Zeedman
Garden Master
That experience & conclusions mirrors what happened in my rural garden. Been gardening there since 2005, and never had trouble with germination - until I started turning under large amounts of organic matter, in an effort to restore fertility. The fertility restoration was successful; but because the decaying organic matter apparently attracted the seed corn flies from nearby farm fields, it has proven to be a double-edged sword. (A triple-edged sword actually, since the weeds too really appreciated the increased fertility .)Seedcorn maggots I had this May. You are correct on emergence speed. I plant May15 weekend every year and this year it was a cooler weather for that weekend. They didn't emerge as quickly as normal years. I havent really had issues until about 2 years ago. And that garden has been around for years. Weird stuff.
The organic matter was necessary to refresh the depleted soil, but maybe I only need to do that every 2-3 years, and add wood ashes, charcoal, and soil sulfur in the off years. I plan to put that to the test this year.
Seed vigor (or the lack thereof) was definitely a factor in seedling emergence. Some of my older soybeans had poor germination when tested; so as I have done in previous years, I just over-planted to compensate. That worked in my home gardens... but in the rural garden, it was just more maggot food. The soybeans which had higher germination rates (and presumably more vigor) fared better. So for the rural garden, I need to make greater use of backup transplants for soybeans (I already start all other beans there as transplants).