Why are the lower leaves of my crop turning pale yellow first while new leaves stay green?

The most likely cause is nitrogen deficiency. Nitrogen is a mobile nutrient inside the plant, so when supply runs short the plant pulls it out of the old lower leaves and sends it to the young growing tips. That is why the bottom leaves go pale and yellow first while the top stays green, and the plant looks generally stunted with thin stems. A classic pattern in cereals is a yellowing that starts at the leaf tip of older leaves and runs down the midrib in a V shape. Before you spend on fertiliser, please confirm with your Soil Health Card or a fresh soil test, because the same yellowing can be caused by waterlogging, root damage, or simply a crop that has run out of feed at a heavy demand stage. Make sure it is not water stress or a root disease first. If a test or your card supports low nitrogen, a topdressing of a nitrogen source such as urea is the usual correction, applied in split doses and ideally with light irrigation so it reaches the roots. A common range is roughly 25 to 50 kg urea per acre per split, but adjust to your soil test, your crop, and your local KVK advice, and do not over-apply because excess nitrogen invites lodging, pests and pollution.