Cucumber leaves yellowing between the veins in bagged potting mix Help My Plant

Check the pH of what you are watering with too. Very alkaline tap water in a container locks up magnesium and iron even when both are present, and you can feed forever without fixing it.

Yellow between green veins starting on old leaves and moving up is classic magnesium, and in a container of bagged mix six weeks in with daily watering it is completely expected: the nutrients that came in the bag are exhausted at around six weeks and magnesium leaches quickly. Epsom salts at about a tablespoon per five litres of water, applied as a drench, will show improvement in a week. But treat the underlying cause too, which is that you have not fed a heavy feeder in a 40 litre pot for a month and a half. A balanced liquid feed weekly from now on.

Worth distinguishing from nitrogen, which also starts on old leaves but yellows the whole leaf evenly including the veins. If your veins are clearly still green, it is magnesium or possibly iron, and iron shows on the new growth at the top instead. Yours is the classic old-leaf pattern.

Daily watering in a well-draining mix also flushes nutrients out the bottom constantly. In containers I feed at quarter strength with almost every watering rather than full strength weekly, and deficiencies mostly stopped.