Thus, when people hypoventilate (i.e. causing an increase in CO2) this causes the pH in your blood to decrease, making it more acidic. (and when hyperventilation occurs, O2 is increased, which causes an increase in pH - making blood more basic).
Pre-emergent applications should be made when soil temperature are still below 50 for the best prevention of crabgrass. Pre-emergent applications are not recommended for areas where new grass seed is going to be planted during the first half of the growing season.
Do not wait until soil temperatures are optimal for crabgrass germination before applying pre-emergent herbicide. Apply it right before it is within crabgrass' optimal range or when soil temperatures hit 55 degrees, according to Danny Lipford.