My crop is yellowing even though I applied urea, what else could be missing?

Yellowing does not always mean nitrogen shortage, so piling on more urea can be a waste or even harmful. Several things can cause yellowing: sulphur deficiency, iron deficiency (common on high-pH or calcareous soils, often showing on the youngest leaves), zinc deficiency, waterlogging or poor root function that stops the plant taking up nitrogen, or the urea you applied not reaching the roots because the soil was dry or it was lost to the air. The pattern helps: nitrogen and sulphur yellowing differ in which leaves are affected, and iron and zinc show their own patterns on younger leaves. The reliable way to know is to look at the leaf pattern with an expert and, where needed, get a soil or tissue test rather than guessing. Once you know the actual deficiency you can correct that specific nutrient, often with a targeted foliar spray for micronutrients. Take a photo and sample to your KVK for diagnosis, base corrections on your Soil Health Card or soil test, and do not just keep adding urea or over-apply.