If you're trying to kill crabgrass, prevention is key. Once it has grown to a certain length, the only natural way to kill it is to uproot it by hand. Luckily, there are many ways to prevent and kill crabgrass naturally before it can get a foothold in your lawn.
Spread more turf seed than usual. This eliminates places for crabgrass to grow. The best time to do this is in the early fall.
Fertilize grass in the fall instead of spring. The first frost kills all crabgrass. Fertilizing after this only benefits the surviving turf grass.
Water your grass more infrequently and more thoroughly. If the soil is allowed to dry between waterings, the crabgrass has more trouble germinating. A more thorough watering encourages the turf grass to grow more deeply and more easily combat crabgrass.
Use the highest blade level setting on the mower. Taller turf grass soaks up sunlight that crabgrass desperately needs to germinate.
Pull any fully grown crabgrass you find. Apart from herbicides, fully grown crabgrass can only be removed by hand pulling. This frees more room for turf grass and gives a head start on next year's work.