How to handle salt in a recirculating irrigation system?

One way to reduce water and fertilizer consumption and environmental pollution associated with ornamental crop production is to use a recirculating system, in which drainage water is captured, supplemented with fertilizer, and reapplied. However, growers who want to use a recirculating system face lots of questions about how to monitor salinity and specific ions (both nutrients and potentially toxic ions), how and when to add fertilizers, and how to manage flushing when necessary (among other issues). This article discusses research related to monitoring a recirculating nutrient solution and deciding how to manage it when salinity gets high.

Ideally, recirculating systems only require additions of fertilizer and water. In the less ideal world of the nursery or greenhouse operator, salinity usually builds up because some constituents of the water accumulate instead of being taken up by the crop. In such cases, the nutrient solution is recirculated until the total salinity or a potentially toxic ion reaches a maximum acceptable concentration. Then the solution must be partially or completely replaced. The water flushed from the system may require treatment to meet water quality standards before it is discharged.

Nutrients in a recirculating system are usually managed by measuring the electrical conductivity (EC) of the irrigation solution and adding fertilizer to restore an EC set-point. There are two problems with this. First, EC does not indicate the concentration of each nutrient, so the nutrient solution could become imbalanced. Second, sodium and chloride in the irrigation water may accumulate over time and contribute significantly to the EC, so nutrients will represent a declining proportion of the EC at the set-point.

An Italian research group has taken an interesting approach to solving these problems (Massa and others 2010). They studied the impact of three closed-loop nutrient solution strategies on tomato plants growing in rockwool slabs in a greenhouse. They started with well water that had a high sodium chloride (NaCl) concentration and an EC of 1.5 dS/m. They added a complete nutrient solution that raised the EC by about 1 dS/m.

Here's the setting. In Method A, they replaced crop water uptake with the full-strength nutrient solution in well water. They expected that the concentration of nutrients would remain relatively constant, and that NaCl would accumulate and raise the EC over time, so they would flush the system and start fresh when the EC reached 4.5 dS/m.

In Method B, they maintained a target nutrient solution EC of 3 dS/m. Since accumulating NaCl represented an increasing proportion of the total EC, they had to dilute the fertilizer concentration when they replaced crop water uptake with the nutrient solution. When the nitrogen concentration fell below 14 ppm, they flushed the system and started fresh.

Method C was similar to Strategy A, except that when the EC reached 4.5 dS/m they replenished the system with unfertilized well water. They continued this strategy until the nitrogen concentration fell below 14 ppm, then flushed the system and started over again.

In Method D, the experimental control, they irrigated with the nutrient solution without capturing and recirculating the drainage water, using a leaching fraction high enough to keep the drainage EC below 3.5 dS/m.

At the end of the experiment, Massa's group found that total plant growth and tomato fruit yields and quality were unaffected by irrigation management strategy. However, all three recirculation management methods used less than half as much water and fertilizer as the freely draining control method. Method B used the least fertilizer and Method C used the least water. The authors conclude by recommending the use of a recirculating system that sets a maximum acceptable EC and allows for temporary nutrient depletion.

However, it is important to note that these results probably depend on knowing the nutrient uptake characteristics of the crop. Massa's group had previously done careful experiments to establish the rates of nutrient and water uptake by the tomato cultivar they studied, and this information enabled them to establish appropriate concentrations of all nutrients in the fertilizer solution.

Without that information, the nutrient concentrations might have strayed from the desired values over time, and yields might have been different. It also stands to reason that managing a recirculation system used for growing a single crop would be much easier than one intended for a mixture of crops.

BY Richard Evans, Cooperative Extension Environmental Horticulturist, Department of Plant Sciences, UC Davis.