I am unsure if adapting an aquarium pH test kit to test soil will yield accurate results. For what it's worth, might as well give it a try and see what you come up with. For example, if you did the aquarium test two or three times and it clearly came back with an alkaline value, then the pH may very well have gotten to high and needs adjustment. I wouldn't trust it to determine the difference between a pH of 6.0 and 6.3, it's my understanding that lab equipment is needed for that. There are home soil test kits on the market, I've never used one and am not sure how well they work. It may be worth the fee to have the RHS do a test so you know the pH. Like Angelika stated, yes nitrogen deficiency can cause yellowing of the leaves, so can an iron deficiency. Soil tests generally never test for nitrogen because it's mobile in the soil and its content is constantly changing as some of it can easily go back to the atmosphere, or wash away with
water. If you do get the soil test done and the pH comes back somewhere in the neighborhood of 6.2-6.7 then it is likely not a pH issue and could very well be a deficiency. The microbes living in the soil could be out of balance and one way to approach this is to add some quality
compost. The
compost will help nurture the microbial life as it's the microbes that make nutrients available for the plants to use and the
compost will add nutrients to the soil as well. Another approach if you don't have compost and like ease & convenience is to apply a fertilizer like liquid fish. I would on all accounts avoid using chemical fertilizers. They make conditions worse for your soil.