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 1997, No. 8

There is a down side to phosphorus immobility. When a grower topdresses a pasture or broadcasts across no-till ground, phosphorus 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. That means fertilizer uptake efficiency by the crop will be less, which requires a higher application rate as compensation. It could also mean lost yield. Ideally, roots should be encouraged to grow deep into the soil so they have the opportunity to absorb both nutrients and water from as much soil volume as possible. This is an important consideration in high yield management. Topdressing doesn't accomplish this.

But, let's 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 topdressed annually on a commercial apple orchard in the state of Washington at a rate of 80 pounds per acre P₂O₅. 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 its movement, especially when that application is to a localized soil area such as is done with drip or micro-sprinkler irrigation. Research in California has shown substantial movement of phosphorus applied through drip irrigation to 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. And even on 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...very much. That means we can build up the phosphorus fertility of our soils and not worry about losing our investment over the winter...even when it is very, very wet.