Know What’s in your Fertilizer

Interpreting the available nutrients in a fertilizer analysis

Whether you are using a commercial fertilizer, organic amendments like manure or compost, or fertilizer by-products from industrial sources, it’s critical to know the nutrient analysis of the product. Analyses for agricultural nutrient sources may come in several different ways:

• Guaranteed minimum analysis, like you’d see on a commercial fertilizer product (N-P₂O₅-K₂O, e.g., 5.5-26-30)
• Organic amendment nutrient tables, as seen in OMAFA’s Agrisuite – Organic Amendment tool or the Agronomy Guide for Field Crops, OMAFA Publication 811
• Manure, fertilizer, compost, or fertigation water testing from a lab

When using a nutrient fertilizer source that isn’t a commercial fertilizer, the gold standard is to submit a sample to the lab for testing. This ensures that the fertilizer value of the product can be accounted for whether there are differences between loads, or in some cases to prove nutrient value for a product purchased.

However, translating the analysis that comes back from the lab can often be challenging, and results showing different forms of the same nutrient can lead to confusion when accounting for macro- and micronutrient content.  For example, the results from a manure analysis will not provide the same information as a compost quality analysis or a guaranteed minimum analysis.

Recently, some lab analyses have come forward as an example of why they can be misinterpreted, and how that can affect a fertility plan.

How do I determine nutrient value from a lab analysis?

Nutrient analysis reports differ in how they display nutrient content, both between laboratories, and may even differ depending on the type of testing done. Nutrient content may be measured in different units as well, all of which affect the end goal of determining nutrient content from the source.

No matter how the report is presented though, for fertility planning, we want to look at nutrient content in terms of fertilizer values in the analysis, meaning a percent N-P₂O₅-K₂O analysis. Nitrogen (N) as well as all other macro- and micronutrients other than phosphorus (P) and potassium (K), are represented as the concentration of the elemental nutrient in the product. Phosphorus is represented by P₂O₅, referred to as phosphate, and potassium by K₂O, referred to as potash, in these fertilizer analyses. Neither P₂O₅ or K₂O exist naturally in the soil or are utilized by plants in this form, but are a carryover from chemical analysis long ago, and continue to be used as industry standards.

Sometimes, nutrients may be reported in different ways on the reports, as in this example from a liquid ammonium sulphate product:

Parameter	Analysis Result	Unit
Nitrogen (total)	3.05	%
Phosphorus	6.52	ug/mL
Phosphorus (as P2O5)	14.92	ug/mL
Potassium	14.05	ug/mL
Potassium (as K2O)	16.92	ug/mL
Sulphur (as SO4)	13.00	%

In this case, at first glance you might assume that the analysis of this fertilizer, due to negligible amounts of P and K would be roughly 3-0-0-13S. However, this is incorrect. The sulphur (S) measurement in this case is measuring the percentage of sulphate, not actual elemental S content. While sulphate is the plant available form of the nutrient, by weight it is only 1/3 actual sulphur. Producers counting on 13% S when determining rate of this product applied may have underapplied their S needs.

In this example, 13% sulphate (SO₄^2-) is equivalent to 4.3% sulphur.

Therefore, the analysis of this ammonium sulphate product would actually be 3-0-0-4.3S.

Depending on the analysis done, nutrients may be represented in different ways. The above example shows P and K analyses in terms of total nutrient, as well as in terms of phosphate and potash analysis. Nitrogen may show up in terms of total N, ammonium-N, nitrate-N, or organic-N, depending on the purpose of the testing.

To calculate fertilizer values from nutrient sources, it’s critical to align results with standard analyses to use recommendations correctly and prevent under or overapplication of fertilizer nutrients.

Calculating fertilizer values from nutrient results

Most labs in Ontario report the amounts of available P₂O₅ and K₂O from fertilizer, manure, or other organic amendments, but you may see a sample reported as %P or %K. Similarly, labs may report S in terms of SO₄^2- or other elements in terms of nutrient instead of standard elemental values.