Lawn grasses have 2 major needs…sunlight and a few common minerals. Like most green plants, grass is able to grow by combining carbon dioxide from the air with nitrogen, phosphate, and potassium from the soil. Special cells in plants are able to use the energy from sunlight to fuel this process using a compound called chlorophyll (the stuff that makes grass green).
Grass needs to grow constantly during the growing season to fight insects and repair aged or damaged blades, so there is a constant need for nutrients. Just like baking a cake though, it needs measured amounts of everything, but after a while, the nutrients in the soil run out.
Fortunately, what grass needs is cheap…sunlight is free, and the 3 major nutrients occur naturally. Nitrogen is abundant in manures, but most of us prefer to use the less-smelly stuff that’s manufactured from oil or natural gas. Phosphate and Potassium (Potash) are rock products and are mined from the ground, crushed, and packaged for you to spread.
Of course, the first lawn fertilizers were part of the lawn mowers…animals grazing on grass trimmed off the tops of the blades, and left behind minerals and urea, a form of nitrogen found in animal waste. Today, in order to grow as thick and even as we like, lawns need more than they can get from nature