EVALUATION OF A HAND-HELD RADIOMETER FOR FIELD DETERMINATION OF NITROGEN STATUS IN COTTON

Authors: Read, John; WHALEY, ERIC - SPECTRUM TECHNOLOGIES; TARPLEY, LEE - TEXAS A&M UNIVERSITY; REDDY, RAJA - MISSISSIPPI STATE UNIV

Submitted to: Crop Science Society of America Monograph
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/7/2003
Publication Date: 12/1/2003
Citation: Read, J.J., Whaley, E.L., Tarpley, L., Reddy, R. 2003. Evaluation of a hand-held radiometer for field determination of nitrogen status in cotton. American Society of Agronomy Special Publication Number 66. p. 177-195.

Interpretive Summary: Changes in the nutrient status of cotton (Gossypium hirsutum L.) can be determined directly in a laboratory by chemical analysis of tissue samples or indirectly in the field by measuring leaf reflectance in narrow wavelengths of light. In part, this is because reflectance in the visible region of the spectrum (300-700 nm) varies as a function of leaf chlorophyll concentration (Chl), which is closely associated with leaf nitrogen (N) status. This study evaluated three commercially available chlorophyll meters, CM-1000, GER 1500, and SPAD-502, for remote assessment of Chl under N-deficit conditions. Treatments comprised control N, 20% of control N at first floral bud (square) stage onward, and 0% and 20% of control N at first flower stage onward. Leaf reflectance (R) values at 700 and 840 nm (+/- 5.0 nm) were used to compute R840/R700, a ratio known to be associated with general plant stress. Restriction of N at first square decreased Chl and N concentrations by about 40%, as compared to control treatment. Within each sampling date, strong linear relationships (r = 0.49-0.93) were obtained between Chl and R840/R700 across various N-stress treatments; however, the SPAD 502 meter was often a better indicator of Chl than R840/R700. In providing a simple and reliable assessment of plant N status, we conclude the CM-1000 is a useful tool for nutrient management in cotton. Although R840/R700 may not accurately estimate leaf Chl concentration, use of this reflectance ratio to monitor crop nitrogen status should be more timely and less expensive than periodic estimates based on plant tissue analysis.

Technical Abstract: Nutrient management in cotton (Gossypium hirsutum L.) often relies on determining plant nitrogen (N) status at different stages of crop development. This study evaluated three commercially available chlorophyll meters, CM-1000, GER 1500, and SPAD-502, for remote assessment of leaf chlorophyll concentration (Chl) under N-deficit conditions. Treatments comprised half-strength Hoagland's solution (control N), 20% of control N at first floral bud (square) stage onward, and 0% and 20% of control N at first flower stage onward. Leaf reflectance (R) values at 700 and 840 nm (+/- 5.0 nm) were used to compute R840/R700, a ratio known to be associated with general plant stress. Within each sampling date, strong linear relationships (r = 0.49-0.93) were obtained between Chl and R840/R700 across various N-stress treatments. In short, reflectance measures agreed well between the CM-1000 and GER 1500; however, SPAD readings were often a more sensitive indicator of Chl than R840/R700. This illustrates the difficulty of measuring leaf reflectance under ambient light levels, as compared to measuring transmitted light at two wavelengths in an enclosed chamber in the SPAD meter. Replicate (n=5) reflectance readings per leaf did not substantially improve the association between Chl and the CM-1000 index. Results indicate the CM-1000 should be useful for nutrient management in cotton. Although R840/R700 may not accurately estimate Chl, this leaf reflectance measure should be more timely and less expensive than periodic estimates of plant N status based on tissue analysis.