Nitrogen Fertilization Strategies for the ‘Hass’ Avocado that Increase Total Yield Without Reducing Fruit Size

Abstract

Effects of nitrogen (N) fertilizer application times and rates on ‘Hass’ avocado (Persea americana) yield and fruit size were determined to resolve whether a single dose of soil-applied N [1x N (25 lb/acre)] at each of the five key stages of tree phenology (January, April, July, August, and November) (control) was as efficacious as soil-applied 2x N (50 lb/acre) at one or two key stages or soil- or foliar-applied 3x N (75 lb/acre) at only one stage. All trees received soil-applied N at 125 lb/acre as ammonium nitrate (NH4NO3) annually; trees receiving 2x or 3x N received the remaining N divided evenly at the same phenological stages (months) as trees receiving five 1x N applications. The importance of supplying N during the summer, when June drop, exponential fruit growth, vegetative shoot growth, and floral initiation occur, was determined by testing soil-applied 0.8x N in July plus August only (40 lb/acre N as NH4NO3 annually). Application time proved an important determinant of total yield. Yield of commercially valuable size (CVS) fruit was correlated with total yield (r = 0.80, P < 0.0001). Four-year cumulative total yields were equal for trees receiving soil-applied 1x N at five key phenological stages and trees receiving soil-applied 2x N in April and 18.75 lb/acre N at the four other stages (months). However, trees receiving soil-applied 2x N in April plus November and only 8.3 lb/acre N in the three other months, in particular July and August, had significantly lower 4-year cumulative total yields (P = 0.0362). Additional evidence of the importance of meeting avocado tree N demand in the summer is that trees receiving only 40 lb/acre N split in July plus August produced 4-year cumulative total yields equal to trees receiving 25 lb/acre N at the five key phenological stages; lower annual N would reduce fertilizer expense and protect the environment.