Q.Why Do Cucumber Plant Leaves Turn Yellow?
the cucumber plant is a patio snacker in a large pot

Certified GKH Gardening Expert
Unfortunately, there are a lot of reasons for why they would turn yellow, depending on how it looks and where it presents itself, exactly. Reasons can include too much or too little fertilizer, pH imbalance, too much or too little water, thick soil, excessive heat, and disease. Luckily, you can test for these things.
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/test-soil-for-pests-and-diseases.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/macro-and-micro-elements.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/making-sure-soil-drains-well.htm