2017 ASHS Annual Conference

Nitrate (NO₃) contamination of groundwater is a significant unresolved environmental issue worldwide. Any Nitrogen (N) fertilizers not taken up by shallow rooted vegetable crops with high demand for N, such as Cauliflower (Brassica oleracea), can leach to the groundwater. Hence, in adopting a preventative strategy to mitigate nitrate contamination of groundwater it is critical to determine and control the amount of nitrate below the root zone by implementing strategies that optimize crop yield while minimizing the soil nitrate leaching index (LI). When combined with the appropriate fertilizers, surface drip irrigation can be a useful tool to help mitigate the nitrate leaching potential of a cauliflower crop. This research focused on determining the N leaching potential for drip irrigated cauliflower grown on a sandy loam, and fertilized at four N rates (0, 75, 150 and 225 lbs N/ac) with organic soybean meal (7-1-2) in a randomized complete block design with five replicates. Field experiments were conducted in 2014 and 2015. The nitrate leaching potential below the root zone of the crop was quantified using soil solution access tubes installed at depths of 12 and 24 inches below the soil’s surface. Soil samples from the top 4ft were taken in 1foot increments and analyzed to for soil NO₃-N concentrations. The nitrate LI was calculated as the ratio of NO₃-N in the 12- 48 inches increment to the total amount in the 0- 48 inches of soil. Marketable yield from the fertilized plots were significantly different (P<0.01) from the control plots which received no fertilizer addition with yield increases of 23% and 59% in 2014 and 2015, respectively. The greatest soil nitrate content occurred within the 12-24 inches of soil for the plots fertilized with 225 lbs N/acre. Soil solution samples collected with the access tubes indicated that there is a high degree of variability associated with the NO3-N concentrations. Therefore, it is advisable that assessment of nitrate leaching potential should not be based solely on soil solution samples, but should be supplemented with data derived from core soil samples. Overall, for the sandy loam soil used in this study, the nitrate LI values would imply that cauliflower growers should not exceed the 150lbs/N acre rate, since the is no significant economic yield increase reported between 150 lbs/N acre and 225 lbs/N acre, but there is a greater potential to leach significant amounts of nitrate into groundwater.