Encapsulated and nano-organic fertilizers for water retention and controlled nutrient release.

Abstract

Declining soil fertility, low nutrient-use efficiency, and heavy dependence on imported synthetic fertilizers remain critical constraints to agricultural productivity in Uganda. This research developed and evaluated advanced organic fertilizers derived from biochar-blended compost (BBC), specifically focusing on two engineered derivatives: Encapsulated Biochar-Blended Compost (EBBC) and Nano-Biochar-Blended Compost (Nano-BBC). An optimized co-composting matrix (60% Tithonia diversifolia and 5.7% rice husk biochar) was established using Response Surface Methodology and Central Composite Design. The quadratic models developed for nitrogen, phosphorus, and potassium were highly significant (F-values of 33.70, 50.64, and 86.60, respectively) and exhibited a non-significant lack of fit (p < 0.05). Model robustness was confirmed by high coefficients of determination (R2 ≥ 0.97) and adjusted R2 ≥ 0.94, and low coefficients of variation (3.24%–6.24%), indicating high reproducibility. Tithonia diversifolia most influenced N and K enrichment, while P availability depended on quadratic effects of both substrates. The enriched mature compost served as the base for enhancements. Nano-BBC synthesis was optimized via high-energy ball milling, and a reduced quadratic model identified the milling solvent mass and ball-to-powder ratio as key factors for particle size reduction. Chitosan–starch biopolymer encapsulation further enhanced performance. Under simulated 20-mm rainfall, EBBC reduced leachate volume to 6.5 mL (65% less than conventional BBC and mineral fertilizers) while eliminating nitrate-N leaching. Nitrogen-release assays showed controlled release: EBBC pellets released 56.9–70% of total N over 30 days via Fickian diffusion, unlike uncoated BBC, which exceeded 100% by day 25 via non-Fickian kinetics. EBBC also improved soil moisture retention in sandy loam to 4.4% at 30-days via hydrogel effects. In semi-field Zea mays L. (Maize) trials under drought, EBBC produced the highest plant height and shoot biomass, outperforming BBC, Nano-BBC, and synthetics. All formulations met FAO/EU heavy-metal thresholds. This scalable, climate-smart framework transforms organic waste into high-performance fertilizers, synchronizing nutrient delivery with drought resilience in Uganda.