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

The management of N fertilization through a method called "dynamic optimization of nitrogen supply" requires the assessment of crop nitrogen status throughout the growing season. A tool for plant analysis, that increased interest recently, is the hand-held chlorophyll meter SPAD-502 (Minolta - Japan), which allows leaf nitrogen status from measurements of chlorophyll content in potato. The data from this instrument is necessary to calculate some threshold values below which N supply is necessary to avoid yield losses. A procedure for the calculation of chlorophyll meter threshold value was set using data obtained in a potato trial 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 conducted on different soil types. The experiment was conducted on 108 vegetation boxes of 4 m2 area (2x2 m) and 0.8 m depth. A randomized complete block design was used with three replicates of six increasing rates of N (0, 50, 100, 200, 300, 400 kg ha-1) factorially combined with 3 soil types (clay, silty-loamy, sandy), in either the presence or absence of residues of the previous crop (wheat straws). Linear-plateau equations were developed for calculating the relationships between SPAD values and yield and this equation allowed the calculation of STV (SPAD Threshold Value) which represents the limit below which a reduction in yield occurs. The procedure described in this work appears to be suitable for supplementary N management in potato crop. It can be used very easily for decision making on time and rate of supplementary N to potato.