Effects of nitrogen fertilizer rate on SPAD of cotton leaves and diagnosis of nitrogen nutrition

Chlorophyll and nitrogen content of each leaf on main stem of cotton plant under different nitrogen application rates and soil properties were investigated. The results showed that nitrogen content of leaves varied with growth stages, being 2.83%, 2.74%, 3.22%, and 3.12% for seedling, bud, flowering, and boll stage, respectively. Leaf nitrogen content increased as nitrogen rate increased for all the treatments.There were significant differences in leaf nitrogen content between the soils of sandy loam and clay loam.The analysis showed that the differences in chlorophyll content were significantly under different nitrogen levels, and it also varied with leaf position.Nitrogen fertilizer increased cotton leaf SPAD values,while there was no marked difference in SAPD value between leaf positions. In addition, leaf color was significantly different between the 4th and the 1st leaf fromthe top,which was correlated with the plant nitrogen content at flowering stage and flowering-boll stage.Therefore, color difference between the 4th and the 1st leaf from the top could be used for diagnosis of plant nutrition status.