Potassium Requirements for Optimum Yield and Fruit Quality of Mature-Green Tomatoes in Florida.

Potassium soil test based fertilizer application is essential in Florida sandy soils with exchangeable soil K ranging from very low to medium. Current UF/IFAS K₂O fertilizer recommendations for tomatoes ranges from 0 to 225 lb/acre for soils testing ‘very high’ to ‘very low’ by ‘Mehlich-1’plus supplemental K₂O fertilizer applications in specific situations, respectively.  However, based on a survey, is not uncommon for tomato growers to use as much as 450 to 650 lb/K₂O/acre.  Therefore, the objectives of this study was to evaluate the effect of K rate on tomato petiole sap, plant biomass, K uptake, yield and fruit quality on spring tomatoes grown in seepage irrigation in a randomized complete block design with four replications. Two fertilizer trials were conducted in the spring of 2010 (very low K) and 2011 (medium K).  Eight fertilization rates were applied ranging from 30 to 930 lb/K₂O/acre.  Data collection consisted on petiole sap, yield, plant biomass and K content, K soil and postharvest quality as internal white tissue (IWT).  Plant petiole sap was analyzed by ANOVA and mean separation by Duncan’s multiple range tests. A yield, plant biomass and K-uptake, and K soil was analyzed by four models linear, quadratic models, linear- plateau and quadratic-plateau. Maximum yields were determined at the intersection of the quadratic and plateau. Based on one year of data in very low soil test K, tomato petiole sap K concentrations were below UF/IFAS sufficiency levels at K₂O rates lower than 360 lb/acre.  Similarly, plant biomass accumulation and tomato yield production increased with added fertilizer K₂O rate to 380 lb/acre. However, the study with medium soil test K indicated no response to added fertilizer K₂O rate. These results would indicate that further K rate studies with seepage irrigation are warranted.