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Fertilizers come in a dizzying array of formulations. Walk through the garden center at the local warehouse joint, and you'll be confronted with dozens of different products, each touting itself as the cat's meow for your lawn. You can cut through the hype and get the right stuff though, if you know what you're looking for.
Fertilizers can be divided into 5 categories: Growth, starter, balanced, complete and special need. These are categorized based on the amounts of Nitrogen, Phosphorous and Potassium in the bag, or the N-P-K ratio.
A growth fertilizer will have a high N content and relatively low P and K content. Ammonium nitrate is the highest Nitrogen fertilizer you can buy, and it is potent stuff. It is usually rated as 33-0-0 or 34-0-0. The bag contains 33% or 34% Nitrogen and the other 66% or 67% is inert material. Other formulations of growth fertilizers will contain 5, 6 or even 7 times more Nitrogen than anything else. Use growth fertilizers very sparingly, applying them lightly during the peak growing season of your grass.
A starter fertilizer is used to help a new lawn become established. They contain very small amounts of Nitrogen relative to the other two nutrients, and you will often see them in a 5-20-20 formulation. New lawns need extra Phosphorous and Potassium to develop strong roots and resist disease, but they also need a tad of Nitrogen for growth. Use these fertilizers as recommended on your seed package, or as recommended in a soil analysis conducted prior to planting.
Balanced fertilizers are just what they sound like--the amount of nutrients contained in the bag are balanced. You will see these as 6-6-6, 8-8-8, 10-10-10, etc. Unless a soil analysis has shown that your soil is deficient in all three nutrients, your lawn will rarely need a balanced fertilizer. These are better suited towards other plants in your landscape, such as trees, shrubs, annuals, perennials and vegetables.
Complete fertilizers have 3 or 4 times as much Nitrogen as Phosphorous and 2 times as much Nitrogen as Potassium. In most cases, this is the stuff your lawn will use best. They are called "complete" because they provide nutrients in a mix that completely satisfies most grass' requirements. If you only make one or two fertilizer applications a year, this is the stuff you want.
Special need fertilizers are formulated for the requirements of specific grass types, such as Centipede, or to correct a soil deficiency problem. 15-0-15 is the only fertilizer you should use on Centipede. 0-0-60 is used to correct severe Potassium deficiencies, and 0-40-0 is used to correct phosphate poor soil. Unless you have Centipede, you should have your soil analyzed by the extension service in your county before buying and using a special need fertilizer.