Phosphorus (P)

What phosphorus does

Phosphorus is central to how plants store and use energy. It supports root development, tillering and early vigour, and in legumes it supports nodulation.

Signs of deficiency

Plants are stunted and slow, with poor tillering. Older leaves can turn dark green, then purplish. Maturity may be delayed. Symptoms are often clearest in young crops and can be hard to see later.

Why phosphorus becomes unavailable

Phosphorus moves very little in soil. Soon after fertiliser dissolves, much of the phosphorus reacts with calcium in alkaline soils, or with iron and aluminium in acidic soils, forming compounds plants cannot easily use. Calcareous soils and soils with a high phosphorus buffering index (PBI) bind more of it. Because it stays close to where it is placed, phosphorus at sowing is usually put near the seed, where young roots grow.

Testing

Soil tests such as Colwell P, read together with PBI, indicate how much phosphorus is available and how strongly the soil holds it. Over time, results show whether soil phosphorus is being built up, maintained or run down.

Other ways phosphorus goes missing

• Runoff and erosion: phosphorus attached to soil particles, or fertiliser still on the surface, can be carried off after heavy rain.
• Soil pH: availability falls in very acidic and very alkaline soils.
• Cold soils: root growth and phosphorus uptake slow in cold soils, so young crops can show deficiency early in the season.
• Seed-row effects: phosphorus fertilisers that also contain ammonium nitrogen, such as DAP, release ammonia that can damage seed at high rates in the furrow.