Study on nitrogen nutrition diagnosis and fertilization recommendation of winter wheat using canopy digital images from cellphone camera

Digital cameras have been successfully used in nutrition diagnosis of crops. Few researches were reported on the application of cellphone cameras on nutrition diagnosis and precision fertilization of crops, though the cellphone cameras have various advantages, such as portability, handleability and universality. In this study, we used Samsung and Xiaomi smart cellphones to photograph winter wheat canopy at jointing stage from four shooting angles, frontlighting looking down, frontlighting overlooking, backlighting looking down and backlighting overlooking. The color parameters of achieved winter wheat canopy images were extracted, processed and analyzed. And their correlations with traditional nitrogen nutrition indexes were analyzed. According to the statistical analysis of the correlation, appropriate color parameters were selected, and the nutrition diagnosis model was established with color parameters and nitrogen nutrition index. Then the model was fitted to establish indicator systems of diagnosis of nitrogen nutrition and recommendation system of fertilization for winter wheat. The aim of the study was to provide references for application of smart cellphone on precision fertilization of crops. The results showed that there was no remarkable difference in the color parameters of canopy images taken with different types of smart cellphones. However, a certain difference was observed in color parameters of canopy images taken from different shooting angles. More than half of color parameters of canopy images taking from the view of backlighting overlooking were significantly correlated with traditional nutrient indexes. Furthermore, the results indicated that the color parameters VARI(G-R)/(G+R-B) and R/(R+G+B) both had outstanding correlations with leaf SPAD and stem nitrate concentration. And VARI was found to be the best, and therefore, selected as the sensitive color parameter for winter wheat nitrogen nutrition diagnosis. The diagnosis model was stem nitrate concentration=1.481×10⁶×VARI^4.987. According to the equation, the nitrogen nutrition status of winter wheat was normal when VARI was between 0.201 3 and 0.250 9. The nitrogen application rates under different VARI values were calculated for different target yields of winter wheat. The results were applied to develop cellphone software for nitrogen nutrition diagnosis and fertilization recommendation of winter wheat. In summary, it was possible and applicable to take photographs of canopy from the view of backlighting overlooking with smart cellphone and extract VARI color parameter to diagnosis nitrogen nutrition status. The results provided technical support for the diagnosis of nitrogen nutrition and recommendation of fertilization of winter wheat based on cellphone camera.