Understanding Fertilizer Numbers and Letters

Fertilizer’s formula usually is represented by 3 numbers that refer to the content of nutrients by weight in this order: N-P₂O₅-K₂O.

• The first number is the percentage of nitrogen (N)
• The second is the percentage of phosphorus (as P₂O₅)
• The third one is the percentage of potassium (as K₂O)

For example, a fertilizer that contains 15-10-5 means it has 15 percent nitrogen, 10 percent phosphorus (as P₂O₅), and 5 percent potassium (as K₂O). According to the fertilizer tag, also soil and leaf chemical analysis reports the nutrient contents in oxide form, and fertilizer recommendations are given in oxide form as well. These oxide forms are not really present in the fertilizers, nor in the soils nor in the plants. For example, potassium does not exist as K₂O in any fertilizer – potassium is present as a salt like potassium chloride (KCl) in MOP fertilizer or potassium sulfate (K₂SO₄) in SOP fertilizer.   Potassium is taken up by plant roots from the soil solution as K⁺ cation. The main reason for the oxide expression is an historical convention: The practice of expressing nutrient content as oxides dates to the early days of fertilizer analysis and production. It was convenient because many fertilizers contained nutrients in the form of various compounds, including oxides, which were easy to analyze and quantify using traditional chemical methods. In old days, plant ash analysis was the major procedure in plant nutrition. Early chemists ashed their samples so that nutrients that didn’t volatilize were oxidized under the high temperatures remained as oxides.

Therefore, it became a practice to express various plant nutrients found in plant ash in oxide form (P₂O₅, K₂O, CaO, MgO).While P and K are expressed in their oxide forms, nitrogen is expressed as elemental N (nitrogen). This is due to the historical methods of chemical analysis. In the past, nitrogen was typically quantified using Kjeldahl analysis because it would turn volatile when the sample was combusted. On the other hand, potassium and phosphorus were quantified from calcined samples, which means the samples were exposed to high temperatures to eliminate all water and carbon before analysis. These ashes resulting from the calcination process contained all the non-volatile elements. When determining in the lab the concentrations of phosphorus and potassium from these ashes, they were measured as P₂O₅ and K₂O, respectively.

Nowadays the labs use modern analytical techniques such as inductively coupled plasma mass spectrometry. Most of the countries have regulations and labeling standards that require fertilizer manufacturers to express nutrient content as oxides. In some countries like Scandinavia, Australia, Argentina and South Africa the fertilizer labels are in elemental forms (N-P-K). Other nutrients like calcium, magnesium and sulfur can also be expressed in oxide or elemental form.

In order to convert oxide form to elemental and vice versa, all you need to know is the atomic weight of each atom in the oxide and calculate the factors (i.e., the percentage of P in P₂O₅). To facilitate these calculations or avoid the need to memorize the factor numbers, we have organized all these conversions in one Nutrient Converter tool. Our Nutrient Converter allows conversions between the two units (oxide or elemental).

Utilize ICL’s nutrient converter to gain a better understanding of fertilizer numbers and letters.