Effects of salinity on substrate grown vegetables and ornamentals in greenhouse horticulture

Since the mid 1970s substrate growing has become popular in the greenhouse industry in The Netherlands. Because of the small rooting volumes that are used in substrate growing, such systems require an accurate fertilization, but at the same time they offer possibilities for precise control and management of the conditions in the root environment.The osmotic potential of the substrate solution in the root environment is often used for improvement of the quality of the produce. For adequate management of the osmotic potential, firstly information about the absorption of water and ions by the crop is essential. Secondly, the effect of the osmotic potential and its interaction with climatic conditions in the greenhouse on crop development must be known. Thirdly, information on the spatial distribution of water and ions in the root environment should be available, because this may strongly affect salinity effects on plants.In studies on effects of low osmotic potentials on crops, both osmotic and specific ion effects should be distinguished. The osmotic effects predominate for most crops and growing conditions. Osmotic effects can be described according to the model developed by Maas and Hoffman. This model is characterized by two parameters, the salinity threshold value and the salinity yield decrease value. In this simple model the EC caused by nutrients is not taken into account separately, though nutrients have a significant effect on the EC of the substrate solution in greenhouse cultivation. So the model needs adjustment for the contribution of nutrients to the EC. Furthermore, effects of EC variations in time and space have been described.Fruit vegetables and cut flowers were used as test crops in experiments with different EC values in the root environment. Comparisons were made between EC effects caused by NaCl and by nutrients. Yield of tomato, cucumber, and sweet pepper were reduced at increasing EC, but most fruit quality characteristics were favourably affected. Blossom-end rot, however, increased with increasing EC. For sweet pepper this was especially the case after NaCl addition. Salinity threshold values for the vegetable crops varied between 2.3 and 3.5 dS m -1 and relative salinity yield decrease values between 2.3 and 7.6 % per dS m -1 . The flower weigths of gerbera, carnation, rose, aster, bouvardia and lily were negatively affected by increasing EC. Salinity threshold values ranged from 1.1 to 4.3 dS m -1 and salinity yield decrease values varied between 2.1 and 16.8% per dS m -1 . For aster such parameters could not be obtained, because the highest EC of 4.2 dS m -1 did not affect production. However, the regrowth of this crop after the first harvest was specifically strongly hindered by NaCl. Bouvardia also exhibited a specific sensitivity to NaCl. This effect was studied in more detail to obtain information about which ion, either Na or Cl, was responsible for this effect. The results showed that bouvardia was specifically sensitive to Na.The response of tomato and cucumber to an unequal distribution of nutrients and NaCl in the root environment was studied with plants grown in a split-root system. Tomato yield was determined by the EC value considered optimal for production if present in one of the rockwool cubes, despite the fact that the EC in the other cube was up to 10 dS m -1 . Tomato absorbed water preferably from the root part with the lowest EC and nutrients from the root part with the highest EC. When the EC in the root parts was raised by nutrients from low to standard values, the nutrient uptake by cucumber was highest from the parts with the highest concentration. In root parts with concentrations of nutrients > 4 dS m -1 the uptake decreased strongly. Nutrient uptake from one root part with high NaCl was also reduced when the NaCl concentration in the other part was low. When both root parts had high NaCl concentrations the plant was able to take up adequate amounts of nutrients. Like tomato, cucumber absorbed water preferably from the root part with the lowest EC. In case no nutrients were supplied in one root part, the water uptake from that root part was reduced.Interactions between salinity effects and climatic conditions and effects of temporal variation of salinity were studied with tomato as the test crop. High EC under low light conditions did not affect yields. In spring and summer yield reductions between 5 and 7 % per dS m -1 were found. In one experiment at very high humidity the yield reduction was about 10 % per dS m -1 . This was in contradiction with the nature of the interaction between salinity and climate in other studies. Obviously the calcium status of the plant had played a dominant role in this experiment. From the experiments with temporal variation of EC it could be concluded that for estimation of the yield reduction not only the lengths of the EC-intervals and the EC-level during the interval but also the light intensity during the interval has to be taken into account.The management of salinity in relation to nutrient supply was discussed. Nutrient absorption of greenhouse crops was studied by determining the total nutrient uptake and the nutrient uptake in relation to the water absorption (the so-called uptake concentrations). The widely published very low external concentrations to achieve optimal yields, are not realistic because of the high flow rate necessary to adequately supply crops with nutrients. External nutrient concentrations corresponding with 1.5 dS m -1 are required for sufficient nutrient supply to greenhouse crops.Required and acceptable external concentrations were defined considering the following aspects. Required external concentrations should not exclusively be related to a sufficient supply of nutrients in order to attain maximum growth or yield, but also to quality demands of the market. Acceptable concentrations should be considered with respect to maximum accumulation of residual ions to a level that does not negatively affect crop production and quality. In this way leaching and thus environmental pollution is minimized.In the assessment of required and acceptable concentrations osmotic and specific ion effects should be clearly distinguished. When no specific ion effects occur, the "space" between the nutrient concentration required for maximal production and the required concentration with respect to the produce quality or the acceptable concentration with respect to maximum salt accumulation can be filled up with any ion available in the system. With specific ion sensitivities the accumulation is restricted by the critical non toxic level to the crop.Required and acceptable concentrations of ions strongly depend on crop and growing conditions. Under cool and humid growing conditions, use of drip irrigation, and CO 2 supply, EC-values in the substrate solution between 3 and 6 dS m -1 seem to be realistic. Such conditions can be realised in greenhouses in North-West Europe from autumn until early spring. For summer conditions the EC-values suggested in this study between 1.5 and 3.0 dS m -1 are more realistic. In the interpretation of EC-values more credit should be given to the consequences of spatial distribution of ions in the substrate. The stable equilibrium established between low and high concentrated spots in the systems, offers excellent possibilities for an osmotic escape by plants. The discussion is concluded with some calculations of environmental pollution as a consequence of different management strategies of irrigation and drain-off.