Nitrate-nitrogen contamination of groundwater continues to be a major concern throughout the USA. These concerns are greatest in areas where groundwater is close to the soil surface and in areas that have irrigated crops with large N fertilizer requirements. Specific objectives of this work were to use the chlorophyll meter to determine in-season crop N status and to correct in-season N deficiencies in irrigated corn (Zea mays L.). Chlorophyll meter readings were used to calculate a sufficiency index [(as-needed treatment/well-fertilized treatment) x 100] and in-season N fertilizer applications were made when index values were below 95%. Using this procedure, maximum yields were attained if early season N levels were adequate to maintain sufficiency indexes between 90 and 100% at the V8 growth stage. However, if the sufficiency index at V8 was below 90%, maximum yields were not achieved with in-season N fertilizer applications because early season N was below that needed for optimum growth and yield potentials had already been reduced. Even in these causes, N applications did increase yields, but not to the maximum. These results did demonstrate that early N deficiencies could be corrected using chlorophyll meters and the sufficiency index approach when they were not severe. Although the objective was not tested in this study, less N fertilizer may be required when in-season monitoring is used as the basis for N application. Use of the chlorophyll meter and sufficiency index should also result in greater N use efficiency and less N being available for leaching to the groundwater since these applications are made when N uptake by corn is greatest.
