Residual Nitrogen and Soil Fertility

One doesn't add oil to an engine without checking the dip stick.  Neither should one add fertilizer products to a field without checking the dip stick by soil sampling.

This fall, soil sample results are another clear indication of the importance of properly taking samples with sound testing results and interpretation.

Residual nitrogen is particularly high on low yielding areas and non-irrigated fields as a result of drought last summer.  Normally, soybeans are excellent scavengers of nitrogen and use up all the residual from the previous corn crop.  With the drought, non-irrigated fields show significant levels of residual nitrogen that was never used this year.  There is as much as 40-60 pounds per acre in the top 8-10 inches alone.  This can be credited to next year's row crop along with the additional legume credit from the soybeans by leaving the soil low on residue.  There will be nitrogen mineralization from the soil organic matter in the spring prior to significant crop removal.

With the low amount of rainfall this fall, the soil moisture is very low at this point.  It would be best to wait to apply nitrogen fertilizer on non-irrigated fields until next spring or wait and sidedress after crop emergence.  This would allow a better assessment of yield potential and adjustment of nitrogen rates.

Even the very high yield irrigated corn fields show some significant nitrogen residuals.  Some areas have high levels of nitrates in the water.  With extensive watering, more nitrogen was applied this year than normal through irrigation water.  The high temperatures also allowed more mineralization last summer and this fall after the corn stopped nitrogen uptake.  No leaching of the nitrate accrued with the drought conditions.  One may be able to apply less nitrogen for 2013 than last year, even though excellent grain yields were removed.

Other nutrients, such as phosphate, may not have dropped significantly, depending on the amount of inherent fertility of the soil.  One cannot make accurate fertility recommendation based on crop removal alone.  Proper testing is the major tool to making cost effective recommendations


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