KNOWLEDGE · APPLICATION
NPK 18-18-18 for rice: how much, and when
18-18-18 is the workhorse grade for lowland rice: high enough in total nutrients for a single basal application, balanced enough not to distort the soil. The questions that decide the result are how much to apply, how to split it, and how to manage water around each application.
Why 18-18-18 suits lowland rice
Rice responds first to nitrogen for tillering, then to phosphorus for root establishment, then to potassium for grain filling and lodging resistance. A balanced 18-18-18 covers the first two stages in one pass and leaves potassium top-dressing optional for high-yield fields. At 54% total nutrients, a 50 kg bag delivers 27 kg of nutrients, so freight and handling cost per nutrient unit stay low.
Rates by growth stage
| Stage | Product | Rate (kg/ha) | Note |
|---|---|---|---|
| Basal (before transplanting) | 18-18-18 | 250–350 | Broadcast and incorporate into puddled soil |
| Tillering (15–20 DAT) | 18-18-18 or urea | 100–150 | Top-dress into shallow water |
| Panicle initiation (40–45 DAT) | Urea or high-N | 80–120 | Split into two doses on sandy soils |
| Grain filling (optional) | MKP foliar | 0.2–0.3% spray | Improves grain weight and uniformity |
Water management around application
Apply basal fertilizer into drained or puddled soil, then re-flood to 3–5 cm. Top-dressings go into shallow water and the field should not be drained for at least 48 hours, otherwise volatilisation and runoff remove a large share of the nitrogen. On light-textured soils, split the nitrogen into three doses rather than two.
What the trial data shows
In a rice trial in Jiangxi province, 18-18-18 at the same nitrogen rate as an imported brand produced 198,024 panicles per mu against 136,197 — a 45.4% difference — with tiller numbers of 16–19 versus 11–12. In Fujian, the same grade produced visibly greener leaves and markedly less algae growth on the water surface, a common indicator of unabsorbed nitrogen in the paddy.
Both trials used the same transplanting date, the same variety and the same water management. Only the fertilizer brand was changed.
Common mistakes
- All nitrogen at basal. Losses to volatilisation and leaching are high; split into two or three doses.
- Ignoring soil pH. Above pH 7.5, ammonium-based nitrogen volatilises faster; incorporate rather than broadcast onto the surface.
- Using chloride-sensitive grades where sulphate is registered. Minor for rice, relevant if you rotate with vegetables.
- Buying on total nutrient price. Compare water-soluble P, because rice planting often coincides with cool or cloudy weather.
Frequently asked
How many bags of 18-18-18 per hectare for rice?
A typical basal application is 250–350 kg/ha, which is 5–7 bags of 50 kg. Actual rates depend on soil test, target yield and the preceding crop.
Can I use 18-18-18 for direct-seeded rice?
Yes, but reduce the basal rate by roughly 20% and bring the first top-dressing forward, because direct-seeded rice establishes faster and can lose basal nitrogen to early flooding.
Is 15-15-15 a substitute?
Yes where soil fertility is moderate and the target yield is below 5 t/ha. 18-18-18 makes more sense at higher yield targets and where labour for extra top-dressings is limited.