Organic and Mineral Nitrogen Fertilization for Processing Tomato in Southern Italy

A four-year field experiment to evaluate the effects of different types of organic fertilizers on yield and quality of processing tomato and on soil chemical evolution, was carried out in Southern Italy during the 2002–2005 period on a siltyclay soil. The fertilization treatments aimed to supply 100 kg ha -1 of N and were: 1) an organic biological fertilizer (BIO); 2) an experimental compost composed by olive residues, sludge and straw (COMP); 3) a mix (MIX) of an organic fertilizer (50%) and ammonium nitrate (50%); 4) a traditional chemical fertilizer (ammonium nitrate, MIN). At harvest the main productive and qualitative parameters were assessed (fresh and dried yield; biomass and marketable yield). Chemical analyses of the soil were made at the start and at the end of experiment. Fruit yield did not differ among the fertilization treatments, but unripe fruit yield was higher in the MIN and BIO treatment; MIN showed also smaller fruit than BIO and COMP. The N availability during crop cycle influenced the mean fruit weight, date, and simultaneity of maturity during the ripening process. Soil characteristics changed during the four years of application for mineral N and among treatments (P2O5 for BIO and COMP and mineral N for MIN and MIX fertilization treatments). Heavy metals: cupper and zinc did not increase their concentration in the soil; instead, lead increased below the Italian law’s limit in COMP. The possibility to use organic fertilizer for processing tomatoes has been evaluated and the conclusions are that organic fertilization is, in same cases, more suitable than mineral one from a productive and soil fertility standpoint of view.