The land grant university Cooperative Extension Services (CES) developed crop soil fertility recommendations with correlation (crop productivity/quality versus soil test) and calibration (crop productivity/quality versus nutrient application rate – at different soil test levels) field studies. Soil test potassium (STK) values have decreased with time across Kentucky, according to data generated from the University of Kentucky Regulatory Services Soil Test Lab. Further, the majority of soybean samples submitted to the UK Plant Diagnostic Center at Princeton diagnosed with nutrient deficiency are low in K. Whether producers are applying less K than recommended or the recommended rate is not sufficient to maintain STK levels is unknown. Soil phosphorus (P) levels do not appear to be declining at the same rate as STK, but these are falling. Additionally, the use of early (and often) in-season plant analysis and/or crop canopy sensors to better detect soybean nutrient stress has been proposed. We propose to continue to test the adequacy of current UK Cooperative Extension Recommendations in a field with low soil test P and K values. We also propose to evaluate the use of early/often plant tissue analysis throughout the growing season to detect soybean nutrition stress. Soybean yield and nutrient removal will be determined in our final evaluation of the several soil fertility programs we examine. An economic analysis of the project will be conducted in order to provide a better understanding of the costs associated with and possible economic repercussions associated with over or under fertilizing.

1. Reevaluate the adequacy of current University of Kentucky phosphorus (P) and potassium (K) fertility recommendations for soybean production in a soybean/corn rotation under dryland conditions.
2. Measure the nutrient content of soybean tissue and grain at different growth stages to determine if they help guide soil fertility programs.
3. Evaluate if the use of banding of P or K at planting is beneficial to soybean yield with and without broadcast nutrient applications at different initial soil test values.
4. Evaluate different potassium fertilizer sources and sulfur sources for soybean yield (separate field).
5. An economic analysis will be conducted in the final year of the project to determine the profitability of UK recommendations and if nutrient rate adjustments need to be considered.

Complete KSPB reports, write Soybean Research Reports, use data for extension presentations and publications as appropriate, and update UK recommendations if needed based on the outcome of this study.

The land grant universities and Cooperative Extension Services (CES) developed soil fertility recommendations for crop production based on field studies that included correlation and calibration work. Soil test recommendations from Land Grant Institutions are often questioned due to being lower than private labs. It has been questioned if higher current nutrient removal rates are not being addressed with adequate fertilizer rates, K in particular. We propose to test the adequacy of current UK Cooperative Extension Recommendations in field with low soil test P and K values. Final soybean yield was collected to determine the effect of the soil fertility programs.

The experimental design for this aspect of the project will: 1) allow us to determine if our current recommendations for P and K are sufficient to provide adequate nutrition for the year of application in low testing soils; 2) determine if adequate P and K are added to build or maintain soil test levels to optimum agronomic levels. Knowing the rate of fertilizer that optimizes crop yield without excessive applications will benefit producers with their soil fertility inputs, especially at low commodity prices.

Materials and Methods

A field at UKREC initially had low and very low P and K fertility. Phosphorus and K fertility plots were designed in a soybean-corn rotation at UKREC, common to Kentucky production conditions. Phosphorus and K were evaluated separately . The average soil test values were used to determine the base rate for each individual trial. The treatments started with individual P and K trials (2), starting with soybean and corn (2) broadcast fertility treatments - no fertility added, the UK recommended rate, 25% below the UK recommended rate, and 25% above the UK recommended rate (4), and the presence or absence of 2X2 fertilizer placement (2) and replicated 4 times (4). The results in previous years were used to guide fertility treatments for the following years for further evaluation of UK CES soybean fertility recommendations.

The rates used to evaluate the P and K recommendations in the soybean part of the rotation were based on averaged recommendations for individual plots within treatments. Phosphorus treatments ranged from 0 to 50 lb P2O5/A. Potash treatments ranged from 0 to 60 lb K2O/A. Soybean was planted at approximately 130,000 seed/A. All other production practices followed UKCES recommendations. Soil samples were collected prior to planting in 2020 for baseline values and then again in 2021 to evaluate the changes in soil test values.

Results and Discussion

No significant differences were present for soybean yield or test weights in the P or K tests (Table 1). This indicates that all broadcast application rates were adequate in this environment. This also supports the previous findings that adequate nutrition was applied as a broadcast application and additional P or K at planting was not beneficial in this environment. The annual application as compared to the residual application (only applied in odd years to plots 100 – 800) did not show a clear trend towards responsiveness with fertilizer additions. However, in other environments smaller, annual applications have been shown to be beneficial to crop yields when compared to larger applications at greater time intervals. The banded K did not relate into a soybean yield increase.

The K was in the low fertility range and required a moderate rate of fertilizer K to be added. The amount of P added to make sure that P was not limiting in the K test was sufficient to raise the STP in this test as well. The STK for the K study was higher in treatments that received the UK K rate or higher (Table 2).However this did not relate to a significant increase in soybean yield. Plant growth can be limited by other factors than soil fertility, such as water availability, insect, disease of weed pressure. Further, STK values can be influenced by environmental conditions at the time of sample collection. This is why it is best to track soil test values over time as long-term trends can be a better indicator of the effectiveness of a soil fertility program compared to a single sample point.

Conclusions from year 3

The soybean yields in this environment were statistically improved with added K at or above the UK rate. Although the UKCES fertility recommendations are sometimes questioned as being too conservative, the results from this data suggest they are currently adequate for dryland soybean production in Kentucky. Once all the data is aggregated at the conclusion of this study additional details may become evident. Additional years of investigation and sites will help confirm this finding or aide in refining it for Kentucky soybean producers.

Excessive fertilizer will increase soil test levels, but nutrient losses to chemical fixation, erosion, and runoff mean that the value of those soil-contained nutrients is reduced with time – a form of negative interest on the nutrient investment. Knowing the rate of fertilizer that optimizes crop yield without excessive application will benefit producers, especially when low commodity prices exists. Intensive plant analysis is being used in several high value crops (vegetables and fruits) and promoted by some utilizing high input or yield contest participants. However, intensive tissue sampling does add a substantial cost to production systems. If intensive tissue sampling is not needed or warranted to a high degree, this money can be saved or utilized for other crop production inputs. The overall objectives of the research presented in this proposal is to maximize soybean yield and profitability by optimizing nutrient inputs and reducing unnecessary inputs and management practices.