Evaluation of Both SPAD Reading and SPAD Index on Estimating the Plant Nitrogen Status of Winter Wheat

Rapid non-destructive estimation of the plant nitrogen (N) status is needed for the precise management of N in small-scale farms. Our objectives were to describe the responses of the soil–plant analyses development (SPAD) measurements to the plant N status of winter wheat, and to evaluate whether the SPAD index is more precise than SPAD readings in estimating the plant N status. During 2009–2011, a field experiment with winter wheat was conducted in the North China Plain. There were eight N supply levels ranging from 0 to 420 kg N ha−1 to create gradients of leaf greenness. The SPAD readings were measured on the topmost fully expanded leaf; the SPAD index was expressed relative to the SPAD readings of sufficiently fertilized plants. Combing over years, significant quadratic responses of SPAD reading to the plant N concentration (PNC) occurred at each/similar Zadoks growth stage (ZGS) but differed largely from ZGS 39/43 to ZGS 58/59, the r2 varied between 0.88 and 0.97; the SPAD readings exhibited a high logarithmic correlation to plant N accumulation (PNA, r2 = 0.94) irrespective of growth stages. Compared with SPAD readings, the SPAD index showed the same precision in response to PNC and PNA at each growth stage in each year, but presented less precision for the combined datasets across years, indicating that the SPAD index is not a more preferable tool in estimating plant N status. In addition, both the SPAD readings and SPAD index demonstrated poor to moderate responses to basal stem nitrate content (BSNC) with large differences among ZGS 39–59 and between the 2 years, indicating that the SPAD measurements could not be effectively used to estimate the BSNC.