Buy compost or make it yourself

Buy compost or make it yourself?

Compost is a valuable part of many potting soils. Here we explain where you can best buy it and how you mix compost yourself. In addition to the well-known peat-containing potting soils, in the garden areas of hardware stores, garden centers, and nurseries, one sees more and more soils that consist to a large extent of compost.

These composts – often in organic quality – declare war on the overexploitation of threatened peatlands. The production of potting soil could thus run again in a recycling economy. This article contains a definition and a differentiation from pure compost, a list of possible secondary components, and also a brief comparison with peat. Finally, you will find a compilation of various products as well as for instructions for mixing your own compost.

Compost or compost soil: what are the differences?

Compost soil and compost are by no means two terms that can be used synonymously: While compost is the pure product of composting, compost soil is a mixture that contains compost. Compost can be divided into different types and degrees of ripeness. Green waste compost refers to compost made from green waste that is rather nutrient-poor, while organic compost is compost made from nutrient-rich organic waste. Composts that are poor in nutrients are significantly less decomposed in the soil and accordingly provide more humus for soil improvement, while nutrient-rich composts provide more plant nutrients.

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Fresh compost has only been composted for four to eight weeks, is nutrient-rich, and is quickly broken down by microorganisms in the soil. Under good conditions, ready-made compost only comes into being after five to six months and offers fewer nutrients, but in return is more stable to microbial degradation. Producing mature compost takes one to two years – it is extremely stable, but hardly provides any nutrients.

Buy compost or make it yourself?

Summary of the differences between compost and compost soil:

  • Compost refers to the pure rotting product, compost means mixtures that contain compost, among other things
  • Compost soil contains secondary components that are supposed to complement the compost properties
  • Organic compost is far richer in nutrients than green compost
  • Fresh compost is young and rich in nutrients, finished compost is about half a year old and less nutrient-rich; Ripe compost is 1 – 2 years old and poor in nutrients
  • The more nutrient-poor a compost is, the more stable humus it contains that is retained
  • The more nutritious a compost is, the less stable humus it contains and the faster it is broken down
  • Ready-made compost and mature compost are the most widely used composts in compost soil

Compost soil: definition and benefits

Compost soils – known as compost substrates by experts – are potting soils that contain a lot of compost. Since composts are often relatively rich in nutrients and therefore salty and also often have very high pH values, they are only very rarely used purely: So-called additives or secondary components in compost soil are supposed to balance the chemical and physical properties of the compost.

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Mineral components such as sand and broken bricks, clay minerals such as bentonite and vermiculite, perlite, lava rock, rock flour, expanded clay, and pumice are used as aggregates. Organic secondary components such as wood fibers, xylitol, bark humus, coconut fibers, or even rice husks can also be used. With the right combination of the main and secondary components, compost should enable optimal growth conditions.

Tip: Organic soils are also based on compost and contain structurally stable green compost with additives such as wood fibers and clay. They are also approved for organic farming and do not require peat.

Excursus: technical terms for substrates

  • A material is said to be “inert” if it does not significantly affect the nutrient content and pH of its surroundings. Chemical reactions with this material also hardly take place at all.
  • A material is “structurally stable” if it is not broken down by microbial processes and its volume and porosity are stable over long periods of time.
  • The air capacity describes how many air-carrying, large pores a material has.
  • The water capacity describes how many water-bearing, medium, and small pores a material has.
  • Nitrogen fixation is possible when organic material contains many carbon compounds but little nitrogen. Microorganisms then deprive their environment of the nitrogen they need to survive to break down the material. As a result, the plants growing in the area lack nitrogen. A deficiency can then be prevented by utilizing “compensatory fertilization” with a long-term nitrogen fertilizer.

Buy compost or make it yourself?

Aggregates in compost substrates

Supplementary components in compost are referred to as additives. The mixing ratio of the compost with a selection of different additives should ensure the best possible properties of the substrate in terms of ventilation, structural stability, water capacity, or weight.

