My potting mix is based on (too keep it really short) a Johnson-Su compost (mainly grass clipping, shredded wood and bark and sawdust with trace minerals and microbial mixes) that I later feed to worms. It’s got a really nice structure, but I was shocked to find that it has a pH of about 9! I must reduce the lime. I am mixing it with coir so it stays light, but that doesn’t bring the pH down like peat does. So it’s still a bit on the alkaline side and some plants (mainly some tomato varieties) are starting to show iron deficiencies like your cucumbers. I could use peat instead of coir, but I would prefer to avoid it.
I am now trying to adjust the pH with citric acid, but i realize I have come to the limit of my understanding of chemistry to workout how much to use. I will start by wetting the compost with a 0,1% concentration or less.
I am really not happy with what is on the market. What is “gw compost”?