The year 2015 was declared the “International Year of Soils” by the United Nations – a welcome occasion for many garden owners to do something good for their garden soil. But also a welcome reason for retailers to market many new fertilizers and soil additives, such as algae lime. It is also natural, like any limestone formed from any organic material through long deposition, but if you are not careful, you will be helping to damage nature again when you buy it. Unfortunately, algae lime is not a new miracle cure, just a lime fertilizer.

What is algae lime and what is in it?

Algae lime is largely ground calcium carbonate, one of the most common compounds on earth, especially in sediments/sedimentary rocks. These rocks are formed by deposits of material on land and in the sea and together with the igneous rocks (cooled magma and its metamorphoses) form the earth’s crust = 0 to 40 km deep, the outermost layer of our planet, with which the gardener has to do.

Sedimentary limestone is formed from calcareous deposits, mostly organic remains. Directly from corals, mussels, snails and sponges that secrete calcium carbonate to build their exoskeletons and internal skeletons, indirectly through phototrophic organisms (algae, land plants, bacteria, fungi, animals) that assimilate CO2 and thus create an alkaline environment in which calcium carbonate fails.

In the case of algae lime, it is the dead deposits of red algae, it contains approx. 70 – 80% calcium carbonate, approx. 6 – 10% magnesium carbonate, approx. 3 – 4% silicic acid and trace elements, more details below.

The advantages of algae lime

Algae lime is said to be beneficial for the garden soil in several ways:

  • Because of the trace elements it contains, it is said to be more beneficial than other limes
  • As a compost additive and in general it should enrich with trace elements
  • Because of a particularly favorable ratio of lime and magnesium, it should benefit the soil
  • ensures good availability of nutrients and healthy, biologically active soil
  • represents all the good properties of lime to a special degree
  • should be able to be used all year round
  • If plants are dusted with algae lime, this should prevent fungal attack
  • Dust on all parts of the plants should drive away insects (also works with flour)
  • It is said to lead to increased resistance to fungal and viral diseases and pests
  • It also improves the shelf life and flavor of fruit and vegetables
  • It is said to act as a fertilizer that supplies the plants with the main nutrient calcium
  • And as an important soil conditioner, it binds acids in the soil
  • By using algae lime, the soil should warm up faster in spring, so that the soil organisms can quickly develop their full activity.

A powerful number of arguments, instead of showing the sometimes strange logic, the following sections explain exactly what algae lime can do.

Application of algae lime

Recommendations for use are given on the algae lime packs, e.g. B. as follows: First application 10 – 15 kg per 100 sqm, subsequent application 3 – 8 kg per 100 sqm.

That is 100 – 150 g or 30 – 80 g per square meter and does not necessarily correspond to the recommended amounts for lime fertilization of soils that require lime:

  • Light soil, pH < 5.3: 150 to 200 grams of lime per square meter
  • Light soil, pH 5.3 – 5.7: 120 to 180 grams of lime per square meter
  • Medium-heavy soil, pH < 5.7: 300 to 400 grams of lime per square meter
  • Medium-heavy soil, pH 5.7 – 6.5: 180 to 250 grams of lime per square meter
  • Heavy soil, pH < 6.3: 400 grams of lime per square meter
  • Heavy soil, pH 6.3 – 6.9: 250 to 350 grams of lime per square meter

These are the values ​​for pure calcium oxide (CaO), carbonated (algae) lime is only about 50% basic, you need more of this to lime too acidic soil (converted according to specific formulas that also take into account the magnesium in the algae lime , a knowledgeable fertilizer dealer should have tables available).

Algae lime is of little or no use in the specified application quantities if the soil is actually too acidic, which is perhaps why it is recommended for the “Area of ​​application: healthy soils”.

It goes without saying that lime must be applied evenly; It should be remembered that when liming individual areas it is advisable to check whether the neighboring plants can cope with the new soil pH value.

All lime is a slightly irritating substance, including mild calcium carbonate, which is used, among other things, as chalk. Chalk is not dusted or mixed around in large quantities, so small children and pets should be kept out of the way and allergy sufferers/asthmatics should not hold their noses in the wind.

