Blueberry yield and soil properties response to long-term fertigation and broadcast nitrogen

The long-term effects of annual applications of nitrogen (N) at varying rates through the broadcast (BROAD) and fertigation (FERT) methods on soil properties and blueberry yield were assessed in British Columbia, Canada. Seven N treatments, including a control (CONT, 0%), were arranged in a randomized complete block design with six replicates. From 2008 to 2012, the N treatments were 50% (BROAD-50 and FERT-50), 100% (BROAD-100 and FERT-100), and 150% (BROAD-150 and FERT-150) of the recommended rate. From 2013 to 2015, as the planting matured, a supplementary 50% was added incrementally to all N rates. The BROAD and FERT methods consisted of 3 and 10 equal-split applications respectively. The FERT rates above 100% increased the NO3-N concentration in the 0-30 cm soil layer by 2.6 times for FERT-150 and by 3.6 times for FERT-200. However, the BROAD-based N above 100% increased the NH4-N concentration in the sawdust layer by 1.9 times. The supply of ammonium sulfate via fertigation reduced soil pH and increased soil EC as compared with BROAD; the lowest soil pH (4.2) was observed under FERT-200 in the 0-30 cm layer, while the highest EC (1516 μS cm−1) was found under FERT-150. The maximum berry yield for establishing plants (2010-2012) was 14,908 kg ha−1 obtained with FERT-150. At plant maturity (2015), the maximum berry yield was 32,444 kg ha−1 obtained with FERT-100 (57% higher than with CONT). However, berry yield only increased by 32% with FERT-150 and by 11% with FERT‐200 as compared with CONT, indicating a negative effect associated with repeated applications of high FERT-based N rates. Correlations between berry productivity and EC showed substantial yield decreases at EC values greater than 760 μS cm−1 in the 0-30 cm layer and greater than 291 μS cm−1 in the 30-60 cm layer, which corresponded to FERT-based N rates above 100%. From these results, we can conclude that fertigating mature plants with ammonium sulfate above the recommended rate is not a sustainable practice for blueberry production, as it reduces yields, increases soil EC above critical levels, and raises the risk of NO3-N leaching to the aquifer.