Aggregate properties Effect in the substrate
sand Quartz grains; slightly acidic, high air capacity, permeable, structurally stable, heavy Improves ventilation and water permeability makes substrates heavier, requires regular watering
Broken bricks Recycled / Broken Bricks; porous, water-storing, structurally stable, light Improves ventilation, water permeability, and water retention capacity (to a small extent), everything from approx. 30% by volume
Bentonite Volume; nutrient-binding, water-storing, easily wettable Increases water retention capacity, stores nutrients and protects against over-fertilization, easy to moisten
Vermiculite Clay mineral; very good water storage, nutrient binding, easily wettable, pH value 8, expensive, contains a lot of magnesium Increases water retention capacity, ensures even moisture, stores nutrients and protects against over-fertilization, easy to moisten
Perlite Weathered, Crushed, Heated Obsidian; inert, pH 7.5, very porous, light Improves ventilation and structural stability (substrate sags less), makes substrates lighter, ensures even moisture (everything from approx. 30% by volume)
Foam lava Volcanic rock; porous, high air capacity, frost-resistant, light Increases structural stability (substrate sags less) from approx. 30% by volume, improves ventilation
Pumice Volcanic rock; porous, high air/water capacity, very light Increases structural stability (substrate sags less) from approx. 30% by volume, improves ventilation, and ensures even moisture
Rock flour Crushing rock; acidic to alkaline, contain plant nutrients, form clay minerals Provides main and trace nutrient elements for plants over long periods of time increases the clay content of soils in the long term, nd improves the crumb structure and water retention capacity
Expanded clay Heavily heated, puffed up clays; porous inside, structurally stable, high air capacity, light Increases structural stability (substrate sags less) from approx. 30% by volume, improves water permeability and ventilation, makes substrates lighter, requires regular watering
Wood fiber Recycled, usually from softwood; high air capacity, not structurally stable, nitrogen-fixing, low water capacity, pH value 5-6, poor in nutrients Improves ventilation and water drainage, is broken down microbially, sets nitrogen in the process, increases the pH value and sags (compensating nitrogen fertilization necessary), requires regular watering
Xylitol / lignite Lignite precursor, crushed; low in nutrients, structurally stable, high water and air capacity, slightly acidic (peat-like properties) Improves ventilation and water retention, can be changed with lime and fertilizer as required, protects against over-fertilization
Bark humus Crushed, fermented bark; medium water and air capacity, contains a lot of potassium, phosphate, and trace nutrients Improves ventilation and drainage, improves water retention to a lesser extent, protects against pH fluctuations
Coconut flour (cocopeat) Coconut shell abrasion; high air capacity, good water capacity, easily wettable, light, sometimes loaded with salt (note the declaration, especially potassium, chloride, sodium), otherwise low in nutrients, structurally stable Improves ventilation and drainage, less water retention, increases structural stability (substrate sags less), makes substrates lighter; possibly strong fertilizing effect, which must be taken into account when fertilizing
Rice husks “Husk” of rice grains; very light, very high air capacity, low water capacity, structurally stable, nitrogen fixation possible, low in nutrients, pH value adapts to other substrate components Improves ventilation and water drainage, requires regular watering, can fix nitrogen (compensating nitrogen fertilization is necessary), usually increases structural stability (substrate sags less)

Compost substrates and peat substrates in comparison

Peat extraction is harmful to the climate, it destroys habitats and disrupts the water balance of entire stretches of land since bogs can absorb and store huge amounts of water. But peat substrates are still the rule, especially in professional horticultural production, and alternatives are only the exception. This is not least due to the undisputed qualities of peat.

Buy compost or make it yourself?

Properties of peat

Its properties, which we list below, make it the ideal starting material for the production of potting soil.

