Determining the proper nitrogen (N) fertilisation rate is important for the economic viability of corn production. A desire for high yields, fueled by low fertiliser N costs, has led producers to apply rates that ensured adequate N rather than risk costly yield losses due to a shortage of N.
Due to water quality concerns and recently high N fertiliser prices, management strategies are needed that can improve the efficiency and profitability of N use. With N application rates based on economic return instead of maximising productivity, producers want to confirm that rate decisions are working adequately. Also, if unexpected N losses occur, help is needed with decisions for rescue N applications.
Corn plant N sufficiency / stress sensing offers an approach to determine crop N status and manage in season fertiliser N application. Adequate time remains after significant corn N uptake to make N rate decisions, apply N, and have the crop respond to that N.
Various sensing tools are available, including the Minolta SPAD 502 chlorophyll meter (CM). This handheld device measures the greenness of corn leaves as reflected by the chlorophyll content and N status. The relationship between leaf greenness and N sufficiency is well documented.
Corn plants will reach a maximum greenness with adequate N. When N stressed, the plant will be less green. The CM can detect N stress in corn, but cannot differentiate between adequate and excess N.
The Minolta SPAD CM is highly portable and provides an instantaneous reading of the corn N status. Through N sufficiency / stress sensing, in-season N fertilisation can connect N application to plant indication of N need each season.
Each CM is provided with a calibration disc to ensure the meter is functioning properly. Always follow the CM use instructions.
Corn takes up N rapidly beginning with the V8 growth stage. Since we want to detect and correct any N deficiency in time for adequate yield recovery, N stress sensing can begin at the V10 vegetative growth stage, and should be completed before tassel emergence, with preference closer to V10.
To approach in-season N management in this way, high clearance equipment is needed to apply additional N. Applications should be completed before the silk emergence growth stage.
A CM reading is a unit-less value, and by itself does not adequately determine N sufficiency/stress. When CM readings are compared with readings from an adequately N fertilised reference area, however, we can evaluate corn N status relative to the 'greenest' corn in the field.
It is critical that each field has reference strips or areas. By adjusting (normalising) CM sensor measurements to reflect the adequately fertilised N reference area, the user reduces the effects of other variables such as hybrid differences or moisture stress.
The article discusses in detail how to collect a CM measurement and create reference strips for comparing CM readings.
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Article made possible through the contribution of Iowa State University.