Iron chlorosis in field grown olive as affected by phosphorus fertilization

• Phosphate fertilization aggravates Fe chlorosis in olive trees on calcareous soils.
• The effect depends on Fe oxides content, available Fe and P and plant characteristics.
• Suggestion I: Keeping the available P slightly above the prescribed critical levels.
• Suggestion II: Avoiding high local P concentrations.

Olive tree is an important crop in Mediterranean-climate regions. In Spain, about 70% of the olive is grown on calcareous soils, most of which have potential to induce iron (Fe) deficiency chlorosis. During the last decades, intensification of olive cultivation across the Mediterranean area has often been accompanied by the application of high rates of phosphorus (P) to the soil. There is no information, however, on whether soil P level affects the severity of Fe chlorosis in field-grown olive. In this work, we assessed the effect of fertilization with P on Fe chlorosis in Arbequina, Ocal, Manzanilla and Picudo olive cultivars during four (three for cv. Picudo) growing seasons in four orchards located in southern Spain with soils that were highly calcareous and poor in poorly crystalline Fe oxides – the main source of Fe to the plant. Application of P to the soils decreased leaf chlorophyll concentration (as estimated via SPAD) and leaf weight, both significant for cvs. Manzanilla and Picudo. These differences between cultivars were probably due to the fact that the Fe oxides content was lower in the Manzanilla and Picudo than in the Arbequina and Ocal orchard soils. So, the extent of the Fe oxides surfaces that were free of phosphate, and thus more capable of releasing Fe, was smaller in the former than in the latter soils. The response of trees to P-induced Fe stress lagged behind P application probably because of built-up tree nutrient reserves. This can also explain why canopy volume, trunk diameter and yield were not affected by P fertilization. The results of this study point to the need to curb on P application to Fe chlorosis-inducing soils, particularly in intensive oliviculture.