Although Ca, Mg, and K are all exchangeable nutrients that are considered plant available, soil chemistry and plant root interactions result in different uptake and bioavailability. Within the soil, Ca and Mg can move with soil water or by diffusion, while the lower K concentrations do not readily move with soil water. This results in differences in uptake for soils with adequate moisture versus those under drought stress. Understanding how concentrations of each nutrient, the soil CEC, and soil moisture content interact is important for giving future nutrient recommendations.

Irrigated and rainfed dry corners in a Delaware field.

The objectives of this study (sponsored by the Delaware Soybean Board) were to sample center pivots in their dry corners and irrigated regions and compare soil nutrient levels and nutrient uptake in the leaf tissue for potential. Soil and leaf tissue samples were obtained at the R2/R3 growth stage in the summer of 2021 and 2022 from full season soybean fields in a total of twenty fields and eighty samples.

Only nutrients that were significantly different (a=0.1) between irrigated and dry corners (rainfed) samples are presented in Table 1. Characteristics without differences included soil pH, CEC, and all other soil macro and micronutrients. All tissue nutrients, regardless of differences, were within sufficiency ranges.

Soil NO3 (ppm)Soil K (ppm)Soil Na (ppm)Tissue N %Tissue P  %Tissue Na %
Irrigated 9.2b123.17b14.8a5.64a0.46a13.6a
Table 1: Soil and soybean leaf tissue nutrients from center pivot fields (Fishers LSD, a=0.1). Soil and tissue nutrients not shown were found to be similar between irrigated and rainfed parts of the fields.

Within the soil, NO3 was greater in the rainfed dry corners when sampled in late summer, which could be due to reduced uptake under a drought (Table 1). However, this makes more sense for corn crops, since legumes can fix their own N. The higher leaf tissue N under irrigation may be related to greater plant health and root development with adequate soil moisture. The same could be assumed for higher P under irrigated conditions since soil P concentrations were similar.

Soil K measured in dry corners was higher, possibly due to reduced uptake by both corn and soybean crops with lower yield. If applicators are not reducing their rates for expected lower yields, it could cause K to buildup over time. There were no differences observed in tissue K (data not shown), which was surprising considering the dry conditions in the corners. However, the increased K concentrations observed in dry corners may have allowed for easier uptake.

Irrigation water can contain salts like Na, which would help explain why both soil and leaf tissue Na concentrations were higher under irrigated parts of the fields (Table 1). Interesting, under rainfed conditions, Na concentrations measured in the soil also correlated to greater nutrient uptake for almost all macronutrients. The same was observed for soil B concentrations. However, in the leaf tissue, higher B often meant lower tissue levels of N, P, and K, while tissue Na also had negative relationships with biomass K, Ca, Mg, and S. Under irrigation, tissue concentrations of Na only had inverse relationships with K. This could suggest that whatever drives greater concentrations of soil B (and possibly Na in rainfed soils) is also influencing availability and uptake of other macronutrients. Since higher B in the tissue is not associated with higher N, P, or K, its concentrations and uptake alone may not be driving their uptake.

This was an observational study without any treatments or additions of fertilizer. Different rates of K on dry corners and irrigated parts of the field may better explain the best rates for efficient application.

This project was sponsored by the Delaware Soybean Board.

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