Dave's approach is generally what has worked for us as well. The first line of attack in all organic weed management is to diversify your crop rotation so that the crops you plant differ in their phenology (fall planted, spring planted, summer planted crops, and perennials). This will diversify the selection pressure on weeds and prevent any species of weeds from completely dominating and its populations from rapidly growing. After focusing on your crop rotation, standard approaches to mechanical weed control in organic soybean production includes 2-5 blind cultivations (tine weeders and rotary hoes) and 2-3 between row cultivations (s-tine cultivators, wide singe sweep cultivators). The blind cultivations should be done the day of planting to try and create a soil dust mulch (loose soil that decreases optimal conditions for weed germination). After at-planting tine weeding or rotary hoeing, try and get in 1-2 more blind cultivations on 3-4 day intervals. Be careful not to blind cultivate when soybeans are in the crook stage (just emerging). They are easily snapped off. Once soybeans are 6-10 inches tall and blind cultivation will begin to do a lot of damage, switch to inter-row cultivators. Try and use an inter-row cultivator that has shields for the first pass or you can move a lot of soil on the row and bury plants. At last cultivation, many growers will use hillers to throw a lot of soil around the base of the soybean plants and bury weeds. An acre is not that much land, so it would not hurt to go out with a pair of clippers and take the tops off the really big weeds before they produce mature seed heads. That will keep weed seed production down and lower weed populations in your crops over time.

Good luck.