This study explores the potential use of extracellular polymeric substances (EPS), commercially known as Kaumera Nereda Gum®, extracted from aerobic granular sludge (AGS), for foliar fertilization in agriculture. Koppert, a company specializing in sustainable agricultural products, is interested in using Kaumera Nereda Gum® for its biostimulating effects, water-absorbing capacity, biofilm formation, and adhesive properties. They are currently spraying leaves with a solution mixed with Kaumera Nereda Gum®, water, and other fertilizers.

Nonetheless, the utility of using Kaumera Nereda Gum® for foliar fertilization can be affected by several factors, such as seawater intrusion in AGS projects like Faro-Olhão in Portugal and the type of acid utilized for the precipitation of the polymer. In certain places like Utrecht, hydrochloric acid is used, whereas in Faro, sulfuric acid is employed. The latter acid is more favorable when considering agricultural applications.

Thus, the chemical composition of Kaumera samples from Faro and Utrecht was compared, and their suitability for foliar fertilization assessed. Parameters such as total solids, volatile solids, carbohydrate content, and protein content were quantified, and FTIR-ATR analysis was conducted to gain a better understanding of each polymer. Conductivity and pH levels were also measured and compared to expert recommendations against the ideal values for foliar fertilization. Additionally, nutrient levels were quantified and compared to regulatory guidelines and nutritional recommendations. An adhesion protocol was also designed to compare the adhesive properties between Kaumera Utrecht, Kaumera Faro, and Kaumera Zutphen (established benchmark).

Results indicated that Kaumera Faro has 0.114 ± 0.005 grams of carbohydrate per gram VS, whereas Kaumera Utrecht has 0.168 ± 0.002 grams of carbohydrate per gram VS (47% more). On the other hand, Kaumera Utrecht has 0.323 ± 0.009 grams of protein per gram VS, while Kaumera Faro has 0.456 ± 0.004 grams of protein per gram VS (41% more). High carbohydrate content could be positively correlated with strong adhesion properties, whereas a high protein content might demonstrate enhanced biostimulating effects. In addition, the pH and salinity adjustments required for applying the Kaumera solution to leaves are determined by its mixing ratios. When preparing the solution, it is essential to consider the low pH of Kaumera Nereda Gum® and evaluate whether washing the polymer to reduce salinity is necessary. Heavy metal concentrations in samples of Kaumera Utrecht and Kaumera Faro remained within permissible limits according to regulatory thresholds when considering the applied dosage of 35 liters per hectare per year (as suggested by Koppert). However, with such dosage, nutrient levels in the Kaumera samples were lower than recommended, confirming that additional fertilizers are to be mixed with Kaumera Nereda Gum®. Furthermore, the adhesion protocol proved to be a valuable screening tool, showing that Kaumera Utrecht has superior adhesive properties compared to the benchmark. In summary, this study confirms that Kaumera Utrecht and Kaumera Faro exhibit advantageous properties in foliar fertilization, but their effective use requires pH adjustment, salinity reduction, and nutrient supplementation.