My paddy plants are stunted with brown rusty spots on lower leaves, which nutrient is missing?

Stunted dull paddy with brown to rusty spots on the older leaves is a common picture of either potassium deficiency or zinc deficiency, and both occur in rice, so they need separating. Potassium deficiency in rice shows yellowing and brown rusty spots starting at the tips and margins of the older lower leaves, with weak plants. Zinc deficiency in rice, often called khaira, usually appears two to four weeks after transplanting as brown rusty blotches on the lower leaves too, but with stunting, delayed tillering and a bronze cast, and it is very common on alkaline, flooded and high-organic soils. Dark unhealthy roots and waterlogging can worsen both and can themselves cause similar symptoms, so please rule out root problems and confirm with a soil test before treating. If zinc is confirmed, zinc sulphate applied to soil before transplanting, or a foliar zinc sulphate spray around 0.5 percent with a little lime on the standing crop, is the usual correction, in a soil-test-based range. If potassium is the cause, muriate of potash per your test is used. Follow your Soil Health Card and local KVK doses and do not over-apply.