Using a Chlorophyll Meter to Predict Mid-season Nitrogen Fertilizer Rate for Cotton.

Applying too little or too much nitrogen (N) fertilizer to cotton lowers profit. The amount of N supplied by the soil is difficult to predict, making it hard to choose the best N rate to apply. Low pre-plant N, followed by a mid-season N application (early square to first flower) offers an opportunity to diagnose N status and adjust N fertilization to the optimal level. Our objective was to develop interpretations for a hand-held Minolta chlorophyll meter to diagnose N status mid-season in support of profitable N rate decisions. Field-length (1100 ft) strips of N fertilizer rates from 0 to 150 lb N/ac were applied and replicated four times. Chlorophyll meter measurements were taken every two weeks from all N rate strips. Before harvest, alleys were cut every 50 feet across the strips, and a plot picker was used to measure yield in every 50 foot area. The experiment was divided into two zones using an NRCS soils map and a soil electrical conductivity map. Zone 1 was sandy and made up 28% of the field, while zone 2 was loamy. All yield data from zone 1 were used to construct a curve showing yield response to N in that zone, and the same was done for zone 2. From these curves, it was possible to calculate the most profitable rate of N in each zone. The experiment was conducted for three years, giving a total of six measurements of the most profitable N rate (3 years x 2 zones/year). We found that chlorophyll meter measurements in the 0 N strips, divided by measurements from the 150 lb N/ac strips, was strongly related to the most profitable N rate. We propose that chlorophyll meter measurements can be used to predict the most profitable N rate for a mid-season N application.