Digging into Phosphorus Recommendations for Corn
Ken Janovicek, University of Guelph & Greg Stewart, Corn Specialist, OMAFRA


Phosphorous (P) fertilizer recommendations and strategies in Ontario have been evaluated by the acquisition and analysis of research trials that looked at grain corn yield response to phosphorus fertilizer. Funding was provided by Environment Canada’s “Lake Simcoe Clean-Up Fund” and the Ontario Ministry of Agriculture, Food and Rural Affairs.

The results from 113 Ontario public research trials evaluating corn yield response to phosphorus fertilizers from 1967 to 2010 were entered into the database.  Of the 113 trials, there were 71 trials which had multiple rates of phosphorus fertilizer where the maximum economic rate of phosphorus (MERP) was calculated. The other trials evaluated single application rates, multiple fertilizer products and/or placement options against a zero P control.

Economic Impact

The analysis suggests that corn yield and economic return potential is currently not limited by phosphorus availability when following OMAFRA phosphorus fertilizer rate recommendations.   Ontario P rate recommendations are often more than adequate for the current year’s grain corn production requirements, particularly when soil tests are in the range of 6 to 12 ppm.  (Note - all references to soil test P refers to the OMAFRA-accredited sodium bicarbonate phosphorous soil test.)

Adjustment of MERP based on changes in the phosphorus:corn price ratio were determined to be relatively insignificant.  For example, in the event that corn price doubles from $4.50/bu to $9.00/bu for a given P fertilizer price, the optimum P recommendation increases by about 9 lb/ac of P2O5.  Adjustments in phosphorus fertilizer rates based on soil test results will have a much larger impact on net returns than attempting to make the minor rate adjustments associated with changes in corn prices and phosphorus fertilizer costs.

Placement and Rates of P

Application of seed-placed fertilizers at P2O5 rates between 11 to 18 lbs/acre increase overall profits about 50% of the time.  The likelihood of observing an economic yield response to seed-placed starter fertilizer was relatively unaffected by soil-test P levels. For example, a profitable seed-placed P fertilizer response was almost as likely to be observed at a soil test of 25 ppm as it was at 10 ppm. 

Applying P fertilizer in a 2 X 2 inch band (2 inches below and 2 inches to the side of the seed) was associated with significantly larger increases in yields and profitability when compared to broadcast-applied phosphorus.  Direct comparisons of banded and broadcast phosphorus fertilizer rarely occurred in the same trial. However trials with phosphorus applied in a 2 X 2 starter band tended to have larger yield increases compared to trials with broadcast-applied P when soil tests were within the range of 8 to 30 ppm.

There are some interesting observations when soil test P is greater than 15 ppm:

Some growers broadcast P at crop removal rates to maintain soil test levels (i.e. 60 lbs P2O5 for a 150 bu/acre corn yield).  This strategy has some practical advantages but the data suggests very little short term economic benefit to this practice, especially as P soil tests climb above 15 ppm.

A need for additional research was also clearly identified as the database was analyzed. This relates particularly to the changes in average grain crop yields over the last 5-10 years.  For example, the database suggests that corn yields of  140 bu/acre were not limited by following an OMAFRA soil test and applying the recommended amount of P (particularly if it was banded); additional P did not improve yields and reduced profitability.  However some of these concepts need to be re-evaluted with corn yields in the 180+ bu/acre range.

Potassium (K) Reminder

The database also incorporated some analysis of other starter fertilizer options. The key finding here was that when soil-test K levels are less than 90 ppm, more consistent and larger corn yield increases can be expected with seed placed or 2 X 2 banded starter fertilizers that also contain K. 



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