MAP as a Starter Fertilizer for Corn

Monoammonium phosphate (MAP) and diammonium phosphate (DAP) are excellent sources of phosphorus (P) and nitrogen (N) for high-yield, high-quality crop production. Research trials at 42 field sites in seven Corn Belt states showed an average corn yield of 162 bushels per acre with MAP and 159 with DAP. MAP (11-52-0) and DAP (18-46-0) contain about 90 percent water-soluble P, which is well above the 60 percent needed for optimum crop growth.

Growers who plan to apply P and part of the N as a starter fertilizer for corn should be aware of the qualities that make MAP a preferred P source.

Both MAP and DAP are effective providers of P and N for corn when applied prior to planting. But when banded at planting time, MAP has the advantage. Growers prefer MAP based largely upon two soil conditions during banding: the possibility of release of free ammonia and the change in pH of the soil solution surrounding the fertilizer particle.

Since DAP contains a higher content of ammonium nitrogen, its granules can release free ammonia as they dissolve in soil solution. In acidic soils, this release of free ammonia can injure seeds if DAP is placed with or near germinating seeds. Also, the release of free ammonia results in a temporary elevation of soil pH near the fertilizer granules, which can lower the availability of certain micronutrients such as zinc and manganese. Ultimately, both MAP and DAP have an acidic effect on the soil due to the acidity created when ammonium ions convert to nitrates in the soil, but since the soil solution surrounding MAP granules remains acidic, there’s no significant release of free ammonia to injure seedlings.

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