Reevaluating the University of Kentucky’s Soil Fertility Recommendations for Soybean Production Year 1
Sustainable Production
DiseaseField management Pest
Parent Project:
This is the first year of this project.
Lead Principal Investigator:
Edwin Ritchey, University of Kentucky
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Brief Project Summary:

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...

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Information And Results
Project Deliverables

Final Project Results

Once the 100 bu/A soybean yield barrier was broken in yield contests, many others have attempted to repeat this feat. Numerous research projects were conducted in Kentucky to investigate the possibility of repeating this result, however none achieved a 100 bu/A yield at the time of this research.
This project did not necessarily attempt to break the 100 bu/A soybean yield, rather investigate mechanisms and practices that have the potential to greatly increase soybean yield that could potentially be coupled with other practices to provide a substantial economic return to soybean production.
Several novel approaches were utilized in a field setting at the University of Kentucky Research and Education Center (UKREC) located in Princeton, KY from 2017 to 2019. The soil type was a productive Crider silt loam. Elevated levels of CO2 have been shown to enhance C3 plant growth and result in an increased yield in greenhouse setting. SoyFACE (Soybean Free Ait Concentration Enrichment) at the University of Illinois in cooperation with USDA ARS, has shown yields can be increased in a field setting by adding CO2 through multiple aerial field applicators, but application in a production field setting in this manner does not prove feasible. The authors wanted to apply a product (eventually a cheap byproduct) to the field that would degrade and release CO2 where the plant would benefit and positively respond. Two products were used, celluslosic based packing peanuts and used newspaper.
Another approach was to increase the oxygen concentration in the rooting zone. This was done in two ways, subsoiling with a conventional ripper immediately below the soybean row and planting over it. The other way was to bury a diffusor hose and inject oxygen to the root zone. Both of these methods were compared to a non-treated check.
Finally, the question of increasing available nutrients during the later part of the growing season was investigated. This was accomplished by sidedressing with a rolling knife rig at late vegetative growth. These treatments consisted of adding N, P, K, and a combination of N, P, K and a double rate of K.
The results of three years are presented below in tables 1-3, but will be described here. The carbon treatment did not increase yields to a level that seemed feasible with the rate of application made in the field (Table 1). Higher rates may have increased yield, but seemed impractical in a field setting. This was not repeated after 2017.
Elevating the oxygen status by subsoiling or injecting oxygen did not improve yields above the untreated check (Table 2). This lead to the conclusion that adequate gas exchange was present in the Crider soil and that a physical restriction of the roots was not present either. None of the three years showed a benefit to this approach.
Finally, adding additional nutrients at late vegetative growth stages showed a slight benefit one of the three years, with one of two of the treatments (NPK), but not consistently (Table 3). Initially, the K treatment in 2017 showed potential promise. This was repeated along with making a 2X K application, which did not increase yields in the other years of the study. It was concluded that when adequate fertility is present at planting and other factors don’t limit nutrient uptake, there is not additional benefit to this application.
In conclusion, when physical, nutritional, or environmental conditions don’t limit soybean growth and yield there does not appear to be a substantial benefit to these added inputs. Following proven production practices will generally result in yields equal to those with great costs or time investment in implementing.

Table 1. Soybean yields resulting for the “Carbon Source” study in 2017.
Treatment Soybean Yield (bu/A)
Check 42.1
Ethephon 42.9
Newspaper 43.5
Peanuts 45.0
No statistical differences were present for the treatments at the 90% confidence interval.

Table 2. Soybean yields from oxygen enrichment or subsoil tillage for 2017, 2018, and 2019.
Treatment --------------------Soybean Yield (bu/A)--------------------
2017 2018 2019
Check 57.1 59.1 77.1
Oxygen 57.1 60.3 74.1
Subsoil 58.8 59.8 75.6
Average 57.7 A 59.7 A 75.6 B
No statistical differences were present for treatments effects at the 90% confidence interval for individual years. The main effect of year was significant (Pr>F = <0.001) and represented as the Average by capital letters.

Table 3. Soybean yields resulting from late-season nutrient applications.
Treatment --------------------Soybean Yield (bu/A)--------------------
2017 2018 2019*
Check 45.6 59.1 77.9 a
N 50.0 59.9 79.1 a
P 46.7 59.4 77.7 a
K 53.8 57.2 74.0 a
KK No data 59.6 74.8 a
NPK 50.4 59.5 85.4 b
Average 47.8 A 59.1 B 78.2 C
*Pr>F = 0.0428 for 2019 where the same lower case letter within the column indicate no differences. No other statistical differences were present at the 90% confidence interval. The main effect of year was significant (Pr>F = <0.001) and represented as the Average by capital letters.

The United Soybean Research Retention policy will display final reports with the project once completed but working files will be purged after three years. And financial information after seven years. All pertinent information is in the final report or if you want more information, please contact the project lead at your state soybean organization or principal investigator listed on the project.