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.

Plant Science for Gardeners by Robert Pavlis

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.

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. 🙁

    Reply
  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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  3. Greetings,

    [N.B. My primary source is Science and Technology of Organic Farming by Allen V. Barker]

    Your use of the phrase “organic fertilizer” describes the chemical use of the phrase “organic,” which is separate from the horticultural use of “organic.” This latter use is tied to e.g. OMRI certification, the USDA National Organic Program, and the Organic Foods Production Act of 1990, but more broadly “organic” is a global movement which began in the 1920’s and ’30’s through a synthesis between modern agricultural sciences and traditional agriculture (especially in India, China, and Japan).

    One concise description of the organic movement is that it aims to treat the stewarded land as an “organic whole.” This the the way Walter James AKA Lord Northbourne, who coined the phrase “organic farming,” used it in Look to the Land and subsequent writings. In other words, organic fertilizer does not need to be derived from organic material: it only needs to relate to the cultivation in a way which serves the whole system in a sustainable, regenerative way. As a rule of thumb for organic fertilizers, they should either be low in nutrients or slow to release their nutrients. This serves the holistic principle that soil ecology is more resilient to slow changes than rapid ones.

    It is true that rock phosphate is not an especially great fertilizer, but it is one tool which some circumstances will warrant, such as “veganic” gardening, which is organic-system gardening with the additional imperative that no fertilizer is of animal origin. Bonemeal is the standard phosphate source for organic fertilizer. “Basic slag” is another Phosphorus source, a by-product of iron and steel production, but is unlikely to be available in most markets. So-called “superphosphate” and “triple phosphate” are synthetic fertilizers derived from reacting rock phosphate with sulfuric acid and phosphoric acid, respectively.

    As an aside, in general, if compost is used as a nitrogen fertilizer, it will also provide for the nutrient requirements of Potassium (K), Calcium (Ca), Magnesium (Mg) and Sulfur (S), but additional fertilizing will be needed to meet the requirements for Phosphorus (P) and the micronutrients.

    Rock phosphate or “hard rock phosphate” is a nonrenewable mined material derived from ancient marine life. (As an aside, this makes it a fossil form of an animal byproduct, perhaps with the important distinction that humans are not the mechanism which converted these animals into fertilizers, so thus we bear none of the blame.)

    “Soft” or “colloidal” rock phosphate is a lower-grade of rock phosphate, being a byproduct of rock phosphate production. Soft rock phosphate is essentially the clay that gets washed off of rock phosphate during processing. These clay particles still have a good amount of residual rock phosphate clinging to them, making colloidal rock phosphate suitable as a fertilizer. But “soft” rock phosphate is not more accessible than “hard” rock phosphate, and all the clay essentially dilutes it as a phosphate fertilizer (containing about 20% P2O5 compared to 30% in “hard” rock phosphate). The available phosphate in rock phosphate is already very low.

    Barker outlines 4 conditions which must be met to succeed with rock phosphate or soft rock phosphate:

    • “The rock must be finely ground and mixed thoroughly with the soil”
    (This facilitates the physical deterioration of the rock phosphate, called weathering, which converts the material into a form available for uptake by plants)
    • “The soil should be acid”
    (5.5 pH recommended, and no less than 5.0 pH, helping to convert rock phosphate into available phosphates)
    • “Organic matter should be added with the rock phosphate”
    (Materials such as compost and manure produce acids and facilitate chelation which help to convert rock phosphate into available phosphates)
    • “Rock phosphate should be applied in amounts that are two, four, or even up to 10 times the recommendations for phosphorus application indicated by soil tests for chemical fertilizers to allow for the low availability of phosphorous in rock phosphate”
    (Available P205 is 3% for rock phosphate, 2% for colloidal/ soft rock phosphate)

    Another limitation of rock phosphate (including soft/colloidal rock phosphate) is that the Phosphorus becomes less available the longer it is in the soil, until years have passed and very large reserves of non-available Phosphorus which have accumulated in the topsoil gradually convert to available forms (this will only happen on sites with little erosion). Rock phosphate should be added to the soil at the time of seeding or transplanting.

    Hope that helps. Happy gardening!

