The present invention is directed to biologic-chemical herbicide compositions (BCHs) for controlling or preventing weeds that include one or more chemical herbicides and microorganisms including gram-negative bacteria and yeast. The addition of a biological microbial component to one or more chemical herbicides significantly increases the efficacy of the chemical herbicide thereby permitting lower amounts to be used to achieve a desired level of weed control. Any chemical herbicide or combination of herbicides can be used, including fatty acid herbicides like Round-UP.RTM.. The BCHs typically also include an optional nutrient component in an amount sufficient to support the growth and replication of the microorganisms. If the BCH is applied to soil rich in nutrients, or if the crops would support the growth of the microorganisms in the BCHs, then the nutrients can be omitted.