Fertilizer components are labeled on the package with a 3 number series indicating the percentage of Nitrogen, Potassium, and Phosphate (abbreviated N-P-K) that is contained by weight. So a 10 pound bag of 20-5-5 contains 2 pounds 10 pounds x 20%) of Nitrogen, and ½ pound each of Phosphate and Potassium. Note that this only adds up to 3 pounds, the rest is just mixed stone dust or salt.
You might notice that some of the newer fertilizer bags are a lot lighter than they used to be. Manufacturers refine out more of this stone and miscellaneous junk, then mix the nutrients with a light weight filler to help it spread better. This cuts down the shipping costs and makes it easier to spread. It also makes it possible to make the fertilizer much more concentrated, but this carries some bad along with the good.
Too much Nitrogen at once can harm your lawn…that’s what happens when grass dies in spots burn from animals pee or that get too much fertilizer at once. At the same time, Nitrogen dissolves in water and can be washed away by rain, so the fertilizer manufacturers created a timed release form of Nitrogen. It adds to the cost, but is well worth the added expense. Look for the label to say that some of the Nitrogen is insoluble or timed release.
Phosphate and Potassium are rock products, so by nature they are slow release and don’t easily wash away, but they are natural salts and too much can dry out and burn any type of plants.
We’ll talk about how much to use on another page, but my proofreader wants me to mention trace elements. All living things need an assortment of minerals, everything from Iron to Magnesium, and Sodium to Copper. I get my trace minerals every day from a vitamin tablet, but your lawn can probably get all that it needs from unsupplemented soil and it’s not a serious concern.
These illustrations are from a pre 1900 publication. Bonemeal has long been used as a fertilizer. As the name implies, its made from crushed bones. Bonemeal is more suited for gardens than lawns. The most common lawn fertilizer from that time was manure. It was sometimes spread manually on lawns, but more often it was left naturally as animals grazed.