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

In this work, agar (Aa)-based hydrogels were developedincorporatingdifferent concentrations of orange peel (OP) particles, pretreatedusing three different methods (hydrolysis, ultrasonication, or simplesuspension in hot water), and 1.5 wt % limonene (LMN), to be testedas active soil conditioners for agriculture. The results of swellingof these hydrogels in distilled and tap water showed that acid hydrolysisor ultrasonication pretreatments of OP are counterproductive and thatthe simplest and lowest-energy method of OP pretreatment resultedin the most efficient hydrogels in terms of swelling. The incorporationof LMN into the Aa-based hydrogels increased their swelling capacityin tap water, while the incorporation of OP particles in the Aa+LMNhydrogels increased the number and decreased the size of their poresbut did not significantly influence their swelling capacity, whichranged from 650 to 700% after 4 h of immersion. The hydrogels developedhere were prepared with up to 50 wt % OP particles due to the increasedwater solubility for higher concentrations. The biopesticide activityof the hydrogels was investigated against the bacterium Pseudomonassyringe pv tomato DC3000 (Psy) and the necrotrophic fungus Fusarium solani f.sp. eumartii (F. eumartii), while the ability ofthe hydrogels as soil conditioners to retain soil moisture under simulateddrought conditions was exploratorily investigated in tomato plants.The results demonstrate that even if the hydrogels did not show biopesticideactivity against the studied microorganisms, the addition of 2.5 wt% hydrogel to the substrate significantly increased the moisture contentof the soil and consequently improved the relative water content oftomato leaves under drought stress. Soil conditionersprepared with agar, orange peel particles,and limonene improve soil moisture and plant water content under droughtstress.