What is compost? Definition & Properties

• What is compost?
• Properties of compost
• Where does compost come from?
• Compost Worms and Other Composters
• Compost from recycling centers
• Make compost yourself
• Put out compost: how to use it correctly
• Compost in agriculture

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What is compost, what is it made of and how is it composed? We reveal everything about the origin, use and spreading of compost in the garden.

Man has been making compost from various types of waste for centuries to fertilize his plants. As early as the 8th century BC, the Greek poet Homer reported that a fragrant heap of dung was growing in Odysseus' farmyard, which was later spread over the fields. But which special and mysterious superpowers make compost so valuable for humans that it is sometimes referred to as the gardener's "black gold"? In this article you will learn more about the formation and origin, properties and application of compost.

What is compost?

Compost is dead organic material that has gone through the process of so-called rotting. Decomposition by air-breathing microorganisms first takes place in the composting process. These break down parts of the organic material in such a way that it escapes into the air in gaseous form as carbon dioxide. The starting materials slowly disintegrate into coarser parts and finally even into individual molecules or atoms. From these "building blocks" something new is then created in the process of "humification" - i.e. the formation of humus - namely the humic acids (or "humus molecules"). These, taken together and chemically combined with clay particles, form visible crumbs and flakes that we can recognize as compost or humus. You may be wondering what exactly the differences are between humus and compost - a very valid question. In fact, the word "compost" comes from the Latin compositum, which roughly means "that which is put together". The name thus refers to the diverse starting materials that the composter - i.e. the human being - uses for the targeted formation of humus. So compost is a type of humus. Humus, on the other hand, is also created naturally and is therefore not the same as compost.

What is compost made of?

Likeeverything that has grown organically - i.e. animals, plants, fungi or even algae - also compostable waste consists of different proportions of carbon compounds. The cell walls of plants, for example, consist largely of cellulose, hemicellulose and pectin. All three are carbohydrates, i.e. carbon compounds that also have oxygen and hydrogen attachments and are connected at the molecular level to form long, stable chains. If a cell wall is eaten and digested by microorganisms, the same thing happens as when we eat carbohydrates, for example in the form of a slice of bread: the carbon compounds contained are converted in cell respiration to generate energy and the end product is carbon as carbon dioxide (CO₂) Exhaled with water.

Summary: What is compost?

• Compost is material that has decomposed and turned into humus
• The term "compost" refers to the various man-made constituents of humus
• Compost consists of humus molecules which alone and in combination with clay particles form visible flocs
• Composting releases carbon dioxide because microorganisms feed on carbon compounds in the compost

Properties of compost

In general, it can be said about compost that - depending on the degree of rotting - it is coarse and fibrous or brownish and crumbly and has a pleasant smell that is often perceived as "woody". It is about twice as heavy as peat and half as heavy as sand. Compost also has many medium-sized pores, which allows it to improve aeration as well as water balance in the soil. Depending on the starting materials, it can be slightly acidic, neutral or slightly alkaline. The nutrient content depends on the raw materials and the duration of maturation: it can be very high or very low. Nitrogen, phosphorus and potassium - the three main nutrient elements of plants - can be in a balanced relationship to each other, but the proportions of phosphorus and potassium are often greatly increased compared to nitrogen. Compost usually contains all the trace nutrients that plants need. The effect that compost fertilization has on the soil and the plants can also differ fundamentally: while a mature, nutrient-poor green compost sustainably improves the soil properties by increasing the humus content, a fresh, nutrient-rich organic compost is better for plant fertilization,because it releases many nutrients, but does not lead to an increase in the humus content in the soil. If you are interested in the properties of compost as a fertilizer, you can find out more in this special article.

Decomposition degree of compost

The rotting degree is the unit of measurement in which the stability of compost against degradation by microorganisms is given. Depending on the degree of rotting, decomposition takes place in the soil to varying degrees once compost has been spread. The stability increases as the degree of rotting and compost maturity increases, while the ability to release nutrients decreases at the same time. We find the reason for this in humification: Because this makes new, stable humus molecules from the organic building blocks released after decomposition, which are immune to degradation. Compost raw material has the lowest degree of decomposition, namely 1, fresh compost has a degree of decomposition of 2 or 3, finished compost has a degree of 4 or 5.

Summary Properties of Compost:

• The properties of compost depend on the starting materials and the degree of decomposition
• Is coarse and fibrous, brownish-crumbly, or a mixture of both
• Pleasant, woody smell
• Heavier than peat and lighter than sand
• Has many medium-sized pores, which explains the good water and air balance of compost
The pH is between 6.2 and 8.4
• Nutrient content from very low up to 50 percent by volume
• Contains a lot of phosphorus and potassium, mostly less nitrogen, also trace nutrients
• Acts as a plant fertilizer or soil conditioner - or both

Where does compost come from?