    Reply
  4. I came across a pallet of ‘Tennessee Black Rock Phosphate’ at an auction once, and brought it home for the same price as lime. I spread it in April on a lower hayfield/pasture that would consistently produce 2t. acre of baled hay in mid-June, then take up the slack for our permanent pastures through mid-October. In other words, a good fertile, well watered field. There was no immediate response, or change in the hay, but immediately after harvest the entire area shot up in alsike clover so thick you couldn’t walk through it without picking up your feet. This was all the more surprising since, while the pasture always would come in as a nice blend of small white clover and grass, NO alsike at all was really noticed before.
    Fortunately, the former owner and I were still in contact and when I told him about it, he told me they’d sown it in an alsike/orchard grass blend over twenty years ago, before the entire field was covered by the reed canarygrass that had pretty much taken over all of the lower fields in that vicinity. When I told the whole story to the elderly retired Ag extension agent who lived nearby, he gave me a big grin and explained the great ‘rock phosphate’ dilemma. Long story, short…

    On mildly (5.9-6.4) acidic upland glaciated soils, the stuff works like a miracle for growing legumes- even more so than direct application of lime and P- and no one knows why, even though there are an awful lot of ‘experts’ who say it can’t be true.

    Now, what the value of that is to you, you can figure out for yourself. I didn’t see any yield increase on the hay, but the cows ate a lot more of it and were happier doing so. The late pasture was definitely thicker, and the cows would graze every inch of that ground twice before they’d venture onto the rest, but I can’t really say I noticed any increase in milk output. I sure did notice the gloss it threw on their coats though, and I’m not kidding. I suspect it’s sort of the difference between how you feel eating ‘X’ amount of protein, calories, vitamins, minerals, and etc. at McDonalds and the same of well grown meat, potato, and vegetables at home.

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  5. As a chemist I can see the logic about your comments on rock phosphate. However as a gardener with a fixation on fungi I have to disagree with your conclusion. Around 1999 scientists came to the realization that at least 90% of higher plants need fungi to extract insoluble phosphorus from soils. The fungi extract the phosphate using oxalic acid only they are equipped to handle and trade plants for carbohydrates. Add rock phosphate to soils and let fungi do what fungi do. For a lengthier explanation go to my website and look up “Why Organic Gardening Works”.

    Reply
    • Went to the web site could not find “Why Organic Gardening Works”.

      Your statement “90% of higher plants need fungi to extract insoluble phosphorus from soils” is not correct, but if you have a good reference that supports this idea, please post it. Most plants do use fungi in normal soil situations, but they grow just fine hydroponically and in soilless mixes, clearly showing they don’t need the fungi.

      But none of this has anything to do with rock phosphate unless you can demonstrate that fungi are able to decompose this rock much quicker than it decomposes in soil on its own. I have not found any source that shows rock phosphate decomposes quickly in any soil, with or without fungi.

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      • Yes, it is true! Fungi ARE able to decompose rock phosphate much quicker than it decomposes in soil on its own.

        EITHER rock phosphate or soluble fertilizers will work equally well in a garden. If you put rock phosphate down in a garden, the rock phosphate will be dissolved by oxalic acid which mycorrhizae fungi produce. The fungi then trade higher plants for carbohydrates they cannot produce.

        To find out some about this, on the website labellegardenclub.org, in the list on the right, click on number 17 on the list “why organic fertilizing works”. It goes into it in some detail.

        Book References: “Mycorrhizae in Crop Production” Hamel and Plenchette editors, and “Mycelium Running”, Paul Stamets (my favorite)

        From Wikipedia “Unaided plant roots may be unable to take up nutrients that are chemically or physically immobilised; examples include phosphate ions and micronutrients such as iron. One form of such immobilization occurs in soil with high clay content, or soils with a strongly basic pH. The mycelium of the mycorrhizal fungus can, however, access many such nutrient sources, and make them available to the plants they colonize.[7] Thus, many plants are able to obtain phosphate, without using soil as a source.”

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        • The gardening club reference has not references to their sources.
          The Wikipedia reference does not even mention rock phosphate. I agree fungi are able to access other forms of phosphate as per the reference, but this does not mean they can access rock phosphate.

          I decided to look into this some more and found this reference from 98: https://page-one.live.cf.public.springer.com/pdf/preview/10.1023/A:1008830129262

          “It was found that direct application of low-grade rock phosphate as a P source in neutral and alkaline soils was found to be of little importance (Rogers et al. 1953; Cooke 1956)”

          “However, it was found that plant inoculation with VA-mycorrhizal fungi increased the phosphate uptake by mycorrhizal compared with nonmycorrhizal controls (Evans & Miller 1990; Ruiz-Lozano & Azcon 1993). Many of these studies were done in either sterilized soil or in a small volume of nonsterile soil. Little is known also about the participation of rock-phosphate-solubilizing fungi and mycorrhizal inoculation on growth and P nutrition of crop plants in unsterilized soil, especially under field conditions, where introduced fungi must compete with the indigenous fungal population”

          The study did go on to inoculate fields with specific fungi and found plants absorbed more phosphate. I don’t have the full document. But this is still not proof that there is significant use of rock phosphate in normal soil conditions.

