Diagnosis of nutrient constraints in citrus orchards of humid tropical India

Citrus growing in humid, tropical India is concentrated in east to northeast India. The region is well known for large-scale commercial cultivation of Citrus reticulata Blanco, cultivar 'Khasi,' mandarin. Extensive surveys were conducted covering as many as 108 orchards from 52 locations representing eight states, namely West Bengal, Sikkim, Assam, Meghalaya, Tripura, Mizoram, Arunachal Pradesh, and Manipur. Expressed in milligrams per kilogram, the optimum values for available nutrients in soil were determined to be as: nitrogen (N) (220.8-240.6), phosphorus (P) (21.2-45.6), potassium (K) (252.2-300.8), calcium (Ca) (278.1-318.6), magnesium (Mg) (67.2-92.5), iron (Fe) (82.2-114.6), manganese (Mn) (21.4-32.8), copper (Cu) (0.82-2.62), and zinc (Zn) (2.18-4.22) using multivariate quadratic regression analysis. The corresponding leaf-nutrient values, expressed in percent, were: N (2.23-2.49), P (0.10-0.11), K (1.86-2.12), Ca (2.12-2.32), and Mg (0.28-0.38), and in ppm were Fe (148.2-179.8), Mn (72.2-84.8), Cu (10.4-18.6), and Zn (24.2-38.8). These reference values of soil and leaf analysis were later observed to be well within the range of values obtained for high-performance orchards (45-62 kg tree(-1)), confirming the hypothesis that soil fertility and leaf nutrient levels maintained under high yielding orchards could be used provisionally as nutrient diagnostics. Nutrient constraints in the form of N, P, Ca, Mg, Cu, and Zn were identified using these diagnostics, which deserve a place in a fertilizer program of mandarin orchards of the region to obtain sustainable optimum fruit yield.