Top Crop Manager

Features Agronomy Fertility and Nutrients
Cutting rates below 15 to 20 pounds P2O5 may restrict availability

November 30, 1999  By Top Crop Manager

With phosphorus (P) prices on the rise again, farmers are once again considering cutting phosphate fertilizer rates. Be forewarned, though: at a certain point, cutting P fertilizer rates means plant roots cannot physically access the P fertilizer granule or droplet. As a result, the starter benefits of faster, more uniform emergence and early vigour are no longer achieved, and the investment in fertilizer P is essentially lost in the year of application.

To illustrate how low rates can go before the plant cannot physically access phosphate fertilizer granules or liquid drops, John Lee of AGVISE Laboratories in North Dakota put together a visual presentation to help farmers understand how the distribution of seed and fertilizer in the seedrow can affect nutrient accessibility. Lee worked with fertility specialist John Heard of Manitoba Agriculture, Food and Rural Initiatives to show how cutting rates can affect P response. Heard developed the calculations and Lee put together the PowerPoint presentation. “The reason we put this information together was when phosphate fertilizer went to $1350 per tonne, farmers were looking at cutting rates. We’re starting to get those questions again on what is the lowest rate of fertilizer that can be applied with the seed and still get a starter effect,” explains Lee.

University research has shown that to get the full starter effect, a fertilizer drop or granule must be within 1.5 to two inches of each seed. Phosphorus is relatively immobile in the soil, moving approximately two square inches in the soil within 17 days after application. Reducing the P2O5 fertilizer rate below 15 to 20 pounds per acre may restrict availability.


To demonstrate the risk of cutting P rates too far, Lee and Heard put together charts and PowerPoint displays showing distribution of seed and fertilizer on a seven-inch row spacing with a narrow, one-inch opener (14 percent Seed Bed Utilization). They showed the distance between fertilizer drops for P2O5 rates of five, 10, 15, 20 and 30 pounds per acre.

They also did the same on 30-inch row spacing for wide-row crops. 

Looking at canola as an example, a rate of 10 pounds of P2O5 per acre would have fertilizer granules 3.8 inches apart. Bumping the rate up to 20 pounds would reduce the spacing to 1.9 inches.

Lee says many agronomists and farmers have a misconception surrounding liquid, in that the product is metered out in a continuous stream. That is not the case. In fact, the liquid drips out in the seed row, producing a distribution pattern not unlike that of granular fertilizer.

With a one-inch-wide opener, increasing row spacing from seven inches to 10 inches (10 percent SBU) or 12 inches (8.3 percent SBU) would result in a small increase in the distribution of granules and droplets. Conversely, using a wider opener would increase the SBU, making distribution even less frequent.

For liquid fertilizer, adding water to the fertilizer to dilute the concentration would increase the distribution of droplets. Doubling the water would double the drops but, of course, that means more hauling of product to the field, more frequent filling of the fertilizer tanks and additional labour; it might not be worth the effort. 

Instead of cutting P rates, Lee cautions this approach might be a false economy, and that if rates are cut too far back, the money spent on P fertilizer will not produce a starter effect for the crop, essentially wasting the cash in the year of application. “I can’t believe the number of phone calls I have had on the PowerPoint presentation. It has been a good demonstration tool to show the risks of cutting back phosphate fertilizer rates too far,” says Lee.

The complete PowerPoint presentation is available on the AGVISE website at . Look for “New! Starter Fertilizer Distribution Display.”


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