The prices of algae lime

The prices are quite different, here is a small list:

  • Cohrs algae lime 25 kg bag €26.50 + €4.90 shipping, final price kg 1.25
  • Cohrs algae lime 5 kg €18.37, free shipping, final price kg 3.67
  • Cuxin Marketing GmbH: Fine algae lime 5 kg €16.00 + €5.95 shipping, final price kg 4.39
  • Algae lime, 500g €9.48, free shipping, final price kg 18.93
  • barf proQ algae lime, 250 g €5.80, free shipping, final price kg 23.20

No, it’s not a joke that algae lime from the pet shop is listed here, see the critical examination.

Algae lime – critical consideration

In connection with algae lime, there are a few things to consider so that you as a gardener and your garden really benefit:

1. Algae lime is sometimes advertised as if it is definitely one of the healing substances that can be used in the garden.
Is not he. Algae lime is a lime fertilizer that only makes sense in the garden if the garden soil needs lime.

Whether the garden soil needs lime must be determined by a pH test/soil analysis before liming. Liming (= raising the pH value towards alkaline) can only make sense if the garden soil is too acidic, which is not that difficult to understand.

However, liming should only ever be used as a single measure to save yourself. In the long term, you should find out why the soil is actually too acidic.

2. Calcium carbonate is calcium carbonate
with the chemical formula CaCO3, regardless of whether it is derived from green non-sulphur bacteria, armored flagellants, diatoms, throat flagellants, calcareous algae, red, green, brown algae, green pseudopods, ciliates, lichens, cyanobacteria, cnidarians, giant green anemones or the Spotted Crosstooth Newt has formed.

An added value of algae lime compared to other limes could only result from the trace elements it contains, we will now look at how they are:

trace elements

Einer der Hersteller, mit einem für den Bio-Anbau nach EG-Öko-VO 2092/91 zugelassenen Algenkalk, hat die Inhaltsstoffe genau deklariert:
81 % Calciumcarbonat,  13 % Magnesiumcarbonat und Spurenelemente in mg pro kg:

  • Eisen 2150 mg, 2,15 g
  • Bor 416 mg, 0,416 g
  • Mangan 160 mg, 0,16 g
  • Jod 40 mg, 0,04 g
  • Kupfer 28 mg, 0,028 g
  • Zink 16 mg, 0,016 g
  • Molybdän 3 mg, 0,003 g
  • Kobalt 1,1 mg, 0,0011 g
  • Selen 0,16 mg, 0,00016 g
  • außerdem Fluor, Brom, Nickel, Aluminium, Silber, Zinn, Indium, Titan u.a., offensichtlich in nicht mehr auszuweisenden Mengen.

Da ist ein Kilogramm Erde vielfach besser mit Spurenelementen ausgestattet, nachfolgend Durchschnittswerte des Gehalts in der Erdkruste:

  • Eisen 57 g
  • Bor 0,007 g
  • Mangan 0,1 g
  • Jod 0,0005 g
  • Kupfer 0,058 g
  • Zink 0,081 g
  • Molybdän 0,001 g
  • Kobalt 0,028 g
  • Selen 0,00005 g

außerdem Fluor 0,46 g, Brom 0,004 g, Nickel 0,071 g, Aluminium 80,7 g, Silber 0,0001 g, Zinn 0,002 g, Indium 0,0002 g, Titan 8,5 g u.a. – bis eben alle 92 chemischen Elemente zusammen sind, die natürlich vorkommen.

Da Calciumcarbonat kein sehr teurer Stoff ist, wird er generell gerne unter Bezugnahme auf Stoffe verkauft, die nach Zusatznutzen klingen. Als Thomaskalk z. B. (Hüttenkalk, Konverterkalk), Abfallprodukt aus der Stahlindustrie, das natürlich auch Spurenelemente enthält. Oder als Magnesiumkalk aus ganz normalem Gartenkalk + Magnesium. Eine Ausbringung von Magnesium ist auch nur sinnvoll, wenn Magnesium fehlt (selten, und wenn, kann es auch an einer Überversorgung mit Kalium = zu viel Dünger liegen).

Erhebliche Preisvarianten

Reiner kohlensauerer Gartenkalk aus Calciumcarbonat, CaCO3, kostet normalerweise um 80 Cent pro kg. Sie können ihn aber mit etwas Glück für mehrere Euro pro kg kaufen, dann vielleicht als Kreide oder E 170.

