Impact of the Drought on Next Year’s Fertilizer Rates

Published: August 31, 2012

Reduced yields can have implications for fertilizer need in the year after a drought. Intuitively farmers recognize that substantially reduced yield can lead to less removal of nutrients from a field. When nutrient use is reduced there is an opportunity to estimate a drought nutrient credit from the stricken crop that can be used to reduce fertilizer need in the year following drought.

Farmers have used a range of strategies to deal with drought stricken corn and soybean. How they managed fields this year will directly affect the amount of fertilizer carryover to the next year. This year, corn and soybean fields have been harvested for grain (with substantially reduced yield), mechanically harvested (as green chop, baled forage or silage), grazed as forage, or been abandoned with no removal of any crop material.

Each of these scenarios has implications for how many nutrients applied from this year can be credited to next year’s crop. In some cases these strategies may actually increase fertilizer need next year by removing more nutrients than would have been in the planned grain harvest.

Table 1 summarizes our expectation for the impact of selected strategies on fertilizer need in next year’s crop. The table assumes that your yield goal for corn was 150 bushel/acre (B/A) and for soybean was 50 B/A. Equations are provided later in the article if you want to calculate drought nutrient credits for different yield goals.

Drought nutrient credits can be important particularly on fields where little or no material was harvested in 2012. Recognize that chopped corn can increase potash requirements for next year’s crop.

Also recognize that there is potential for nitrogen estimated in by the drought nitrogen credit to be lost through leaching or denitrification if we have excessively wet conditions between now and next-year’s crop. The final section of this article, “Other Considerations”, details strategies to manage this and other risks associated with using drought nutrient credits.

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).

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.

To predict the impact of harvesting forage one needs to compare nutrient status of the plant when harvested for forage compared to what it would have been if it had made a full grain crop.

Table 3 compares the nutrient harvest index of the grain crop with nutrients accumulated as the percentage of total nutrient uptake at early reproductive growth stage (R1 for corn and R3/4 for soybean).

When the values are equal there is no difference between the amount of nutrient removed in the grain and the amount of nutrient removed in the harvested forage. For example, when you harvest corn at R1 you remove the same amount of nitrogen as if you had harvested the full crop as grain.

When the harvest index value is greater than early reproductive content than fewer nutrients are removed by forage harvest; there will be drought nutrient credit for the crop. For example, when you harvest whole corn plants at R1 your remove less phosphorus than you would have with the full grain crop.

When the harvest index value is less than early reproductive content than more nutrients are removed by forage harvest; harvesting the forage will remove more nutrients than the grain crop. For example, when you harvest whole corn plants at R1 your remove more potassium than you would have with the full grain crop.

In soybean the harvest index is always greater than the percentage of total nutrient uptake at R3/4 so there is always a potential drought nutrient credit if the crop is harvested as forage at this stage of growth.

An example:
A soybean field had phosphate and potash applied based on a 50 B/A yield goal. The field was harvested as green chop with limited pod set.

Drought phosphate credit: [1-(40/80)] X 50 B/A X 0.8 lbs phosphate/B = 20 lbs phosphate/A
Drought potash credit: [1-(37/58)] X 50 B/A X 1.25 lbs potash/B = 40 lbs potash/A

Note that less nitrogen was removed by the soybean plant than with grain harvest; but no nitrogen fertilizer was applied to the soybean crop. So it is not appropriate to take a drought credit for nitrogen. We do recommend taking the rotation nitrogen credit for soybean of 30 lbs N/A despite the drought if corn is grown following soybean.

