Grain crop fertilizer phosphate (P2O5) and potash (K2O) maintenance rate recommendations in AGR-1 (Ritchey and McGrath, 2020) have not been reexamined since their inception - 1992. Other UK extension faculty (G. Schwab, pers. comm.; B. Lee, pers. comm.) have reported that soil test phosphorus (P) and potassium (K) levels were declining in Kentucky row-crop acres, even when AGR-1 (Ritchey and McGrath, 2020) fertilizer P2O5 and K2O rate recommendations are followed. This analysis was caused by those observations. The declines imply either that: a) there has been an expansion in row crop acreage to areas with lower initial soil test P and K levels; or b) that P2O5 and K2O row crop maintenance rate recommendations are not adequate.

First, there was a need to verify soil test P (STP) and/or K (STK) changes with time. The UK soil test lab provided STP and STK data for the 1990 to 2022 period. The data was sorted according to the commodity to be fertilized, as noted on the sample submission sheet, and then by year. Corn, soybean, and winter small grain (barley, canola, oat, rye, wheat) soil test data were separated from other soil test information. There was considerable fluctuation in annual sample numbers, but the average annual sample number was around 9300.

Across all grain commodities, STK has declined over the entire period (Figure 1a). The annual STK mean values were determined using all values remaining after removal of individual STK values greater than one standard deviation above the mean - to remove samples from manured fields or soils naturally high in STK. The portion of samples removed each year ranged from 9.6 to 15.7%, averaging 12.4%. Using the remaining samples, average annual STK values fell about 1.6 lb STK per acre per year. Over the past three decades, STK has fallen by about 47 lb STK per acre.

Across the grains, STP has also declined (Figure 1b) over the time period. As was done for STK, the annual STP mean values were determined using all values remaining after removal of individual STP values greater than one standard deviation above the mean - to remove samples coming from manured fields or soils naturally high in STP. The portion of samples removed, per year, ranged from 7.1 to 12.4%, averaging 9.8%. The decline was modest, about 0.2 lb STP per acre per year. Over 33 years, STP has fallen by 7 lb STP per acre across this group of samples. For University of Kentucky (UK) soil test lab users, STP and STK have been falling for several decades.

After a close look at the soil test data for corn and soybean, there was little support for the idea that soybean area expansion into less fertile fields caused the temporal decline in STP and STK values. This does not preclude the fact that recent expansion in both corn and soybean acreage has contributed to some decline in STP and STK values, but the amount of that contribution was not easy to separate.

It was known that STP and STK declines might be related to increasing grain yield and, coincidently, greater grain P and K removal. Kentucky’s annual average corn, wheat, and soybean grain yield data for 1980 to 2022 were gathered from the National Agricultural Statistics Service (NASS, 2023). Because grain P and K removal are the product of grain yield and grain P and K concentrations, we also needed to update our grain P and K concentration data. A recently published analysis of corn, soybean and wheat grain P and K composition, from the nearby state of Illinois (Villamil et al. 2019), was used (Table 1). The existing AGR-1 grain composition data (Ritchey and McGrath, 2020) was at least 25 years old. Comparing the grain P and K concentrations, recent corn and wheat grain values are lower, while recent soybean grain values are higher (Table 1).

The annual yield data from NASS was combined with the recent grain P and K concentration data to estimate annual average P and K removal for corn, full-season soybean, wheat and double-crop soybean. As an example, Figure 2 illustrates how rising full-season soybean yield was driving grain P and K removal.

Current AGR-1 grain crop P2O5 and K2O rate recommendations are shown in the three tables that constitute Figure 3. The maintenance portion of the recommendations is contained in the red boxes within each table. The Mehlich III STP and STK values are in lb per acre.

The new grain crop P2O5 and K2O rate recommendations are shown in the three tables contained in Figure 4. The expanded maintenance portion of the recommendations is contained in the green boxes within each table. As in Figure 3, Mehlich III STP and STK values are in lb/acre. Note that there is no proposed change to the Mehlich III STP and STK values at which no fertilizer P2O5 or K2O are recommended (60 lb STP/acre and 300 lb STK/acre, respectively). The recommended fertilizer P2O5 and K2O rates for STP and STK values below those associated with the newly expanded maintenance P2O5 and K2O rates also remain unchanged.

Adjusting for modern grain P and K concentrations and increasing yield-driven nutrient removal, we raised corn, soybean and wheat fertilizer P2O5 or K2O maintenance rates by 10 to 20 lb P2O5 and 10 to 30 lb K2O per acre, depending on the individual crop.

A maintenance fertilizer rate recommendation is intended to 'maintain' a level of soil-based nutrition that minimizes the possibility of nutrient deficiency. This kind of ‘insurance’ recommendation does not imply that there is a good probability of an economic benefit to the fertilizer recommendation in the year of application. Grant Thomas (pers. comm.) wrote: “The soil bank account does not pay interest. In fact, losses to fixation, erosion, etc., cause negative interest. Chemical and biological uncertainty make the soil fertilizer bank much less valuable than those dollars left in a bank. Doses of needed fertilizer are more efficient than doses of maintenance fertilizer.” In certain situations, careful and annual soil testing can better ensure adequate, and more economical, soil-based nutrition.

Some challenges remain. There is a need for grain composition data on other important winter crops, including canola, barley and rye. Continuing yield growth with time necessitates ongoing review of crop P and K removal values every 5 to 10 years.

McGrath, J, and E. Ritchey. 2022. 2020-2021 Lime and Nutrient Recommendations, AGR-1. Univ. Kentucky Coop. Extn. Svc., Lexington, KY.

National Agricultural Statistics Service (NASS). 2023. Data and Statistics. Quick Stats. USDA. Washington, D.C. https://www.nass.usda.gov/Statistics_by_State/Kentucky/index.php

Villamil, M.B., E. D. Nafziger, and G.D. Behnke. 2019. New grain P and K concentration values for Illinois field crops. Crop Forage Turfgrass Manage. 5:180090. doi:10.2134/cftm2018.11.0090

Citation: Grove J., Ritchey E., 2025. Grain Crop Phosphate and Potash Rate Recommendations: AGR-1 Updates. Kentucky Field Crops News, Vol 1, Issue 3. University of Kentucky, March 14, 2025.

 
Dr. John Grove, UK Soils Research & Extension 
(859) 568-1301 jgrove@uky.edu

Dr. Edwin Ritchey, UK Extension Soil Specialist 
(859) 562-1331 edwin.ritchey@uky.edu