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# Impact of the drought on 2013 fertilizer rates

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Calculating the drought fertilizer credit: grain crop harvested

The simplest cases are fields where a grain crop was planted and the field is harvested for grain only at a reduced yield.

Phosphorus and potassium calculations for corn and soybean are similar. The drought credit will be the difference between the expected phosphate and potash removal based on the yield goal used to determine fertilizer rate and the actual yield.

Use Table 2 and the example to estimate nutrient removal values for corn and soybean.

The nitrogen calculation for corn is slightly different. The credit should be calculated as a percentage of applied fertilizer in 2012. Soybean has no fertilizer N applied so there is no drought N credit if it is harvested for grain.

An example:
A corn field had phosphate and potash applied based on a 150 B/A yield goal. The drought affected yield was 70 B/A. The crop received 145 lbs of fertilizer N.

Drought phosphate credit: (150 B/A – 70 B/A) X 0.32 lbs phosphate/B = 26 lbs phosphate/A
Drought potash credit: (150 B/A – 70 B/A) X 0.25 lbs potash/B = 20 lbs potash/A
Drought nitrogen credit: = ((150-70)/150) X 145 lbs nitrogen/A = 77 lbs nitrogen/A

These results estimate that reducing the 2013 fertilizer recommendation for phosphate by 26 lbs/A will maintain soil phosphorus status as if there had been no drought.

Similarly, potash can be reduced by 20 lbs/A and nitrogen by 77 lbs/A. This represents a potential savings of almost \$20/A in P and K. If a nitrogen needing crop such as corn is planted next year there is an additional potential savings of approximately \$35/A. Actual savings will depend on fertilizer prices.

If nothing was harvested from the field in 2012 the entire removal rate could be credited to next year’s crop (phosphate = 150 B/A X 0.32 = 48 lbs/A; potash = 150 B/A X 0.25 = 38 lbs/A; nitrogen = 145 lbs N/A).

 Table 2.  Estimated removal values for selected crops. Crop Nitrogen Phosphate Potash -  -  -  -  -  -  lbs/A  -  -  -  -  -  - Corn 0.74 0.32 0.25 Soybean 3.5 0.8 1.25

Calculating the drought fertilizer credit: green chop, baled or silage harvested

Many farmers have harvested drought-stressed soybean and corn crops as a forage crop. There are no easy rules about nutrient removal in corn and soybean forage compared to what was expected with the grain crop. Harvesting your corn or soybean grain crop as forage can lead in some cases to removal of nutrients in excess of what would have been removed by the grain crop and for others a drought nutrient credit.