Synthetically produced fertilizer is not necessarily the dream material for gardens: Sensible application only after soil analysis, helpful and tolerable application for the environment and plants only after complicated quantity calculation; In general, fertilizer, the production of which consumes tons of petroleum and promotes climate change, is not exactly the latest craze among responsible gardeners – the trend is clearly towards fertilizing with natural materials. Nature offers a lot, including rock powder, the only question is whether you can really fertilize with stones.

The mysterious “primeval” of primary rock powder

Rock flour needs less explanation; but where the primordial rock for the primordial rock dust comes from might be of interest to many a gardener.

The matter is less spectacular than it sounds: with an age of between around 4 billion and 200 million years, every rock is a primordial rock, primordial rock powder can theoretically be ground from any rock.

But it won’t, because not every rock flour makes sense in the garden:

“Rock” consists of minerals (rock formers, the most common 30-40 of the approximately 5,000 minerals in the world), 1-10% components, less than 1% accessories (often eponymous despite the small proportions) and water, crystal water or pore water.

The composition and structure (the “structure”) of the rock determine whether it can be easily processed into flour, with a grain size that is good for the soil. Therefore, some rocks or primordial rocks are better than others to grind into flour and sprinkle in the garden.

Which primary rock flours are offered?

“Primary rock flour” is flour from certain parent rocks, possibly a mixture:

  • Base rock with a lot of calcium results in alkaline (basic) bedrock powder
  • Base rock with little calcium is called “sour” primary rock flour
  • The pH values ​​of the primary rock dust lie in a range between 6.5 and 13
  • A garden soil is acidic below pH 6.5, neutral between 6.5 and 7.5, basic from 7.5, with a maximum value of pH 10 that is compatible with plants
  • Even “acidic” primary rock powders are not acidic, but are close to the alkaline limit
  • Which does not mean that the few percent of lime lower the pH value, you use garden lime with 95% calcium carbonate
  • The most commonly used source rocks are diabase and basalt
  • Only stones with a loose structure are ground, not only because it is easier:
  • Optimal soil has a “crumb structure” that supports roots and keeps water in the soil
  • Loosely structured stone particles promote the development of the crumb structure

So the structure is already useful, now it’s a question of whether the substances contained can fertilize plants. That depends on what plants need for their nutrition and whether the corresponding substances can be found in the bedrock meal:

Plant nutrition in “natural plant life”

Plants without fertilizer live off what they find in the soil, water and air, and these are made up as follows:

1. Earth’s crust, numbers rounded, each concise

  • 47% oxygen in oxides
  • 28% silicon dioxide (main part of the earth’s crust + mantle)
  • 8% Aluminumoxide
  • 5% iron oxide
  • 4% Calcium oxide
  • 3% Sodium oxide
  • 3% Potassium oxide
  • 2% magnesium oxide

That would be 100%, but as I said just a little, some titanium oxide, phosphorus oxide, hydrogen and percentage fractions of trace elements (which are so called because they are only involved in traces) are also included, in decreasing order carbon, manganese, sulfur, barium , chlorine, chromium, fluorine, zirconium and nickel.

2. Water
Water is H2O and therefore a chemical compound of oxygen (O) and hydrogen (H).

3. Air
Air, gas mixture of the earth’s atmosphere: nitrogen (almost 79%), oxygen (almost 21%), a little argon, carbon dioxide, traces of other gases and 1.3% water vapour.

4. Light Light
energy is used in photosynthesis.

What is not automatically available in the environment must be admitted, e.g. B. by fertilizer:

Plant nutrition by fertilizer

It took a while to research which substances plants really need. Aristotle was followed around 350 BC until the 17th century. Chr. set up humus theory, according to which plants feed on soil. In the Age of Enlightenment, plants and potting soil were weighed before and after growth and it was found that a plant can gain biomass by the kilo, but the soil only decreases by the gram, so “eating humus” could not be the case.

In further vegetation experiments, the mineral nutrients that are crucial for plant nutrition were explored. The main nutrients (essential in large quantities for growth and biomass formation) are the four “organic” basic elements carbon, hydrogen, oxygen, nitrogen, and phosphorus, potassium, sulphur, calcium and magnesium, the main nutrient elements, must also be available.

