Plant Biomass Ash and Nitrogen Fertilization Raise the Soil pH, SPAD Index and Growth of Urochloa brizantha

Tropical pastures form the basis of livestock production in Brazil; however, their productive potential is limited by soil acidity and low nutrient availability. The objective of this study was to evaluate the interactions between biomass ash and nitrogen application rates on the soil pH, SPAD index, leaf area, shoot dry mass, and root dry mass of Marandu grass (Urochloa brizantha cv. Marandu) grown in an Oxisol. A randomized block design was used in a 5 × 5 factorial scheme, consisting of five ash rates (0, 10, 20, 30, and 40 g dm−3) using Eucalyptus-derived plant biomass ash and five nitrogen rates (0, 100, 200, 300, and 400 mg dm−3), with four replications. The treatments with ash significantly increased the soil pH, whereas the nitrogen treatments promoted leaf expansion and increased the SPAD index. Significant interactions were observed for the SPAD index, leaf area, and shoot biomass. The results indicated that plant biomass ash acted effectively as a soil acidity corrector, increasing the soil pH from 4.3 to a maximum of 6.8. The root dry mass increased independently with ash application up to 28 g dm−3 and with the nitrogen supply up to the estimated dose of 222 mg dm−3. The data were subjected to statistical analysis using R software version 4.1.2. Plant biomass ash corrects soil acidity and positively affects nitrogen fertilization to enhance the morphophysiological development and root growth of grass, indicating that it is a sustainable fertilizer for pasture management.