Alginate-starch hydrogel capsules with hydrolysed anaerobic digestate for controlled micronutrient fertilisation

Sodium alginate was used as the network-forming biological macromolecule in hydrogel capsules with corn starch and hydrolysed anaerobic digestate (AD). The formulation series was used to determine how AD and starch contents affected capsule integrity, mechanical strength, swelling, micronutrient sorption, and release. AD contents above 40% caused capsule disintegration, whereas increasing starch content from 0 to 10% increased the maximum compression force from 34.6 ± 1.7 to 61.4 ± 3.1 N and the equilibrium swelling degree from 1.52 ± 0.07 to 7.44 ± 0.13 g/g. The selected ALG2/S10/H40/D formulation was subsequently loaded with Cu, Mn, and Zn by sorption. Equilibrium data were best described by the Sips model for Cu and Mn and by the Langmuir model for Zn, with maximum sorption capacities of 3.04, 1.29, and 2.63 mg/g, respectively. Release in 1% NaNO₃ was burst dominated and approached an early plateau, indicating strong micronutrient retention within the alginate-starch matrix. Preliminary pot trials with cucumber showed element-specific responses, with Cu and Zn treatments reducing stem length, whereas Mn treatments did not. Overall, the results demonstrate formulation-level composition-property relationships in an alginate-starch macromolecular matrix containing hydrolysed AD and support its potential as a biopolymer-based carrier for Cu, Mn, and Zn fertilisation.