Soybeans are often grown with a grass such as buckwheat. Soybeans do best on a well-drained soil, with a moderate amount of moisture, but will tolerate poor drainage. Cover with an inch of soil. Use buckwheat, a broadleaved non-legume annual, to quickly smother summer weeds. Buckwheat builds organic matter and adds phosphorus to the soil.
Buckwheat grows in most soils, including infertile and acidic ones.
Plant 5 ounces per 100 square feet or 3 pounds per 1000 square feet in the spring or summer. Sweet clover is the best legume for improving poor or disturbed soils that aren't acidic. It adds phosphorus, nitrogen, and large amounts of organic matter to the soil. Its deep roots loosen the soil and draw nutrients from the subsoil, which return to the top soil when you turn the green manure under. Sweet clover requires a well-drained soil with a pH near neutral.