Chestnut Response to Organo-mineral and Controlled-Release Fertilizers in Rainfed Growing Conditions

In the context of climate change, chestnut fertilization is receiving great attention in the Mediterranean basin, due to the increase in the unpredictability of the pluviometric regime which makes it difficult to determine the optimal timing of fertilizer applications. The purpose of this work was to assess the suitability of fertilizers with mechanisms for nutrient protection on the increase of tree nutritional status and crop productivity. Four fertilizers with restricted nutrient solubility were tested: Bioscape 5:14:7 (32.8% organic matter (OM)); Humix 13:3:5 (80.1% OM); Exactyon AG 6:15:8 (37.7% N and 33.1% P encapsulated); and Exactyon AG 18:5:13 (47% N encapsulated, 28.8% N as urea coated with ammonium sulfate). The trials were carried out for four years in Moimenta and Meixedo, NE Portugal, which also received a non-fertilized control. In Moimenta the ground was managed by tillage and in Meixedo by a cover of natural vegetation. Exactyon AG 18:5:13, being the most concentrated in N and B, gave significantly higher cumulative (2015-2018) nut yields (146.6 kg tree(-1)) than the control (52.3 kg tree(-1)) in the Moimenta trial. The results were explained by the increase in leaf N and B levels, which in the control treatment were in the deficiency range. In Meixedo there were not found significant differences among treatments, likely due to the competition by cover crop for resources. The results showed that the response of the trees to the fertilizers relied mainly in their content in nutrients limiting to the agroecosystem and less in the mechanisms of nutrient protection.