If you are trying to prevent crabgrass without chemicals, liquid corn gluten meal is a great way to do that.  Corn Gluten Meal has been shown to suppress crabgrass and other annual weed seeds.  Liquid Corn Gluten meal provides the same benefit without all the bulk.  3 Small Hose End Sprayers will cover the same square footage as one 50 lb bag of dry Corn Gluten Meal, so the liquid is a bit easier to apply and much less expensive to ship. 

Each container will cover 1000 square feet of grass or mulch bed area.

Liquid Corn Gluten Meal (Bonide Maize Weed Preventer) will only suppress plants sprouting from seed.  It will not keep existing dandelions or other weeds from blooming.  If you have Dandelions, you'll want to try something like ADIOS or FIESTA to help minimize their appearance. 

One final note - Liquid Corn Gluten Meal is a great way to help suppress crabgrass and other weeds without using chemicals.  In addition, it is really helpful to mow and water your lawn properly.  Here are some good mowing and watering instructions for maintaining an organic lawn.