AGRI-BRIEFS
  AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Fall 1998, No. 2

PHOSPHORUS FIXATION-THE GOOD AND BAD OF IT

Laws of nature…unlike laws of man…can’t be broken. We have to live with them. So it is with phosphorus fixation, or more precisely, phosphate fixation, since this is the form of phosphorus in virtually all fertilizer products. No matter what fertilizer we add to the soil, whether it is a dry product such as concentrated superphosphate (0-46-0), diammonium phosphate (18-46-0), or monoammonium phosphate (11-52-0), or a solution product such as ammonium polyphosphate (10-34-0), the fate of phosphate is ultimately the same. When applied to the soil, the phosphate in each of these products precipitates…or as we say…is fixed.

Phosphate precipitation happens very fast. Therefore, fixation occurs at the point of application, and the phosphate tends to move very little…usually just a fraction of an inch in any single season.

The bad of it is fixation results in only about 10 to 30 percent of the applied phosphate fertilizer being utilized by the crop during the first season. Some will be utilized in subsequent years, but in progressively lesser amounts. Also, if the phosphate is not placed in the vicinity of developing roots…such as topdressed and not incorporated…then it may be positionally unavailable.

Soil pH also affects phosphate availability. In acid soils, phosphate is fixed by iron and aluminum. Aluminum is most active fixing phosphate at a pH of 5.0 to 5.5. Iron is especially active below pH 4.0 where phosphate is very strongly fixed. Calcium is primarily responsible for phosphate fixation in alkaline soils where fixation peaks around pH 8.0. Of the three fixation processes…precipitation by iron, aluminum, or calcium…phosphorus is relatively more available when phosphate is fixed by calcium. Therefore, fixation is actually of less concern in alkaline than in acid soils.

We can’t break laws of nature, but by understanding them we can sometimes work them to our advantage. For example, we can minimize phosphate fixation by adjusting soil pH to an optimum level for phosphate availability. Phosphate fixation is minimized relative to soil pH when the pH is just slightly acidic, between 6.0 to 7.0. This is one of several benefits to liming acid soils.

We can also reduce fixation by banding phosphate in the root zone. This does two things: 1) It concentrates phosphate in a localized area that minimizes contact with soil constituents that cause fixation; and 2) concentrating phosphate maximizes its solution concentration for increased uptake by plants.

That is the bad of it…so what is the good of phosphate fixation? Phosphate does not normally leach…at least very little and only over a long period of time. So it is not a hazard to contaminate groundwater supplies. Much has been written about the hazards of nitrate and how to minimize its potential hazard. It is of concern because nitrate remains highly soluble in the soil and moves with the water. Most environmental issues with agricultural phosphate relate to erosion of topsoil that carries phosphate on soil particles into surface streams and lakes. Stopping erosion is the best way to minimize movement of phosphate off our agricultural lands.

Another positive aspect of phosphate staying where it is placed is there is no guesswork as to where it might have migrated to several weeks or months after application. Growers can place the phosphate exactly where it is most available for the crop and it will stay where it is placed.

So phosphate fixation is not necessarily bad. There is, in fact, a good and a bad to it. We can optimize the good by understanding the laws of nature and utilizing them to our advantage. Phosphate fixation is a law of nature, so let’s understand it and manage it to our advantage.


—AEL—

For more information, contact Dr. Albert E. Ludwick, Western Director, PPI, P.O. Box 970, Bodega Bay, CA 94923. Phone (707) 875-2163.  E-mail: aludwick@ppi-far.org.
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