How can I be sure my crop's yellowing is a nutrient deficiency and not a disease or waterlogging?

This is the right question to ask before spending, because nutrient deficiency, disease and water stress can all yellow a crop and the wrong treatment wastes money. Use these checks. First, the pattern across the field: deficiencies usually appear in patches that follow soil type, such as the sandy or high lime parts, and they often spread evenly within a plant by leaf age, old or new. Waterlogging yellowing follows the low wet spots and the plants there are dull with dark unhealthy roots. Disease often spreads in a spot that enlarges over days, may have visible lesions, spots, mould, ooze or a foul smell, and can jump plant to plant. Second, the leaf age rule: a clean old-leaves-first or new-leaves-first pattern with green veins or uniform yellowing leans nutrient, while random spots, rings, mosaics or wilting lean disease or pest or virus. Third, dig and look at the roots, and check recent watering and rain. The dependable confirmation is still a soil or tissue test plus, if disease is suspected, a sample shown to your KVK or plant clinic. Please do that before applying anything. If a test confirms a deficiency, correct that nutrient per your Soil Health Card and local KVK in the recommended range and do not over-apply.