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

Winter 2001, No. 3

THE HILLS AND VALLEYS OF PHOSPHORUS FIXATION

Phosphorus fixation is one of those things that happen to some degree no matter what we do. Once phosphorus is applied to soil, regardless of the fertilizer brand or chemical composition, it will indeed be fixed.

The very nature of the word…fixation… sounds negative. And in the extreme, it is. But as with many things practiced in moderation, there are benefits to fixation. Because phosphate (the chemical form of phosphorus in fertilizer) is precipitated in soil (fixed), it leaches very little and only over a long period of time under most agronomic conditions.

So how do we ensure that phosphorus fixation is moderate and not extreme? A good place to start is with soil pH. Across the range of soil pH there are three peaks of high phosphorus fixation. The two highest peaks occur in the acid pH range, where phosphate precipitates with iron and aluminum. The strongest fixation is with iron. This fixation occurs primarily below pH 4.0. It is very difficult to supply sufficient phosphorus for crop needs when phosphorus solubility is being controlled by iron.

At a little higher pH, around 5.0 to 5.5, phosphate precipitation by aluminum is especially strong. The fixation peak is high, but not as high as with iron. None-the-less, phosphate availability to crops is exceedingly limited when it is being controlled by aluminum. Another problem when the soil pH is below 5.5 is direct aluminum toxicity to plants.

Liming is the cure for acid soils. The high peaks of fixation with iron and aluminum disappear and phosphorus fixation is minimized when the soil pH is moved toward neutrality. The valley (area of lowest fixation) occurs between pH 6.0 and 7.0. This is the ideal environment for phosphorus and for optimum crop growth. Precipitation does occur, but not to the extent that most of the applied phosphorus is rendered unavailable to crops. And as previously stated, the positive aspect of moderate fixation is that the phosphorus stays in the root zone, moving very little in most soils.

The final peak of phosphorus fixation occurs in alkaline soils around pH 8.0. It isn’t much of a peak…more of a hill…compared to those in the acid pH range. It is possible to lower soil pH by applying acid-forming amendments, but this is seldom done if phosphorus availability is the only issue. Phosphate is precipitated primarily by calcium in alkaline soils. The fixation is relatively weak, and it is generally more economical to apply a few more pounds of fertilizer phosphate than to acidify the soil.

Remember, the fact that phosphate rapidly precipitates in soil is not necessarily a bad thing. As a result of precipitation, phosphate seldom leaches. The exception is in fields with a history of heavy and repeated applications of manure where mobility is increased. But that is another story.

Understanding the hills and valleys of phosphorus fixation is important in managing this essential plant nutrient for maximum benefit.


— 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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