Understanding Fertililizer Tags

   Understanding Fertilizer Tags  

fertilizer_tag.jpgYou'll notice that fertilizer tags always contain a number such as 10-10-10, 0-35-0, 3-18-18. Most people know that these numbers represent an "NPK Value", so represents the Percentage of Nitrogen (N), Phosphorus (P) and Potassium (K) within the fertilizer in that order.  However, A lot of people believe that the NPK value is the percentage of pure, nitrogen Phosphorus and Potassium. This is incorrect.  Actually fertilizer tags are calculated based on Nitrogen, P205 (phosphate) and K20 (potash). You'll notice then that the number does not represent elemental phosphorus or potassium but an oxidized compound of each element. This is true even when there is no p205 or k20 compounds within the fertilizer.

So why do they do this?  In the olden times, chemists used a technique of weighing after ignition to determine the phosphorus and potassium content of fertilizers. This method meant that the phosphorus and potassium was oxidized in the form of phosphorus oxide (P2O5) and potassium oxide (K2O). This has stuck and the amounts of P & K in fertilizers are still expressed in these forms to this day. Nitrogen on the otherhand is always expressed as elemental N.

To confuse matters more, it is standard for soil lab tests to report nutrients in the soil in its elemental form. So thus, if someone does not understand that the nutrients on the fertilizer bags and the nutrients reported from the soil is not the same, they may apply completely skewed applications of fertilizers to their soils which could be detrimental to achieving high quality produce.

So how do we convert a fertilizer tag value from a phosphorus and potassium compound into a percentage that represents the pure form of these major nutrients that are present in the bag and also correspond to the reading given by a soil labs elemental analysis? Also how do we convert back from an elemental soil lab reading to a fertilizer NPK analysis?

1) For phosphorus you multiply the P205 reading on the fertilizer tag by 0.44. This will get you the actual P reading and not just P205.
To do it the other way round you multiply elemental phosphorus by 2.29 to get P205 as written on a fertilizer tag.

2) For potassium you multiply the fertilizer tag (k20) by 0.83 to get actual K. To convert back you multiply by 1.2.

This is based on the atomic weight of the elements within the compounds. In other words when the compound is P2O5, the two Phosphorus atoms represent 44% of the atomic weight of the compound whereas the five oxygen atoms represent 56% of the compound.

fertilizer_tag_s.jpgAs an example, a 50lb bag of 10-10-10 fertilizer will contain 5lbs Nitrogen, 5lbs P2O5 and 5lbs K2O. Now when you convert the fertilizer number to the actual elemental plant nutrient it will still contain 5lbs of Nitrogen but only 2.2lbs of elemental phosphorus (5lbs x 0.44) and 4.15lbs of elemental potassium (5lbs x 0.83). The same fertilizer tag if it were reporting in elemental form would read 10-4.4-8.3.

You may also see fertilizer tags with more than 3 numbers to indicate when you see fertilizer tags that have a significant amount of a secondary nutrient such as Calcium, Magnesium or Sulfur. In this situation the secondary material is reported in its elemental form  such as 10-10-10-1S to represent 1% by weight of elemental sulfur.
Now when looking at the fertilizer tag of granular fertilizer to add to the soil, remember that it will not all be available to the plant all at once. Theres a solubility factor and only a small percentage, maybe even less than a percentage at any given time depending on soils and water levels that is available throughout the growing season. However a liquid fertilizer like our nutritional sprays is instantly available and 100% soluble, so  they have to be diluted greatly otherwise the concentration of fertilizer  may burn the leaf or simply be wasteful. That is why we recommend such high dilution rates. 
The last point about fertilizer tags is to become educated about the source of each element. Some fertilizer sources can burn plant rootlets, put bacteria out of business, cause sterile soils and an environment that is impossible to grow top quality flavorful foods. We will tackle fertilizer sources in a future E-Newsletter. 

Martin Capewell

Agriculture Solutions LLC