Research progress on nitrogen nutrition diagnosis method based on SPAD for main grain crops

This paper reviews the research progress on N nutrition diagnosis of corn, wheat, rice, and other crops using chlorophyll meter. The review focused on the relationship between SPAD, plant leaf chlorophyll, and N concentration. Firstly, indicators such as “relative SPAD value” , “nitrogen saturation index” , or “normalized SPAD” could eliminate or reduce errors between varieties, growth periods, and regions interannual. Secondly, leaf is the ideal indicator suitable for the diagnostic target at different growth periods. Thirdly, the relationship between SPAD differences among various layers of leaves and N nutrition is more stable and reliable. We summarized the identified problems and shortcomings in the technical specification of crop nutrition diagnosis. Further, we recommend fertilization based on SPAD, building and screening SPAD values of different crop species and their derivative parameters, stability and universality of models, and the response relationship between other nutrients besides N and SPAD. On this basis, the paper put forward further research directions on the diagnosis of plant N nutrition and fertilization using a chlorophyll meter. 1) There is a need to scale up technical specifications for different crops N nutrition diagnosis based on SPAD. 2) There is a need to consider the determination of N sufficiency and deficiency indexes of leaf SPAD. 3) Fertilization models based on leaf SPAD should be established. 4) A fertilization decision support system based on SPAD should be developed. 5) There is a need for further research on the relationships between other nutrients related to chlorophyll synthesis such as K, Mg, Fe, and Mn and SPAD values.