The Grass Doctor

what do the numbers mean when purchasing fertilizer?

I just laid down sod in my front yard. I was not sure if I needed to give it any fertilizer, since i have not done so prior to laying the sod. It has been 3 week since I have grass. I now have what to appears to be crab-grass, but a neighbors tells me patience, you will conquer that problem later. But he started going off on types of fertilizers with no mention of meaning for the numbers...can someone tell me what it means. Thanks.

Public Comments

1. The three numbers refer to the % of nitrogen, phosphorus and potassium in the bag. The remaining amount is filler/carrier. So a 10-16-20 fertilizer would be 10% nitrogen, 16% phosphorus and 20% potassium or potash. The higher the first number, the nitrogen (N) is found in many grass fertilizers. If the second number is higher, the phosphorus (P) it is often a flower or vegetable type fertilizer as their nitrogen requirements aren't as high as a lawns. Organic fertilizers will be very low in %: 2-1-1 for example......which means only 4% total is nutrients, the rest of "organic fill." Actually that isn't really bad as in organic fertilizers it is probably a good organic matter source.
2. Fertilizer is listed with three numbers the first listing the percentage of nitrogen(N), the second phosphorus(P) and the third is potassium(K). For example, 20-10-10 is 20% nitrogen. Nitrogen is what greens up your lawn, phosphorus and potassium are more for root development and growth. Ideally, you should have put down a light dressing of fertilizer on the soil before you put down the sod, but since you didn't you can do it now. If you laid sod, how can you possibly have crab grass? Sod is usually pretty pure....unless you didn't kill the weeds and crabgrass before sodding??? I am not sure that is even possible for them to get up through the sod this quickly. Usually, established grass is fertilized with a higher nitrogen and small amounts of P and K, say 20-0-10. Phosphorus is left out of many fertilizers now because it is often not needed and so it doesn't end up running off into the lakes and streams where it causes algae problems. In your case look for a fertilizer that is for new grass, and it will have some phosphorus in it. Good luck