Yellowing between veins on the OLDER lower leaves while veins stay green, which nutrient is short?

Interveinal yellowing on the older lower leaves, with green veins and sometimes reddish or purple tints later, most likely indicates magnesium deficiency. Magnesium is mobile, so unlike iron it shows on the old leaves first, not the new ones. That is the key separation: iron chlorosis is on the youngest top leaves, magnesium chlorosis is on the oldest bottom leaves, and both have green veins on a yellow leaf. Magnesium shortage is common on light sandy acidic soils, on soils that have had heavy potassium application, and under heavy leaching. Please confirm with your Soil Health Card or a soil test before treating, since the pattern can overlap with other issues. A quick correction on a standing crop is a foliar spray of magnesium sulphate (Epsom salt), often around a 1 to 2 percent solution sprayed two or three times on the affected leaves. For longer term correction, magnesium can be added to soil through magnesium sulphate, or dolomite where the soil is also acidic and needs liming. Adjust doses to your soil test and local KVK advice, and avoid over-applying potash on these soils, because that can worsen magnesium uptake.