Applying the economically optimum nitrogen rate (EONR) to corn requires the detection and correction of nitrogen deficiencies during the growing season. Such deficiencies result from splitting nitrogen application between before and after corn emergence, and from excessive leaching and/or volatilization of previously applied nitrogen caused by adverse weather.
Applying only the amount of in-season nitrogen needed by the corn crop for maximum yield should result in greater nitrogen-use efficiency. Also, yields and profits should be improved, nitrogen fertilizer costs should be lowered, and nitrogen available for surface and groundwater contamination should be less.
Research results posted in the late 1990's demonstrated that chlorophyll meter readings can be used to indicate the nitrogen status in corn leaves during the growing season. They also showed that chlorophyll meter readings and corn grain yields responded similarly to nitrogen fertilizer applications. Thus, these readings were a good indication of nitrogen status in corn during the growing season. These results indicated that this plant-based technology might also be used to determine if and how much in-season nitrogen fertilizer is needed for maximum yield.
Scharf and colleagues in the midwestern US (Agronomy Journal, V. 98:655-665, 2006) and Varvel and colleagues in Nebraska (Agronomy Journal, V. 99:701-706, 2007) recently published results from research that describes the use of chlorophyll meter readings for detecting and correcting in-season nitrogen deficiencies in corn. Scharf’s results are a summary of sixty-six nitrogen rate experiments conducted in seven midwestern states over a four-year period. The Nebraska results are summarized from ten years of research in that state.
Use of chlorophyll meter readings to determine amount of after-planting nitrogen needed for maximum corn yield resulted in less nitrogen being recommended than was indicated by the traditional yield goal approach. Chlorophyll meter readings were much stronger indicators of EONR than were soil nitrogen indicators.
The EONR for continuous corn and for corn following soybeans as indicated by chlorophyll meter readings was nearly identical. Thus, monitoring the plant to determine corn nitrogen requirement gave credit for the additional nitrogen available to corn following soybeans.
Obtaining accurate chlorophyll meter readings for predicting in-season nitrogen fertilizer need in corn requires that corn growing in a high-nitrogen reference area in a similar environment be available to establish the meter reading associated with nitrogen-sufficient corn. Readings from these plants are used to calculate relative readings (meter reading from area being diagnosed divided by meter reading from the well-fertilized reference area) that are better predictors than the actual numerical readings from the instrument. The applicability of the calibration should be extensive, since a wide range of conditions (hybrids, soil types, weather, management practices, previous nitrogen fertilizer applications) was represented in the studies cited above.
Once an area of well-fertilized corn has been established, results indicate that chlorophyll meter readings can be taken between about V7-8 and V10-12 to get accurate data for calculating relative readings. Readings taken after silking probably will be useful only for determining what nitrogen rates would have been optimal.
This procedure is a tool that producers can use to determine if in-season nitrogen is needed, and if so, how much to apply to obtain maximum economic yield. Moreover, if relative chlorophyll meter readings are collected from specific field areas, nitrogen can be applied in different amounts to those discrete areas. This methodology should reduce nitrogen losses due to leaching and denitrification because nitrogen will be applied only in the amount needed during the period of highest demand by the corn crop.
In summary, relative chlorophyll meter readings are related to corn nitrogen requirement and corn yield, and can be used to determine the amount of in-season nitrogen needed for maximum corn yields. They are strong predictors of when and how much nitrogen should be applied during the growing season to achieve maximum yield. Establishment of economically optimum nitrogen rates for midsouth corn will enhance the use and application of this new technology.