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 in some areas. 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 hypothesize that higher current nutrient removal rates are not being addressed with current fertilizer rate recommendations. We propose 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 and crop sensor technology 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) allow us to determine if our current recommendations for P and K are sufficient to both build and provide adequate nutrition for the year of application in low testing soils
2) determine if adequate P and K are added to maintain soil test levels and adequate crop nutrition to give optimum yields. Knowing the rate of fertilizer that optimizes crop yield without being excessive will benefit producers, especially with low soybean prices
3) allow us to demonstrate the amount of field variability contained within a given field and how current UK CES recommendations handle this variability
4) give us a field research platform for the evaluation of new ideas regarding intensive plant analysis and/or crop sensors for soybean nutrient management evaluations.

Updated November 3, 2021:
The land grant university Cooperative Extension Services (CES) developed soil fertility recommendations for crop production based on field studies that included correlation and calibration work. Soil test potassium (STK) values have decreased over 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 Disease Diagnostic Center at Princeton with a nutrient deficiency were diagnosed low in K. The question is whether higher nutrient removal rates are not being addressed with adequate fertilizer rate recommendations, especially for K. The experimental design for this aspect of the project will: 1) allow us to determine if current recommendations for P and K are sufficient to provide adequate nutrition for the year of application; 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.

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 to avoid confounding of fertility work in the plots. The average soil test values were used to determine the base rate for each individual trial. The treatments were individual P and K trials, starting with soybean and corn broadcast fertility treatments - no fertility added, the UK recommended rate, 25% below the UK recommended rate, and 25% above the UK recommended rate, and the presence or absence of 2X2 fertilizer placement, replicated 4 times. Fertilizer was preplant broadcast with specific treatments being hand-applied. Triple super phosphate (0-45-0) and muriate of potash (0-0-60) were used for P and K applications. The band applications were 26 lbs P2O5/A as ammonium polyphosphate (10-34-0) and 15 lb K2O/A as potassium nitrate (14-0-46). The P and K recommendations were based on averaged recommendations for individual plots within treatments. Soybean was planted at approximately 130,000 seed/A and corn was planted at approximately 32,000 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.

No significant differences were present for soybean yield or tissue concentration in the P test. 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. There was no increase in soybean plant tissue P concentration due to the band application, but there was a significant increase in plant tissue K with the main banded potassium. Any fertilizer K rate increased tissue K concentration above that of the untreated check. The banded K did not relate into a soybean yield increase.

The changes in soil test values were investigated from the time of initial application (spring 2020) to one year out (spring of 2021) prior to another crop being planted. For the K studies, a significant 8 lb/A increase in STP was observed, however no increase (1 lb/A) increase in STK was observed. The year by interaction for STK fail to show an increase with added fertilizer from 2019 to 2020. This can be due to STK values being influenced by environmental conditions at the time of sample collection. Therefore 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. The P tests show that STK did not increase over time, however P did. This is also seen with the increase at individual rates for STP.

Conclusions from Year 2: The soybean yields and tissue K concentration in this environment were statistically improved with added K. Soil test values did increase with most of the phosphorus applications from 2020 to 2021, but not for potassium. Although the UKCES fertility recommendations are sometimes questioned as being too conservative, the results from this single year of data suggest they are currently adequate for dryland soybean production in Kentucky. Additional years of investigation and sites will help confirm this finding or aid in refining it for Kentucky soybean producers.

Introduction The land grant university Cooperative Extension Services (CES) developed soil fertility recommendations for crop production based on field studies that included correlation and calibration work. Soil test potassium (STK) values have decreased over 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 Disease Diagnostic Center at Princeton with a nutrient deficiency were diagnosed low in K. The question is whether higher nutrient removal rates are not being addressed with adequate fertilizer rate recommendations, especially for K. The experimental design for this aspect of the project will: 1) allow us to determine if current recommendations for P and K are sufficient to provide adequate nutrition for the year of application; 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. 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 to avoid confounding of fertility work in the plots. The average soil test values were used to determine the base rate for each individual trial. The treatments were individual P and K trials, starting with soybean and corn broadcast fertility treatments - no fertility added, the UK recommended rate, 25% below the UK recommended rate, and 25% above the UK recommended rate, and the presence or absence of 2X2 fertilizer placement, replicated 4 times. Fertilizer was preplant broadcast with specific treatments being hand applied. Triple super phosphate (0-45-0) and muriate of potash (0-0-60) were used for P and K applications. The band applications were 26 lbs P2O5/A as ammonium polyphosphate (10-34-0) and 15 lb K2O/A as potassium nitrate (14-0-46). The P and K recommendations were based on averaged recommendations for individual plots within treatments. Soybean was planted at approximately 130,000 seed/A and corn was planted at approximately 32,000 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 tissue concentration in the P test (Table 1). 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. There was no increase in soybean plant tissue P concentration due to the band application, but there was a significant increase in plant tissue K with the main banded potassium (Table 1). Any fertilizer K rate increased tissue K concentration above that of the untreated check. The banded K did not relate into a soybean yield increase. The changes in soil test values were investigated from the time of initial application (spring 2020) to one year out (spring of 2021) prior to another crop being planted. For the K studies, a significant 8 lb/A increase in STP was observed, however no increase (1 lb/A) increase in STK was observed (Table 2). The year by interaction for STK fail to show an increase with added fertilizer from 2019 to 2020. This can be due to STK values being influenced by environmental conditions at the time of sample collection. Therefore 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. The P tests show that STK did not increase over time, however P did. This is also seen with the increase at individual rates for STP. Conclusions from year 2 The soybean yields and tissue K concentration in this environment were statistically improved with added K. Soil test values did increase with most of the phosphorus applications from 2020 to 2021, but not for potassium. Although the UKCES fertility recommendations are sometimes questioned as being too conservative, the results from this single year of data suggest they are currently adequate for dryland soybean production in Kentucky. Additional years of investigation and sites will help confirm this finding or aide in refining it for Kentucky soybean producers.