UPTAKE OF NUTRIENTS BY LETTUCE ON NFT WITH CONTROLLED COMPOSITION OF THE NUTRIENT SOLUTION

An automatically controlled NFT system was developed consisting of a growth unit and a measuring and control unit. A preliminary experiment with lettuce (Lactuca sativa, cv. Sitonia) was carried out. Unfortunately, the measuring and control system failed, so that manual control and dosage was necessary. The percentage dry matter of the heads decreased from 6.86 (week 1, 20/6/88) to 3.89 (week 6, 25/7/88). The shoot/root ratio increased from 0.50 (week 1) to 6.47 (week 6). The time courses of dry weight and cumulative uptake of macronutrients were well described by exponential equations. The cumulative uptake determined from the loss of nutrients from the nutrient solution exceeded the cumulative uptake determined from plant analysis. Cumulative uptake against cumulative daily radiation was well described by a logistic equation. Some constant plant uptake ratios between micro- and macronutrients were observed: Cl was related to N, K, P or Mg, and B to K. Dry weight as a function of transpiration was also well described by a logistic model, with an average dry mass production of 3.1 g per liter transpiration. Transpiration as a function of radiation was well described by a logistic model, with an average transpiration of 5.81 per unit (kJ cm-2 increase in radiation.