity (requires more lime), resulting in a much higher lime rate than the surrounding area.

The results can be seen in Figure 3, where pH has been grouped into categories with a range of 0.5 pH units. In 2022, much of the field had a pH less than 5.5, so variable rate lime applications were successful in brining most of the field between 6.0 to 6.5. The linear irrigation portion of the farm remained lower because the pH was already close to target in 2022 and received a lower lime rate.  On the north side of the pivot, areas that were highly acidic in 2022 were not brought above a pH of 6.0, so more lime may have been required. The area outlined by the square had a buffer pH that predicted less lime but may not have been accurate. More lime may have been warranted in that case.

Figure 3: Soil pH measured in a) 2022 and b) in 2024, one year after variable rate lime application. Target pH was 6.2.

Another issue that arises with lime application is the availability of most micronutrients, which drops with higher soil pH (Figure 4). An index which includes manganese (Mn) concentration and soil pH can be used to determine the potential issues that may arise due to reaching the desired soil pH. At our research farm, the southwest corner of the pivot had lower Mn concentrations, and liming put us into a “Watch” category, meaning Mn deficiencies may occur. This may be another reason to consider variable rate liming, to ensure that regions of the field with lower micronutrients to not become less available. However, decisions can also be made to concentrate additional fertilizer to make up for potential deficiencies.

Figure 4:  The a) concentration (ppm) of manganese (Mn) across the field compared to b) the availability of Mn after variable rate liming occurred. Mn availability is related to both its concentration and the soil pH. The category “Watch” means to tissue test, while “Need” means an issue will occur. Most of the field is considered “OK”.
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