Digging into phosphorous recommendations for corn

RECENT ANALYSIS BRINGS CLARITY

sifting through the best fertility management plans for the farm can be an overwhelming task. There are a multitude of articles and recommendations that can be challenging to distill into practical management practices for the farm. Fortunately, we have recently evaluated phosphorous fertilizer recommendations and strategies by acquiring and analyzing the results from 113 Ontario public research trials from 1967 to 2010 evaluating corn yield response to phosphorous fertilizers.

economic impact
The analysis suggests that corn yield and economic return potential is currently not limited by phosphorus availability when following phosphorus fertilizer rate recommendations from the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). Recommendations are often more than adequate for the current year’s grain corn production requirements, particularly when soil tests are in the range of six to 12 parts per million (ppm).

(Note – all references to soil test phosphorous refers to the OMAFRA accredited sodium bicarbonate phosphorous soil test.)

Adjustment of the maximum economic rate of phosphorous (MERP) based on changes in the phosphorus to corn price ratio were determined to be relatively insignificant. For example, in the event that corn price doubles from $4.50 per bushel to $9.00 per bushel for a given phosphorous fertilizer price, the optimum recommendation increases by about nine pounds per acre of actual phosphorous. 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
Application of seed-placed fertilizers at rates between 11 to 18 pounds per acre of actual phosphorous increase overall profits about 50 percent of the time. The likelihood of observing an economic yield response to seed-placed starter fertilizer was relatively unaffected by soil test phosphorous levels. For example, a profitable seed-placed fertilizer response was almost as likely to be observed at a soil test of 25 ppm as it was at 10 ppm.

Applying phosphorous fertilizer in a two by two inch band (two inches below and two 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 two by two starter band tended to have larger yield increases compared to trials with broadcast applied phosphorous when soil tests were within the range of eight to 30 ppm.

ongoing management
This analysis showed interesting observations when soil test phosphorous is greater than 15 ppm. There was little economic benefit from applying phosphorus fertilizer to the current year’s corn crop at rates exceeding 18 pounds per acre of actual phosphorous. In addition, studies show that phosphorous should be applied as a banded or seed-placed fertilizer to maximize the efficiency of phosphorus fertilizer use by corn.

Some growers broadcast phosphorous at crop removal rates to maintain soil test levels (i.e. 60 pounds of actual phosphorous for a 150 bushel per acre corn yield). This strategy has some practical advantages but the data suggests very little short term economic benefit to this practice, especially as phosphorous 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 five to 10 years. For example, the database suggests that corn yields of  140 bushels per acre were not limited by following an OMAFRA soil test and applying the recommended amount of phosphorous (particularly if it was  banded); additional phosphorous did not improve yields and reduced profitability. However some of these concepts need to be re-evaluated with corn yields in the 180-plus bushel per acre range.

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

Funding for this project was provided by Environment Canada’s “Lake Simcoe Clean-Up Fund” and OMAFRA. •