Fertilization with N and K increases oil and water content in olive (Olea europaea L.) fruit via increased proportion of pulp

• Fertilizer (i.e., N and K) application generally increases fruit oil content in olive.
• In our work, fertilization increased pulp/pit ratios, but not oil concentration in the pulp.
• This resulted in an apparent increase in oil content in the fruit, since most of the oil is in the pulp.
• Reanalyzing data from the literature supported this finding.
• When studying fruit composition, pulp (i.e., not fruit) concentrations should be considered.

N and K fertilization usually increases olive fruit oil content as well as fruit size and pulp to pit ratio. The objective of this work was to investigate whether the increase in fruit oil content occurs via increased oil concentration in the pulp, or via increased fraction of pulp in the fruit. In 2009 Leccino olive trees were fertilized with the equivalent of 180 kg of urea-N, split in three equal doses (April, June, September). In April, K2O fertilizer (potassium sulphate) was also applied for an equivalent amount of 100 kg of K2O per hectare. Control trees were not fertilized. Fresh and dry weight of fruit, pulp and pit, fruit and pulp oil and water content, pulp firmness and fruit maturity index were measured throughout fruit growth, on fruit samples from both fertilized and control trees. At all sampling dates, fertilized plants had significantly greater fruit weight, both on a fresh (+23% on average) and dry (+17% on average) basis. Pulp/pit ratio also increased significantly with fertilization (+23%, both on fresh and dry basis). Fruits of fertilized plants also had significantly higher oil content on dry matter basis (+9% on average). However, pulp oil content did not differ significantly between treated and control plants. The same was found for water content, which was significantly higher in the fruit of fertilized plants (+5% on average), but not in the pulp. Fertilization did not affect fruit maturation (i.e., color and pulp firmness).