The addition of wood bottom ash to an agrowaste anaerobic digestate has been proposed for improving the carbon to nutrient ratio of the soil organic amendment (C/N/P < 100/10/1). The ash-based treatment also aimed to improve properties of the anaerobic digestate as controlled-release fertilizer by decreasing the availability of nitrogen, carbon, and phosphorus. Treated (39.86 ± 0.94 g) and untreated (33.51 ± 0.84 g) digestates were incubated for 7 h at 20 °C and 100 rpm, in a 250-mL chamber with a 0.11 M sulfuric acid trap of 4.38 ± 0.02 mL. By the end of the incubation, the blend of 13.12 ± 2.87% ash and 86.88 ± 2.87% digestate released 8.95 ± 2.19 times more ammonia than the digestate alone. This was in agreement with the lower content of water-soluble ammoniacal nitrogen in the treated digestate (93.35 ± 21.00 mg/kg) than in the digestate incubated without the addition of the ash (357.64 ± 54.10 mg/kg). As per the very low levels of water-soluble nitric nitrogen found in the treated (0.16 ± 0.05 mg/kg) and the untreated (0.06 ± 0.02 mg/kg) digestate, the emissions of nitrous oxide were discarded. The availability of carbon increased 1.43 ± 0.82 times and the availability of phosphorus decreased approximately 100 times due to the addition of the ash to the digestate. The ash-based treatment was successful in improving the C/N/P of the digestate from 121.51 ± 57.18/9.94 ± 4.21/1 to 42.05 ± 9.88/2.48 ± 0.58/1.