313 Nutrient Concentrations of Plant Tissues of Greenhouse Crops as Affected by the EC of the External Nutrient Solution C. Sonneveld Nijkerk The Netherlands W. Voogt Wageningen UR, Applied Plant Research Division Glasshouse Horticulture Naaldwijk The Netherlands Keywords: EC, nutrient uptake, osmotic effects, osmotic adjustment, uptake concentration Abstract This paper reports osmotic effects from a series of experiments where green- house crops were grown in substrates with different EC values of the nutrient solu- tion. Results of yield characteristics were published elsewhere: in the present paper, the nutrient uptake in relation to the EC will be discussed in the light of growth of lily, lettuce and kohlrabi. The crops were grown in substrates within a circulation system and thus, the water absorption of the crops could be precisely determined. The nutrient absorption was determined by tissue analysis. For lettuce whole heads were sampled and analysed; for lily, bulbs and leaves and for kohlrabi, tubers and leaves were sampled and analysed separately. The results showed a strong increase of plant nutrient concentrations in the EC-domain until the optimum growth response of the crops. Higher EC values did not affect the plant nutrient dry matter concentrations seriously, except for K concentrations of the lily crop. Calculations showed that the K concentration in the plant sap played an important role in the osmotic adjustment of the different crops to high external concentrations. The uptake concentrations of the different crops were presented and discussed in relation to the EC value in the external solution. External nutrient concentrations and plant nutrient concentrations in relation to optimum yield are presented. Calculations are made of the nutrient efficiency of a system with drainage to waste compared with a system with reuse of drainage water. INTRODUCTION Results of a study of osmotic effects on greenhouse crops grown in substrate (Sonneveld et al., 2004) outlined the relationships between the yields of a series of crops and osmotic potentials in the external solution. In the experiments, differences in the osmotic potential of the external solution were achieved by addition of nutrients. In the present paper results of nutrient uptake in three crops, in relation to the osmotic potential of the external solution, will be discussed. These include a tuber crop, a leaf crop and a bulb crop, represented by kohlrabi, lettuce and lily, respectively. METHODS AND MATERIALS The crops were grown in sand and as well in granulated rock wool substrates placed in a 0.15 m thick layer in basins with a size of 0.8 m x 1.6 m. Nutrient solutions of different concentrations were prepared and added with the aid of a sprinkler system. The irrigation time, the quantity of water and the concentration of nutrients in the water were varied according to the crop, the growing conditions and the osmotic potential aimed at. The leaching fraction in the growing system was focused on 0.25 and 0.50 and the drainage water was reused in the system. Six treatments were laid out in four parallel-randomized blocks in which the EC in the substrate solution roughly varied between 1 and 8 dS m -1 . The composition of the nutrient solutions used was tuned to the need of the crop according to the recom- mendations for growers (Sonneveld and Straver, 1994). The different EC values were achieved by addition of various amounts of major elements. NH 4 and P, however, were kept constant, since NH 4 may influence the pH of the nutrient solution (Sonneveld, 1991) Proc. IS on Growing Media Ed.: J.-C. Michel Acta Hort. 779, ISHS 2008