If cost is truly zero, consider these legally free soil microbiology resources (high quality, peer-reviewed):

These won’t replace Subba Rao’s unique focus on tropical and Indian agronomy, but they can supplement your learning.

While not a full download, Google Books often previews the first 50–80 pages of the 5th edition. This is enough to read key sections on soil biota, carbon cycle, and microbial interactions. Amazon’s “Look Inside” feature also gives you high-quality image renders of tables and figures.

The paperback 5th edition is reasonably priced in South Asian markets. You can find legitimate used copies for as low as $8–$12 USD on:

Search for ISBN 978-8120416558 to ensure you get the correct edition.