Potash improves turf grass "hardiness". It is one of the three major nuitrients (NPK). Although there are some studies that indicate a possible downside to excessive K applications, a 0-0-7 application will not have a harmful effect. A growing number of jurisdictions will not allow some staight pre-m to be sold to a user without an applicator's license unless the bag contains fertilizer, so it is suspected manufacturers are adding the K to get around this.

Thanks, grass1950. Wonder why they insist on more chemicals?

No one is insisting on more chemicals. Government is attempting to protect the enviorment by prohibiting the sale of herbicides to persons who are not licensed. However, legislators are aware that homeowners want to prevent crabgrass. You may have noticed that it is very difficult to find a pre-m that doesn't contain N fertilizer--especially at the big box stores. In order to please homeowners the government cut an exception: An unlicensed person can by pre-m if it has fertilizer in it.
Now the catch. Manufacturers are aware that more and more research is showing early spring N fertilization is detrimental to turf. That it is best to add straight pre-m w/o fertilizer. They are also aware that more and more homeowners are slowly learning this from turf specialist (like your extension office)- a growing customer base.. So manufacturers are evidently doing a work around by adding K (a fertilizer component) to their straight pre-m so the homeowner can buy a pre-m w/o N. It's an unintended loophole.

The Dimension Pre-M that I use is exactly what you are referring too (0-0-7). I've used it for the past couple of years with very good effect. Basically you just need to prevent nitrogen from being applied and feeding the weeds. The small amount of potassium is not going to do anything for the weeds without the nitrogen.

Get it down ASAP! The mini-heat wave is upon us and could sprout some crabgrass if enough sunlight can heat the ground to germination temperatures. I put mine down on Saturday morning (Tenacity) and am glad I did as even though the weekend temperatures were not quite as warm as they had thought, lots of new weeds were sprouting and flowering.