The edges of my crop's older leaves are scorched and browning like a burn, which nutrient?

Marginal leaf scorch on older leaves, where the edges and tips go yellow then brown while the centre stays green, most likely points to potassium deficiency. Potassium is mobile, so the symptom appears on the older lower leaves first, and you often also see weak stems that lodge, poor grain or fruit filling, and lower disease resistance. The scorched margin pattern is the key clue. Before correcting, please confirm with your Soil Health Card or a soil test, because salt injury, drought scorch and some leaf diseases can look similar, and you should rule out water stress along the field edges. If the test confirms low potassium, the usual correction is muriate of potash (MOP) or sulphate of potash applied to soil, and a foliar potash spray can help a standing crop in mild cases. Soil doses depend strongly on the test value and the crop, since some crops are heavy potassium feeders, so follow your Soil Health Card and local KVK recommendation rather than a fixed number, and do not over-apply, because excess potassium can interfere with magnesium and calcium uptake.