Understanding Fertilizer Tags
You'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.
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
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.
As 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.
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