You wonder if you should sprinkle compost with lime?
And if so, with what lime and how often it should be done?
Then you’re in the right place, because in this article I will answer all your questions about lime compost.
Compost can, but does not necessarily have to be sprinkled with lime. If, then algae or dolomite lime (or limestone flour) is suitable for the compost to neutralize the pH value, to combat stench or to accelerate the compost.
Do you have to lime the compost?
As is so often the case in the garden, when it comes to the question of whether you need to sprinkle the compost with lime, the motto is: everything can, nothing must!
More precisely, compost does not necessarily have to be limed. On the contrary, a balanced, intact compost normally manages without lime.
Nevertheless, there are some advantages to liming compost or situations where it makes sense. We’ll look at all of this together in the next chapter.
Sprinkling compost with lime: when it can be useful
There are a total of four situations in which it makes sense to lime the compost. I will explain why in the following chapters:
- Lime helps to neutralize the compost
- Lime helps against stench on the compost
- Lime can prevent mold growth on the compost
- Lime can speed up the compost
lime helps to neutralize the compost
Lime has the property of neutralizing compost, or raising the pH, because lime (calcium carbonate) reacts with acids in the compost, which are split by the lime and thus neutralized.
If you want to know more about the chemical, physical and biological effects of lime, check out this article.
For a classic vegetable and flower garden, a relatively neutral soil (pH ±7.0) is needed, with which most plants do very well.
Why should the compost be neutral?
Since the finished humus from your compost will end up back in your garden, you should make sure that your compost is not too acidic (pH less than 7.0) but also not too alkaline (pH greater than 7.0).
Depending on what you compost and in what quantities, your compost may become slightly acidic.
Since most compost waste is in the neutral range it is almost impossible to get the compost into the highly alkaline range.
Some compost wastes, some of which are more common, can acidify your compost:
- Leaves (especially oak leaves)
- Pine needles
- Lawn clippings
- Fruit peels or fruit residues
- Coffee grounds (only in very large quantities but not in household quantities; learn more about coffee grounds in the compost in this article).
So if you compost, for example, a lot of oak leaves, pine needles, lawn clippings or fruit scraps, it can be easier for your compost to become a bit acidic.
In this case, it can be useful to lime the compost a little from time to time to bring it back into the neutral range.
Is it necessary to neutralize my compost with lime?
Compost should be limed if a neutral humus is desired, but the compost is currently acidic. You can find out if the compost is acidic with a pH test.
The pH test determines the pH value, i.e. the acid-base ratio, of your compost. You can do this either with a classic pH soil quick test or with a digital soil tester.
Here you can get the classic pH soil test from Neudorff* or here a digital soil tester*.
If the pH value is in the acidic range although you need a neutral soil or humus for your garden, then you should lime the compost.
pH soil test Neudorff
lime helps against stench on the compost
Lime on the compost can help reduce or neutralize bad odors by binding excess moisture that is responsible for the stench.
This is because bad odors are usually created on compost because it is too moist or wet, which causes it to start rotting. You can read more reasons why your compost stinks in this article and here what helps against it.
Since lime has a large surface area due to its fine and porous structure, it can bind a lot of moisture so that the rot and the smell in the compost are reduced.
lime can prevent mold growth on the compost.
Sprinkling lime on the compost can help prevent or reduce mold growth.
Although a healthy compost should not have any problems, if it is too moist, mold may appear along with bad odors.
Again, lime can help soak up the moisture and counteract mold growth.
lime as a compost accelerator
Lime can be used as a compost accelerator because it contains many minerals and nutrients that are needed by the microorganisms in the compost to survive.
More specifically, the microorganisms, protozoa and also worms in the compost need food to reproduce. The more living organisms there are in the compost, the faster the decomposition process takes place and the compost rots.
If the compost is sprinkled with lime, the living organisms get more food, which leads to a faster reproduction and thus an acceleration of the rotting process.
When should you lime the compost?
Compost can be limed at different times, and the different methods have different advantages and disadvantages.
On the one hand, compost can be limed whenever large amounts of acidic waste (e.g., lawn clippings, leaves) are composted. Alternatively, you can either sprinkle a handful of lime on a compost layer from time to time, or you can lime the finished humus when you take it out of the compost
|Time for liming the compost||Advantages||Disadvantages|
|Whenever large quantities of acidic waste are composted||Needs to be done only a few times a year|
Additionally accelerates the rotting
|Total amount difficult to estimate without weighing it out|
|From time to time a handful||Lime is very well distributed in the compost and layers|
Additionally accelerates the rotting
|It is necessary to remember regularly|
Total amount difficult to estimate without weighing it out
|When removing the finished humus||Must be done only once|
One keeps a good overview of the amount of added lime
|Lime can not accelerate and support rotting|
Personally, I would always prefer the first or second variant – lime compost with large quantities of acidic materials or from time to time with a handful.
