The Crucial Role of Soil Organic Matter in Satisfying the Phosphorus Requirements of Olive Trees ( Olea europaea L.)

Under high organic matter content, even under low extractable soil P concentrations, sufficient or over-sufficient foliar P levels may be found. This multi-year study aimed at examining the effects of organic matter content and irrigation management on (a) soil fertility, (b) P-cycle related soil enzymes (acid and alkaline phosphatase, pyrophosphatase) and (c) foliar nutrient concentrations. Irrigated and non-irrigated groves of fully productive trees of the cultivar “Chondrolia Chalkidikis” with low organic matter (LOM < 1.5%), medium organic matter (1.5% < MOM < 2.5%) and high organic matter (HOM > 2.5%) were selected for the experimentation. It was hypothesized that olive groves receiving high inorganic fertilization and irrigation inputs (usually with medium to relatively low organic matter content) would show higher soil and foliar P concentrations compared to the non-irrigated groves with higher organic matter content receiving manure applications. Most of the soil variables (including the three enzymes’ activities) were affected by differences in organic matter content. However, organic matter content did not show a significant influence on foliar nutrient concentrations. Olive trees, especially those cultivated in soils with high organic matter content (receiving organic fertilization), can over-satisfy their P nutritional needs, even though soil analyses show low soil extractable P concentrations, probably due to the high enzymatic activity of acid and alkaline phosphatases. The practical conclusion of this study is that P fertilizer recommendations should be primarily based on foliar P rather than on extractable soil P.