Spectral characterization and prediction of nutrient content in winter leaves of litchi during flower bud differentiation in southern China

Nutrient availability can affect the cracking rate of the litchi fruit tree, and the nutrient content of the canopy can be used to monitor availability and fertilizer needs of the litchi orchard. In this study, we analyzed the correlation between calcium (Ca), magnesium (Mg) and potassium (K) in canopy leaves and four indices of in situ spectral data of litchi canopy. These were reflectance (R), reciprocal-logarithm-transformed reflectance (log(1/R)), the first-order derivative of reflectance (dR) and the first-order derivative of reciprocal-logarithm-transformed reflectance (dlog(1/R)). The results showed that the spectra of the litchi canopy have common spectral characteristics. The correlation between the selected nutrients of the litchi canopy and R or log(1/R) was weak, that between the nutrients and dR was significant and with dlog(1/R) it was the most significant. The 1262 nm wavelength for dR and 1018 nm one for dlog(1/R) had the most significant correlation with Ca. The 1293 nm wavelength for dR and 1601 nm one for dlog(1/R) had the most significant correlation with Mg. The 1686 nm wavelength for dR and 1337 nm one for dlog(1/R) had the most significant correlation with K. Therefore, those wavelengths were chosen to create the regression equations for prediction. The linear regression equations performed the best when predicting canopy nutrient content of litchi for Ca at 1018 nm, Mg at 1601 nm and K at 1337 nm. Further, the formulated application of K2O played an important role in reducing the amount of K2O applied (50.6 g/plant on average) and in increasing the yield of the litchi tree by 4.4 kg/plant on average. These results are relevant for implementing a precision agriculture approach for litchi production and for environmental protection in South China or in other litchi production areas of the world. Other nutrients that affect litchi growth need to be studied in the future.