Phosphorus Fertilisers

What phosphorus actually does

Phosphorus is the P in NPK, quoted throughout British horticulture as P₂O₅ rather than as the element. It is central to energy transfer inside the plant, to root establishment and to flower and seed formation, which is why it matters most at two moments: when a plant is going in and when it is setting bud. It is the least mobile of the three major nutrients — it barely moves in soil — so getting it into the root zone at planting is worth more than surface-dressing it later.

An honest word before you buy any of this

Most established British garden soils are not short of phosphorus. Decades of general-purpose feeding, bonemeal and manure have left many beds with more than the plants can use, and adding more does nothing except cost money. Worse, excess phosphorus interferes with the uptake of zinc and iron, so an over-phosphated soil can produce deficiency symptoms in a plant surrounded by nutrients. If you have not had a soil test, that is the cheaper first step. Where phosphorus genuinely earns its place is on new ground, on thin or sandy soils, in containers, and at planting time.

The slow-release mineral

Rock Phosphate is micronised so that the particle size gives it far more surface area than the coarse rock phosphate sold agriculturally, which is what makes it usable in a garden timeframe. It also carries a substantial calcium fraction. It is slow — measured in seasons rather than weeks — and it is at its best forked into a bed before planting or mixed through the soil from a planting hole. It does the planting-hole job most gardeners reach for bonemeal to do, from a mined mineral rather than an animal by-product.

The blended feeds

• PK Fertiliser 0-5-5 with Calcium. No nitrogen at all, so it feeds flower and fruit development without pushing leafy growth — useful late in the season, and useful where a plant is already growing strongly and does not need more nitrogen.
• Bloom Fertiliser 2-8-4. The high-phosphorus blend, for flowering plants through bud formation.
• Fruit & Vegetable Fertiliser 4-5-6. A balanced feed with a meaningful phosphorus fraction, for the veg plot and the fruit bed.

Common questions

How do I know if I need phosphorus?

A soil test is the only reliable way. Visible phosphorus deficiency is uncommon in British gardens and easily confused with other things: stunted growth, dull dark-green or purple-tinged foliage on older leaves, poor flowering. Purple leaves in particular are far more often cold soil in spring than a phosphorus shortage, and they correct themselves as the ground warms.

Is rock phosphate the same as bonemeal?

No. Bonemeal is an animal by-product; rock phosphate is a mined mineral. They do a similar job at planting.

Why is your rock phosphate quoted as elemental P and the blends as P₂O₅?

The mineral is described by its actual analysis; the blended fertilisers follow the advertising convention that UK gardeners will recognise from every other bag on the shelf. To convert, multiply elemental P by 2.29 to get P₂O₅.

Can I use these on acid-loving plants?

With care. Rock phosphate carries calcium and will nudge pH upwards over time, which is the wrong direction for ericaceous plants. See hydrangea and ericaceous feed instead.

The nitrogen equivalents are in nitrogen fertilisers and the potassium ones in potash and high potassium fertilisers.