Wood Ash and Vegetable Gardening

I moved into my current place in 2003. I live in Kincardine along Lake Huron, Ontario, Canada. When I first moved in, I put in some more soil and expanded an existing garden. The first couple of years I had excellent results. The soil I put in was sandy and it had a lot of lamb's quarters seeds, however I grew an excellent garden. The last couple of years I have had all kinds of problems. I have major problems with germination with plants like beets, purslane, and green soya beans. My first few years here, I planted purslane from seeds I purchased from Richters and got marvelous results. I used to get a huge onion crop, and now they are stunted and very woody. In the past my peas would grow about 4' (1.2m) and be ready by mid July, and my cherry tomatoes would be ready by August. Last summer (2010)my peas only grew about 2' (60cm) and were not ready till August, and my tomatoes were not ready till about September. I put lots of organic matter in the soil and I put wood ashes in the soil, except ! where I grow acid loving plants like tomatoes. I use fertilizer, but try not to over fertilize. I have noticed that I now have major problems with crab grass around Aug, and after I weed that, about 2 weeks later I get hit with wild millet. Could this be a lack of some trace element or am I perhaps not using enough fertilizer? I have heard a number of people say they have had a lot of problems with their gardens the last couple of years. I look forward to hearing your words of wisdom on this issue.
P.S. I still get excellent carrots and >string beans.

I have a suspicion that your wood ashes may be the culprit. You say you put lots of it into the garden and that may be overdoing it. Onions like a pH of about 6.2 to do best and do not do well at all at a pH o over 7.2. I presume your pH is so high that many nutrient are unavailable to onions and they therefore are stunted. Also trace elements are leached out of open soil fairly quickly and may be lacking by now. Therefore tomatoes don't do as well as before.
Another problem with tomatoes may be the heat available in the summer. In cool, wet summers they do very poorly -even here in zone 4-5. You are colder yet.
Crab grass and wild millet have more to do with contamination of the soil with their seed then with your fertilizing. They are aggressive and if the seeds are there and they receive light they will germinate. Digging the soil exposes new batches to light, just as with lamb's quarters. To limit their number you have to try to religiously weed out every one of them before they go to seed or else their prolific seeds will make your gardening a lot less fun next year. To limit the number of seeds that get light and therefore germinate use a heavy mulch. These seeds (as well as ragweed) are distributed by wind and birds(when their digestion does not manage to break down the seed coat) and even a neighbor feeding the mixed seed available in stores to the birds could give a start to these weeds in your garden.