Effects of organic fertilisation on ‘Valencia late’ orange bearing trees

In a study realised over a three year period on orange bearing trees (Citrus sinensis (L.) Osbeck) ‘Valencia late’, grafted on sour orange (C. aurantium L.), the effect of organic fertilisers (OF) on plant nutrition and performance was verified. In a randomized block experimental design, four treatments were compared, namely: mineral fertiliser (MF) treatment adopted as control, citrus byproduct compost (CB), poultry manure (PM) and livestock waste compost (LW). The trees, with the exception of (MF) treatment, were organically grown since 1994 in the experimental farm of CRA-ACM in Lentini, Sicily, and received the same N input every year. Significant differences for micronutrients (Fe, Mn, Zn) were noticed in leaf analyses, whereas no difference was found between treatments for leaf macronutrient content. The δ15N detected in leaves, proteins of pulp and amino acids of juice showed the lower level in MF, an intermediate value in CB and the highest level in animal derived fertilisers treatments (PM and LW). Fruit of the CB treatment showed values of total soluble solids and total acidity significantly lower than other treatments. Orange peel Chroma C* in CB and MF was higher than in PM and LW treatments. Discriminant analysis of the leaf and fruit analytical data set successfully separated treatments. First discriminant canonical function explains the 96,9% of the variability, with highly significant Wilks’ lambda. Cross validation classified correctly all MF and CB samples, whereas PM and LW in few cases were mixed up.