Remote sensing algorithms for castor bean nitrogen and pigment assessment for fertility management

Judicial management of nitrogen (N), a major nutrient affecting several facets of crop development, growth, and finally yield, require developing tools to monitor N and pigment status in the field. An experiment was conducted under out-door pot-culture conditions to determine N deficiency effects on castor bean (Ricinus communis L.), cv. ‘Hale’ leaf N, pigments and reflectance properties, and to identify N- and pigment-specific wavebands for N management in the field. Castor bean plants were grown in 12-L pots with complete Hoagland's solution from emergence to 34 days after sowing (DAS). Then, the treatments imposed were complete Hoagland's nutrient solution (control, 100N), reduced N to 20% of the control (20N) and withheld N from the solution (0N) from 34 to 66 DAS. Leaf N, pigments and reflectance properties were measured twice weekly. Plant height, total leaf area, and above-ground dry matter were measured at the end of the experiment, 66 DAS. Even though, all plant growth parameters declined in the 20 and 0N treatments compared to 100N, the reduction in leaf area was greater than reductions in stem lengths and above-ground biomass at the end of N treatments. Leaf N, total chlorophyll and carotenoid concentrations declined linearly over time in all treatments except for leaf N in the 100N treatment. The variations in leaf N ranged from 3.3 to 1.2 g N m−2 while the concentrations of chlorophyll and carotenoids declined from 0.427 to 0.256 g m−2, and 0.094 to 0.055 g m−2, respectively during the course the experiment. The reductions in pigments increased leaf reflectance at 555 and 715 nm, and caused a red-edge (680–750 nm) shift to shorter wavelengths. Reflectance ratios of 455/605 and 505/605 nm were highly correlated with leaf N on dry matter (r2 = 0.93) and area-based (r2 = 0.90) estimations, respectively. Similarly, reflectance ratios of 715/505 and 705/675 were highly correlated with total chlorophyll (r2 = 0.94) and carotenoid (r2 = 0.80) concentrations, respectively. The N- and pigment-specific wavebands provide rapid, non-destructive estimation of leaf N and pigments for N management strategies of castor bean plants in the field.