As noted above, compost is a human-compounded form of humus. As a result of the processes roughly described above, humus is also produced at many natural sites where there is sufficient organic waste and microorganisms. A special rule of thumb applies here: If the living conditions for the microorganisms involved are tolerable but not quite optimal, much more humus is produced than nutrients are released. And here we have an explanation for thick layers of humus in forests and moors: At these locations it is usually too acidic or too wet for microorganisms - there are also many other such humus-promoting locations. Humification can also take place in your garden if you take care of it a littleKnowledge of the control of humus-promoting conditions. In our special article you will find a lot of information about how you can make your own garden soil more humus step by step through humus management, even without spreading hummus.

Compost Worms and Other Composters

You have probably already got the right impression that composting is by no means a purely mechanical or chemical process, but rather a biological process. This is carried out jointly by various members of the soil flora and fauna. Especially in the compost, the number of organisms per volume is incredibly high, because here you will find a real feast. In one cubic meter of compost there is a whopping 10 kilograms of living organisms! Groups involved are bacteria, fungi and other protozoa, roundworms (nematodes), insects and their larvae, mites, earthworms of various genera, isopods, centipedes and snails. Depending on the prevailing living conditions, flora and fauna can be composed differently. The decomposition, humification, chemical change, mixing and degradation of organic pollutants are their tasks. Of course, they don't do this to please us: rather, they come to feed and reproduce when they find optimal living conditions. And this includes the presence of organic matter, sufficient moisture and oxygen, a slightly acidic or slightly alkaline pH value and temperatures that are as warm as possible. Once all the organic matter has been converted to humus, they die or migrate. If you would like to read more about compost worms and vermicomposting, you can do so in our special article.

Tip - Effective microorganisms (EM): EM is used to describe various commercially available mixtures of common microorganisms (such as yeast and bacteria ) which are added to compost, during Bokashi fermentation or in sewage treatment plants to accelerate the necessary metabolic processes. For use, the mixtures offered as a powder are mixed with a sugar solution and kept warm for a long time. After that, the microorganisms are called “activated” because in the sugar solution they were able to develop from the initially inactive permanent forms into active microorganisms and multiply rapidly. They are then poured over the material to be processed with the sugar water. Unfortunately, the effect failedThe use of EM has not yet been proven, and there are still reasonable doubts as to whether the observed results are actually due to the microorganisms or not simply to the sugar solution. For this reason we cannot give a professionally justified recommendation for EM in composting.

Compost from recycling centers

Compost that is sold at recycling centers is recycled from green waste and organic waste from the bio bin. Depending on the range, nutrient-poor and nutrient-rich materials are composted and sold together or individually. Green compost is poorer in nutrients, while organic compost is richer in nutrients. There are also various composting systems, some of which process huge amounts of compost. These range from traditional heap composting to permanently moved so-called "moving heaps" to pressurized composting towers or drums with volumes of up to 1000 cubic meters, which constantly and dynamically circulate the compost. Of course, not every recycling center operates such a system - they are only used in areas where a lot of material needs to be processed quickly. You can read more about compost from recycling or recycling centers in this article, which deals with the fertilizing effect of compost. Also, if you want to learn more about buying compost, you can find all you need to know in the dedicated article.

Tip - Hot and cold rotting: If a lot of compost material accumulates at once, as in recycling yards, a new pile, i.e. a Compost heap placed in one go. The consequence of this is that the decomposition processes do not take place in layers (like on the compost heap at home), but in all the material together. As a result of the life processes of the microorganisms, a lot of heat is generated. In addition, a large compost heap has its own insulation, which can result in core temperatures of 60 to 80 °C. Since this phase of intensive rotting lasts for several weeks, the entire volume can be strongly heated once by systematically moving it. This kills pathogens and weed seeds, so the compost becomes sterile. A private compost heap only reaches much lower temperatures due to the layered preparation and can therefore unfortunately carry seeds and pathogens of plant diseases.

Make compost yourself

Even without youHumus is constantly being formed in nature. But you can take advantage of the underlying processes and produce a high-quality organic fertilizer or soil conditioner yourself and relieve your organic or residual waste bin at the same time. We have summarized in a few keywords the steps you need to take to get your own compost. You can also find all the necessary details for correct composting in this special article.

1. Select a composter: compost rental, quick or thermal composter, roll composter or worm box.
2. Find a suitable spot: partially shaded, sheltered, in the garden on open and loose, he althy soil.
3. Prepare the compost: pile up the compost; coarse, solid material alternates with soft, nutrient-rich material. Lime or nitrogen fertilizer may need to be sprinkled in.
4. An occasional layer of finished compost or a pot of compost tea will have the same accelerating effect as using compost starters or compost accelerators.
5. Either layer the compost very carefully or - if necessary and possible - turn it over once a year to achieve good mixing and aeration.
6. Under the best conditions, fresh compost is ready after four to eight weeks and can be used as plant fertilizer. After at least six months you get a finished compost that has both fertilizing and soil-improving properties. After two to three years you will receive mature compost, which improves the soil.

