Determination of SPAD Threshold Values for the Optimisation of Nitrogen Supply in Processing Tomato

This work deals with the use of SPAD-502 (Minolta - Japan), as a tool to assess the crop N status across the growing season and to manage N fertilization of processing tomatoes. The SPAD-502 permits prediction of plant N status from measurements of leaf transmittance. For practical use of this instrument, it is necessary to develop threshold values, below which decrease in yield would be expected. A procedure for the calculation of SPAD threshold values (STVs) was set using data obtained in a trial on processing tomato conducted in 2002 at the Experimental Farm of the University of Padova at Legnaro, Italy. The trial was included in a long-term experiment, aimed at comparing the effects of different type of fertilization on crop yield and soil fertility. Three ways of crop residue management (incorporation of previous crop residues, incorporation of the residues plus addition of 1 t ha-1 of dried chicken manure, removal of the residues) were factorially combined with 5 increasing rates of N (0, 60, 120, 180, 240 kg ha-1). The SPAD measurements were carried out at 2-5 days-interval. At harvest, fruits were collected and weighed. Linear-plateau equations were developed for calculating the relationships between SPAD values and relative tomato yield. The abscissa of the break-point in the linear-plateau regressions, corresponding to the SPAD value below which yield decreases, was defined ‘SPAD Threshold Value’ (STV). This paper describes the procedure step-by-step and gives STVs for tomato cv. Perfectpeel.