The trace elements iron, manganese, zinc, copper, chlorine, boron and molybdenum are essential for plant nutrition, and research is still being carried out – there are probably many other elements that are necessary, useful or usable. It is conceivable that vital nutrient elements can be replaced by other elements – these subtleties of plant nutrition are not easy to research, since some elements are growth-promoting in very low concentrations and some plant species “like” certain elements in particular. The whole impact of bioeffectors like soil organisms and plant substrates is “recent research”, really we still know very little about the details of plant nutrition.

One thing is certain: Nitrogen, phosphorus and potassium are not usually available to plants in excess. These three elements are the core nutrients and components of every complete fertilizer (NPK fertilizer).

In Annex 1 of the Fertilizer Ordinance, the substances that are suitable as fertilizers according to the current state of knowledge are legally fixed: nitrogen, phosphorus, potassium, traces of boron, cobalt, copper, iron, manganese, molybdenum or zinc, the secondary nutrients calcium, magnesium, sulfur – and nothing more, only appropriately mixed products may be sold as fertilizer.

All of these substances must be clearly identified on the packaging; anyone who can read can unmask any miracle fertilizer as a mixture of precisely these chemical elements (or as a substance that has nothing to do with fertilizer).

The composition of primary rock flour

According to a publication by BIO Austria (organization of Austrian organic farmers), diabase contains as main components (average from 60 samples):

  • 46.6% silica
  • 13.4% aluminum oxide (alumina)
  • 12.7% iron oxide
  • 7.3% Calcium oxide
  • 6.5% magnesium oxide
  • Small amounts of potassium oxide, sodium oxide, phosphorus oxide, tiny amounts of trace elements

All diabases in this world have a very similar composition, and the basalts also look similar:

  • silica = by 50%
  • Alumina = by 20%
  • Iron oxide = by 10%
  • Calciumoxid = a 10%
  • Magnesium Oxide = by 10%
  • Small amounts of potassium oxide, sodium oxide, phosphorus oxide, tiny amounts of trace elements

According to the solids analysis, Eifelgold Lava rock flour contains in the company’s own declaration of conformity for organic farming:

  • 40-50% silica
  • 11-15% Aluminumoxide
  • 10-12% iron oxide
  • 10-15% Calcium Oxide
  • 6-8% magnesium oxide
  • 3-4% potassium oxide; Sodium oxide, phosphorus oxide and trace elements certainly too (so little that they do not have to/may not be listed)

“Lava” rock flour is made from basaltic lava, chemical composition like basalt, only more porous because it solidifies on top of the earth and not in it. In the case of zeolite primary rock powder, a tectosilicate was also ground (aluminosilicate with calcium, potassium or sodium, a common component of the earth’s crust; mostly as clinoptilolites, the most common zeolites).

All of these primary rock flours are chemically very similar to the earth; in order to pass as fertilizer, the stone flour (sand) is missing a lot. It is therefore classified as a soil additive in the Fertilizer Ordinance, expressly limited to a much lower total nutrient content than fertilizer (so that nothing is mixed in outside of the approval).

According to Section 2 of the Fertilizers Act, soil additives are substances with no significant nutrient content; a soil additive is thus defined by law as a substance that does not have any fertilizing effect. This is not how retailers explain it to consumers:

Primary rock flour in the trade

“Primal rock flour” as a term that is not legally protected makes the products from the stone mill sound so “primeval” that it makes sense to stimulate consumer fantasies in many ways:

