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: Recirculation (closed) type hydroponics, despite its comparative eco-friendliness and cost-effectiveness, is less popular in greenhouse agriculture due to the need for close monitoring of nutrient availability within the fertigation cycle. As a practicable and low cost measure, the uptake volume based supplementation of plant nutrients at high concentrations was studied in tomato ( Solanum lycopersicum L.) plants at different stages of maturity using the nutrient film technique (NFT). The plants of four age groups or maturity stages (MS), 5, 4, 3 and 2 weeks old at transplanting (MS1, MS2, MS3 and MS4, respectively), were grown in a complete hydroponic solution (Enshi), keeping the electrical conductivity (EC) set-point at 1.8 dS m-1. EC of the stock solution was maintained at 1.6 dS m-1 until the flowering stage and then changed to 1.1, 1.2, 1.6 and 1.7 dS m-1 in treatments MS1 through MS4 respectively. Young leaves were removed in MS1 while old leaves were removed in MS4 during the last two weeks to create substantially different nutrient demands among the MS treatments. The depleted volume of the supply solution was supplemented daily during the 1-2 week recirculation cycles. EC of the supply solution was monitored and the pH was adjusted to 6.5 daily. EC did not change appreciably with relatively matured stage transplants (MS1 and MS2) at the beginning due to high nutrient and water uptake rates, while in relatively young stages (MS3 and MS4) uptake concentrations (except for sulphur) were low. Under different water uptake rates, as determined by the prevailing evaporative demand, plants in all MS treatments regulated the uptake rates of macro nutrients (me day-1) by adjusting respective uptake concentrations (me L-1). Time of reaching reproductive stage and growth rate were not affected by MS at transplanting or variable EC management. Hence, N, P, K, S, Ca and Mg uptake rates of NFT grown tomato are basically governed by water uptake rate and further regulated by the uptake concentration. Keywords : Electrical conductivity, nutrient film technique, nutrient uptake, recirculation hydroponics system, water uptake. Doi: 10.4038/jnsfsr.v39i2.3174 J.Natn.Sci.Foundation Sri Lanka 2011 39 (2):139 - 147

Plant nutrient uptake in recirculation culture of tomato under growth stage based electrical conductivity adjustments