Potassium is an exchangeable nutrient considered to be plant available but depends on soil moisture to be carried to plant roots. With higher fertilizer costs and fields under both rainfed and irrigated production, accurate fertilizer recommendations for K are necessary in Delaware. In prior work observing soybeans grown under the pivot compared to
dry corners, K uptake was slightly higher under irrigation in 2021, with no differences in 2022. Additional work under controlled irrigation and varying starter rates can help determine if these results are more than just an observation. Additionally, we will observe if any additional K is remaining after the season under rainfed conditions and any interactions with other nutrient cations (Ca, Mg). In our previous work, higher K remained in the rainfed dry corners, which may require lower K rates.

1) Plant soybeans with three starter rates to observe K uptake.

2) Use the subsurface drip variable rate irrigation to measure differences in K uptake.

3) Compare the effects of starter K and irrigation on yield.

Soybeans will be planted in 15” rows at a rate of 120,000 seeds acre-1 and three starter rates (0, 20, 40 lbs K2O acre-1) will be applied at planting. There will be two zones (irrigated/rainfed) under the SDI with five replications of each starter*irrigation combination, producing thirty total plots.

Soil tests will be taken pre- and post-plant, while tissue samples will be taken at vegetative (~V3) and reproductive (~R2) stages. Samples will be dried and sent 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 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.

Updated July 28, 2023:
Plots were laid out at the Warrington Irrigation research farm in April 2023. They were randomized within blocks, but blocks were placed over known high and low CEC parts of the field. Soil samples were taken after plots were established. Then, using a Valmar spreader, potassium was applied to the necessary plots where CEC and irrigation would be varied. Following this soybeans were planted within the plots. Tissue samples were taken at both V3 and R2 stages.

Updated February 7, 2024:
Tissue samples from the V3 and R2 growth stages were submitted to the soil testing lab for analyses, where the data was included in a presentation on CEC at the Mid-Atlantic Crop Management School. Plots were harvested in November 2023 and final soil samples were submitted to the UD soil testing lab for analyses.