Commercial lawn fertilizers are labeled with three numbers that represent the ratio by weight of nitrogen (N), phosphorus (P) and potassium (K). Lawn fertilizers often have more nitrogen than phosphorus or potassium. Nitrogen is especially important for lawn growth and greening; potassium is necessary as an aid to photosynthesis; plants that flower and fruit need more phosphorus. Lime is sometimes added to lawns to correct soil pH, a measure of its acidity.
Lawns need to have a soil test to determine the soil pH. A soil test will determine what ratio of nitrogen to potassium in a fertilizer will work best. Soil tests also measure soil acidity on a scale that ranges from 1 to 14. Low soil pH is acidic; high soil pH is alkaline. Most lawns grow best if the soil pH is between 6.5 and 7. Lime is added to soil to increase its pH and make help make more nutrients available to grass.
Fine grinds of lime, calcium carbonate, correct soil pH quickly, but they can cause lime burns on the grass and the wind can blow them where they are not wanted. By increasing soil pH, lime can prevent toxic buildup of aluminum, iron and manganese in lawns. Lime can help make copper, phosphorus and zinc more available to grass, an aid to their growth. Dolomitic limestone can help soils that lack magnesium. Lime in the form of pellets can be scattered by hand and do not blow in the wind.
Nitrogen is the basis for proteins in plants and is contained in chloroplasts, molecules in grass that perform photosynthesis. Adding nitrogen increases the proteins in the chloroplasts, making the grass turn greener. Decaying plants and animal dung contain nitrogen, which is why composts are used as organic fertilizers. Commercial fertilizers contain nitrates, which grass absorbs easily and quickly or ammonium that bonds with particles in the soil and grass absorb more slowly.
Potassium helps the process of photosynthesis in grass, needed to produce nutrients. A deficiency of potassium can cause grass leaves to curl and turn yellow. Potassium helps grass resist disease and extremes in cold and heat.
Fast Release Fertilizer
Fast-release lawn fertilizers, including ammonium nitrate, ammonium sulfate and urea, put nitrogen into the soil quickly with fewer applications. They are relatively inexpensive, make lawns turn green quickly and work in cold soil. Their disadvantages are that they can cause nitrogen burn on grass, and since they are ordinarily sprayed on lawns in liquid form, they are more easily easily lost through leaching and evaporation. They also have to be applied frequently.
Slow-release lawn fertilizers, also called controlled release fertilizers, include activated sewage sludge, IBDU (isobutydine diurea), water insoluble nitrogen, manure composts, methylene urea, sulfur-coated urea, ureaform, and alfalfa, bone and soybean meal. These fertilizers all put nitrogen into the lawn slowly and do not work as well as fast-release fertilizers on cold soil. They are ordinarily applied less often than fast-release fertilizers.