  • Loose and air-permeable (white peat even with water saturation)
  • Very light and easy to transport when dry
  • Can store water many times its own weight
  • Very low in nutrients, so it can be fertilized according to the needs of each plant
  • Structurally stable, so does not sag and maintain ventilation (white peat is more stable than black peat)
  • Low pH, i.e. acidic; it is easier to adjust it to the desired pH value with lime than to change a substrate with a high pH value to a lower value

Properties of compost

Compost as a substrate is much more difficult to use than peat:

  • It is so variable in many properties, depending on the starting materials, that the production must be closely monitored to obtain a defined result
  • Young compost is not structurally stable, can be very rich in nutrients, and is then not suitable for growing young plants or for cultivating herbs and other lean plants that react very negatively to substrates that are too “fat”
  • The water storage capacity is high but not as high as that of peat, which can mean that such a substrate must be watered more frequently
  • Compost is chemically less stable; it reacts to a fluctuating pH value and nutrients by changing its properties
  • The pH value of compost is variable on the one hand – depending on the raw materials it contains and on the other hand, compost cannot be so easily influenced to the desired pH value, since lowering it is more difficult than increasing it

Buy compost or make it yourself?

Of course, these difficulties can be avoided to a large extent: Through the always the same, targeted production of mature compost from the same raw materials, which are as low in nutrients as possible, one obtains a structurally stable, low-nutrient compost with the desired pH value. The use of ready-made compost – which is somewhat richer in nutrients and less stable – is possible and reduces the amount of fertilizer to be added. The water storage capacity and structural stability can be significantly improved with xylitol – also called lignite. In general, aggregates can bring a compost substrate very close to the properties of peat substrates – the difference is then still in the production effort, in the purchase price, and an adapted use in professional horticulture.

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Advantages of compost as a substrate

Before you get the impression that the use of compost in potting soil only has disadvantages, let us now give you the decisive arguments that are also gradually forcing producers and substrate manufacturers in Germany to rethink.

  • Can be recycled over and over again from by-products of industry and waste
  • Doesn’t destroy the habitats of rare plants and animals – as peat extraction does
  • Does not fuel climate change, as no bound greenhouse gases are released – as is the case with peat extraction
  • Possibility of fertilizer savings when using more nutrient-rich compost, which not only saves costs but also reduces the degradation of fossil-based mineral fertilizers
  • In theory, it can be produced in horticultural businesses – as was common a few decades ago – and also in private gardens according to one’s own needs and then used to mix potting soil
  • Production is not tied to a fixed location: there are no long transport routes, as is common with peat

Buy compost or make it yourself?

Use compost

Compost soil – i.e. potting soil with compost components – can usually be used like any other plant substrate. There are nutrient-poor and nutrient-rich soils available for various applications, as well as those with different pH values and different long-term fertilizing effects. Depending on the additives, they differ in their stability and water holding capacity. So if you decide to use compost, it is better to take another look at your plants when using it until you know exactly how it is best for them.

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Conclusion: buy compost or make it yourself?

Both buying compost and mixing it yourself have advantages and disadvantages.

Buy compost soil: advantages

  • Purchased compost soil has a defined quality; the nutrients it contains, the pH value, and the suitability of the soil can be easily read on the packaging
  • The entire effort of producing or procuring compost as well as the procurement of aggregates is eliminated

Buy compost or make it yourself?

Buy compost soil: disadvantages

  • High price and at the same time small quantities
  • Ready-mixed compost soil is often intended for a specific area of application, which can make the use of the substrate inflexible and the purchase of additional mixtures necessary
  • Small packages mean a lot of packaging waste and the aggregates used have some long transport routes behind them, both of which are not really environmentally friendly

Making compost yourself: advantages

  • Mixing your own substrates makes you independent of substrate manufacturers and their prices since your own production cost a fraction of the price of finished soil
  • Because you can control yourself which aggregates are used, you know exactly what is contained and can make your soil extremely environmentally friendly and of high quality

Making compost yourself: disadvantages

  • Composting is quite a hassle, but you can avoid it by getting compost cheaply from a local composting facility
  • Overall, you never know exactly what properties your own mixtures have, as you have to dive deep into the subject of correct composting to be able to roughly assess your compost quality

In conclusion, it can be said that minimalist hobby gardeners are better advised to buy ready-made compost. This is especially true if the quantities are not that large or if the time for gardening is already less than the budget. Only gardeners with a really green thumb and the will to put in a certain amount of effort can benefit from the advantages of even mixed soils. If you belong to the hard-boiled group, you can find out more about making your own compost here.

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