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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

Spring 1996, No. 3

CORN RESPONDS TO SEED-PLACED PHOSPHORUS

The use of fertilizers placed with the seed is a practice frequently used by corn growers. The effectiveness of seed placement is particularly high for supplying phosphorus to corn in cold soils or soils testing low in phosphorus. Recent research at the University of Guelph in Ontario, Canada, has confirmed the benefit of seed-placed fertilizer, and has shown that these benefits extend to soils testing high in phosphorus as well.

Experiments conducted at Elora, Ontario, in 1993 and 1994 utilized a liquid ammonium polyphosphate fertilizer (10-34-0). The fertilizer was placed with the seed at a rate of 13 pounds of P2O5 per acre. A comparison treatment was placement in a band 2 inches to the side and 2 inches below the seed. The effect of seed placed phosphorus was evaluated at soil phosphorus levels ranging from low to very high (6 to 70 pounds per acre by the Olsen soil test).

Averaged over two years, placement of phosphorus with the seed boosted yields by an average of 9 bushels per acre. The response to seed placement was independent of soil test level. Plant tissue concentrations of phosphorus, at a growth stage when the fourth to fifth leaf tips were emerged from the whorl, were much higher with seed placed fertilizer than without. The increased phosphorus supply at this growth stage was the primary factor that increased yields.

Studies in 1995 compared other fertilizer materials for seed placement at a wider range of locations. Again yield responses of 6 to 11 bushels per acre were attained. No differences were found among the liquid fertilizers 10-34-0, 6-24-6, 8-19-3 and granular monoammonium phosphate (13-52-0). One site with a soil test rating of very high (104 pounds per acre) showed no response to seed placed phosphorus, indicating there was an upper limit at which the soil alone would supply adequate phosphorus for seedling growth.

Essentially all of the yield response was attained at fertilizer rates as low as 5 pounds of P2O5 per acre. Rates two to four times higher were shown to delay emergence, even though the yield response was still positive. When soil tests were low or medium, optimum yields were attained only when seed placed phosphorus was applied in addition to that broadcast or banded. The low optimum rates indicate that seed placed phosphorus should be viewed as a supplement to rather than a replacement of typically recommended phosphorus applications. The amount that can safely be placed in contact with the seed is far less than the amounts required to maintain soil phosphorus levels when crop removal is considered.

These results emphasize the importance of phosphorus availability during early seedling growth. Even though the planting dates in these experiments were not early, the growth of seedlings depended strongly on phosphorus. While colder soils respond the most to phosphorus supply, warm air temperatures can often increase shoot/root ratios, leading to increased demand for nutrients from each unit of root. If nutrient supply is good, the crop canopy can be established faster and more efficiently.

Take advantage of high shoot/root ratios. Supply phosphorus with care and your crops will take care of you!

-TWB-
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