Soil and foliar nitrogen and boron fertilization of almond trees grown under rainfed conditions

Rainfed orchards have huge social importance in territories of low agro ecological potential. Research on almond is limited for dry-farmed orchards in particular for crop nutrition and fertilization. This makes difficult to implement a cropping practice adjusted to the ecological constraints of these agrosystems. It is known that nitrogen (N) and boron (B) are determining nutrients for dicot tree crops such as olive and vineyards grown under these environmental conditions. Thus, this study aims to test at what level the crop responds to soil-applied N and B, and whether the foliar sprays can supplement or replace the application of the nutrients to the soil. It is also important to check if the sufficiency ranges set for the almond, which have been based on irrigated orchards, are adjusted for rainfed farming. The N rates applied to the soil were 0 (NO), 25 (N25), 50 (N50) and 100 (N100) kg hm(-2) and those of B 0 (BO), 1 (B1), 2 (B2) and 3 (B3) kg hm(-2). The foliar sprays consisted of three annual applications of N (two in 2017) and two applications of B (one in 2017), the last ones of 2015 and 2016 applied at post-harvest. Foliar N and B sprays were respectively applied at the concentrations of 0.5% N and 0.036% B. The experiment was arranged as a split-plot design with soil applied N or B assigned to the main plots and foliar N or B as subplots. Kernel yield showed a marked alternate-fruiting, with two years of good crops (2015 and 2017) and a year of practically no production (2016), likely due to adverse ecological conditions for almond growth and cropping practices incorrectly performed. Soil-applied N significantly increased kernel yield in comparison to the control but the differences were not significant in N rates higher than 25 kg N hm(-2). The application of B to the soil improved tissue B concentrations but did not increase productivity. In general, foliar applications of N or B failed to improve plant nutritional status and kernel yield. The current sufficiency ranges for almond seem to be unnecessarily narrow for several nutrients such as N, potassium (K), calcium (Ca), manganese (Mn) and iron (Fe), since several results were found to be out of the sufficiency range with no apparent negative consequences for the trees. The laboratories carrying out soil testing and plant analysis should take this into account in diagnosing the nutritional status of almond orchards.