Last updated on October 23rd, 2023 at 08:24 pm
A healthy soil lives from its humus layer. Compost provides the garden soil with valuable nutrients and improves moisture retention. But when it comes to compost, questions quickly arise: What can be put on the compost? What is the difference between fresh compost and ready-made compost? Where is the best place to put compost? We explain to you how to compost properly and what is important in the production and use of this important organic fertiliser.
Compost offers many advantages
Mixed compost usually contains all the main and trace nutrients that are vital for plants. If you compost properly, it can therefore keep up with commercial fertilisers. In particular, the inputs of phosphorus (P), potassium (K), magnesium (Mg) and calcium (Ca) through compost are often quite sufficient. Phosphate and potassium are rather abundant in many garden soils anyway, as is nitrogen (N).
Compost is an organic fertiliser that comes with many benefits:
Adding humus, increasing soil fertility.
Promotion of soil life
Supplying nutrients and liming the soil
Promotes and maintains soil structure
Suppresses pathogens in the soil
Degradation of environmental toxins
Nitrogen is mineralised only slowly in compost, i.e. it takes some time for it to be converted into a form that is available to plants. In impoverished soils or with highly productive perennials, this is not enough. Here you should apply additional nitrogen, for example in the form of horn shavings. Crops with medium nutrient requirements, however, which have been regularly supplied with compost fertiliser over the years, often manage without additional fertiliser applications.
Proper composting: The basic rules
To avoid drying out or rotting, the place for composting should be in partial shade. It makes sense if the subsoil consists of soil, so that small organisms have the opportunity to migrate from the soil.
The compost should never be allowed to dry out completely so that the rotting process can take place undisturbed. You should therefore water during dry periods. During heavy, prolonged rainfall, on the other hand, a cover protects the compost.
Shred woody materials before composting, for example with a shredder. This way they can be broken down more quickly.
A diversely composed compost provides both nitrogen and carbon to the decomposers. Green plant parts and woody material are therefore always turned.
Adding mature compost has an accelerating effect on rotting, as does the use of compost starters.
Loosen the compost once a year in spring, or turn it over. This ensures optimal mixing and aeration.
The degree of rotting is decisive
Since the content of available nutrients in compost decreases with longer storage, you should use compost selectively. Compost is divided into five degrees of maturity or rotting. The degree of rotting refers to the rate of decomposition of the organic substances. However, this speed also depends on other things, such as water content and air temperature.
Fresh compost has a rotting degree of II or III. It is not yet fully matured, but can be used after four to eight weeks under good conditions. Its structure is coarse, many parent materials are still visible – it provides nourishment for the numerous microorganisms it contains. Important: Fresh compost should only be used as mulch and should only be worked into the soil superficially. In deeper layers it can rot and cause plant damage. Fresh compost stimulates the activity of soil organisms, its organic matter decomposes quickly, releasing nutrients in the process.
Finished compost is produced after at least six months of rotting and has a rotting degree of IV to V. It is fine-crumbled, like a layer of soil. It is fine-crumbly like soil and completely mature. Apart from some woody components, the structure of the original materials is no longer recognisable. Mature compost can be added directly to planting holes and seed furrows or used for potting soil mixtures; it is a slow-acting fertiliser. Above all, it improves the soil structure in the long term due to its high permanent humus content. Stable crumbs are formed that protect against silting and crusting. The pore volume increases, the soil becomes looser and aeration improves. In addition, water and nutrients are better stored.
The cress test
Compost is only used when all substances harmful to plants have been broken down. A test with sensitive cress provides clarity about the degree of maturity.
And this is how it works: Fill a container with a garden soil-compost mixture (1:1), moisten well, sprinkle on cress seeds, press down and cover with foil or glass. Place in a bright and warm place and observe. If the seeds germinate quickly and the seedlings continue to grow without any problems, everything is fine. Hesitant or uneven emergence or yellow leaf colouring indicate that the compost should be allowed to ripen.
When and how to apply the compost?
Apply the compost during the main growth phase, from early spring to the end of summer. This is when the plants can best utilise the nutrients released. This reduces the risk of leaching into the groundwater and you can use compost as a fertiliser without any problems.
Proper composting: It’s the dosage that makes the difference
A lot does not necessarily help a lot. You can also overfertilise your garden with compost. An exact dosage is only possible after a prior nutrient analysis of the soil and the compost. On the basis of average values, however, the following guidelines can be given:
Ornamental shrubs: Mix up to one third of mature compost into the excavated soil. The compost should not go deeper than 30 cm into the soil. This will be sufficient for the first few years.
Perennials: Put a shovelful of mature compost directly into the planting hole. When planting a new bed on impoverished soil, up to 40 l/m² of compost can be spread and worked into the top 10 cm of soil. This will last for about three years.
Ornamental shrubs: 1 l/m² compost annually, on humus-poor soils additionally for example 30-50 g/m² horn shavings. Take care with acid-loving plants such as rhododendrons or other bog plants. Compost is usually quite alkaline and therefore unsuitable in these cases!
Perennials: You can enrich ornamental and bedding perennials with 2 l/m² compost annually, supplemented with 50-100 g/m² horn shavings. Wild and forest edge perennials usually only need 1 l/m² of compost. Alpine perennials and rough pastures do not receive compost.
Rule of thumb: 2 l of compost per m² correspond to a layer of 2 mm. The compost should cover the soil in gaps.
Mix compost with soil
Seeding soil should be mixed with compost up to a maximum of 20 percent by volume. Use only well-matured compost.
For balcony and pot plant soil, an admixture of about 30 percent by volume is possible. Suitable mixing components are garden soil, sand, peat or peat substitutes such as wood or coconut fibre.
What can go on the compost?
In order to be able to compost properly and end up with high-quality humus, it is crucial that only suitable materials go on the compost. In principle, of course, only organic, biodegradable substances can be composted. Coarse plant residues such as tree cuttings or wood chippings should have been sufficiently shredded beforehand. Leaves and lawn clippings, or dead potted plants together with the soil, can also be composted, as well as fruit and vegetable residues (fruit peelings, however, are best only in organic quality, as they are otherwise often heavily coated with pesticide residues). Coffee grounds should have been dried beforehand; they provide many nutrients, but also increase the acidity.
Fruit and vegetable waste, herbaceous plant residues and lawn clippings make compost with high nutrient contents. If plenty of wood chippings, woody shrub stems and autumn leaves are added, the values drop.
If you have a lot of leaves in autumn, you should make a separate leaf compost, as the pH value of leaf compost depends on the type of tree. Oak leaves, for example, are highly acidic.
Things that attract rats, such as cooked food scraps or eggshells, should not be put on the compost. Plants treated with pesticides, wood ash or the remains of barbecue charcoal are taboo. You should be especially careful with weeds if they contain seeds – some of them can spread through the compost. There are also plant diseases that can survive composting and infect plants through the soil, for example certain bacterial pathogens or pests such as the chestnut leaf miner or the boxwood borer. Most viral and fungal diseases, on the other hand, are harmless and not long-lived enough to live on in the compost.