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.

Update:
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.

Update:
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.

Update:
The final progress report is attached. An early version article was published here: https://sites.udel.edu/agronomy/2024/02/15/potassium-applications-in-delaware-soils/

View uploaded report

The objectives of this study were to plant soybeans in low and higher zones with and without K application and examine yield and nutrient uptake. Soybeans (Axis 3922E3) were planted in 15" rows at the Warrington Irrigation Research Farm in Harbeson, DE on June 6, 2023 at 120,000 seeds per acre. The planting was performed in a subsurface drip irrigated field and placed into zones that were classified as high CEC (>4 meq per 100g soil) and low CEC (<4 meq per 100g), based on a previous grid sampling. Within these zones, 10 foot wide plots were established receiving either no K application or 60 lbs K2O (100 lbs 0-0-60), applied with a Valmar spreader prior to planting (June 2, 2023). Prior grid sampling shows very low (<50 ppm K) concentrations across this field. Soils were obtained pre-plant and post-harvest, while trifoliate leaves were collected at the V3 and R2 growth stages.

In this study, it was the CEC that drove yields, averaging 70 bushels in high CEC zones, regardless of K applications. The lack of response from K may be associated with the mineralogy of Delaware soils, which have additional K not measured by traditional soil testing.Lower CEC (<4) was associated with a 30-bushel loss, potentially tied to lower nutrient concentrations (K, Ca, or Mg). However the differences in those nutrients were not that great, and we believe that lack of water may be the culprit. These soils received subsurface drip irrigation, which may have difficulty wicking up through sandy soils, providing limited benefit. Overhead irrigation may have produced different results for this study, in terms of yield and nutrient uptake.

Water stress appears to have increased the uptake of metals, Fe, Mn, and Al into the leaf tissue, although the mechanism cannot be determined from this study alone. The low CEC soils also had lower concentrations of Fe, Mn, and Al, so concentration cannot explain uptake. Regarding K applications, even with lower CEC’s, the addition of K helped yield, probably by reducing Mg uptake. The Mg on these soils was higher than recommended for the CEC (>20%), although there are no set recommendations for % nutrients on the CEC. However, uptake of any nutrient is limited when competing with space on the CEC, which appears to have affected both Ca and K in this case. Studies in additions of Ca or higher K rates could help determine if that is the problem here. The leachable anions N, S, and B were associated with greater yield when taken into the plant. All three are associated with greater organic matter, which was higher in the greater CEC zones. However, at the end of the season, NO3 was almost twice as high in the >4 CEC, which may be due to nutrient holding as well. Examining the addition of supplemental N, or greater assistance to rhizobia, could be beneficial on low CEC soils.