First let me say that my entire soul floods with love for the people on this site and for what you are doing...so much kindness and generosity. It's fucking beautiful.
I never tire of reading about and learning from other peoples' grows. Would love peoples' thoughts on this rambling geeky post...
oldskol4evr--Reading about your particular constraints and all the work you put in to make it work on a fixed income compels me to hazard a suggestion: composting materials from horse stables’ manure piles. Anyone please weigh in!
Although I do not use this mulch on my own beds, I am a relentless reader of all sorts of research. I am also a horse owner so I know first hand what goes into manure piles at horse stables. Each day, an individual horse’s stall is emptied of waste materials consisting of manure and urine, hay, and stall bedding. Stall bedding is, except in rare cases, either wood shavings (untreated pine pulp), sawdust, or straw. So you can see why I suggest composting materials from stables. Horse manure piles are one of the most extensively studied by biochemists and agriculture scientists because they start as the right mix of woody/fibrous material and manure. You can look to any university or governmental agricultural extension for confirmation and/or further detail.
Extensive analysis has been done on the particulars of optimal composting of a wide span of differing percentages of manure, hay, and bedding. So much analysis has been done on composting these waste materials that OG content, C:N ratios, pH, N, P, K, mineral salts and micronute levels are all well studied. So the work has in large part been done for you. Find any horse stable and they will be thrilled to have you take as many yards as you want as often as you want. They will generally just point you to the pile and be just fine with you coming and going whenever you wish. You will want to take the freshest material home to compost, because this will have the optimal starting C:N ratio, OG and moisture content, and because it will have the lowest levels of pathogens. You can mulch with routinely turned and wetted compost as soon as three months.
The mixture of manure, hay, and type of bedding is key of course. Wood shavings and straw are best. Sawdust is too fine for adequate porosity. The wood shavings and straw of course increase the amount of carbon that has to be broken down, so you will have to add a nitrogen source—fresh grass clippings or dried leaves so conveniently left all bagged up for you on curbsides are perfect. pH levels of the compost materials themselves will stay pretty much where you want them.
I particularly liked reading that someone else is a fan of letting whatever vegetation that volunteers take over the beds during the off-season. I agree that the root structures stabilize the soil--and I bet you agree that it also keeps a healthy rhizosphere in the beds. Plus the higher the diversity of species is across the site, the higher the diversity and rigor of predator-prey food chains. High biodiversity also makes ecosystems less susceptible to the spread of disease. Sorry to geek out…