Just Right Rates for Phosphorus Efficiency

December 6, 2013 06:32 PM
Just Right Rates for Phosphorus Efficiency

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.

Laboratories and land-grant universities use the extraction methods that work best for soils in their geographic areas, Ferrie notes. For an explanation of several extraction methods and
numerical ratings, he recommends farmers read "A General Guide for Crop Nutrient and Limestone Recommendations in Iowa" from Iowa State University. You’ll find it by searching the Internet for "PM 1688."

How much to apply? After you have determined your soil P levels, plan the rate to apply. "Consider your short- and long-term goals," Ferrie advises.

With P, you’re applying nutrients to hold soil in the optimum range. It’s different from nitrogen, in which a larger portion will be used the same year you apply it.

"Because little of the P you apply this year will be taken up by the crop, you’re looking at where you want your soil test levels to be in three years or 10 years," Ferrie says. "You may want to build soil P levels up, or you may want to draw them down.

"If you’re on an annual lease and don’t know whether you’ll still have the farm next year, or if a field is slated for development, be cautious about building up soil P levels," Ferrie adds. "You can overcome low soil levels by efficiently timing and placing your P fertilizer. If you apply less than the remo­val rate and the soil level is optimum or lower, consider applying most of your P as starter or in the strip, if you strip-till."

Based on crop removal rates, efficient timing and placement are also the keys to farming highly calcaric soils. "On such soils, you can never achieve optimum soil test P levels because too much of the fertilizer you apply becomes tied up and unavailable," Ferrie says.

"If you just purchased land and plan to keep it and pass it to your heirs, building soil P levels into the optimum range increases the value of your asset," Ferrie says. "Build soil P levels slowly; they are fairly easy to build, but if they get too high, they can be difficult to draw down."

Advice for your area. You might see different recommendations about how much phosphate fertilizer it takes to build up soil P levels. "If you enter a build-up program, follow the recommendations of your local land-grant university," Ferrie says.

Ferrie’s experience suggests soil P values climb faster when manure is the main source. "Be careful not to apply too much manure," he advises. "If you build P levels too high on fields close to home, those fields won’t be available for manure application in the winter, when you need them the most.

"If you have a field where soil P values are above optimum, you will want to draw them down for both financial and environmental reasons," Ferrie says.

If you have to choose where to allocate financial resources, consider the soil’s pH value as well as its P level. "The more acidic the soil, the less microbial activity there will be to make P available—it will be tied up by iron and aluminum ions," Ferrie says. "Applying P to soil with a pH of 5.3 or 5.4 will be a waste of money. Instead, spend your money on lime to build up pH and make P more available."

Whatever programs you follow, continue soil testing to make sure P levels are moving in the right direction. "Adjust your plan if soil levels change too fast or too slow," Ferrie says. "Knowing where to hold soil P levels is part of a sound business plan." 

You can e-mail Darrell Smith at dsmith@farmjournal.com.

Dynamics of Soil Phosphorus

There’s a lot of phosphorus (P) stored in most soils, but only a fraction of it is available to your crop at any given time.

"Most soils have 2,000 to 3,000 lb. of P stored in the top 6" of an acre of soil," says Farm Journal Field Agronomist Ken Ferrie. "But about 80% of that is in the organic form, which cannot be taken up by plants. Temperature, moisture and microbial activity influence how much P is converted to the inorganic form, which plants can use.

"Soil microbes are constantly moving P from unavailable to available states and back again," Ferrie continues. "On a typical day, only 0.01 ppm [parts per million] to 0.1 ppm of P—0.02 lb. to 0.2 lb. per acre—is available in the soil solution. Plants need to take up P throughout their vegetative stages. So you need your soil to release P into the soil solution on a continual basis. It’s like a drip IV system for plants."

If your soil is unable to release enough P for your crop—because of physical problems in the soil or a high-calcium content, for example—consider a more efficient application method, such as starter fertilizer and band or strip-till application.

Learn and Profit from Nutrient Navigator


The Nutrient Navigator series focuses on efficient, environmentally sound management of nutrients. The goal is to provide practical knowledge that helps drive yields and profits higher. www.FarmJournal.com/nutrient_navigator

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