May need to get the soil tested to see what it is like. Also, city water that uses chloramine instead of chlorine may contribute, but I doubt it would be that drastic of a decline. And if that's not enough, horse manure can have it's drawbacks as well. See link.

tj

Here is a link that might be useful: Manure dangers

A soil made up on one-half manure has an awful lot of manure. My guess would be that the manure is still slowly decomposing, and the bacteria doing the decomposition are consuming all the nitrogen in the soil, binding it in their cells. This is what can happen in a composter which is why adding nitrogen fertilizer kick-starts the composter, allowing more bacteria to grow. Yellow leaves are a classic sign of nitrogen deficiency. If my guess is right then things will sort themselves out when the decomposition is mostly finished, or you could try adding a high-nitrogen fertilizer to some plants and see if that makes a difference. Or get the nitogen level tested. While your iron and manganese content might be high for well-water for human consumption, there is a lot of iron and manganese naturally in soil, and they don't usually pose any problem for plant growth. You would be far more likely to encouter problems from iron deficiency in soil, not from an excess.

For many years I have watered my garden with well water that is high in iron and manganese without any problems, so I doubt that is the issue. Like the posters above, I also recommend getting your soil tested, both for texture and nutrients. When I have had soil tested there are recommendations of what the nutrients and texture should be. You could go back to the source of your soil and see if they had any other reports of problems with it. Another question would be did you only water when the beds needed it? Did you check the moisture level below the surface? With that much organic matter in the beds, they will retain water, and your soil may have been too wet to grow veggies well since they typically don't like waterlogged soil. Stunting and yellowing could be symptoms of too much water. Fertilizing at 20/20/20 once a week for three weeks, depending on the rate and type of fertilizer used is pretty extreme.

Count this year as part of your learning curve, spend the winter researching the results of your soil test and good luck next year.

Karin, I have to ask the question, "why do you use well-water for your garden"?
Without informing as to how well your well is, I have to assume there is salt being used in the system to soften the water and usually for such use it can be an expense that need not be used.
Most well water systems have one or two taps off-line---that is, a tap that is not on the softening system, usually outside in the backyard or front yard where landscaping is watered, is not part of the softening system and ergo, is not an expense.

I have to conclude that given the fact that salt as part of such system is explicitly not good to use on houseplants, it follows that such use on vegetables is also not the thing to do.
You have given percentages that seem to fall into guidelines that suggests no harm follows with the use of your well water. I'm perplexed why the tester didn't also include the salt percentage.

If you are using a provincial testing laboratory...or maybe even a plumber, such person can better tell you if there is any high degree of any chemical imbalance that should be looked at. From reports by plumbers, the use of salt in softening systems can often be replaced by the use of potassium. Potassium is clearly not an issue when given to plants.
Why not call your local plumber and ask his advice.