Gardening glossary
Phosphorus
Phosphorus is the middle figure in NPK and the nutrient most associated with roots, flowers, seeds, and fruit. Inside the plant it shows up in ATP (cellular energy), nucleic acids, and the phospholipids that build every cell membrane. Symptoms of deficiency are sneakier than for nitrogen: older leaves develop a dark, dull, sometimes purple-tinged appearance, growth slows, and flowering is delayed or stunted.
Phosphorus behaves very differently from nitrogen in the soil. It binds tightly to soil particles and barely moves with water, which has two practical consequences for gardeners:
1. **Surface-applied phosphorus stays at the surface.** To reach deep roots, it must be worked in or banded into the planting hole. 2. **Phosphorus rarely leaches out the way nitrogen does**, so once a bed is well-stocked, you usually do not need to add more every season.
Common phosphorus sources:
- **Bone meal** (3-15-0 typical) — slow-release, easy to band into transplant holes. - **Rock phosphate** — slow but long-lived; useful for new beds. - **Composted manure** — moderate phosphorus with the benefit of organic matter. - **Triple superphosphate (0-46-0)** — concentrated mineral form when a targeted dose is needed.
Soil pH heavily affects availability. Below pH 6 phosphorus locks up with iron and aluminium; above pH 7.5 it locks up with calcium. The 6.0–6.8 sweet spot most vegetables prefer is also the most phosphorus-available window — see our [soil pH guide](/glossary/soil-ph) for adjustment strategies.
A common myth: "bloom-boost" fertilisers loaded with phosphorus (the 10-50-10 or 0-10-10 numbers) will give you more flowers. In most home soils, phosphorus is not the limiting factor. Excess phosphorus actually suppresses beneficial mycorrhizal fungi and can run off into waterways, causing algal blooms. Test your soil before piling on phosphorus.