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Just Right Rates for Phosphorus Efficiency

December 7, 2013
By: Darrell Smith, Farm Journal Conservation and Machinery Editor
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To be efficient with phosphate fertilizer, rates should be calculated for each field or soil type. Also, rates should take into account the operator’s yield objectives and field management challenges.  

Soil and tissue testing key to phosphorus efficiency

Applying too little phosphate fertilizer can draw down your soil "bank account" and reduce yield, especially if the crop is stressed by weather or other factors. But building soil phosphorus (P) levels too high wastes money and might allow P to be lost, where it can cause problems such as algae blooms in water supplies.

So applying the ideal rate of P on every field and soil type benefits your bottom line and the environment. Calculating the right rate is a three-step process, says Farm Journal Field Agronomist Ken Ferrie.

Start by finding out how much P is available in your soil and making sure that plants are taking it up. Second, understand some basics about your soil test to prevent misapplication. Third, consider your goals and objectives for each field.

"Your first concern is whether you are applying enough P," Ferrie explains. "Is the crop getting what it needs to perform to its potential? Plant tissue analysis and soil testing will provide the answer."


Tissue analysis. Visual symptoms, such as a purple color in corn, are your first clue to having a P deficiency. Those symptoms call for a plant tissue analysis to learn whether your plants contain sufficient P.

When having plant tissue analyzed, use the same laboratory. "Different laboratories may use different extrac­tion methods," Ferrie says. "They may have different numerical rating sys­tems and different interpretations of low, medium and optimum P levels.

"Follow your lab’s sampling, handling and shipping procedures. It’s easy to contaminate samples, even just by touching them, and contamination will render the test meaningless. Most laboratories will not want you to wash plant samples because that can remove nutrients," he adds.

If plant P content is low, find out why. "A soil test will reveal whether your soil is making sufficient P available," Ferrie says. "If your test reads optimum but plants are deficient, causes could include cold spring weather, which prevents soil microbes from releasing nutrients, soil compaction, rootworm damage, herbicide carry­over and others."

Soil score. A soil test provides an indication of how much P the soil can release for your crop, depending on the conditions.

Rather than telling you exactly how much P is available in the soil for your crop, the number you see actually indicates how many pounds of P (on a per-acre basis) were released by that lab’s extraction method.

The lab uses that number to assign a high, medium or low value. Agronomists, consultants and dealers can use the number in an equation that determines how much phosphate fertilizer to apply and then adjust that recommendation based on their knowledge of a field and the operator’s goals. "Actually, the soil test is just a starting point," Ferrie says.

Because laboratories use different P extraction methods and different numerical values for high, medium and low soil P levels, "farmers, agronomists and consultants can run into trouble if they mix soil analyses from different laboratories," Ferrie says.

"For example, Laboratory A’s extraction method might produce a rating of 55 lb., while Laboratory B’s extraction method produces a rating of 25 lb. On the two laboratories’ respective rating systems, each number may be considered optimum. But if you plug Laboratory A’s rating into an equation designed for Laboratory B, you will apply the wrong amount of fertilizer," Ferrie says.

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FEATURED IN: Farm Journal - December 2013

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