NPK Fertilizer Calculator
The NPK numbers on a fertilizer bag represent the percentage by weight of nitrogen (N), phosphorus (P₂O₅), and potassium (K₂O). A 10-10-10 fertilizer contains 10% of each nutrient by weight.
What each nutrient does
- N, Nitrogen: promotes leaf and stem growth; makes plants greener. Deficiency causes yellowing leaves. Excess promotes lush foliage at the expense of fruit and flowers.
- P, Phosphorus: supports root development, flowering, and fruit set. Deficiency causes purple or red discoloration. Important at planting time.
- K, Potassium: improves disease resistance, fruit quality, and stress tolerance. Deficiency causes brown leaf edges.
Fertilizer ratios for common plants
| Plant type | Recommended ratio | Notes |
|---|---|---|
| Leafy vegetables (lettuce, spinach) | High N, low P/K | e.g. 20-5-5 |
| Fruiting vegetables (tomato, pepper) | Balanced or low N | e.g. 5-10-10 |
| Flowering plants | Low N, high P | e.g. 5-15-5 |
| Lawn grass | High N | e.g. 30-10-10 |
| Root vegetables | Low N, moderate P/K | e.g. 5-10-10 |
Application tips
- Always water before and after applying granular fertilizer to prevent root burn
- Over-fertilizing is as harmful as under-fertilizing, follow label rates
- Conduct a soil test every few years to identify actual nutrient deficiencies
UK regulations and organic fertilizer options
In England and Wales, farms near rivers and aquifers are designated as Nitrate Vulnerable Zones (NVZ) under the Nitrates Directive. In NVZs, strict limits apply to how much nitrogen can be applied per hectare and when (no applications between September and January). Home gardeners are not legally bound by NVZ rules, but the principle, not over-applying nitrogen near watercourses, is good practice everywhere.
Widely available organic alternatives in UK garden centres:
| Product | Approx. NPK | Notes |
|---|---|---|
| Blood, fish and bone | 5-5-6 | Balanced slow-release; good general feed |
| Bonemeal | 3-11-0 | High phosphorus; ideal at planting |
| Pelleted chicken manure | 4-2-2 | Slow-release; low odour |
| Seaweed meal | 1-0-2 | Low NPK but rich in micronutrients and growth hormones |
The RHS recommends a full soil nutrient analysis (£25-35 from accredited labs) before starting a fertilizer programme, to avoid applying nutrients already present in abundance.