What you need to know about fertilizing your garden with phosphorus

Phosphorus energizes root growth and helps plants as they approach maturity and begin to flower. Here's what you need to know about the fertilizer, as well as some materials you can use to get the nutrient to your plants.

What you need to know about fertilizing your garden with phosphorus

What phosphorus does

Unlike nitrogen, phosphorus persists in the soil and moves very little.

  • When applied in liquid form, it remains in the top inch or so of soil; in dry fertilizers, it remains near the fertilizer particles.
  • For this reason, make sure you mix phosphorus into the soil so roots can benefit.

What a lack of phosphorus looks like

Plants suffering from lack of phosphorus sometimes develop a bluish cast.

  • Leaves may also turn a dark, dull green on top and bronze-purple on the undersides.
  • Stems remain thin and may turn purplish.

For a quick boost

Spray plants weekly as needed with fish emulsion; it has about five per cent phosphorus that is immediately available to plants.

Phosphorus and springtime

Phosphorus becomes less available to plants in acidic and cold, wet soils.

  • This is critical in spring, when young roots are trying to get established.
  • Watch for symptoms of deficiency and add phosphorus as needed. You can quickly sweeten acidic soil — and supply needed phosphorus — with a light sprinkling of wood ashes.


Bonemeal is a slow-released source of phosphorus, remaining in the soil for up to a year.

  • Buy steamed, crushed bonemeal that is finely ground so that soil microorganisms can break it down more readily.
  • Bulbs always appreciate a bit of bonemeal mixed into the soil beneath them, where their roots are sure to find it.

Rock phosphate

Rock phosphate is washed, crushed limestone and contains about 33 per cent phosphorus.

  • It releases the nutrient slowly and will work in acidic soil.
  • Collodial phosphate is the residue left from washing limestone and comes in small particles. It contains about 20 per cent phosphorus, some of which is available immediately; it works best in neutral soil.

If you go overboard and end up with excess phosphorus, work in extra nitrogen and potassium to balance it out.

Sources of phosphorus

Bird guano

  • Source: Dried seabird manure
  • Content: 8%
  • Application rate: 1.5 kg per 9 square metres


  • Source: By-product of meat processing
  • Content: 20%
  • Application rate: 1.5 to 2.5 kg per 9 square metres

Fish emulsion

  • Source: By-product of fish processing
  • Content: 5%
  • Application rate: 1 part to 20 parts water

Rock phosphate

  • Source: Washed, crushed limestone
  • Content: 33%
  • Application rate: 4.5 kg per 9 square metres
The material on this website is provided for entertainment, informational and educational purposes only and should never act as a substitute to the advice of an applicable professional. Use of this website is subject to our terms of use and privacy policy.
Close menu