Optimizing potassium application in organically-grown high tunnel tomato (Solanum lycopersicum) in the northeastern United States

In the northeastern United States, high tunnel production became widespread in the 1990s, and has increased dramatically in the past decade as federal conservation initiatives incentivized installation of new tunnels. The most important crop grown in high tunnels in the region is tomato, which is usually grown in-ground, i.e., directly in soil. Both conventional and organic high tunnel tomato growers rely primarily on pre-plant application of organic fertilizers to provide nutrients for the crop. Thus, high tunnel soils in the region are generally characterized by high levels of organic matter and soluble salts. Despite this, growers frequently experience fruit quality issues that we hypothesized to be associated with low potassium (K) levels (e.g., yellow shoulder (YS), uneven ripening). While precise fertilization regimes exist for other production methods, such as hydroponic production, similar information is lacking for organic high tunnel in-ground production. To improve the efficiency of organic fertilization in high tunnel tomato, our goal was to determine pre-plant soil K levels required for optimum yields and quality. We conducted experiments in three sites over three years; by first depleting soil K levels and then establishing low to high soil K gradients by applying potassium sulfate prior to planting tomato. We collected yield data, assessed yellow shoulder incidence and severity, and monitored leaf tissue and soil K levels throughout the three years. We found that 1) even extremely high levels of applied K were depleted quickly from high tunnel soils, 2) in general, soil K levels were positively correlated with marketable fruit weight, and negatively correlated with incidence and severity of YS and percentage of cull fruit. However, even very high K levels did not reduce YS to acceptable levels.