Up to 10 different soybean fields with center pivot irrigation will be sampled within Delaware. In each field, two locations which receive irrigation and two locations without irrigation will be sampled for both soil nutrient and soybean tissue. This will result in 40 soil and 40 tissue samples for analyses. Fields will include both full season and double crop soybeans.
Soil and tissue samples will be taken at late vegetative/early reproductive stages. Samples will be dried and sent to the University of Delaware for analyses. Soils will be analyzed for the total nutrient suite, as well as pH and organic matter content. Plant tissues will also be analyzed for the full nutrient suite.

Yields, when available, will be correlated to nutrient content of both soils and tissues. Total nutrient uptake of Ca, Mg, and K will be compared among rainfed and irrigated samples to observe differences in soil nutrient vs soil moisture effects on uptake. A final report detailing the results, as well as potential future research projects examining how to
improve nutrient uptake in either rainfed or irrigated fields will be produced.

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 30, 2021:
Fields were selected for sampling, but sampling was delayed as we have had a wetter spring and early summer, which reduced stress and the need for center pivot irrigation. Fields sampling is expected to be done the first two weeks of August.

Updated January 1, 2022:
Field sampling was completed for all sites in the late summer by project personnel. Tissue and soil samples were submitted for testing to the University of Delaware Soil Testing lab and should be completed in January.

Updated March 12, 2022:
Final report attached.

View uploaded report

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. The objectives of this study 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. To do that, soybean fields 10 center pivots in Delaware were sampled in irrigated and dry corners. Upper leaf tissue and soil samples were obtained at the R2/R3 growth stage and tested for soil characteristics and nutrient contents.

Irrigated soybeans had higher concentrations of N, P, Fe, and Na in the upper leaf tissue, with K and Cu being slightly higher, but not statistically significant. This is not necessarily an issue for yield, as all measured leaf tissue nutrients were within their sufficiency ranges, except Ca which was just below the 0.8% threshold in both rainfed and irrigated soybeans.

Nitrogen also had no significant negative correlations in irrigated fields, with concentrations above threshold values (5.7%), so its possible N was not limited in these scenarios and would not produce negative correlations with other elements. This was not the case in rainfed samples, where N uptake was inversely correlated to Ca, Mg, and Al uptake, potentially due to stress when moisture was limited.

Greater Na concentrations in both irrigated soil and tissue samples was most likely related to salts in irrigation water, with Na having very few relationships to other elements in leaf tissue samples. Iron was also higher in irrigated tissue samples but had more correlations to tissue nutrients in rainfed samples. This may mean that Fe uptake occurred with higher soil moisture but was not related to any other nutrient uptake. Alternatively, Al uptake in rainfed beans may also reflect excess uptake, but under stressed conditions, where it positively correlated with many other tissue nutrients, but had no positive correlations with irrigated soybeans.

All these relationships are based on 2021 rainfall, which was sufficient for most of the early summer in Delaware, and not deficient until late July. It is certainly possible to have different relationships in doughtier years, which includes greater differences (lower N, P, and K in rainfed) between regions or fields with different moisture contents.

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.