The emergence of high-throughput, long-read DNA sequencing has enabled recovery of microbial genomes from environmental samples at scale. However, expanding the terrestrial microbial genome catalogue has been challenging due to the enormous complexity of these environments. Here we performed deep, long-read Nanopore sequencing of 154 soil and sediment samples collected during the Microflora Danica project, yielding genomes of 15,314 previously undescribed microbial species, recovered using our custom mmlong2 workflow. The recovered microbial genomes span 1,086 previously uncharacterized genera and expand the phylogenetic diversity of the prokaryotic tree of life by 8%. The long-read assemblies also enabled the recovery of thousands of complete ribosomal RNA operons, biosynthetic gene clusters and CRISPR-Cas systems. Furthermore, the incorporation of the recovered genomes into public genomic databases substantially improved species-level classification rates for soil and sediment metagenomic datasets. These findings demonstrate that long-read sequencing allows cost-effective recovery of high-quality microbial genomes from highly complex ecosystems, which remain an untapped source of biodiversity.