KNOWLEDGE · SELECTION
15-15-15 or 18-18-18? The cost-per-nutrient question
Both grades are balanced. The decision is not which is "better" but which delivers the required nutrients per hectare at lower total cost, given your soil, your yield target and how many times you can afford to send labour into the field.
The arithmetic that decides it
18-18-18 carries 54% total nutrients per tonne; 15-15-15 carries 45%. Per tonne of product you move 20% more nutrient with the higher grade, but you also pay a premium — typically USD 40–80 per tonne FOB. Divide the delivered cost per hectare by the kilograms of nutrient actually applied and the two grades often land within 5% of each other. The differences that remain are agronomic, not financial.
| Factor | Favour 15-15-15 | Favour 18-18-18 |
|---|---|---|
| Target yield | Moderate (below 5 t/ha rice) | High (above 6 t/ha) |
| Soil fertility | Good, with manure or residue | Depleted or continuous cropping |
| Labour for top-dressing | Available, 2–3 splits possible | Limited, fewer applications |
| Freight cost per nutrient | Higher | Lower |
| Risk of over-application | Lower | Higher on light soils |
Where the higher grade actually earns its premium
When transport to the farm is expensive, when storage space is limited, or when a single basal application is the only practical option, the 54% grade wins clearly. On smallholder plots with cheap labour and access to split applications, the 45% grade is usually the better economic choice.
Physical quality matters more than the grade
Two products with identical analysis can differ in granule hardness and in water-soluble phosphorus. A soft granule produces fines in the bag and in the spreader, and the fines are often the most soluble fraction — so the effective analysis at the field edge is lower than the label. Ask for granule hardness and water-soluble P for the lot, not the standard.
Field comparison
In two vegetable trials in Guizhou, 15-15-15 against an imported brand at the same grade produced 4,525 versus 3,578 kg/mu on leafy vegetables (+13.75%) and +18.75% on chilli, with more continuous fruit set and straighter fruit. The message is that grade selection sets the ceiling; product quality decides how close you get to it.
Frequently asked
Can I blend 15-15-15 with urea to imitate 18-18-18?
You can raise nitrogen, but you cannot raise phosphorus or potassium without adding DAP or MOP. Blending also introduces segregation risk unless the granule sizes match.
Is 18-18-18 too strong for seedlings?
At standard rates and with proper placement, no — but keep fertilizer out of direct contact with seed or roots, and prefer a nitrate-based source for sensitive nursery crops.
Which grade is more common in Southeast Asia?
Both are widely registered. 15-15-15 dominates the price-sensitive segment; 18-18-18 is preferred where labour is scarce and yield targets are higher.