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 fertilitySeedcorn 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.