Closed water loop in greenhouses : effect of water quality and value of produce

An increasing fraction of protected cultivations is on systems where drain water is recollected and re-used for irrigation (closed systems). Such systems have a direct reward for growers in saving on fertilizers (and water) costs and in a higher value of the product, caused by a better control of the root environment. In view of the benefit of the prevention of leaching of chemicals in the environment, the application of closed irrigation systems is being increasingly made compulsory. However, when irrigation water contains non-nutrient salts, such salts will accumulate in the closed loop. Since a high salt concentration is known to cause yield loss (though the effect is specie-specific), first or later salts will have to be leached out, by draining the loop. Indeed, even in Holland (where environmental rules are possibly the strictest) growers are allowed to leach the system whenever a specie-specific ceiling of sodium concentration is reached. It can be shown that in a crop cycle, the fraction of water that is leached is nearly proportional to the ratio between the concentration of the critical salt of the irrigation water and the concentration at which the system is leached. In principle, therefore, the ¿optimal¿ EC-ceiling can be calculated, that balances marginal costs of water and fertilizers with marginal yield loss. By using a number of yield response curves, in a couple of different cases (Holland and Mediterranean basin), we show that, with realistic prices both of resources and of produce, the optimal EC is very near to the value that ensures maximal yield. That is, there is no advantage to the grower in maintaining a closed loop whenever the quality of irrigation water is poor. Therefore, closed systems are financially viable only in two cases: a. in regions with good water or b. with high-value crops that offset the costs of ensuring good water (such as rain collection or desalinization). We will show that the latter is the case in most protected cultivation in Europe. On the other hand, there is no way that low-value crops in poor-water-quality regions may still be profitable under stricter environmental rules. This means that local authorities, seriously planning to enforce such rules, should either provide incentives for growers to switch to less sensitive or more valuable combinations of crops, or contemplate developing other economic activities than agriculture