Corn crops in particular have been harvested as forage at various points during the growing season. Some have harvested the crop soon after silking (near R1). Others have waited until the crop dropped in moisture content to a point it could be ensiled or baled. There are others who are realizing that there is little grain in a mature field and are baling it as a source of roughage to be mixed with other higher quality feeds such as distiller’s grains. Table 3 also reports the growth stage of corn and beans for N, P and K where above-ground nutrient removal as forage equals nutrient removal from the full grain crop. This point is within a growth stage or two of early reproductive growth, with the exception of potassium in corn. This emphasizes the advanced the stage of growth when harvesting corn or beans as forage, the less appropriate a drought nutrient credit. The one clear effect of removing corn or beans as forage is that this practice will increase potash removal from the field. Green chopping a corn at R1 for a field fertilized based on a 150 B/A yield goal increases potash removal by 62 lbs/A ([1-(90/34)] X 150 X 0.256 = -62; the negative sign indicates increased fertilizer demand compared to the grain crop).

Calculating the drought fertilizer credit: grazing

If a drought stricken field is harvested by grazing cattle most of the nutrients in the crop will remain in the field. Nearly 100% of the nutrients consumed in the soybean or corn forage will be excreted back onto the pasture. Consequently, the drought credit for phosphate and potash will be similar to a field where no harvest took place (see discussion above for calculation).

Nitrogen availability will be much reduced compared to no harvest. Over 50% of the nitrogen will be excreted in the urine and prone to volatilization. The remaining nitrogen will be in organic compounds that will not be fully available to the crop. In pasture systems a conservative estimate is that 37% of the excreted nitrogen can have fertilizer value. Consequently the drought nitrogen credit on grazed grain fields will be just over one third of what was applied to the field that year.

The nutrient value of grazed nutrients is dependent on animals doing a good job of distributing those nutrients across the field. Better distribution of manure is facilitated by practices such as strip grazing that forces animals to fully graze an area within a day before giving them access to another strip of feed.

Other considerations

Some farmers may question if the calculated drought credit is really available. This can be a real concern, especially for nitrogen where there is the potential for the excess nitrogen from this year’s drought stricken corn to be lost over winter or in spring from excess moisture. Given the exceptionally dry condition of the soil in late August it is less likely we will have substantial leaching of nitrogen over winter. The potential for losing the drought nitrogen credit to excess moisture is much lower if the next crop is wheat compared to corn. A cover crop may also serve to limit nitrogen losses under high moisture conditions.

Farmers may also be concerned that fewer nutrients were taken up in the crop by a specific growth stage because of the drought. For example, potassium deficiency is sometimes observed in drought stressed corn which would indicate reduced uptake. If this occurs our estimate of the drought nutrient credit will be low. Failing to take the phosphate or potash drought nutrient credit or under-estimating the credit will lead to an increase in soil test level in the field that, if significant, will be accounted for next time you soil sample your fields.

Crop uptake of nitrogen during a drought will be less affected than phosphorus and potassium. Soil nitrogen enters crops primarily as an ion as mass flow with soil water entering the roots. The continued uptake of nitrogen coupled with the failure to make grain leads to the concentration of nitrate in the stalk that leads to danger of nitrate poisoning in livestock eating forage corn or beans.

If farmers are planning to grow wheat or corn and think there is a drought nitrogen credit but want a quantitative test to measure the nitrogen in the soil they can use the Soil Nitrogen Test. This test is fully explained in the MU Extension Publication Preplant Nitrogen Test for Adjusting Corn Nitrogen Recommendations (MU Guide G9177). This test requires taking multiple cores to a depth of at least two feet in spring.

In Missouri many farmers rotate from corn to soybean. Such a rotation will not effectively capture the value of any unused nitrogen from this year’s corn crop. The value of nitrogen in the soil can be substantial on fields where little grain and no forage were removed. Many factors must be balanced when considering growing corn after corn including the expected 12% or greater yield penalty for continuous corn. This yield penalty will be equal to or greater than the value of any residual nitrogen in the soil. There is significant value in crediting residual soil N after drought years if you grow corn after corn next year; but the decision to grow corn or beans must be based on an analysis that considers factors beyond the potential value of excess fertilizer nitrogen in the soil.