Examining High Phosphorus Prices and UNL Recommendations

Examining High Phosphorus Prices and UNL Recommendations

May 9, 2008

With MAP and DAP prices nearing or breaking $1200/ton, the question can be asked, "At what price are University of Nebraska phosphorus fertilizer recommendations not profitable?" This is a serious concern. Over the last few years we have added an economic component to our nitrogen fertilizer recommendations; but have not accounted for phosphorus fertilizer price in our phosphorus recommendations for corn.

This article reviews recent Nebraska Soil Fertility Project (NSFP) data to determine if our recommendations are still profitable. The NSFP study was conducted statewide from 2002 to 2004 on irrigated corn sites over a range of soils and climatic conditions.

Over the entire database of 34 site-years, the treatment that received 40 pounds of P2O5 yielded, on average, 226 bu/acre compared to the no phosphorus check which yielded 223 bu/acre. However, the Bray-P1 soil test values for phosphorus ranged from below 10 to over 80 ppm.

At soil test levels between 20 and 25 ppm there were 10 site-years and the average yield was 229 bu/acre where P2O5 was applied and 228 bu/acre where P2O5 was not applied. Clearly there was no benefit to phosphorus fertilization at these soil test levels, as was the case when soil test phosphorus was above 25 ppm.

On May 1, 2008, DAP (18-46-0) was selling for $1230 per ton. When the nitrogen is credited at $ 0.60 per pound (360 lbs N x 0.60=$216), the phosphorus component costs $ 1014/ton, which comes to $1.10 per pound of P2O5. The price of DAP does not include delivery and dealer mark-up so in reality, future costs will be higher.

Any yield increase will need to pay for the DAP. A 40 lb phosphorus application will cost about $44 per acre or more. A nine bu/acre yield increase is needed to pay for the phosphorus application if corn price is $5/bu. Based on the results of the 34 high yield trials, the yield increases from applying 40 lb/acre of phosphate are not sufficient to cover the cost of application if corn is following soybean, but probably are sufficient if corn is following corn.

Other factors also influence response to phosphorus, such as soils where substantial reserves of subsoil phosphorus negate phosphorus fertilizer response even when surface Bray-P1 levels are very low. To test response in your fields, include an unfertilized check strip.

Since yields were not increased at Bray-P1 levels above 20 ppm, phosphorus fertilization at these soil test levels is not profitable. However, at the 225 bu/acre yield level, corn is removing about 80 lb P2O5 per acre per year. If unfertilized, soil test levels will gradually decrease. In our NebGuide, Fertilizer Suggestions for Corn, (http://www.ianrpubs.unl.edu/sendIt/g174.pdf) the following formula is included as a footnote to Table IV. Phosphorus Recommendations for Corn:

If Bray-P <= 25 ppm: P-rate (lb/acre) = (25 - Bray-P1) x 4

The formula is designed for variable rate applications and suggests applying 4 lbs of P2O5 for each Bray-1P below 25 ppm. Use this calculation to avoid depleting soil phosphorus and maintain soil levels at or above 15 ppm.

UNL Department of Agronomy and Horticulture Soil Fertilty Workgroup:
Charles Shapiro
Richard Ferguson
Gary Hergert
Dan Walters
Charles Wortmann
UNL Soil Scientists