Impact of the drought on 2013 fertilizer rates
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).
|Table 3. Estimates of the nutrient harvest index (HI), percentage of above-ground nutrient uptake at early reproductive phase of growth and an estimate of the phase of growth where above-ground uptake equals nutrient exported in the grain for non-drought stressed corn and soybean.|
|HI¹||Early reprod. Uptake²||Uptake = HI³||HI¹||Early reprod. Uptake²||Uptake = HI³||HI¹||Early reprod. Uptake²||Uptake = HI³|
|- - % - -||- - % - -||- - % - -|
¹ Above-ground nutrient harvest index (nutrient in the mature grain/nutrient in whole crop X 100%)
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.
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.
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