          Unless you can find more recent work, it seems there is at best limited proof that fungi have much of an effect on rock phosphate.

          Another reference: https://link.springer.com/article/10.1007%2FBF02377067?LI=true, did similar testing and found that “However, none of these variables (uptake of P by plants) were found to be significantly correlated with the fungal populations”.

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          • https://www.sciencedirect.com/science/article/pii/S0944501316303834

            Phosphate dissolving fungi: Mechanism and application in alleviation of salt stress in wheat

            The present investigation reveals the solubilization efficiency of tri-calcium phosphate (TCP), Udaipur rock phosphate (URP), aluminium phosphate (AP) and ferric phosphate (FP) by Aspergillus niger (ITCC 6719) and Trichoderma harzianum (ITCC 6721) as function of carbon concentrations.

            The two fungi employed different mechanisms to reduce medium pH for release of P from TCP, AP and FP. However, URP was solubilized solely through fungal production of citric, succinic, propionic, malic and acetic acid.

            Use of A.niger as bio-inoculant could be a sustainable approach to improve soil P availability, promote plant growth and alleviate adverse effect of salt stress

            (The guy was correct)

          • I gave it a quick review.

            1) It is all lab work, so evidence of what rock phosphate might do in real soil.
            2) Table 2 seems to show the amount of phosphate available over time for different phosphate sources. Udaipur rock phosphate did not increase phosphate levels over 15 day period. The value for time zero is missing, and no stats have been applied to the data, so we really don’t know if Udaipur rock phosphate increased phosphate levels.
            3) Fig 1 seems to indicate that Udaipur rock phosphate added very little P, but again there is no control where no P was added so we can’t tell if the levels are above a control.

            I don’t find the paper very convincing to show how quickly rock phosphate is made available in a garden soil.

    • Not sure I understand the question, but if you need to increase phosphate levels, use a water soluble form of phosphate, not rock phosphate.

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  6. This article seems like personal opinion based on a basic understanding of the phosphorus cycle and chemistry. A cursory search of the relevant academic literature shows that almost all of the phosphate in rock phosphate is available over time. A number of studies showed that rock phosphate was competitive with acid treated, soluble forms within 10 years of initial use. This may seem like a long time, but to a family farm that measures success in decades, it is a worthwhile investment.

    You are right to mention the connection between acidity and phosphate availability, but I’m not sure where you came up with a pH of 5.5. Much of the literature suggests that the phosphate is made available by microorganisms over time at a normal growing pH, that is 6.3-6.8. Heavy liming made the phosphorus less available, but small amounts increased availability. Adding rock phosphate and elemental sulfur to a compost pile is regarded as an effective way to speed up phosphorus mineralization.

    Rock phosphate is accepted in the NOP because it is not synthetic and has not been treated by acids as in super phosphate and other more available forms.

    Gardeners would not use rock phosphate because at the scale at which most people garden, they can get their phosphorus from compost or bagged fertilizer mixes. This isn’t economically feasible on a commercial production scale. Rock phosphate is affordable (<$400/ton delivered) and slow release, which farmers who want to limit field passes, organic or otherwise, find valuable. That doesn't make it a myth, just as the countless other products and techniques that farmers use and gardeners don't aren't myths. They are fundamentally different practices driven by different goals.

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    • Can you provide a reference to support the idea that rock phosphate is made available in 10 years?

      This blog is written for gardeners, not farming operations. As you conclude this is not a suitable product for gardeners, but many companies are promoting the product for them.

      Reply
  7. RE: “It contains no animal or plant products of any kind and it not a renewable resource.”
    Could you comment on this please:
    My understanding is that is the two primary ‘minerals’ that make up phosphorite (rock phosphate) are hydroxyapatite and fluoroapatite. According to this wikipedia page https://en.wikipedia.org/wiki/Phosphorite “Ca5(PO4)3OH or Ca10(PO4)6(OH)2, which is often dissolved from vertebrate bones and teeth, whereas fluorapatite can originate from hydrothermal veins” and according to Table 2 of ‘Investigation of Cd contents in several phosphate rocks used for the production of fertilizer’
    https://www.researchgate.net/publication/225274804_Investigation_of_Cd_contents_in_several_phosphate_rocks_used_for_the_production_of_fertilizer the primary source of rock phosphate from Florida*, USA, is primarily composed of Hydroxyapatite.

