Upcycling Orange Peel Agricultural Waste for the Preparation of Green Hydrogels as Active Soil Conditioners

In this work, agar (Aa)-based hydrogels were developed incorporating different concentrations of orange peel (OP) particles, pretreated using three different methods (hydrolysis, ultrasonication, or simple suspension in hot water), and 1.5 wt % limonene (LMN), to be tested as active soil conditioners for agriculture. The results of swelling of these hydrogels in distilled and tap water showed that acid hydrolysis or ultrasonication pretreatments of OP are counterproductive and that the simplest and lowest-energy method of OP pretreatment resulted in the most efficient hydrogels in terms of swelling. The incorporation of LMN into the Aa-based hydrogels increased their swelling capacity in tap water, while the incorporation of OP particles in the Aa+LMN hydrogels increased the number and decreased the size of their pores but did not significantly influence their swelling capacity, which ranged from 650 to 700% after 4 h of immersion. The hydrogels developed here were prepared with up to 50 wt % OP particles due to the increased water solubility for higher concentrations. The biopesticide activity of the hydrogels was investigated against the bacterium Pseudomonas syringe pv tomato DC3000 (Psy) and the necrotrophic fungus Fusarium solani f. sp. eumartii (F. eumartii), while the ability of the hydrogels as soil conditioners to retain soil moisture under simulated drought conditions was exploratorily investigated in tomato plants. The results demonstrate that even if the hydrogels did not show biopesticide activity against the studied microorganisms, the addition of 2.5 wt % hydrogel to the substrate significantly increased the moisture content of the soil and consequently improved the relative water content of tomato leaves under drought stress.