Effect of the nutrient solution potassium concentration on tomatoes grown on new or reused pumice

Tomato plants were grown over a 5 month period (from October till February) under unheated glasshouse conditions in double-faced (black and white) polyethylene bags containing 20 1 of pumice, new or reused. On each bag two tomato plants were placed, while the bags were laid in paired rows giving a plant density of 2.7 plants/m(2). Complete nutrient solution with three modified levels of potassium concentration (K-1=200ppm, K-2=290ppm and K-3=340ppm) were applied. Each potassium treatment was applied on 36 bags containing pumice as new material and on 36 bags with pumice used for second consecutive tomato cultivation period. Total yield/plant was affected by K concentration only for the reused pumice plants, while the new material gave considerably higher yield than that of the reused one only for the higher potassium treatments (K-2 and K-3). The yield of first class fruits (over 150g of weight) increased significantly with increasing K concentration for the new substrate plants, while the opposite was found for the reused ones. Early yield increased with increasing solution K concentration, but only for the new substrate plants.