Crossposted to Soil, compost and Mulch

It's aimed at organic growers, but is applicable to everything from a small backyard to 2500 acres of soybeans ...

The weed seed bank is the reserve of viable weed seeds present on the
soil surface and scattered throughout the soil profile. It consists of
both new weed seeds recently shed, and older seeds that have persisted
in the soil from previous years.

http://articles.extension.org/pages/18528/manipulating-weed-seed-banks-to-promote-their-decline 

http://articles.extension.org/pages/18527/manage-the-weed-seed-bankminimize-deposits-and-maximize-withdrawals