Availability of iron in hydroponic nutrient solutions for rose crops

This study investigated the effect of light and agitation on the availability of iron in hydroponic nutrient solutions. In recycled hydroponic nutrient solutions used for the cultivation of roses, iron is added usually in the form of chelate fertilizers, such as Fe-EDDHA. In practice, the concentration of iron in the nutrient solution in those systems reaches low critical levels very rapidly tin less than 2 d). Fe-chelates are unstable in light and can form soluble or insoluble precipitates. Iron contained in soluble precipitates can be recovered in the solution by agitation. In contrast, iron contained in insoluble precipitates cannot be recovered. The tank of the closed-loop fertigation system used to grow a rose crop was filled with fresh nutrient solution every 10 d. Immediately after preparing each solution, four 11 plastic bottles were filled with the solution and kept in light or in the dark (two bottles of each), at the same temperature. For the next 10d the concentration of iron in the tank was measured every 2d, without any agitation before measurement, and in the bottles before and after agitation. Incident solar radiation in the greenhouse was recorded continuously. It was observed that absorption by the plants and light exposure did not affect the availability of iron in the nutrient solution. Ion concentration was not different between the samples kept in light and those kept in the dark. Agitation increased the quantity of iron available compared with the non-agitated solution. The increase was greater in the solution kept in light (+59%) than in the solution kept in the dark (+35%). The pattern of iron concentration in the recycled nutrient solution was similar to that measured without agitation in the solution exposed to light. It was concluded that in a closed hydroponics systems, agitation is very useful for keeping the concentration of available iron at acceptable levels for longer periods and thus minimizing further addition of iron, while exposure to light does not affect Fe availability.