“My biggest problem is that squash and such seem to do well until just about the time fruits start to develop, then they weaken and produce poorly or not at all. Same for melons.”
Zuke & Cukes will benefit from a scoop (about a cup) of crushed eggshells and a scoop of Bone Meal in the planting hole. What you’re probably seeing with Zukes, Cukes and Melons is most likely a nutritional issue, versus your soil. Give it a try this season. I’ll bet you’ll have much better results. I give the crushed eggshells (Calcium) and Bone Meal to my Tomatoes and Peppers, too. You can buy liquid Calcium if you haven’t been saving eggshells all winter. :)
I put crushed eggshells out there, but not in that quantity. Plus some Super Phosphate to encourage blooms (although sufficient blooms don’t seem to be much of a problem anyway.) But I’ll give a shot at fruit formation time a try - maybe the Super Phosphate all gets taken up by the time the fruits are starting to form.
My Rural King store has bone meal, 3 lbs. for $7. But that won’t go far if you are recommending a cup for each plant?
I also have about 5 gallons of (wood ) ash.