Nitrogen Loss and Residue Breakdown Potential for Fall-Applied Diammonium Phosphate.

Many growers apply diammonium phosphate (DAP) in the fall to replenish the soil phosphorus levels.  Along with phosphorus, DAP also contains 18% nitrogen.  There is debate on how much, if any, of this nitrogen is available for the spring crop.  The nitrogen component of DAP may leach through the soil profile and contribute to water quality problems.  There is also debate on whether or not the nitrogen component of DAP, liquid urea-ammonium nitrate (UAN), or a bacterial breakdown agent can contribute significantly to corn residue decomposition.

This study found that after applying 336 kg/ha of DAP in the fall, there was no remaining nitrogen from the DAP application the following spring to a depth of 0-30 cm or 30-60 cm. These results demonstrated that fall-applied nitrogen from DAP had begun to leach through the soil profile within one month of application, and it was lost entirely by April.

Residue breakdown contribution was evaluated through stalk integrity and weight loss over time as compared to an untreated control.  Variables tested were 224 kg/ha of DAP, 28 l/ha of UAN, and 28 l/ha of UAN + Breakdown (bacterial agent).  As expected, stalk integrity decreased and weight loss increased over time.  However, there was no significant difference in stalk integrity or weight loss among treatments at any point in time or in any soil type.  None of the treatments decomposed faster than the untreated control group.