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

Summer 2002, No. 1


Phosphorus applied in the field to benefit crops can cause trouble in the stream when it sets off algal blooms. But the field and stream differences are not irreconcilable. A recent study in two Ohio watersheds draining into Lake Erie shows that soils – while building up in phosphorus – were losing less.

Between 1975 and 1995,the load of soluble reactive phosphorus in rivers from these two watersheds declined by 60 to 89 percent. In the meantime, farmers increased production. They now remove 20 percent more phosphorus in crop harvests. They do this with less input of fertilizer and manure, but on balance they still supply a little more phosphorus to the soil than the crops remove. In theory, soil tests should have been rising, and indeed they did.

If soil tests were still rising,how could the load to the rivers have decreased so much? The answer is found in the chemistry of how phosphorus behaves in the soil.

Phosphorus reacts with common soil substances to form minerals. Whether applied as fertilizer or manure, a fraction is always kept away from plants by reactions with calcium, aluminum, and iron. A deficient soil must usually be fertilized with considerably more than the plants remove. Over the years, the fractions that remain in the soil gradually build up and begin to contribute a greater proportion of the plant’s needs. Eventually, phosphorus no longer needs to be added in amounts greater than plant removal. Building up the soil test is a necessary condition for efficient fertilizer use, where the amount applied doesn’t have to exceed the amount removed.

Within reasonable limits,a soil built up in phosphorus doesn ’t lose that much more. The phosphate mineral fractions don’t easily move from the soil. Other factors – including erosion control, and application methods for manure and fertilizer – impact losses to a greater degree. The authors of the study suggest “erosion control programs have reached a considerable portion of the critical phosphorus source areas in these watersheds.” Decreased erosion – resulting from conservation tillage practices – explains part of the reduction, but it doesn’t explain why losses of soluble phosphorus decreased more than particulate forms.

Changes in management practices may offer a more thorough explanation. While manures and fertilizers used to be applied broadcast in the fall, producers now put on manure in the spring to capture more benefits from its nitrogen. They more frequently band-place fertilizer to increase its availability to young seedlings. The shift in management practices avoids putting soluble phosphorus on the soil surface during the critical late fall and early spring runoff periods. The effect shows up as a particularly sharp decline in loss during those periods.

A soil test summary conducted last year showed that 53 percent of Ohio soils currently test medium or less,in the range where they could benefit from building up phosphorus. The proportion of soils in that category increased slightly since 1995, suggesting that the phosphorus balance has shifted to a deficit position. The good news is that investment in fertility can raise soil test levels to the point where producers can manage for optimum yield while reducing their impact on water quality. Using best management practices, phosphorus use can benefit both field and stream.

For more information, contact Dr. Tom Bruulsema, Eastern Canada and Northeast U.S. Director, PPI, 18 Maplewood Drive, Guelph, Ontario N1G 1L8, Canada. Phone: (519) 821-5519; E-mail:
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