Why are the young top leaves of my pulse crop yellow while older leaves are still green?

When the youngest top leaves yellow first in pulses while old leaves stay green, the likely candidates are sulphur, iron or manganese, because all three are immobile and hit new growth first. In pulses, sulphur deficiency is a common one and gives a uniform pale yellow-green on the young leaves with thin plants and poor nodulation, while iron gives sharp green veins on a yellow new leaf, especially on high pH soils. Another thing to rule out in pulses: poor root nodulation can mimic nitrogen-type yellowing, so check if the roots have healthy pink nodules. Please confirm with your Soil Health Card or a soil test before treating, and make sure it is not waterlogging or root rot, which pulses are sensitive to. If sulphur is confirmed, gypsum or single super phosphate worked into the soil supplies it, and proper Rhizobium seed treatment plus a starter dose of phosphorus supports nodulation and overall vigour. If iron is the cause, a foliar ferrous sulphate spray around 0.5 to 1 percent helps. Adjust all doses to your soil test and local KVK recommendation and do not over-apply.