Weeds, if they grow, can contribute directly to the malnutrition. Weeds like crabgrass and dandelions are quick to absorb nutrients, and many of them, hardly leaving any for your growing weed. In order to keep your turf alive, you will need to be diligent in removing weeds.
Weed chemicals are toxic humans, animals and other plants, not to mention generally bad for the ecosystem. Instead, choose a natural option for killing weeds. Vinegar can be applied directly to the leaves of weeds to kill them. Just make sure you only spray the weed and not your entire lawn. Techniques like pour boiling water over weeds are equally effective. Some have found success using corn gluten meal as a natural herbicide—wmixed hen with soil, it can prevent the growth of new weeds—But the results may be inconsistent. If in doubt, you can always dig up the weeds directly (which may be more effective for very rooted dandelions).