Do Critical Soil Phosphorus Concentrations Vary in Space and if so, Why?

Approximately 70% of agricultural service retailers offer variable rate nutrient application, with much of this likely based on grid soil sampling. However research indicates that grids greater than 1/4 acre cannot be accurately interpolated to make variable rate prescriptions. Furthermore, not only does soil phosphorus (P) concentration vary across fields, but it is likely that threshold soil P concentration for crop response (the critical level) varies as well. This project investigates spatial and temporal variability in soil P critical concentrations and P supply for grain crop production as they relate to soil chemistry, soil biology, and rhizosphere interactions that influence crop P requirements.

The methodology relies on numerous sample points covering the variation within fields and between fields and across time. To date, variable rate P management relies on the interpolation of grid sampled soil data and the relationship between covariance and distance, where it is assumed that the greater the distance between points, the less similar their behavior or the lower their covariance. This project evaluates the covariance of yield response at both field and regional scales in order to evaluate fertilizer recommendations for different soil test values. Small plots were randomly established in each field site and then split into subplots that either received P or did not receive P. The difference in yield between the fertilized and unfertilized subplots provides the spatial distribution of yield response at various soil P levels across both the field and sites.

Two field sites were established in Kentucky during 2016. At both sites the the soil P concentration in the majority of plots fell below the University of Kentucky critical level, where a response to P fertilizer would typically be expected. Regardless of soil P concentration, a yield response to P fertilizer application was observed in about 50% of the plots. These preliminary results seem to support the hypothesis that critical level varies spatially across fields. In 2017, an additional field site was added near Blacksburg, Virginia.

In 2018, three additional sites are proposed, one located in Ohio and two in Texas. To date the focus has been on yield and biomass response to P. The next phase of the project focuses on more detailed soil and rhizosphere analysis to understand the mechanisms influencing differential response within narrow soil P concentration ranges.