Natural weed killer

Weeds are succession pioneer plants that protect the soil when land is disturbed (flood, fire, clear cutting, etc.)  As they grow and die they add organic matter and allow the next stage of plants to grow. Most yards are kept in a perpetual disturbed state, which encourages weeds. Chemical fertilizers and pesticides kill the soil organisms in the soil food web which could have kept weeds in check. Herbicides are a quick fix, but not a long term or effective solution. The soil is kept in a bacterial dominated state, the preferred habitat for weeds. By restoring the proper balance of beneficial bacteria and fungi, plant succession is allowed to proceed, and it becomes easier to grow the plants you want to grow. [Read the section on Building Soil under Resources] When your soil is out of balance / beneficial bacteria vs fungi, the weeds dominate because they are in their ideal environment. Change the balance (increase the beneficial fungi), the plants you want to grow thrive and it is the weeds that are weakened and struggle to grow.

The most effective weed killer is hand pulling or a hoe, then mulch, or covering the area with cardboard or at least 6 pages of newspaper, then mulch.
but if you are going to use Roundup, try this first:

1 gallon vinegar
2 cups Epson salt
1/4 cup original blue Dawn dish detergent

Mix and spray it on leaves in the morning after the dew is gone.
Kills any plant whose leaves are coated. Be careful it is not a windy day.

Pro:  Non-toxic, breaks down into plant nutrients.
Con: The salts will harm soil life, so use sparingly.
·        (Killing weeds in pavement cracks is a fairly benign use)
·        The vinegar alone or with detergent will kill many weeds.
·        May need to re-treat to get the roots.

Monsanto claims Roundup is safe because it only affects plants, not animals.
WRONG. Tiny quantities of the residue also kill our gut bacteria and probiotic flora that live in us. We have more* of those symbiotes than human cells in our body, and most are critical to good health. Roundup preferentially kills beneficial bacteria, allows the overgrowth of pathogens, and other negative effects. Glyphosate (the active ingredient in Roundup) residue is in almost all non-organic food and does not wash off.
Get informed about the safety issues of Roundup and Glyphosate.
Even the World Health Organization has confirmed that Glyphosate poses a cancer risk.

* A person has about 37 trillion human cells and about 10 times that number of non-human microbial cells.