Fall-Applied Phosphorus: Efficient And Effective

Is fall a good time to apply phosphorus, and will the applied phosphorus be available for spring-planted crops? Those are key questions many retailers and growers ask when considering fall-applied phosphorus.

This was the subject of CropLife Media Group’s recent Webinar “Fall-Applied Phosphorus: Efficient And Effective,” presented by Dr. Barney Gordon, agronomic consultant and Professor Emeritus, Kansas State Uni­versity, and Jake Sanders, Special­ty Fertilizer Products’ (SFP) vice president of market development. The two speakers discussed the advantages and benefits of fall phosphorus application, as well as some innovative tools that help increase its availability to spring-planted crops.

Maximizing Yields With Phosphorus

Phosphorus (P) is essential for crop growth and development, Gordon said during the Webinar. “Availability of adequate P, especially early in the growing season, has a profound effect on crop yield in both corn and wheat,” he said.

While application of P to the soil is necessary to ensure plant productivity, the recovery of applied P by the crop plants is often low due to a lack of P movement, or diffusion. Gordon said there are several factors that affect phosphorus diffusion in the soil. They are:

  • Volumetric water content: Increasing soil moisture level increases rate of diffusion.
  • Soil bulk density: Changing bulk density affects the ability of nutrients to diffuse to the root surface.
  • Soil buffering capacity: Ability of the soil to maintain nutrient concentration.
  • Nutrient concentration: Initial amounts of nutrients in the soil.
  • Temperature: Increasing the temperature increases concentration in the solution.

Studies show when soil temperature is reduced from 70°F to 58°F, corn root growth decreased five-fold and P uptake by corn roots decreased four-fold, Gordon said. “The colder the soil, the more problems we have getting phosphorus to the plant,” he said.

Gordon said he’s found little difference in yield when P was applied in the fall or the spring, providing growers with much-needed flexibility.

“Weather can cause delays in the spring, which makes it difficult to get planting done in a timely manner,” he said. “Fall application of fertilizer has the advantage of taking some of the workload off of producers. We could put our fertilizer on in the fall and still have it available and working for us in the spring.”

In addition to corn, wheat also tends to be highly responsive to P input, Gordon said. “P is generally the second most limiting nutrient in wheat production, behind N (nitrogen),” he said. “And in some areas, P is more limiting than N.”

Early-season P deficiency limits yield potential in wheat production, Gordon added. “The most critical period is that first five to six weeks after emergence in the fall,” he said. “P has major impacts on tillering and rooting. So it’s critical we have adequate amounts of phosphorus in the fall when the wheat crop first germinates and starts growing.”

Greater Efficiency With AVAIL

One of the most significant problems associated with P is that it is extremely inefficient, SFP’s Sanders said. “It becomes tied-up, or fixed to the soil,” he said. “This happens because phosphorus has a negative charge and other elements in the soil have a positive charge. When they are active in the soil, they come together like a magnet and severely limit the crop’s access to the nutrient.”

Research shows that 75% to 95% of applied P is lost during the season of application. “In the best-case scenario, we are losing 75% of our phosphorus, which is an incredibly inefficient way to do business,” Sanders said. “We firmly believe that a solution to that problem is AVAIL.”
AVAIL P fertilizer enhancer is a water-soluble polymer with a very high negative charge, Sanders said. When that polymer is used as a coating on granular P or mixed into fluid P fertilizers, it diffuses out in the soil and interfaces with those positive charges that would typically tie up the P.
“Now instead of losing 75% to 95%, we keep a majority of that phosphorus available to the crop,” he said.

Sanders shared results from several corn yield studies that examined fall application of P with and without AVAIL. “We’re seeing the same positive effect whether we apply phosphorus with AVAIL in the fall or in the spring,” he said. “And when we’re talking about time savings, that’s a big deal. We can save a lot of time by getting that phosphate out early, protecting it, making it available to the crop and receiving the greatest return on investment for the grower.” 

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