need to spray the field twice. If you decide to plow instead, you'll have to smooth the field (disc and harrow) afterwards. After tilling you should wait before planting so you can spray new growth that will inevitably spring up.
The prairie grass and wildflower seeds you plant must be in contact with bare soil in order to take root and grow. If you're planting in a crop field, or you've plowed the site to eliminate existing sod, you've got this covered. A site where sod was sprayed to kill grass may or may not provide sufficient bare soil. Bare ground must be visible between the dead blades or stems. If dead vegetation is too thick, options include using a harrow, light discing or burning to expose more soil.
If planting in the spring, when winter's freezing and thawing won't assist you, or if the soil is particularly hard and compact, disturbing the soil surface is probably a good idea.
Disturbing the soil will very likely increase weed growth, but if you don't mind a little more mowing, it may be worth it. If you work the site with a disc or tiller, you may need to follow up with a harrow or other type of drag (a weighted section of chain-link fence will work) to level and smooth the site.
If you are planting into tilled ground, or if site preparation involved soil disturbance, rolling the site will improve the rooting medium for germinating plants. It also helps press the seed into the soil and reduces the likelihood of the seed (and soil) being moved off-site by wind or rain.
Rolling is especially important when broadcast seeding in the spring. If using a seed drill, roll the seedbed before planting. Large plantings are typically rolled with a culti-packer. A lawn roller, the tires of any number of vehicles from lawn tractors to a pickup, or even the stomping feet of neighborhood kids can compact the soil on modest-size plantings.
Plan on using from 10 to 15 pounds of seed per acre, split about equally between grasses and wildflowers. If the seed is from a commercial supplier, 10 pounds should be sufficient. Use closer to 15 pounds if you collected the seed.
For small plantings (less than an acre), broadcast the seed by hand or with a lawn fertilizer spreader. Mix the seed with an equal or slightly greater volume of moist sand, sawdust or similar material