What is a sensible whole-season fertiliser plan so I stop guessing every year?

A good plan removes the guesswork and usually cuts cost. Build it in clear steps. Step one, test your soil and get a Soil Health Card, so you know what is actually low, medium or high, instead of assuming every nutrient is short. Step two, decide the crop and its nutrient need, then take the card's crop-wise recommendation as your target quantities of nitrogen, phosphorus, potash and any micronutrients. Step three, plan placement and timing: put the immobile nutrients (phosphorus and most of the potash) and a starter dose of nitrogen into the soil at sowing, ideally drilled or banded near the roots, not broadcast on the surface. Step four, split the remaining nitrogen into 2 or 3 top dressings tied to the crop's growth stages, applying each to moist soil or before irrigation in the cool hours. Step five, correct any specific deficiency the card flags (zinc, sulphur and others) with the right product and method, keeping zinc and phosphorus from being banded together. Step six, build soil organic carbon over years with FYM, compost, residue and biofertilisers, so the soil itself supplies more and you depend a little less on bags. Step seven, write the plan down with quantities and dates and tick off each application so you never double up. Convert the card numbers into actual bags with your KVK or block officer, base everything on your soil test, and do not over-apply any nutrient.