The inefficient use of nitrogen is due to the loss of nitrogen (N) in the soil caused by a range of mechanisms, including the volatilization of ammonia, the leaching of nitrates, the runoff of soil, and autotrophic reactions. In this work, an effort was made to keep nitrogen from being lost during the urea treatment by making use of frass produced by black soldier fly larvae (BSFL) on decanter cake (DC) and palm kernel expeller (PKE). Thirty days of soil incubation was carried out to study the effects of varying concentrations of BSFL on the amount of nitrogen retained from varying concentrations of urea. In the incubation of the soil, the treatments were as follows: 300 g of soil only (T0), 300 g of soil + 4 g of urea (T1), 300 g of soil + 40 g of frass (T2), 300 g of soil + 30 g of frass+ 3 g of urea (T3), and 300 g of soil + 20 g of frass + 2 g of urea (T4). The treatments were organized using a complete randomized design (CRD), and each replication was performed three times. Incorporating BSFL frass into the soil increased pH, total carbon, organic matter, available nitrate and exchangeable ammonium, and cation exchange capacity in the soil. This is due to BSFL frass being rich in base cations. Amending urea with BSFL frass significantly increased soil cation exchange capacity (CEC) and pH because high pH contributes to a low concentration of H+ ions in the soil and increases the possibility of NH4+ cations binding on the negative sites on the surface of BSFL frass. In conclusion, pH was an important regulator of net nitrification during BSFL composting.
