Understanding N-P-K

Understanding how and when to fertilize is an important part of hortilculture.  Each bag or bottle of fertilizer is labeled with a set of numbers representing three of the macro-nutrients used by plants.  These numbers are always in the same order and correspond to the specific plant nutrients nitrogen, phosphorous, and potassium (N-P-K).  If you hadn’t guessed, the symbols N, P, and K are the same as used on the Periodic Table of Elements. An exception to this method of labeling is found in Australia where a fourth letter is assigned for Sulfur (S).
N-P-K value labeling is highly regulated by the country or state in which the fertilizer is intended to be sold.  The value listed on fertilizer packaging for N is calculated by the percentage by weight.  For example if the N-P-K reads 12-1-1, then the product is 12% nitrogen by weight.  The labeling methods used for phosphorous and potassium are more complicated and do not represent the raw elemental values.  Phosphorus is measured by its content of oxides while potassium is measured by its soluble compounds. This is important to understand because some fertilizers may contain much more of these nutrients than their manufacturers are allowed to print on the packaging.  It should be noted that some fertilizer companies circumvent regulations by printing soluble and insoluble nutrient percentages on their labels.

Nitrogen (N)

Most of what you breathe is nitrogen, an odorless, colorless gas that constitutes over 75% of Earth’s atmosphere.  Despite the overwhelming presence of atmospheric nitrogen, there are only a few kinds of plants in the legume family that can make use of it.  Most indoor plants are not legumes, so nitrogen must be supplemented.
Synthetically-produced nitrogen fertilizer is made in the form of anhydrous ammonia from natural gas. Anhydrous ammonia is then usually converted into ammonium nitrate for storage and ease of application.  Once granulated, the ammonium nitrate is ready for use as a cost-effective and easy-to-use plant food.  Natural sources of nitrogen are numerous and include; blood meal, manure, bird and bat guano, among many others.
Because of its crucial role in photosynthesis, fertilizers high in nitrogen are used during the ‘vegetative’ phase of growing flowering plants. Nitrogen is key to both chlorophyll and protein production and used throughout photosynthesis for the conversion of carbon into sugars and other organic compounds.

• Grow fertilizers are high in nitrogen
• Used for rapid growth
• Used to build chlorophyll and promote a healthy green color
• Improves volume and health of plant foliage

Phosphorus (P)

Phosphorous is a volatile and reactive element, which is why it is probably never found in its pure elemental form in nature.  Because of this volatility, fertilizer packaging is actually showing a measure of its oxide (P₂O₅).  Phosphorous is commonly refined from stone called phosphotite or mined from ‘guano islands’ comprised primarily of bird and bat dropping. A common biomass source of phosphorus is found in ground bone (bonemeal).
Fast flowering plants use phosphorus throughout their growing cycles but at elevated levels when flowering. As the plants’ nutrient requirements change, the feeding schedule is adjusted to include ‘bloom foods’ with higher levels of phosphorus and lower levels of nitrogen.  Because plants also use Phosphorus in root formation, low doses of it are often found in cloning solutions.

• Bloom foods are higher in phosphorous
• Improves flower production 
• Aids root development
• Involved in oil and glucose production

Potassium (K)

Potassium is an alkali metal that reacts violently with water and oxidizes quite rapidly when exposed to air.  This unstable nature requires that it be added to fertilizers in the form of potassium compounds known as potash.  Not all forms of potash are suitable for plants; K₂SO₄ , K2SO₄, 2MgSO₄, and K₂Mg₂(SO₄)₃ are the three that seem the best suited for a plant’s needs. It should be noted that potassium oxide (K₂O) is never used for fertilizer as it is caustic and hygroscopic. Like phosphorus, potassium is mined from mineral-laden ore.
Like N and P, potassium (K) is a key component in the metabolic functions of plants and plays a large role in the production of proteins and photosynthesis.  Potassium also helps strengthen the marijuana plant’s immune system, which helps the plant cope with environmental stresses, including insects and diseases.  Bloom formulated fertilizers often include increased levels of potassium because it helps improve flower quality.

• Found in both ‘Grow’ and ‘Bloom’ fertilizers
• Helps plants immune system
• Increases flower quality