Compost Tea

Compost tea is prepared in a similar way to activating the Effective Microorganisms (EM) mentioned above. However, the source of the microorganisms is not a purchased mixture, but a few grams of compost. Sugar or syrup is mixed with compost in water and then left at 25°C for about a day. It is important to have a good supply of oxygen, which is ensured by blowing in air or by using a stirring system. After all, the microorganisms contained in the compost are supposed to multiply quickly in the broth. This can then be used to inoculate the soil or compost to promote biological activity. Spraying the plants with compost tea is also recommended in various internet forums, but the intended benefit for the plants is rarely or only very superficially mentioned. Microorganisms living in the soildo not find a habitat on the plant and would not colonize it by themselves, which is why a positive effect initially seems unlikely. However, the use as a compost starter and to increase microbial activity in planting areas with a simultaneous input of organic matter (such as mulch) can have a positive effect. This applies in particular to newly prepared composts from very poor soils with low biological activity. It is questionable whether spreading the compost yourself would not have at least as great an effect.

Make compost tea yourself:

1. Pour 100 liters of rain or well water into a clean container (e.g. a rain barrel). If tap water is used, it must be left to stand for about seven days beforehand; stir occasionally
2. Heat water to about 25°C with an immersion heater (100 to 150 W)
3. Dissolve 500 g sugar beet syrup in water, stir in 250 g rock flour, stir in 500 g compost or hang in a large-volume net
4. Switch on the pond aerator pump (if possible with a bubbler)
5. Leave for 12 to 18 hours, then drain, filter if necessary, dilute with rain, well, or stale tap water (1:5 ratio for soil treatment, 1:1 for compost treatment) and apply within 4 hours
6. Clean the compost tea bin thoroughly

Tip: Since many of the described effects of compost tea and EM have not yet been definitively proven, not many commercial companies in agriculture and waste management use one of these methods. However, products for larger-scale manufacturing for companies are already on the market. It's more of an alternative approach so far, though, and we'd advise against relying on anything its proponents tout. In any case, over years of personal and scientific applications, spreading compost has been proven to improve soil.

Summary: Where does compost come from?

• Humus occurs in various natural locations to varying extents, it can also find its way into your garden in the form of potting soil - the best example of this is peat
• Compost is always created through the activity of countless, diverse microorganisms
• You can buy compost at recycling centers or produce it yourself on your own compost heap
• Effective Microorganisms orCompost teas are brewed to propagate beneficial microorganisms and then diluted and applied to soil and compost. So far, however, the effect has hardly been defined or proven

Put out compost: how to use it correctly

Compost has many beneficial properties that improve soil and plant growth. When spreading, however, always bear in mind that compost is a collective term for various mixtures of differently decomposed and humified material and that its properties are therefore not constant. There are basically the following possible applications:

• Use of finished compost to fertilize plants and soil
• Mulch and fertilize with fresh compost
• Shallow spreading of finished or mature compost to improve beds
• Mix with soil or other additives to make your own potting soil
• Lawns and shrubs benefit when they are supplied with compost

Depending on the exact area of application, a different type of compost with different properties is ideal. For detailed advice on using compost, see this special article on the fertilizing properties of compost.

Compost in agriculture

Compost can also be used in agriculture. The positive effect on soil he alth and yields has already been proven in various studies. One should really ask oneself why not much more of the "black gold" ends up on pastures and fields. Like any other fertilizer, it is subject to the Fertilizer Ordinance (DüV, supplement to the Fertilizer Ordinance DüMV), which regulates which quantities may be used at what time on which areas and for the cultivation of which crops.

Farmers, garden centers and also (fruit) tree nurseries have to keep records of their fertilization. In particular, the nitrogen inputs are precisely recorded and randomly checked. Agricultural holdings determine for the determination of fertilizer requirements:

• The Deprivations of Harvest Crops
• The nitrogen content in the soil
• Replenishment of nitrogen from soil organic matter
• The subsequent delivery of nitrogen from crop residues from previous crops
• Replenishment of nitrogen from previous green manures
• The nitrogen replenishment from organic fertilizers in the previous three years

This lineup willcompared with the nutrient requirements of the crop and further fertilization is adjusted accordingly. Nitrogen inputs and nitrogen withdrawals are offset against each other annually in the "nutrient comparison" and should balance out as far as possible or at least not exceed specified limit values.

Composts are treated as follows in the newly drafted Fertilizer Ordinance of 2022: In the first year after application, 4% of the total nitrogen content is taken into account when determining the fertilizer requirement, in the second and third year only 3% each. This results in the released nutrients that should be available to the crop plants. In the nutrient comparison, however, 100% of the nitrogen content over the three years is taken into account. Adequate supply of the plants with compost would therefore only be possible if the limit values applicable in the comparison of nutrients were also exceeded. However, anyone who exceeds these limits must expect pen alties. This discrepancy in the fertilizer ordinance makes the use of compost in agriculture more difficult and is currently still being regulated by individual agreement with the fertilizer authority, but will hopefully be resolved soon.

Summary of compost in agriculture:

• Fertilising with compost has been shown to have many positive effects on soil he alth and yield
• Compost is currently very difficult to calculate when determining fertilizer requirements and comparing nutrients
• This currently makes the use of compost in agriculture even more difficult

As you can see, compost has many benefits. With a few more ingredients, you can also mix your own compost soil yourself. In our special article you will find instructions on how to mix your own compost-based potting soil.

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