I'm not finding anything about using soap and baking soda to control crab grass. If you have links to what you have read post any here, but those articles you read should have had the application method outlined.
Crab grass is an annual grass that is pretty easily controlled with a good mulch that covers the grass really good, or by pulling the newly emerged seedlings from the soil.

I'm the one that has been spreading that news. Several years ago I had heard of this approach. I presented it here and on other forums but nobody ever tried it. Finally Cactus tried it and took a lot of pictures. I'd link you to the forum where he presented it but that's not really polite and I'm pretty sure GW blocks links to that forum.

Here's a link to the youtube video that really shows how to do it. How much soap and molasses has been figured out by Cactus.

Suffice it to day baking soda will kill crabgrass in a day or two; HOWEVER, it might also kill your grass. The results are not in for all grasses but for St Augustine, it works GREAT! St Aug is only slightly affected by the baking soda. Subsequent photos posted by Cactus show the St Augustine recovering nicely after 30 days. After Cactus' report hit the forum, Andy tried it on his grass in MA. He reported that the baking soda test killed about 75% of his northern mix grasses. So there is good news here for northern gardeners. If you are not worried about the surrounding grass, like in a flower bed, baking soda will kill crabgrass every time. The reason for the soap and molasses is to pretreat the hydrophobic blades of crabgrass so the baking soda will stick.

If the crab grass is growing in a flower, or vegetable, bed either covering it with a mulch (once that grasses access to sunlight is halted it will die) or simply pulling it out is better then spraying it with anything, especially anything that will also kill other plants.
If the crab grass is growing in the lawn, any spray that will kill that will also kill any other grass the substance touches, leaving large brown spots for some time which could allow more crab grass seeds to germinate, or other "weeds" to grow to fill in that bare spot.
Since most all crab grass is an annual understanding how it grows and then fixing the conditions where it could grow is a better solution then spraying any poison around, and if those products do kill plants they are poisons.

I think the reason the baking soda works in a St Augustine lawn is that St Aug is extremely tolerant of salt.

I agree that mulching it in a flower bed would be preferred to anything else.

Hi everybody..I live in the Palm Springs, Ca area and I was glad to find a forum on this subject because I suffer from major crabgrass issues in my new vegetable garden. Just wanted to ask how much mulch should I put in to smother them little buggers? I am scared to try the soap or baking soda as I do not want to hurt my very first veggie plants...:-) Thanks everybody!

Lisa, you need enough mulch to deprive whatever plant you do not want growing there to not get the sunlight they need. That means a minimum of 4 inches of shredded leaves in my experience, or a layer of newspaper or cardboard down first and covered with another, better looking, material to hold the paper down. The newspaper or cardboard simply allows you to use less material and get the same results since about 2 inches of a mulch, shredded leaves, wood chips, straw, etc. is enough with that paper down.