Utilizing aqua ponics may allow brackish water aquaculture producers remediate wastewater while producing a marketable product. Kale (Brassica oleracea) has been shown to tolerate brackish water up to 15 ppt salinity, albeit with reduced growth and harvest weight. Many plants have established ranges of required nutrients for growth but in brackish water such ranges are unknown and uptake rates are likely impacted by the high levels of sodium present in the water. Some nutrients influence sodium tolerance, particulary transition metals like iron, zinc, and manganese. Increasing the levels of these nutrients may allow for increased plant production in brackish water . Selecting a form of each nutrient that is bio-available to the plant in brackish water conditions is also critical. This presentation covers two trials, one testing different iron containing compounds and the other differnet iron concentrations on kale performance in 15ppt brackish water.
The first study examined the effectiveness of three iron containing compounds on kale performance, along with an iron containing foliar spray made from fermeted dandilion (Taraxacum officinale) leaves. The iron compounds used were EDTA, iron sulfate (FeSu), and iron citrate (FeCi) and were added to each tank to reach 3mg/l total iron. Twenty four 18L tanks were assigned in triplicate to 8 treatments, 4 without a foliar spray: NoFe (No aqueous iron), EDTA , FeCi , FeSu ; and 4 with a foliar spray: NoFe/FS, EDTA/FS, FeCi/FS, FeSu/FS. All tanks contained four plants on a raft , were acclimated to the target salinity of 15ppt using a low cost salt mixture, and fertilized using a hydroponics fertilizer made without iron. The second study examined the effectivness of elevated levels of iron on kale performance. Identical systems as the first study were used with 12 tanks , 4 treatments , and 3 replicates each. The treatments were 0mg/l, 3mg/l, 6mg/l, and 9mg/l, representing the aqueous iron (EDTA) concentation. Water quality parameters (Temperature, O2, pH, Salinity, Total Ammonia Nitrogen, Nitrite, Nitrate, and Iron) were measured weekly in both studies. Harvest took place after 28 days in both studies , at which time plant performance metrics were measured (Height, weight, roots weight, CCI, etc.).
Results from the first experiment showed no significant differences in kale performance between the different iron compounds, however average kale harvest metrics were all slightly higher in the EDTA treatment. The foliar spray showed modest improvements when combined with aqueous iron citrate, but no difference in the other treatments. EDTA tanks maintained a higher and more consistant level of aqueous iron compared to iron sulfate and iron citrate. Results from the second study showed that iron levels of 3mg/l or higher were acceptable for kale at 15ppt salinity and only minor increases in average kale growth were shown at 6mg/l of aqueous iron. Further research will investigate other forms of chelated iron and their effects on kale performance, along with additions of other elements.