Modern intensive agriculture leads to significant decline in soil K status due to crop removal without compensating through K fertilizer. Most of the K ores suitable for commercial K fertilizer production are distributed in few countries in the northern hemisphere (Canada, Russia, Belarus and Germany). India is completely dependent on foreign countries since its whole consumption of K fertilizer is imported. So, self-sufficiency in commercial K fertilizer is a major issue in developing countries like India. In this context, alternative sources of K have a promising future in the developing countries where commercial K fertilizers are imported for crop production. There are some low-grade but indigenous resources of K-bearing minerals which can be exploited as an alternative of the expensive imported K fertilizers. Direct application of these indigenous K minerals as well as bio-activation through potassium solubilizing microorganisms could be a quite promising K source. In this chapter, we have discussed the scope of naturally occurring K minerals (low-grade K minerals, silicate minerals and greensand) and indigenous sources of K (crop residue, manures, wood ash and seaweed) as well as K biofertilizer as a potential substitute of commercial K fertilizer for sustainable agriculture in the developing countries.