Can soil biology tests improve phosphorus and potassium corn fertilizer recommendations?

Authors: SVEDIN, JEFFERY - University Of Missouri; Kitchen, Newell; Ransom, Curtis; Veum, Kristen; ANDERSON, STEPHEN - University Of Missouri

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/20/2022
Publication Date: 8/18/2022
Citation: Svedin, J., Kitchen, N.R., Ransom, C.J., Veum, K.S., Anderson, S. 2022. Can soil biology tests improve phosphorus and potassium corn fertilizer recommendations? Agronomy Journal. 114(6):3457-3472. https://doi.org/10.1002/agj2.21180.
DOI: https://doi.org/10.1002/agj2.21180

Interpretive Summary: Soil tests have been used for nearly a century to aid farmers with knowing how much fertilizer to apply on their crops. These inorganic chemistry tests were developed to estimate nutrient availability from the soil to the plant during a growing season. Today however, “soil health” is also promoted by private and public organizations as a way of appraising soil nutrient availability. An assessment of soil health typically includes soil biological tests in addition to fertility tests. Current soil fertility tests do not include tests of soil biology, so many have asked “Can soil biological tests be used to improve fertilizer recommendations?” The purpose of this research was to explore whether integrating soil fertility and soil biological tests would improve corn phosphorus and potassium fertilizer recommendations. Our investigation on 84 producer fields in Missouri conducted over three growing seasons found that including biological soil health tests did not more accurately identify those sites where adding fertilizer increased corn yield. Further, the current soil fertility tests for phosphorus and potassium recommended by the University of Missouri performed best at predicting when crop yield increased from fertilization. Soil health and biological tests may be helpful for other agronomic and environmental evaluations, but our study shows that other factors (e.g., current management practices, cropping systems, and soil factors) may need to be considered to help farmers improve soil fertility recommendations.

Technical Abstract: Integrating soil health (SH) biological properties with soil fertility (SF) evaluations offers a unique opportunity to potentially refine fertilizer recommendations. The objectives of this research included: 1) evaluating current University of Missouri phosphorus (P) and potassium (K) fertilization recommendations for corn (Zea mays L.), and 2) assessing whether SH biological metrics are effective indicators of yield response to P and K fertilization. In Missouri, 446 monitoring sites (148 m^2) were implemented on 84 production corn fields over the 2018-2020 growing seasons. For each monitoring site, SH and SF samples were collected prior to planting, followed by application of non-replicated single-rate fertilizer treatments: 1) unfertilized control; 2) 112 kg/ha K2O; and 3) 112 kg/ha P2O5. At monitoring sites below recommended critical soil test values, P (n=152) and K (n =86) fertilization increased yield at 42% and 36% of the sites respectively, with average yield increasing 10% and 11% for P and K, respectively. At the lowest fertility ratings, P and K fertilization increased yield at only 52 and 56% of sites, respectively, highlighting inherent inconsistency that exists with current fertilizer recommendations. However, integrating SH biological properties with SF indicators through random forest algorithms failed to improve prediction of yield increases from P or K fertilization. Further, variable importance ranking confirmed that current physiochemical SF tests remain the most effective factors identifying when fertilizer nutrients are necessary. Although SH metrics offer insight into other agronomic or environmental benefits, established SF evaluations remain the most effective tool for guiding P and K fertilizer decisions in Missouri corn production.