The use of SPAD-502 chlorophyll meter for dynamically optimising the nitrogen supply in potato crop: A methodological approach

The management of N fertilization through a method called "dynamic optimisation of nitrogen supply" requires the assessment of crop nitrogen status throughout the growing season. A tool for plant analysis, that aroused interest recently, is the hand-held chlorophyll meter SPAD-502 (Minolta - Japan), which permits prediction of leaf nitrogen status from measurements of chlorophyll content in potato. For practical use of this instrument, it is necessary to develop threshold values, above which no increase in yield would be expected. A procedure for the calculation of chlorophyll meter threshold value was set using data obtained in a trial on potato conducted in 1997 at the Experimental Farm of the University of Padova at Legnaro, Italy. The trial was included in a long-term experiment on nitrogen fertilization carried out on different soil types. The experiment was carried out on 108 vegetation boxes of 4 m2 area (2x2 m) and 0.8 m depth. According to a randomised complete block design with 3 replicates, 6 increasing rates of N (0, 50, 100, 200, 300, 400 kg ha-1) were factorially combined with 3 soil types (clay, siltyloamy, sandy), in either presence or absence of residues of the previous wheat crop. Linear-plateau equations were developed for calculating the relationships between SPAD values and relative tuber yield and this equation allowed the calculation of STV (SPAD Threshold Value) which represents the limit below which a reduction in yield occurs. This paper describes the procedure step-by-step and gives STVs for potato cv. Primura.