Using a chlorophyll meter for nitrogen management on corn

The chlorophyll meter is a non-destructive diagnostic tool that can be used to improve nitrogen management on corn. The test relies on adequately fertilized N reference plots for its proper calibration.

The chlorophyll meter (for example, the Minolta SPAD 502) is a small, handheld device that instantly measures the greenness of plant leaves. The leaf greenness is related to the chlorophyll content which, in turn, is closely associated with the nitrogen (N) content. Most of leaf N is contained in the chlorophyll molecules.

When calibrated correctly, this meter can be used as a tool for detecting N deficiencies on corn. The major advantage is that it is a non-destructive test and permits random, repeated measurements instantly throughout the growing season. A major drawback that has limited its widespread use is that leaf greenness can vary between hybrids and due to other plant and environmental factors that are unrelated to N availability. This necessitates the establishment of high N reference plots in the same field. Meter readings mean very little by themselves unless they are compared to the adequately fertilized reference plots. It is difficult, if not impossible, to specify a critical SPAD reading that will suit all situations.

Another consideration is that plants only produce as much chlorophyll as needed. At luxury consumption, chlorophyll level reaches a plateau regardless of how much extra N is taken up. Therefore, the chlorophyll meter is only effective at detecting N deficiency fields. Meter readings from the corn field and reference plot are compared and a N sufficiency index is established for each field. Most universities recommend that the N sufficiency index should not be allowed to drop below the 90 percent value.

The meter readings have to be taken only after the plants have reached the six leaf stage and be continued until the ear leaf at tassel stage. Using the meter early will permit additional N application if necessary using traditional sidedress or high-clearance equipment. The late measurements will provide data for fine tuning N practices in future years. It should be emphasized that the chlorophyll meter be used as another tool to complement, but not replace other best practices such as soil nitrate testing to fine-tune N management and reduce potential N losses to the environment.

The references listed below provide excellent information on the chlorophyll meter and some of its special uses.

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