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

Summer 1996, No. 3

PHOSPHORUS DOESN'T MOVE IN SOIL...OR DOES IT?

Phosphorus is a good news/bad news story. The good news is that it is immobile in the soil. So when a grower fertilizes with phosphorus it stays where it is placed. While nitrogen may be leached below the root zone of crops and could threaten groundwater in extreme cases, phosphorus resides in the root zone. It is not of environmental concern unless it is moved on soil particles by erosion of the soil itself.

Phosphorus is immobile because it is only slightly soluble in soil and rapidly forms insoluble compounds with aluminum in acid soils and calcium in alkaline soils.

The fact that phosphorus is immobile is also the bad news. So when a grower top-dresses a pasture or broadcasts phosphorus on no-till ground, it remains at the soil surface. This isn’t necessarily a problem if the crop has sufficient feeder roots near the surface. But without those feeder roots or when the soil surface dries out, phosphorus becomes positionally unavailable. The phosphorus is available all right...at least chemically available...it’s just that the roots can’t get to it. At the least it means fertilizer uptake efficiency by the crop will be less, which requires a higher application rate as compensation. But it could mean lost yield. Ideally, roots should be encouraged to grow deep into the soil so they have the opportunity to absorb nutrients and water from as much soil volume as possible. This is an important consideration in high yield management. Topdressing for many crops doesn’t accomplish this.

But let’s look a little closer at phosphorus...and not over-generalize. More accurately, phosphorus does move in soil...but usually it is so slow and in such small amounts that for all practical purposes we simply say it is immobile. Here is an example of some, but not a lot of movement: phosphorus was top-dressed annually on a commercial apple orchard in the state of Washington at a rate of 80 pounds of phosphate per acre. The soil was a fine sandy loam. After 17 years...and 17 applications...the zone of greatest enrichment by far was the top one-inch of soil and accumulation below 12 inches was minor. Some movement of phosphorus was detectable to two feet. But this isn’t much mobility compared to nitrogen that would be mostly gone after just one growing season.

Management makes a difference. Applying phosphorus in irrigation water enhances movement of the nutrient, especially when that application is to a localized soil area such as with drip or micro-sprinkler irrigation. Research in California has shown substantial movement of phosphorus applied through drip irrigation...10 or 12 inches deep after just one season on a clay loam soil. This is enough movement to greatly enhance phosphorus availability to the growing crop, but not enough that it is in danger of leaching below the root zone. Other research has shown similar results. On a sandy soil, movement was reported to only 36 inches deep. This is certainly good news both agronomically and environmentally.

So, phosphorus doesn’t move in soil...well, just a little.


-AEL-
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