Chlorophyll Meter and Stalk Nitrate Techniques as Complementary Indices for Residual Nitrogen

Improved N management techniques are needed to prevent excess N applications while maintaining yields. Environmentally, development of these improved techniques has become critical for crops such as irrigated corn (Zea mays L.), which require large N fertilizer inputs to maximize yield. Our objective was to determine if some current N management techniques could be used to guide and improve future soil sampling strategies that would result in improved N fertilizer recommendations and reduced adverse environmental effects. Chlorophyll meter readings and end‐of‐season stalk nitrate‐N concentrations were taken from an irrigated corn study at Shelton, NE, from 1992 through 1994. Residual soil N samples were collected to a depth of 5 ft after the 1992 and 1993 growing seasons. This study included continuous corn and soybean [Clycine max (L.) Merr.]‐ corn rotations with four corn hybrids and five N fertilizer rates. Chlorophyll meter readings taken at several growth stages reached a maximum at sufficient N levels for maximum yield and then plateaued. Similarly, end‐of‐season stalk nitrate‐N concentrations exceeded established thresholds (−2000 ppm) above the N fertilizer rate where maximum yields were obtained in both cropping systems. Together, these techniques provided additional criteria to help partition and separate fields into areas with potentially different levels of residual soil N. This information can then be used to guide soil sampling and to develop or improve site specific N fertilizer recommendations which should decrease environmental risk by reducing the amount of nitrate‐N available for leaching. Research Question Improved N management techniques are needed to reduce nitrate‐N contamination of groundwater while maintaining yields. These techniques are especially critical for crops such as irrigated corn, which require large N fertilizer inputs. Our objective was to determine if some of these techniques could be used to guide and improve future soil sampling strategies that would result in improved N fertilizer recommendations and reduced adverse environmental effects. The techniques included using chlorophyll meters to monitor crop N status during the growing season and collecting end‐of‐season stalk samples for nitrate‐N determinations. Literature Summary Several studies have shown that chlorophyll meters can effectively monitor the N status of corn throughout the growing season. Others have shown that the end‐of‐season stalk nitrate‐N test may provide some indication of excess crop N at maturity. Little research is available that compares in‐season and end‐of‐season measures of N availability. Study Description An irrigated monoculture corn and soybean‐corn cropping systems study with cropping system as the first split, corn hybrid as the second split, and N fertilizer rate as the final split in four replications was used. Four commercially available Pioneer brand corn hybrids (3162, 3379, 3394, and 3417) were evaluated in both cropping systems. Nitrogen fertilizer as ammonium nitrate was broadcast at rates of 0, 45, 90, 135, and 180 lb N/acre in 1991 and 1992 and 0, 56, 112, 168, and 224 lb N/acre in the 1993 and 1994 growing seasons. Applied Questions Did chlorophyll meter readings taken throughout the growing season reflect the N status of corn? Chlorophyll meters proved to be an effective way to discern whether sufficient N was available to corn throughout the growing season in both cropping systems. It did not indicate how much additional N was needed in deficient plants when the readings were low or how much excess N was available once the readings had reached a maximum. Chlorophyll meter readings increased to a plateau at the optimum N fertilizer rate and then did not increase with additional N fertilizer applications (Fig. ). Do stalk nitrate‐N concentrations reflect the N status of irrigated corn at the end of the growing season? End‐of‐season stalk nitrate‐N concentrations reached established thresholds (−2000 ppm) at the same N fertilizer rate where maximum yield and chlorophyll meter readings were obtained and then continued to increase with application of additional N (Fig. ). Nitrogen deficient plants had stalk nitrate‐N values below the threshold level. Can the chlorophyll meter and end‐of‐season stalk nitrate‐N techniques be used to improve residual N sampling strategies? Our data indicated when chlorophyll meters and the end‐of‐season stalk nitrate‐N test are used in conjunction they provide additional criteria to help partition and separate fields for future sampling into areas with potentially different levels of residual soil N. Improved sampling strategies will result in better site‐specific N fertilizer recommendations and decrease environmental risk because the amount of nitrate‐N available for leaching is reduced. For example, in Fig. , residual soil N levels started to increase at the same fertilizer rates that yield and chlorophyll meter readings reached a maximum and stalk nitrate‐N concentrations exceeded −2000 ppm, indicating excess nitrate‐N was available for leaching. Recommendation Chlorophyll meters and stalk nitrate‐N measurements can be used concurrently to determine areas where residual soil N levels may be either excessive or potentially N deficient. This research provides data that can be used to improve future N recommendations for irrigated corn because it identifies additional criteria to help partition and separate fields into areas with potentially different levels of residual soil N. Use of both methods can improve N fertilizer recommendations and concurrently reduce environmental risk by reducing the amount of nitrate‐N available for leaching. Chlorophyll meter readings at VT growth stage (a), stalk nitrate‐N concentrations (b), grain yield (c), and residual soil N (d) as affected by N fertilizer in continuous corn and soybean‐corn rotations. Arrows denote N fertilizer rate at which maximum values were obtained.