    *Florida a primary source for rock phosphate in the USA “Florida is presently providing approximately 75 percent of the nation’s supply of phosphate fertilizer and about 25 percent of the world supply.” http://www.fipr.state.fl.us/about-us/phosphate-primer/other-phosphate-deposits

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      • Your claims that ” “It contains no animal…products of any kind…”
        Just wanted to know if you consider “dissolved from vertebrate bones and teeth” to be – contains no animal products.

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        • At what point do you no longer consider something to be organic? If it was organic millions of years ago, is it still organic, if it is now a pure compound? Since rock phosphate no longer contains organic molecules, I don’t consider it to be organic. It is now no different than any other mineral.

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          • The simple fact that it is formed in a natural cycle over a long period of time seems good enough reason for me to consider it organic but hey I am not an expert.

  8. Robert, we just put on 2000 lbs/acre based on our soil test. I remembered Calphos as being granular, but it’s not, it’s a powder and looks like a yellow limestone. I agree with you, to just throw material at the garden or crop without a soil test is a waste. This application was based on our chronically low P levels. Our soils hold K like crazy so we put minimal amounts of that on….but that P and Ca!!!! Enjoying your comments!

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  9. I think that there are 2 types of rock phosphate. One is hard rock phosphate (not very useful for produce) and the other is soft rock phosphate-the soft rock that we apply is called Cal-Phos and I think that it’s 0-3-0 (3% available and 20% total). It is a brown, granular material which softens up when wet. I think that it also contains a total of 20% Ca too. Our soils are very low in P and Ca so we apply the Cal-Phos for a slower but more sustained release at high rates (1000-2000 lbs/acre), as well as 180 lbs/a of 11-52-0 (MAP) for a quicker shot of P. It works for us!

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      • I worked for a company in Alberta Canada that processed Rock Phosphate by the tons.and sent most of it to the US farmers. the process was quite ingenious. They actually broke the raw Phosphate down to about 4 or 5 microns so it was actually like water. and when mixed with water and applied to the soil ,went straight to the roots.
        just saying

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        • 5 microns is still quite large compared to the size of atoms. Phosphate, once in solution actually moves very slowly in soil, about an inch a year, so it does not “go straight to the roots”.

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  10. I once observed a grass field where half was treated with rock phosphate and the other half was not. The treated side was obviously greener and taller than the untreated side.

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    • What does that tell you? Unless you also know that other details for these fields it does not tell you much. Assuming we know that all other conditions for both fields were identical, then it is possible that the fields were short of phosphorus and the pH was low. In that case, some of the P would be released.

      Are you sure it was not treated with super phosphate? That dissolves much more quickly and is a common fertilizer.

      Reply
    • The term ‘Rock Phosphate’ may be a poor choice for the product given many of the new products on the market–but it has been called this for a long time. Much of the phosphate used in agriculture is mined, and the minerals they mine is called Rock Phosphate. The product contains up to 30% phosphate which is quite high. The ‘rock’ part is not important–it is just a good source of phosphate.

      I am not familiar with ‘glacial rock dust’ but have looked at a number of such products. There are now many claims of magic happening from mineral deposits from ancient volcanoes or glacial deposits. I decided to have a look at this product.

      If you look at the ingredient list on the product web site it has 1.4% calcium, 0.6% magnesium, virtually no cobalt, 1% iron and 1.3% sodium. Sodium is probably not good for the plants. Plants need very little iron and cobalt and soil usually has enough of these nutrients. Plants do need more calcium and magnesium, but again most soil is not deficient of these nutrients. If you soil does need either of these, adding 1% is not very much. There are many cheap sources of calcium (bone meal) and magnesium (Epsom salts).

      The levels on nutrients are so low as to be almost nonexistent.

      the comment ” Long-lasting: Good for the whole season ~ Slowly available nutrients ” is important. The product is ground rocks. Rocks don’t dissolve very easily. What this means is that it will take many years before the plants can use these nutrients. For one product I reviewed it was expected that it would take 100 years before the product was available to plants.

      “Our Glacial Rock Dust lets the soil re-create the colloids (minerals and humus) which are needed to improve soil structure, moisture holding properties, nutrient availability and bacterial action.” This statement is just wrong. The product does not contain any humus and adding a bit of minerals will not create humus. This product will have almost no effect on soil structure or moisture holding properties.

      For more information on humus see this post: https://www.gardenmyths.com/what-is-humus/#more-1424

      To understand how soil structure is improved visit: https://www.gardenmyths.com/what-is-the-real-value-of-organic-fertilizer/#more-1407

      In summary, this product adds very little value to the garden. It won’t hurt your plants and if you need more calcium and magnesium, it will help a bit.

      Reply

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