Corn Gluten as a weed killer

From here:

Corn gluten acts by inhibiting the growth of the roots of seedlings, causing them to die by dehydration when the soil dries out. It acts on sprouting plants but does not affect more established ones. In fact, due to its nitrogen content, it causes larger plants to grow more vigorously [...] High cost is one of the main drawbacks to using corn gluten. It is more expensive to apply at the recommended strength than chemical herbicides 

 See also:

• Iowa State U page - lots of good resources