Why apply any shampoo to a lawn or garden?
As a general rule the shampoo (soap) helps reduce the surface tension of water so the water flows better. Using shampoo, or soaps, is usually recommended by people that know not much about soils in an attempt to get by without adding organic matter to soils but still getting water into that soil.
If water soaks into the soil quite well, and it will if there is adequate amounts of organic matter in that soil, spraying shampoo, or soap, will be largely a waste of time, energy, and money.

Chueh,
The use of shampoo on the lawn is for lawns that basically are repellling water.
If you have extremely hard soil, by using shampoo it enables the soil to begin to drain.
THEN you add organic matter to the soil, increasing the soils nutrients.
Shampoo is only good for soil that really needs it.
It is an alternative to severe aeration, especailly if you have some grass that is alive on this soil that is repelling water.
You never have to use it, otherwise.
It is a desperate measure, AND IT WORKS WELL.
I USE IT when necessary, which isn't often.
SO, unless your soil has NO DRAINAGE, and lacks organic matter, don't do it.
WHen you read the threads, really read the threads.
It isn't meant for someone who has drainage in the soil.

A 20 oz bottle of the baby shampoo is about $1. It will cover about 5,000 sqft for one application. Applying it with a hose end sprayer takes little time or effort. Any soil can develop dry spots where water is not absorbed. Improving wetting capability and penetration is good for any soil and the shampoo will not harm anything.

Adding the milk and molasses in a seperate spraying effort is also pretty darned cheap. I have not found a good explanation as to why the milk makes improvements. However, I am pretty well convinced that it improved my lawn. There is something in it that the microbes like. A quart of milk will cover 5,000 sq ft. Pretty cheap to try and make your own observations. It does not need more than two applications per year, spring and fall. Putting a Tbsp or two of molasses per 1,000 sq ft in that app could encourage microbe multiplication. Don't over apply the molasses though as you will a population boom and bust.

Since these are all relatively inexpensive and annot harm anything, why not give them a try and make your own observations.

In over 50 years of gardening what I have seen, and learned, is that soils that repel water lack adequate amounts of organic matter. While spraying any soap, and shampoos are soaps, will reduce the surface tension of water so it flows into the soil easier it is not much more than a quick fix.
Spraying milk and/or molasses on soils that repel water will not do anything since both require an active Soil Food Web to be converted into anything plants can use. If the soil contains adequate levels of organic matter the milk or molasses will do little except maybe stimulate the soil microbes a bit.

This is an old thread, but the first question about whether the shampoo will harm the earthworms hasn't been answered yet. Diluted dish washing liquid dish is often used to flush out esrthworms from the ground because it irritates them, hence the concern.

Detergents are antibacterial. Don't use them. The milk provides proteins, also called amino acids, and sugar. Molasses is sugar. These bloom bacteria. Kimmsr is not verbose enough in this instance, and I would add that one should look up their soil type (1 of 12) so that at least a general understanding of soils in the area can be the basis for amendments.