How to build up humus in the garden

The secret of healthy, vigorous plants is a high humus content in the soil. Here you can find out why humus is so important in the garden, how you can build up humus and why humus build-up contributes to climate protection.

Humus is the total dead organic matter in the soil, consisting of plant remains and the remains or excretions of soil organisms. In terms of quantity, carbon is most abundant in this substance, so that after humus buildup, soils are in principle huge carbon reservoirs. What at first sounds unspectacularly theoretical is enormously important for the soil, the plants and also the climate. The organic substance decisively determines the soil structure as well as the soil properties and thus the plant growth. In addition, humus binds huge amounts of the greenhouse gas carbon dioxide (CO2). This means that a high humus content is not only important in agriculture with its huge areas, but also in the garden, where you can consciously build up humus.

The humus content of the garden soil has a very great influence on its fertility. In contrast to the mineral content, which can only be changed with a costly soil exchange, it is quite easy to increase the humus content of your garden soil. All you have to do is what happens in the wild in forests and meadows. There, all organic waste, whether autumn leaves, dead plant remains or animal droppings, eventually falls to the ground, is decomposed into humus by a wide variety of organisms and is incorporated into the upper soil layer.

Humus build-up in the garden: Tips in brief

Compost, mulch, green manure, manure, old potting soil and organic fertilizers from the trade are all suitable for building up humus in the garden. Mulching is especially important to build up a humus layer. It is also advisable to use peat-free or peat-reduced soils. Drainage of peatlands and humus decomposition result in the increased release of CO2.

How does humus build-up and decomposition in the soil work?

The build-up of humus or humification is a dynamic process, the biomass in the soil is subject to constant decomposition and build-up, the content of organic matter can therefore remain stable, increase or decrease. Certain components remain in the soil as nutrient humus for only a few months, while others remain as permanent humus for centuries or even millennia. The decomposition of humus is called mineralization, whereby in extreme cases without regular humus supply, only the mineral soil components remain – the soil is depleted.

Microorganisms break down the easily degradable components of organic matter, such as sugars and proteins, within a few months, and the degradation products are released into the soil as water, nutrients and volatile carbon dioxide, and into the air or atmosphere. For the plants valuable nutrients are released, for your garden soil a good aeration, water and nutrient storage. This so-called nutrient humus makes up a good 20 to 50 percent of the biomass. Complex components of organic matter, such as cellulose or lignin (wood), are only gradually broken down into permanent humus. This is because soil organisms cannot, after all, use all the constituents for their own purposes. What remains is, among other things, humic substances that form the basis for permanent humus, which is then firmly integrated into the soil structure.

The actual nutrient humus content always depends on the organic starting substances, how active and lively the soil is and, of course, on the air and water content of the soil. Compost has already undergone the rotting process and is therefore particularly valuable for the soil structure and soil life.

Why is humus content important for the garden?

Soil organisms break down the biomass in the garden soil into plant nutrients and store the remainder as permanent humus, the humic substances of which build up clay and mineral particles into permanently stable, so-called clay-humus complexes. These keep the garden soil loose like a giant timber-framed structure. But you should also build up humus for other reasons:

• Humus is the basis of all life in the soil, and thus for soil fertility and plant growth.
• Humus provides nutrients that are not, or hardly, washed out.
• By building up a humus layer, you promote the soil’s water-holding capacity, but also its ability to percolate – the garden soil will not become waterlogged.
• When you build up humus, the soil becomes pretty loose.
• A high humus content protects against erosion from heavy rains.
• Biomass in soils buffers pH value fluctuations.

How to build up humus in the garden?

Since humus is continuously broken down in the soil and biomass also leaves the garden as harvested material, it must be continuously added to the garden and also to agriculture. If you want to build up a humus layer, compost, green manure, manure, mulch and even old potting soil come into question for this, but also organic fertilizers from the trade. These granulated fertilizers, however, have a comparatively small share in humus building, but certainly a measurable one. Their strength lies in the short-term supply of nutrients to the plants, in addition, the organic fertilizers keep the soil life in good spirits and promote the humus build-up. Mulching is particularly important for building up a humus layer, as mulch protects the soil from drying out like a sunshade and keeps the soil life or the entire soil biology happy.