Simply for the reason that in these cases the lime is already evenly distributed in the compost and can support the rotting.
Which lime for compost?
So-called algal lime is the best type of lime to spread in the home garden and on the compost. This is because algal lime is a fairly mild lime that is easy to use and still brings all the benefits of other types of lime.
Algal lime is available both as a very fine powder (for example, this one) or in larger quantities as a slightly coarser bedding (like this one).
The name algal lime hints at the origin of the lime, because algal lime is a 100% natural raw material derived from dead coral sediments (specifically red algae).
Depending on the type and manufacturer, algae lime consists of approx. 80% carbonic acid lime (calcium carbonate), approx. 10% magnesium, approx. 5% silicates and other trace elements, such as boron or iodine.
Algal lime has the following properties:
- Algal lime is very fine and porous and therefore has a large surface area.
- Algal lime can therefore absorb and neutralize organic acids (such as tannic or resin acid) well
- Algal lime can be applied all year round
- Algal lime releases nutrients rather slowly but constantly
- Algal lime inhibits fungal spores from germinating
- Alternative lime varieties to algal lime for compost
In addition to algal lime, limestone flour (or also called dolomite flour) is a possible alternative to spread on the compost.
This is available either in 20kg bags here from Neudorff* or in 25kg bags e.g. from Hamann*.
Limestone flour consists largely of primeval coral reefs and has a similar composition to algal lime.
However, limestone flour or dolomite flour is more difficult to dissolve in water than algal lime and is usually spread more in autumn and winter.
While limestone flour is usually somewhat less expensive than algal lime*, it is not available everywhere.
The above quantities also apply to limestone powder and approximately 1kg of lime should be used for 3-5 cubic meters of compost per year.
How much lime to spread on compost?
The amount of lime to spread on compost depends on the type of lime and the size of the compost.
If you lime with algal lime*, you can use about 1 kg of lime for 3-5 cubic meters of compost per year as a rule of thumb. For a standard compost of about 800 liters volume (0.8 cubic meters), this corresponds to about 160-260 grams of lime per year.
Sometimes more lime is advised, but this is not normally recommended. If you want to lime your compost without first measuring the pH, you should generally use lime more sparingly at first.
Once you know the pH of your compost, you can then increase or decrease the amount of lime accordingly.
In the table below, I have calculated lime requirements for the most common compost sizes.
The lower range shows a lower amount of lime (so if you would use about 1 kg of lime for 5 cubic meters) if you have a not so acidic soil or less acidic waste, while the upper range shows if you would use about 1 kg of lime for only 3 cubic meters (so with more acidic waste, for example).
|Volume compost||Amount lime for compost|
|Lower range (approx. 1 kg lime per 5 m³)||Upper range (approx. 1 kg lime per 3 m³)|
|400 Liter (0,4 m³)||Approx. 80 g||Approx. 130 g|
|600 Liter (0,6 m³)||Approx. 120 g||Approx. 200 g|
|900 Liter (0,9 m³)||Approx. 180 g||Approx. 300 g|
Alternatives to lime for compost (home remedy)
In addition to algal lime or limestone powder available for purchase, there are also two well-known “home remedies,” eggshells and ash, that you can use to lime your compost.
Eggshells as an alternative for lime on the compost
Eggshells contain a lot of lime, but they rot very slowly. Therefore, always make them as small as possible and spread them well on the compost.
If you want to know how to make plant fertilizer from eggshells, just take a look right here
Ash as an alternative for lime on the compost
Hardwood ash from a tiled stove is mostly calcium and about 25-45% quicklime. Since wood ash is highly alkaline and easily soluble, very little fertilizer should be used and care should be taken with it. You should also make sure that it is completely untreated wood. You can learn more about the use of ash as fertilizer here
Algal lime and dolomitic lime should not be confused with calcium cyanamide!
Calcium cyanamide is often used as a long-term fertilizer and has a strong effect, which can also be partly not completely harmless because calcium cyanamide can have a strong corrosive effect if used incorrectly.
Therefore, I would recommend only algal lime or limestone flour / dolomite lime to sprinkle on the compost.
Buy lime for compost (price comparison)
You can buy lime for your compost classically at your local hardware store or garden center, or order it online from one of these retailers or Amazon.
The biggest differences are in structure, so whether it’s powdered lime or coarser flour/granules, and which manufacturer it is.
Dolomite lime/ground limestone is generally less expensive than algal lime. Algal lime is most expensive, especially in small quantities and as a powder.
However, the advantage of algal lime powder is that you can use it for compost as well as for boxwoods, for example.
Which lime you buy for your compost is entirely up to you. In the following table I have only made a small sample with different types of algal and dolomitic lime that are suitable for compost and can be purchased.