Plants need at least 16 chemical elements in order to grow. Most of these elements are abundant enough that plants will get all they need without human intervention. However, plants consume a few elements in such large amounts that soil will need replenishing if you want it to keep producing healthy plants. These elements provide the basis for the notation "NPK" commonly seen on fertilizer packages.
Agronomists classify elements essential to plant growth as either "mineral," meaning plants get them from the soil, or non-mineral, meaning they come from somewhere else. The non-mineral elements are hydrogen, oxygen and carbon; plants get these from air and water. Most of the mineral elements are plentiful in most soils, so plants don't have trouble getting them and gardeners usually don't have to fret about providing them. The exceptions are the 3 "primary nutrients" -- nitrogen, which goes by the chemical symbol N; phosphorous, which goes by P; and potassium, whose symbol is K. The "NPK" nutrients occur naturally in soil, but the more times soil gets planted, the more they get depleted. That's why you often need to add fertilizer.
Plants need nitrogen to build proteins and to produce the green pigment chlorophyll, which is essential to the photosynthesis process, in which plants use water, carbon dioxide and sunlight to produce their own food. More than 3/4 of the air around you is nitrogen, but plants can't get their nitrogen from the air; they need to get it from the soil. Plants are genetically programmed to absorb as much nitrogen as possible, so it gets pulled out of the soil fairly quickly.
Phosphorous and Potassium
Phosphorous is essential to photosynthesis and promotes both healthy root growth below the ground and the production of blooms and seeds above it. Potassium also plays a role in protein production and photosynthesis, as well as helping control the plant's metabolism and building its resistance to disease.
A bag of fertilizer will typically have a 3-number NPK ratio printed on it -- for example, 10-18-10 or 5-5-5. This tells you what percentage of the fertilizer is made up of substances that include the primary nutrients. So a 100-lb. bag of 10-18-10 fertilizer would have 10 lbs. of nitrate, a nitrogen compound; 18 lbs. of phosphate, a phosphorous compound; and 10 lbs. of potash, a compound of potassium. If it were a 20-lb. bag, it would have 2 lbs. of nitrate, 3.6 lbs. of phosphate and 2 lbs. of potash.
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