What is the meaning of N – P – K in fertilizers? And how to apply it in the field?


Fertilizer is any material of natural or synthetic origin that is applied to soils to supply one or more plant nutrients for plant growth. Plant nutrients are known also as essential elements. These are chemical elements that are required for plant growth. They must be present whether the system is organic or not.

Several criteria must be met for a chemical element to be considered as a plant nutrient: (1) The element must be required for plants to complete their life cycles. Deficiency of an essential element will result in abnormal growth or premature death of plant. (2) The requirement for these elements are universal among plants. If some plants require an element and others do not, the element is not considered to be a plant nutrient but a beneficial element. (3) No other element will substitute fully for an essential element. Plant nutrients consist macronutrients, the nutrients required in large amounts (in percentage or %) and micronutrients, the nutrients required in small amount (in parts-per-million or ppm).

There are 23 plant nutrients that have been proven essential and can be categorized in 4 main groups. The list of essential elements below is not static and may be expanded in future as experimental techniques improve to purge elements from the environment in which plants are grown. Macronutrients derived from air and water: carbon (C), hydrogen (H), oxygen (O). Primary macronutrients in soil: nitrogen (N), phosphorous (P), kalium (K) or potassium. Secondary macronutrients in soil: calcium (Ca), magnesium (Mg), sulfur (S). Micronutrients in soil: iron (Fe), molybdenum (Mo), boron (B), copper (Cu), manganese (Mn), sodium (Na), zinc (Zn), nickel (Ni), chlorine (Cl), cobalt (Co), aluminum (Al), silicon (Si), vanadium (V), selenium (Se).

The most common labeling fertilizers show the amount of primary macronutrients nitrogen, phosphorous, and potassium or N – P – K fertilizers. According to Barker (2010) in his book Science and Technology of Organic Farming, about 70% of the time increase in crop yields are recorded after soils are fertilized with nitrogen. About 40% to 50% of the time, increased yields occur after fertilization with phosphorous and potassium. Nitrogen aids plants in making proteins that are used for healthy leaf growth. Phosphorous aids in strong root growth as well as flower development. Potassium aids in overall plant health through the activation of several enzymes.

Fertilizers that are in the market, whether organic or chemical, for example “All Purpose” fertilizer labeled 20 – 20 – 20 contain 20% available nitrogen (N), 20% available phosphoric acid (P2O5), and 20% available potash (K2O). No correction is needed to express nitrogen as actual nitrogen. A correction of 0.436 times the available phosphoric acid is needed to express the actual phosphorous. A correction of 0.830 times the available potash is needed to express actual potassium in a fertilizer. So, a fertilizer with 20% N – 20% P2O5 – 20% K2O converted to its actual nitrogen, phosphorous, and potassium concentration is 20.00% N – 8.72% P – 16.60% K by weight.

In the case study of crop with high demand for nitrogen such as corn with 120 lbs N per acre removed in one season (about 3 months), if we want to use the “All Purpose” fertilizer 20 – 20 – 20 in a bag of 25 lbs, how many bags do we need? As per bag contain 20% N X 25 lbs = 5 lbs N. So, per acre corn field will need 120 lbs N : 5 lbs N = 24 bags.

Corn is also crop with high demand phosphorous and potassium with 30 lbs P and 100 lbs K per acre removed in one season, respectively. Using the same “All Purpose” fertilizer 20 – 20 – 20 in a bag of 25 lbs, per bag contain 8.72% P X 25 lbs = 2.18 lbs P and 16.60% K X 25 lbs = 4.15 lbs K. So, using the same calculation as for N, per acre corn field will need 30 lbs P : 2.18 lbs P = 13.76 or 14 bags to supply P. And 100 lbs K : 4.15 lbs K = 24.09 or 24 bags to supply K. In this case, using 24 bags “All Purpose” fertilizer 20 – 20 – 20 is correct to supply N and K only but it will be too much to supply P because the P application only need 14 bags “All Purpose” fertilizer. In this case “Customized” fertilizer rather than “All Purpose” fertilizer is needed to save the cost.

The example above is getting more complicated and less realistic because there is no single generic fertilizer for corn. The nutrient requirements of corn depend on yield goal. New hybrids and high-yielding corn varieties produce 230 bushels per acre or about 14.5 ton per hectare will need a higher nutrient demand. And corn requires different nutrient rates at different stages of its growth. So, the timing of nutrient application (mainly N – P – K fertilizer) for corn is critical as illustrated in the photo above from the courtesy of Smart! Fertilizer Management.

– Bintoro Gunadi


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