Deliverable 9. Cascade cropping  systems as a tool for increasing water use efficiency in protected horticulture (UNIPI)

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In the last years, the spread of soilless culture techniques has been increasing in the Mediterranean countries with the main purpose of saving labour and avoiding soil-borne diseases. It is well established that for reducing environmental impact and for saving water and fertilisers, the closed-loop system should be adopted. Indeed, in some countries countries, such as The Netherlands and France, the law has imposed at the growers to recycle the drainage solution (van OS 1998).

In closed-loop hydroponics the control of crop mineral nutrition generally relies on the recirculation of a feeding solution with relatively high nutrient concentration, that is maintained by injecting stock-solutions on the basis of the readings of electrical conductivity (EC). This procedure is efficient only when it is available an irrigation water with low concentration of ions that are not or scarcely absorbed by the crop, such as Na and Cl.

On the contrary, when a saline water is used, as it occurs frequently in the Mediterranean countries, there is a build-up of salts and this makes it necessary to replace more or less frequently the solution with consequent waste of water and fertilisers. Generally, the nutrient solution is completely or partially replaced when the values of EC and/or the concentration of ions such as Na or Cl exceed a given threshold, which is dependent on crop tolerance to salinity.

The amount of nutrient solution run-off is related the crop transpiration and to the water quality; in particular, the use of rain water or reverse-osmosis purified water can reduce hardly the need of leaching. Nevertheless, in Mediterranean areas rainfall is quite scarce and reverse osmosis treatment is not economically sustainable due to the small dimensions of farms.

Drainage solution not only contains non-essential ions, but also nutrients that may be exploited by other crops. Indeed, the practice to feed soil cultures with drainage water is quite common. This paper illustrates the possible application of cascade cropping system, performed with crops of different salt tolerance grown in semi-closed hydroponic systems, in order to reduce the environmental pollution related to the production of drainage exhausted nutrient solution.