My crop has purple and reddish older leaves and stems, which nutrient is missing?

The most likely nutrient is phosphorus. When phosphorus is short, plants often show a purple or reddish-purple colour on the older leaves and stems because sugars build up and form purple pigments, and you usually also see stunted plants, delayed maturity and weak root systems. The purpling shows on older leaves first because phosphorus, like nitrogen, is mobile and moves to the young growth. One important caution: cold weather alone can cause temporary purpling even when soil phosphorus is fine, because cold roots cannot take up phosphorus well, and the colour fades as it warms. Purpling can also come from certain hybrids genetically, or from root damage. So please do not assume it is a deficiency from colour alone. Confirm with your Soil Health Card or a soil test before buying anything. If the test shows low phosphorus, the correction at sowing is a phosphatic fertiliser such as DAP or single super phosphate placed near the seed where roots can reach it, since phosphorus moves very little in soil. Doses depend heavily on your soil test value and crop, so follow your card and local KVK recommendation and do not over-apply, because excess phosphorus can lock up zinc and iron.