Oder eben als Algenkalk, auch ein ganz normaler Kalkdünger nach Anlage 1 Abschnitt 1 Nr. 1.4.1 DüMV, der bei Herstellung aus Meeralgen mindestens 65 % CaCO3 enthalten muss, für 1,25 € bis 23,20 € pro Kilogramm.
Ob 1,25 € oder 23,20 € pro kg, all diese Algenkalke werden aus Ablagerungen der gleichen Alge gewonnen:

  • Cohrs Algenkalk: Kohlensaurer Kalk aus Meeresalgen = abgestorbenen Korallalgenablagerungen (Rotalgen)
  • Oscorna = Cohrs
  • Manufaktum/Deutsche Cuxin Marketing GmbH: Meeresalgenkalk der Alge Lithothamnium calcareum mit ca. 70 % Calciumcarbonat
  • Algenkalk: 100 % Lithothamnium calcareum
  • Barf proQ Korallen-Algenkalk: Lithothamnium calcareum (Korallenalgenkalk) aus calzifizierten, wilden Rotalgen

Auch der Algenkalk für 23,20 € besteht also aus den gleichen Rotalgen, Sparpotenzial für Algenkalk fütternde Hundebesitzer. Wenn ein Händler den höheren Preis damit verteidigt, dass man dieses CaCO3 ganz besonders gereinigt hat, wäre die Nachfrage angebracht, wie ein einheitlicher, im Meer abgebauter Kalkstein behandelt wird, um ihn „reiner“ zu machen (wenn farbliche Abweichungen ausgewaschen werden, werden möglicherweise gerade die Spurenelemente entfernt?).

Calcium ist kein Bestandteil eines normalen Volldüngers

Der normale Volldünger, „Mehrnährstoffdünger“, enthält die Kernnährelemente Stickstoff, Phosphor und Kalium (NPK-Dünger), die eifrig wachsende Pflanzen ohne Zugabe durch den Gärtner aus Erde, Luft, Wasser nicht ausreichend erhalten.

The plant can get everything else from its environment in a naturally cultivated garden, calcium e.g. B. is present in soil, air and water and does not need to be added separately to garden soil with a healthy pH (6.3 to 6.8). On the contrary, if the soil contains too much calcium, it tilts into the alkaline range and the uptake of nitrogen, which is extremely important for growth, is inhibited by calcium.

environmental Protection

A specialist article by the Research Institute for Organic Farming (FiBL) in Switzerland on “Algae products in organic farming” (, search for algae products, 1st entry in the result) has evaluated algae lime and warns against some algae lime:

Calcareous algae (Lithotamnium calcareum) grow very slowly, it takes an infinitely long time until their skeletons become algae lime (similarly slowly to how high moors become peat). The maërl banks on which they grow are important for the juvenile stages of many sea creatures and thus important for aquatic ecosystems as a whole.

Raised bogs would probably never have been damaged by gardeners if the gardening industry had not discovered and sold the peat; the horticultural industry has now also discovered the algae lime and is partially mining it on living Maërl banks (with large vacuum cleaner ships), which leads to more or less severe destruction of the banks and severe damage to the ecosystems. There is also mined fossil algae lime, the extraction of which is regarded as ecologically unproblematic. Maërlbanke have been protected in the EU since 2012 and dismantling is prohibited. You should therefore find out exactly where an algae lime comes from and where it was mined before you buy it and ask for a corresponding certificate.

Incidentally, the authors have not found any scientific results for the fact that algae lime is said to be more advantageous for fertilization than other limes and expressly warn that it is more expensive than ordinary lime fertilizers.

Algae lime can be used as lime fertilizer if you know where it comes from and how it was mined. It contains trace elements, but on average not much more than ordinary soil and, like any other lime fertilizer, can affect the pH of the soil. It is not a miracle cure, but a very specific form of straight fertilizer. The fungal infestation, of course, is prevented when plants are “dried out” by pollination, but can only be used if the soil really lacks lime. It is doubtful that the fruits of the garden taste better with algae lime, but it is certain that you can buy lime cheaper.

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