Dealing with Na accumulation in soilless systems with recirculation of drainwater : A case study with sweet pepper (Capsicum annuum)

Due to the small buffering, soilless systems are prone to salinity. Strict requirements for the quality of irrigation water have therefore been set, in particular if drainage water is being recycled. Since water qualities of resources do not always meet these requirements, a certain rate of leaching or discharge of the drainage is inevitable. The question is how this rate can be limited to avoid the loss of valuable water and nutrients. Evidently Na is the bottle-neck element, therefore we explored for various crops the very limits for the Na concentration in the root environment. The approach was to make use of the gap in the EC-value between the maximum recommended EC and the minimum required EC needed for the nutrients, and make use of this for Na accumulation. We also tried to increase the tolerance to high Na by adjusting the ratios of nutrients. The results for sweet pepper trial with increasing Na concentration up to 10 mmol L-1 in six treatments combined with a low and a high K/Ca ratio is worked out in this paper. No significant effects on yield or blossom end rot (BER) were found. The Na uptake concentration increased linearly with the Na in the root environment, though on a very low level. The K/Ca ratio did not affect the results, although at low K/Ca the K depletion in the root environment became serious. The results showed that the tolerance for Na worked out to be much higher than has been believed up till now and will make crop growing with zero liquid discharge possible. It also opens perspectives for soilless culture with water of lesser quality.