Answer: Have you considered planting a cover crop? A cover crop can help crowd out weeds and can be tilled under to help amend your soil. Legumes such as cowpeas, soybeans, annual sweetclover, sesbania, guar, crotalaria, or velvet beans may be grown as summer green manure crops to add nitrogen along with organic matter. Non-legumes such as sorghum-sudangrass, millet, forage sorghum, or buckwheat are grown to provide biomass, smother weeds, and improve soil tilth. Of course, if you allow these cover crops to go to seed, you'll be dealing with their relatives for a long time! Planting a cover crop means you'll be rototilling a couple more times before you seed your lawn.
The only other way to keep it weed-free is to cover the soil with a thick layer of mulch (which you can rototil in before planting grass), or covering the area with moisture permeable weed block fabric.
I would avoid using chemicals to retard weed growth - the chemicals might build up in the soil and prevent your lawn from growing.
Hope this information is helpful!
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