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. In particular K and Mg have antagonistic relationships in both corn and soybeans, with over application of either nutrient suppressing uptake of the other. 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.

1) Sample irrigated and rainfed soybean fields, with a focus on dry corners in center pivot fields.
2) Compare Ca, Mg, and K uptake in the tissue vs soil nutrient contents and cation exchange capacity (CEC).
3) Determine the need for future projects to improve uptake of essential nutrients in both rainfed and irrigated fields.

Updated July 23, 2022:
Fields have been selected in Sussex, Kent and New Castle counties for sampling tissue, following the current statewide stresses due to drought conditions. Some fields in Sussex have been sampled for tissue and soils, the remaining fields will be sampled through the beginning of August and sent to the UD soil testing lab for analyses.

Updated December 26, 2022:
All fields were sampled for soil and tissue over the summer of 2023. Data analyses was performed by combining the 2021 and 2022 samples together to get a larger focus. A final report of the data has been drafted and results were shared through the Delaware Agronomy Blog.

Updated April 6, 2023:

View uploaded report

This study is observational in nature and designed to observe what research projects can be pursued for Delaware farmers related to fertility and irrigation combinations. The known issues with K uptake under variable moisture conditions (rainfed) are well known (Karlen et al., 1982; Fernadez et al., 2008; Wijewardana at al., 2019), and show some potential here. Although soybean tissue levels of K did not vary between rainfed and irrigated parts of the field, soil concentrations did. One potential explanation is that blanket potash applications continue to raise the levels of K in dry corners, while yields and uptake are lower in drought years. Higher remaining K in the fields may help with K uptake under dry conditions.

It is not clear why NO3 is higher in rainfed parts of the fields as well since leachable anions like B and SO4 was similar. We could hypothesize that NO3 remains due to reduced uptake, but that does not explain why so many other soil nutrients were similar. The higher Na observed in irrigated part of the field could be related to salt additions from irrigation water. This would also explain the higher Na observed in soybean leaf tissue under irrigated conditions. The only other soybean leaf tissue nutrients with observable differences were N and P, which were both higher under reduced stress irrigated parts of the field.

Concentrations of B in the soil had positive influences on many soybean tissue nutrients under both rainfed and irrigated conditions but had negative relationships between tissue B tissue macronutrients. This may indicate that conditions giving rise to higher B concentrations are influencing nutrient uptake or plant health, but higher B in the tissue is not related to this same phenomenon. In particular, B up to about 0.5ppm may assist with S uptake into the soybean plant, with a plateau about that point. Tissue Na concentrations were also negatively influenced by soil B concentrations, but only had a positive relationship with tissue B under rainfed conditions. Boron in the soil can be tied to organic matter mineralization which we wouldn’t expect to be similar between rainfed and irrigated parts of the field. Also, NO3 was different by soil moisture contents, but B was not. This certainly warrants further study.

Higher tissue Na levels had negative relationships with macronutrients under rainfed conditions, another potential indicator of stress. Particularly since higher soil Na had positive relationships with many of these same nutrients. It is obvious that complicated relationships between nutrient uptake and environmental conditions exist, making fertility studies difficult under varying soil moisture.