In California, lettuce (Lactuca sativa) production generates over $1.8 billion in revenue. As a leafy green vegetable, Nitrogen (N) fertilizer applications can reach as much as 280 kg N/ha. The relative shallow root systems and frequent irrigation can enhance Nitrate (NO3) Leaching Potential (NLP) when the crop is grown on Sandy Loam soil. Because of economic, health, and environmental concerns, growers are exploring the option of using organically approved fertilizers such as water-soluble N derived from soy protein hydrolysate (GS) as an alternative to Urea Ammonium Nitrate (UAN). The objective of this study was to compare the NLP of equivalent rates of UAN and GS applied to a (1) soil-column study (Phase 1) and (2) greenhouse lettuce crop with 3 Irrigation regimes x 2 Fertilizers x 4 Rates as a factorial experiment replicated four times (Phase 2). In the Phase 1 study, approximately 2,150 ml of water containing a chloride (Cl) tracer and fertilizers at rates of 0, 56, 112, and 168 kg N/ ha were applied to a fixed volume of soil at a bulk density of 1.35 g/cm3. The chloride concentrations in the leachate from both fertilizer treatments were similar, and the water balance accounted for 95% of the solution applied. The concentration and amount of NO3 in the leachate collected from soil treated with GS were lower or at least equal to that obtained from the soils treated with UAN. In contrast, the amount of residual soil NO3 was significantly higher in the soil receiving the UAN, ranging from 2 to 4 times as the application rates increased from 56 to 168 kg N/ha, respectively, implying a relatively higher NLP for the UAN. In the case of the lettuce experiment (Phase 2), there were no significant differences in the chlorophyll content based on the choice of fertilizer or application rates. Lettuce yields, expressed on a dry matter basis, were not significantly (p=0.11) different for fertilizer type, with similar fertilizer rate response curves being obtained for both UAN and GS. Finally, the mean %N and %C in the lettuce leaves were 3.22% and 40%, respectively, regardless of whether the lettuce was grown with synthetic UAN or organically derived soybean hydrolysate (GS). These preliminary findings justify the need to investigate further the impact of higher irrigation rates and even saturated conditions on the NLP of vegetables subjected to the two fertilizers used in the current study.
