Rock Phosphate Fertilizer

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Robert Pavlis

Rock phosphate is a recommended phosphorus fertilizer that adds phosphorus to your organic garden. As one web site says, it breaks down slowly but you have time–don’t you!

Is rock phosphate good for your plants?

Phosphorus Cycle
The Phosphorus Cycle

What is Rock Phosphate?

Rock phosphate is a mined rock that contains limestone and clay as well as a high concentration of phosphorus (P). The actual composition varies depending on it’s source but it usually contains 16 to 20% P.

Is it an Organic Fertilizer?

Wikipedia defines organic fertilizers as “fertilizers derived from animal or vegetable matter”.

Rock Phosphate is recommended as an organic fertilizer. It is not clear to me why this is considered to be organic, when other mined fertilizers are not organic? There is nothing organic about this product. It contains no animal or plant products of any kind and it not a renewable resource.

In any event the organic experts consider it to be an organic fertilizer.

Is it Good for Plants?

The first question to ask is, does your soil need more Phosphorus? Most garden soils contain plenty of phosphorus and adding too much can be detrimental to the microbe population in the soil. You should only consider adding more P if a soil test shows that you need it.

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So let’s assume your soil is deficient and you decide to add some rock phosphate to fix the problem. What happens in the soil?

Nothing!

Unless your soil pH is below 5.5, which is unlikely, the rock phosphate is completely insoluble. That means it does not mix with water, and it is not available to plants. One source I found suggests it starts to break down in 100 years. I can’t wait that long for my tomatoes!

This is certainly true of any rock phosphate mined in North America. Apparently there is a source in Africa that degrades a bit faster–starts to degrade in 2 years–but most sources in the world are not suitable as a soil amendment.

The interesting thing to me is that this is not news. Scientists have known this fact since the 1950’s, but ‘Organic Enthusiasts’ keep recommending it.

Phosphorus Chemistry

In the spoil, phosphorus occurs as three forms:

  • soluble P–is dissolved in water and is available in very small quantities
  • labile P–is held loosely by soil particles
  • stable P–makes up the majority of non-organic P in the soil and is held strongly by soil particles

Plants can access the soluble and labile P through their root system but it is easiest for them to use the soluble form. As they use up the soluble P, some of the labile P is converted to soluble P so that there is always some available to plants.

Commercial fertilizer it is mostly soluble P (in the form of phosphate). Within 24 hours this soluble P starts being converted to labile P and eventually to stable P. What this means is that most of the phosphorus fertilizer you add to the soil is available to plants for a short period of time, and then it gets locked away in the soil. Slowly, it will be made available to plants as it moves from being stable P, to labile P and finally to soluble P.

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The problem with rock phosphorus is that it does not become part of this cycle. It remains as rock for a very long time. Don’t add it to your garden.

References:

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

31 thoughts on “Rock Phosphate Fertilizer”

  1. Disappointing to read this, as I’ve just been given a 50kg drum of fine ground phosphate rock (packed in 1982 according to the label!) & I was thinking it would be of some use. 🙁

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  2. Hi, I am a beginner and I wanted to ask that if we add rock phosphate powder that you get in market in some water and use lemon juice or any other organic acid to it, to reduce its pH till lets say 6.6 or 5.5; will that solubilize more additional phosphate (from rock phosphate) to the water? And can then that water be used for plants as more phosphate is available? What will happen?

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