Regular mulching of garden soil

Regular mulching is one of the most important measures for building up humus in the garden. Basically all organic materials and garden waste are suitable as mulch, from autumn leaves to dried lawn clippings and chopped shrubs to classic bark mulch. With materials that are very low in nitrogen, such as bark mulch and chopped wood, you should work about 100 grams of horn shavings per square meter flat into the soil before mulching. This will prevent the microorganisms from extracting too much nitrogen from the soil as the mulch decomposes, which the plants then lack to grow. Experts also call this phenomenon nitrogen fixation, often recognizable by the fact that the plants suddenly take care of themselves and show typical nitrogen deficiency symptoms such as yellow leaves.

Mulching the ornamental garden with organic material is basically the same as area composting in the vegetable garden, where the beds are completely covered with vegetable waste. In addition to increasing the humus content, the mulch layer has other beneficial influences: It prevents weed growth, protects the soil from drying out and from sharp temperature changes.

Higher humus content with garden compost

Compost is a particularly rich humus. It not only enriches the soil with organic substances, but also provides all the important nutrients. You can spread compost every spring as a basic fertilizer in the ornamental and vegetable garden, between one and three liters per square meter, depending on the nutrient requirements of the respective plant species. However, be careful with strawberries and heather plants such as rhododendrons: garden compost usually has a relatively high lime and salt content and is therefore not suitable for these plants.

If you want to enrich the soil in the rhododendron bed with humus, it is best to use composted autumn leaves that have not been treated with a compost accelerator. It forms a particularly coarse-textured, durable humus that provides a loose soil. You should collect the autumn leaves in special wire baskets in the fall and let them rot for a year before using them as humus. Turning it over after six months encourages rotting, but it is not absolutely necessary. Even half-decomposed leaves can be used as raw humus for mulching or soil improvement.

Fertilize plants organically for humus soil

Organic fertilizers, such as horn shavings, provide not only nutrients but also humus. However, due to the small amounts needed for fertilizing, they do not lead to a noticeable increase in the humus content of the soil. The situation is quite different with manure: cow manure in particular is an excellent supplier of nutrients and humus that can also be used in rhododendron beds without any problems, especially for soil preparation when planting new plants.

Important with all types of manure: allow the manure to rot well before spreading it on the soil, fresh manure is too strong and rather harmful, especially for young plants. To prepare vegetable beds in the spring or new beds in the ornamental garden, you can work the rotted manure shallowly into the soil. In perennial crops, simply spread the manure thinly on the soil and possibly cover it with leaves or bark mulch. You should avoid working it in so as not to damage the roots of the plants.

Which plants do not like humus?

Not all garden plants welcome a humus-rich soil. Some Mediterranean herbs and ornamental plants, such as rosemary, cistus, gaura, sage, or lavender, prefer low-humus, mineral soils. Observations repeatedly show that these species are even much more resistant to frost damage on well-drained, winter-dry sites. So the water-retaining humus in the soil tends to do them a disservice here.

Why does the build-up of humus counteract climate change?

Plants take CO2 that is harmful to the climate out of the air, store it and thus remove it from the air, thus slowing down climate change. After humus buildup, this CO2 is therefore stored in the soil.

The possibilities for CO2 fixation through humus content are therefore limited, because humus decomposition releases the CO2 previously removed from the atmosphere. In fact, everything that was bound in the last decades or even centuries. Marshlands, where the high water content actually slows down humus decomposition due to a lack of oxygen, are quite extreme in this respect. However, if the peat soils are drained, they are aerated and humus decomposition begins. And with it the release of CO2 in large quantities. If you buy peat-free or even peat-reduced potting soils, you no longer provide substrate companies with an incentive to use peat soils that have already been drained or even to drain new peatlands. By rewetting already drained peatlands, its mineralization can be stopped, which some soil manufacturers are already doing.