How to Read a Fertilizer Label: What Do NPK, SO3 and ME Mean?
Short answer: The three numbers on a fertilizer bag (e.g. 15-15-15) show, in order, the percentage by weight of nitrogen (N), phosphorus (as P2O5) and potassium (as K2O) in the fertilizer. A 50 kg bag of 15-15-15 contains 7.5 kg N, 7.5 kg P2O5 and 7.5 kg K2O. SO3 expresses the sulphur in the fertilizer (as sulphur trioxide); 20% SO3 corresponds to about 8% elemental sulphur. ME stands for micronutrients (in Turkish, "mikro element"); it indicates that the fertilizer also contains nutrients needed in small amounts, such as zinc.
The bags standing side by side at a fertilizer dealer or in a warehouse look alike at first glance: numbers, abbreviations and expressions in brackets... Names such as "15-15-15+(20SO3)+ME", "NP 20-20-0" or "CAN 26" are in fact the fertilizer's identity. A farmer who reads the label correctly gives the crop the nutrients it really needs and avoids unnecessary spending. In this article we explain every piece of information on a fertilizer label in plain language.
What Is NPK? The Meaning of the Three Numbers
The most basic information for mineral fertilizers is the NPK value. These three letters represent the three primary nutrients that plants need most:
- N – Nitrogen: Green growth, shoot and leaf development.
- P – Phosphorus: Root development, flowering, seed and fruit formation.
- K – Potassium: Fruit quality, water balance, resilience.
The numbers are always written in the same order: N – P2O5 – K2O. They show the percentage by weight of each nutrient in the fertilizer.
An important detail: P2O5 and K2O
The phosphorus and potassium values on the label are given not as the pure element but in oxide form (P2O5 and K2O). This is an established standard in the fertilizer industry. If you want to convert to the pure element, approximately:
- P2O5 × 0.44 ≈ elemental phosphorus (P)
- K2O × 0.83 ≈ elemental potassium (K)
In practice, fertilizer recommendations and analysis reports are also usually given in terms of P2O5 and K2O, so this conversion is often unnecessary. What matters is making sure the recommendation and the label use the same unit.
What does a zero mean?
A 0 means that the fertilizer does not contain that nutrient. For example:
- DAP 18-46-0: 18% nitrogen, 46% P2O5, no potassium.
- NP 15-36-0: 15% nitrogen, 36% P2O5, no potassium.
- NP 20-20-0: 20% nitrogen, 20% P2O5, no potassium.
For straight (single-nutrient) fertilizers, usually only the relevant nutrient is stated: Urea 46 (46% N), CAN 26 (26% N), Potassium Chloride (61% K2O) and so on.
Additional Terms on the Label: SO3, ME, CaO, S
SO3 – Sulphur
Expressions in brackets such as (20SO3) show the fertilizer's sulphur content. Sulphur is involved in protein synthesis and helps the plant use nitrogen efficiently; it is especially important for cereals, oilseed crops (sunflower, canola) and legumes.
On labels, sulphur is sometimes written as SO3 (sulphur trioxide) and sometimes as S (elemental sulphur). Conversion:
- SO3 × 0.4 ≈ S
- Example: 20% SO3 ≈ 8% S; 15% SO3 ≈ 6% S; 10% SO3 ≈ 4% S.
So when comparing two fertilizers, check whether the unit is SO3 or S. For example, on the Ammonium Sulphate label, sulphur is given as 24% S.
ME – Micronutrients
ME stands for "micronutrients" (Turkish: mikro element). These are elements that plants need in very small amounts but whose deficiency causes serious problems: zinc (Zn), boron (B), iron (Fe), manganese (Mn), copper (Cu), molybdenum (Mo). If "+ME" appears in the fertilizer name, it contains one or more of these elements. Which micronutrient is present, and at what level, should be stated in the detailed part of the label. For example, NPK 15-15-15+(20SO3)+ME contains 1% zinc (Zn).
CaO – Calcium
In some fertilizers, calcium is stated as CaO (calcium oxide). For example, CAN 26 contains 8% CaO in addition to 26% N. Calcium is part of the cell wall structure and is of particular importance on acidic soils.
Example: How to Read "15-15-15+(20SO3)+ME"
Let us read this expression piece by piece:
| Term on the label | Meaning | Amount in a 50 kg bag |
|---|---|---|
| 15 (first number) | 15% nitrogen (N) | 7.5 kg N |
| 15 (second number) | 15% phosphorus (P2O5) | 7.5 kg P2O5 |
| 15 (third number) | 15% potassium (K2O) | 7.5 kg K2O |
| (20SO3) | 20% sulphur trioxide (≈ 8% S) | 10 kg SO3 (≈ 4 kg S) |
| +ME | Contains micronutrients (1% Zn in this product) | 0.5 kg Zn |
In other words, this fertilizer is a balanced NPK compound fertilizer supplying the three primary nutrients in equal proportions and also containing sulphur and zinc.
