I'd go 30-X-X or above or as close to that as you can (that first number is your N "Nitrogen" percentage by weight) find a bag and it will list N-P-K. Example triple 14 is 14-14-14 which is 14% Nitrogen (N), 14% Phosphorus (P) and 14% Potassium (K) in this case it doesn't matter what your P and K are because it is the Nitrogen that's going to be killing the dallisgrass for the most part. Urea will be 46-0-0 would be ideal but Im not sure big box stores sell plain urea. But anything around that 28-x-x 35-x-x something like that would work fine. I was told this "trick" years ago and it absolutely works for clumps that are well defined throughout the yard. if dallis is mixed all in the lawn it isnt going to work. Quick and easy. Put it all in one small scoop pile in the center don't spread it out. It will do the work for you. Ace, HD, Lowes any garden store will have something like this.