My maize lower leaves are yellowing in a V from the tip down the midrib, which nutrient?

A yellow V starting at the tip of older maize leaves and running back down the midrib is the textbook sign of nitrogen deficiency in maize. Because nitrogen is mobile, the plant strips it from the oldest leaves first, and in maize that withdrawal forms the characteristic tip-to-midrib V, with the rest of the leaf and the younger leaves staying greener. You usually also see stunted, pale, spindly plants. Please confirm with your Soil Health Card or a soil test before topdressing, and check it is not waterlogging or a poor root zone causing the plant to be unable to take up the nitrogen that is there. If low nitrogen is confirmed, the correction is a topdressing of a nitrogen source such as urea, given in splits at key growth stages with light irrigation so it reaches the roots. A typical split might be in the range of roughly 25 to 50 kg urea per acre, but adjust to your soil test, the maize stage and your local KVK recommendation, and do not over-apply, because excess nitrogen causes lodging and pollutes groundwater.