  • Rock flour is correctly declared as a soil improver, but classified in the fertilizer category
  • The “optimal composition” is praised, the only question is why, a stone is usually optimally composed like a stone
  • Silicon is emphasized as a “plant-strengthening silicic acid”, but siliceous silicon, such as in rock flour, is unfortunately not available to plants.
  • Trace elements should be absorbed and stored by the plants – yes, of course, plants always do that when they grow.
  • Primary rock flour in the organic waste bin is supposed to hold back odors and rot – that’s right, any sand can do that
  • Slugs are repelled naturally – yes, by any wall made of natural material, eggshell, ash, rock dust or sawdust…
  • Supposed to accelerate rotting in the compost – probably not, rock flour is only broken down faster in the compost than in the soil and absorbs moisture/nutrients, which it later releases to plant roots (works better with clay minerals, which swell more)
  • Apply compost as fertilizer humus – hopefully that’s true, but only if it’s composed correctly (with or without rock dust).
  • “Antifungal” – true, sucks moisture from densely overgrown plants, but usually such overgrowth is corrected by pruning
  • “Magnesium and iron promote the greening of the leaves – that’s right, if they are missing, they have to be added with fertilizer

So stone flour is not magic, but has physical properties that make it part of a healthy garden soil as it gradually weathers. Until then, it loosens up compacted soil so that the remains of soil life that is still present have better chances and “dilutes” polluted/over-fertilized soil so that plants that are already dying may have new opportunities. There are also a few trace elements, but they are in almost every soil (and in every sand); Rock dust cannot save a garden soil that is no longer soil.

Buy bedrock meal

Rock flour is in vogue, and what is trending is traded enthusiastically, especially when you can earn a fortune with the little stuff as well as with the miracle fertilizer “especially for (any plant)”.

Since the raw material is quite cheap, primary rock flour is traded at quite variable prices:

  • 25 kg €11.54 + €5.95 shipping, €0.70 per kg
  • 20 kg €15.99 + €3.90 shipping costs, €0.99 per kg
  •, from €39 free shipping, then €0.80 per kg
  • 10 kg €7.39 + €3.80 shipping, €1.12 per kg

Comparing different pack sizes of the same product shows how lucrative repackaging can be:

  • Diabas Urgesteinsmehl Big Bag 1.000 kg 199,00 €, 0,19 € pro kg
  • Diabase primary rock flour package 10 kg €10.90, €1.09 per kg

Is it cheaper at the building materials dealer, e.g. B. at crushed sand (diabase) 0-2 mm (usually better for the structure of the garden soil than very fine flour), a ton for €12.79, €0.013 (1st .3 cents) per kg.

Which bedrock meal for what?

Theoretically, there are as many different rock flours as there are stones in our world, every stone can be ground into flour. This is also done with many stones, is called sand and is sold by the building tradesman.

Each stone contains a wide variety of substances that plants need, because earth consists of weathered stones. If you want to loosen up your garden soil a little, it doesn’t really matter which stone sand you use – how much of the finest trace elements it contains depends on hundreds of environmental conditions and that could only be analyzed with considerable effort.

If you buy primary rock flour from a retailer, the retailer should at least be able to reveal the composition of silicon dioxide, aluminum oxide, iron oxide, etc. (e.g. in the certificate for use in organic farming). If the composition is not specified in the product description, you should keep your hands off it, someone could also have ground old shoe soles here.

The builder’s merchant can hardly afford to grind anything else. After all, this sand is used to build, the consequences of adhesion to a sand that does not correspond to the product description could quickly ruin it. You can also ask him where his diabase comes from.


The short formula for using bedrock powder is:

The more broken the floor, the more.

This only makes sense with coarse primary rock dust; with really damaged soil (rock-hard, over-fertilized, etc.) only as part of a complete soil remediation, the whole package with green manure and Co.

If you have a magnificent, loose, well-rooted garden soil, it will be happy about a few minerals from primary rock flour – the more diverse, the better, say not only the people with the many 100-year-olds, whose microbiome is twice as species-rich as that of the western average people.

The “official” application recommendation is 100 to 500 g per square meter once a year, preferably in spring, dry by spreading or wet by adding water. You would certainly have noticed that water gets wet and the dry method generates a lot of dust; the amount to be applied depends so much on the condition of the soil that grams are only a guide.

Prehistoric rock dust can be helpful in soil remediation and soil care, combined with gardeners according to good professional practice with crop rotation, mulching and other soil care methods. The prices increase a lot with smaller packages, for larger areas it can make sense to ask the nearest building materials dealer, maybe he can tip you off a few hundred kilograms.

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