How compost helps your garden soil

How compost helps your garden soil

Tips for homemade compost
30 March 2022

Also known as “gardener’s gold,” or “black gold”, compost is best when it comes from your own garden. The rewards of composting include healthy, nutrient-rich soil. Martina Kolarek, a biochemist and hot compost specialist, provides expert advice.

What are the benefits of composting?

Martina Kolarek: Composting rewards you with fertile soil and healthy plants – and less watering, fertilizing and pest control. Compost adds nutrients to the soil. By nourishing the soil with organic materials, we create a habitat for myriad organisms and microorganisms that work to keep the soil healthy.

What makes soil fertile?

MK: Soil is fertile when it contains many diverse living organisms. They create the soil structure that stores air, water and nutrients and makes them available to plants to grow and thrive. Adding fertilizer to soil without structure does not help: the soil cannot absorb it, so it goes directly into the groundwater, and the plants cannot access the nutrients. Besides improving the soil, compost helps to protect water from pollution by reducing erosion. It also improves the climate: plants absorb carbon from the air and release it into the compost-rich soil, which stores the carbon in its vital underground structures.

"Composting rewards you with fertile soil and healthy plants – and less watering, fertilizing and pest control."

What are signs of good, fertile soil?

MK: Soil that contains an adequate amount of humus is dark brown, moist and smells good. If the soil has too little humus, it looks pale and dry and doesn’t have any smell at all. It’s like a desert because nothing can grow there.

Compost and humus – how are they related?

MK: Humus is the organic part of the soil, the product of soil organisms. They digest and convert dead plants, animals and microorganisms into molecules. Fallen apples, pears, and leaves will naturally decompose and turn into humus over time – compost speeds up this process. Similar processes of decomposition and transformation can turn your garden, fruit and vegetable waste into compost. The thin, top layer of fertile soil always comprises a mixture of organic and inorganic matter. Adding mineral-rich, volcanic rock dust to the compost therefore helps to optimize the humus.

Compost Heap
Compost Heap
Inspecting Compost

Compost ingredients – dos and don’ts

MK: Don’t add cooked or seasoned leftovers. No bread, no pastries. Add citrus peels sparingly; they attract green mould. Do add green garden cuttings and weeds, small branches and twigs, and fallen leaves in moderation. And, of course, all the unwanted raw fruits and vegetables from the kitchen. Banana peels are also good, as are coffee grounds and tea leaves – in small, household quantities.

"Compost improves soil, protects waterways from pollution and helps the climate."

Good compost needs a balance of structural substances, nutrients and contact with the environment. Compost is much more than fertilizer – its crumbly, sponge-like structure can simultaneously store nutrients, air, and water. It also provides a habitat for a diverse community of small flora and fauna, who keep the soil fertile and resistant to wind and water erosion.

How do I use the finished compost?

MK: Depending on the nutrient requirements of the plants you are growing, work 1 to 3 litres of compost into the top layer of your garden soil. Hot compost is very rich in nutrients and structure, making it less suitable for growing seeds. It’s better to use it for plants whose roots are already fully formed.

Building your compost pile: helpful tips

Making compost isn’t hard, but there are a few things to keep in mind. A hot compost pile is best built in one day. Mineral rock dust and a homemade herbal extract enrich the mix and stimulate the composting process. Layering the materials quickly and in the right combination enables the compost to heat up to the right temperature. If you are a composting beginner, follow the instructions carefully. It gets easier and faster to prepare the more you do it.

One cubic metre of organic waste

For hot compost, you need at least one cubic metre of garden waste and kitchen scraps. Ideal garden ingredients include grass clippings, plant cuttings, weeds, twigs, branches, and some fallen leaves. Kitchen ingredients include raw vegetable and fruit scraps, coffee grounds and tea leaves. Cow and horse manure and compostable materials like brown cardboard can also go into the compost.

Things you’ll need to build your compost pile:

- A suitable bin for the kitchen scraps

- Rock dust/mineral powder

- Herbal extract

- Natural fibre insulation

Build your compost layer by layer

A single compost layer can comprise up to nine different materials: a mix of garden waste, kitchen scraps, wood chips and sawdust, cow or horse manure and other compostable products, like shredded cardboard. Kitchen scraps provide essential nutrients, while garden waste creates structure.

Chop or shred the materials into small pieces, then add them one after the other, mixing them well with the other materials of the same layer right away. Add the mineral powder to this mixture. Then pour in the herbal extract and add a handful of garden soil. The proper composition and having enough air and moisture are crucial for compost quality. Once you’ve built your own compost, you will quickly get a feel for the correct ratio of ingredients.

How soon will the compost be ready?

Your completed compost pile should be 1 to 1.5 metres high. After two days at the latest, it will become hot, reaching a temperature of 60 to 65 degrees Celsius. After three to eight days, you can remove the insulation materials so the compost can cool down a bit. The compost heats up when the microorganisms become active, rapidly multiply, and quickly break down the organic substances.

However, they only do this in optimal living conditions – with enough food, water, air and space. These microorganisms also serve as food for other soil organisms that later migrate into the compost and rebuild the soil for us. Certain herbs can stimulate microbial activity, so it helps to include a bit of herbal extract to each later. Composting is not a secret; it is an art.

Should I make compost in winter or summer?

Hot compost prepared in summer will be ready in about 3 months. In winter, it takes about six months. Hot composting has advantages over conventional cold composting or using industrial compost. It does not need to be turned, does not attract vermin, and is easy to make in diverse settings. That’s especially important for composting food waste and saves labour, time and space.

Learn more about soil fertility and soil health.

Martina Kolarek 

Expert in Agricultural Soil Science and Sustainable Nutrition

Martina Kolarek is an expert in agricultural soil science and sustainable nutrition. She is a biochemist and studied applied ecology at the University of Natural Resources and Applied Life Sciences in Vienna. In her workshops, she shows how to create good compost.