Recognising Fertilizer Types from the Label
| Fertilizer group | How it appears on the label | Example AKFE products |
|---|---|---|
| Nitrogen | Single number or name, % N | Urea 46, CAN 26, Ammonium Sulphate |
| Phosphate (NP) | K value is 0 | DAP 18-46-0, NP 15-36-0, NP 20-20-0+(12SO3)+ME |
| Potash | % K2O | Potassium Chloride, Potassium Sulphate |
| NPK compound | All three numbers filled | NPK 15-15-15+(15SO3)+ME, NPK 25-5-10, NPK 13-24-12, NPK 12-25-12 |
| Liquid | % N or content, litres | UAN 32 + ME, Phosphoric Acid |
| Soil conditioner | Humic/fulvic acid content | Humic Acid |
Calculating the Rate from the Label: A Simple Formula
A soil analysis and an agronomist's recommendation usually give you a requirement in the form "X kg of pure nitrogen (or P2O5, K2O) per decare". To convert this into an amount of fertilizer:
Fertilizer required (kg/da) = Pure nutrient required (kg/da) ÷ (Nutrient percentage in the fertilizer ÷ 100)
Example calculation (only to illustrate the method): Suppose the recommendation is 6 kg of pure nitrogen per decare.
- With Urea 46: 6 ÷ 0.46 ≈ 13 kg/da of urea
- With CAN 26: 6 ÷ 0.26 ≈ 23 kg/da of CAN
- With Ammonium Sulphate (21% N): 6 ÷ 0.21 ≈ 29 kg/da of Ammonium Sulphate
As you can see, different amounts of different fertilizers are needed for the same amount of nitrogen. That is why, when comparing fertilizer prices, it is more accurate to look not at the bag price but at the cost of 1 kg of nutrient.
For compound fertilizers, the rate is usually set first according to the most critical nutrient, and any shortfall or excess in the other nutrients is balanced with straight fertilizers. We recommend doing this calculation together with an expert.
What Else to Check on the Label
Besides the NPK values, you should expect to see the following information on a reliable fertilizer label:
- The name and contact details of the manufacturer or importer,
- The fertilizer type and name,
- The guaranteed analysis (nutrient contents) and their forms (e.g. urea, ammonium or nitrate form of nitrogen),
- Net weight or volume,
- Batch/lot number and production date,
- Usage, storage and safety warnings.
The form of nitrogen also matters: the urea form becomes available to the plant after conversion in the soil and can be lost if left on the surface; the nitrate form acts quickly but is prone to leaching; the ammonium form stays in the soil longer but has an acidifying effect on the soil.
Common Mistakes
- Thinking the numbers are "kilos": 15-15-15 does not mean there are 15 kg of nitrogen in the bag; it means 15% nitrogen.
- Confusing SO3 with S: 20% SO3 is not 20% sulphur, but about 8% sulphur.
- Looking only at the bag price: Not calculating the cost per unit of nutrient.
- Choosing the fertilizer with the "highest numbers" without analysis: Paying for nutrients the crop does not need.
Conclusion
A fertilizer label is in fact a nutrient prescription. Once you know that the NPK numbers are percentages, that phosphorus and potassium are expressed in oxide form, and that SO3 means sulphur and ME means micronutrients, you can compare different fertilizers knowledgeably and make the choice that suits your soil analysis.
Need help choosing the right fertilizer? Contact the AKFE Gübre team with your soil analysis results, and we will prepare a quote for products that meet your needs. Delivery to all 81 provinces of Turkey — contact: +90 462 325 05 41 / +90 531 474 78 85, info@akfe.com.tr.
Frequently Asked Questions
What does 15-15-15 on a fertilizer mean?
15-15-15 shows that the fertilizer contains, by weight, 15% nitrogen (N), 15% phosphorus (P2O5) and 15% potassium (K2O). A 50 kg bag contains 7.5 kg N, 7.5 kg P2O5 and 7.5 kg K2O.
What does NPK mean?
NPK is the abbreviation for the nutrients nitrogen (N), phosphorus (P) and potassium (K). On a fertilizer label they are always written in the order N – P2O5 – K2O and as percentages by weight.
What does SO3 mean on a fertilizer?
SO3 expresses the sulphur in the fertilizer as sulphur trioxide. To convert to elemental sulphur, the SO3 value is multiplied by about 0.4; for example, 20% SO3 corresponds to about 8% sulphur.
What does ME mean on a fertilizer?
ME stands for micronutrients. It shows that the fertilizer contains at least one of the elements plants need in small amounts, such as zinc, boron, iron, manganese, copper or molybdenum. Which element is present, and at what level, is stated in the details of the label.
What does a 0 on a fertilizer label mean?
A 0 means that the fertilizer does not contain that nutrient. For example, DAP 18-46-0 contains no potassium.
How do I calculate how much fertilizer to apply per decare?
Divide the required amount of pure nutrient (kg/da) by the nutrient percentage in the fertilizer divided by 100. For example, for 6 kg/da of nitrogen, about 13 kg/da of Urea 46 is needed. The required amount should be determined by soil analysis and expert advice.
What is a compound fertilizer?
A compound fertilizer contains more than one primary nutrient (usually N, P and K) together in a single granule or a single product. NPK 15-15-15 and NPK 25-5-10 are examples of compound fertilizers.
