The role of chemical amendments on modulating ammonia loss and quality parameters of co‑composts from waste cocoa pods

Authors

• Rokotamana Vitinaqailevu ¹
• B. K. Rajashekhar Rao ¹

• Department of Agriculture, The PNG University of Technology, Lae, Papua New Guinea

Abstract

Purpose Nitrogen loss as ammonia (NH₃) in a waste cocoa pod co-composting process was determined following the addition of three potential chemical amendments that could lower NH₃ loss. Further, amended compost products were assessed for quality and possible phytotoxicity in a pot experiment.
Method Pod borer damaged cocoa pods were co-composted in lab-scale vessels under ambient conditions with double superphosphate, phosphoric acid, and gypsum at 2%, 5% and 1% of fresh weight, respectively. NH₃ emitted during composting was trapped in standard H₂SO₄ and determined titrimetrically. Composting mixtures were sampled on days 0, 7, 56 and 90 to monitor pH, total C, total N, and C:N ratio. Mature co-composts were incorporated to the soil at 20 t ha⁻¹ and growth response of Chinese cabbage plants (Brassica rapa chinensis) assessed for the phytotoxicity.
Results The chemical amendments were ineffective (p > 0.05) in preventing loss of NH₃ but the quality of composting mixtures were significantly (p < 0.0001) improved. The manure compost generated had acceptable parameters (C, 31.2–31.9% and C:N ratio, 9–10:1) and were free from phytotoxicity for soil application as revealed by the response of Chinese cabbage crop.
Conclusion Chemical amendments used in the co-composting process mainly modulated pH without any detrimental effects on quality. The results also demonstrated that the NH₃ loss is insignificant from the cocoa waste-composting process. Further studies are required with increased rates of acidifying amendments like phosphoric acid to further augment the quality of cocoa pod composts.

Keywords

• Phytotoxicity
• C:N ratio
• Cumulative NH3 loss
• Volatilization