Should I treat seed with a biofertiliser like Rhizobium or PSB and how?

Yes, biofertiliser seed treatment is cheap and helps, especially in pulses, oilseeds and legumes. Rhizobium fixes nitrogen from the air for legume crops (gram, soybean, groundnut, pulses), PSB (phosphate solubilising bacteria) makes soil phosphorus more available, and Azotobacter/Azospirillum help in cereals. Apply them last in the treatment order, after any fungicide and insecticide and after that coat has dried, because strong chemicals can kill the living bacteria. Make a slurry as per the pack, mix it gently and evenly over the seed, dry it in shade (never in direct sun, which kills the culture), and sow the same day. Use the right Rhizobium strain for your crop. Store the packets in a cool place and check the expiry date, because old or heat-damaged culture does not work. Follow the dose and method on the pack, and confirm the right strain and rate for your crop with your KVK.