No matter how big or small your yard and what ornamentals or edibles you grow in it, you need humus. Humus is what transforms sterile dirt into fertile soil so plants can grow. In nature, the addition of humus to soil happens on its own as plant debris, dead animals, and other organic matter decompose.
In wooded areas, the soil is naturally rich in humus and fertile whereas in areas with sparse vegetation, such as along the coastline or in the desert, the soil contains very little humus and is infertile. In most people’s yards, there is not a lot of vegetation left to natural decay that will produce humus.
Find out why adding humus to the soil is so important and what you can do as a gardener to increase the humus in your soil.
What Is Humus
Humus is the substance that is left over after plants and animals have undergone a long process of thorough decomposition done by earthworms, bacteria, fungi, and other microorganisms. It is found in the top few inches of soil. The color of humus is brown or black, and it has a loose, crumbly, and spongy texture.
Humus contains the elements necessary for plant growth: nitrogen, phosphorus, sulfur, calcium, magnesium, and potassium. Depending on the original plant and animal material, the chemical composition of humus varies.
Why Humus Matters
There are several reasons why humus is crucial for soil health. Humus contains important nutrients needed for plant growth, including nitrogen. And it’s not just that humus contains nitrogen, it provides it in a form that is highly usable for plants.
Humus also gives soil the necessary structure and a crumbly and loose texture so oxygen can get in and reach the plant roots. And humus lets water move through, which is the key characteristic of well-drained soil, and at the same time absorbs and holds water.
The humus content in soil can easily get out of kilter because growing food crops, turf grasses, and ornamental plants rapidly depletes humus. It needs to be replenished but in our backyards and gardens, it’s popular, but often not necessary for horticultural reasons, to remove any plant residues such as grass clippings, dried plant parts, leaf litter, instead of leaving them in place to decay and turn into humus. Also, sun exposure, wind, and strong rains, as well as removing topsoil when weeding can further reduce the humus content of the soil. To keep the soil fertile and loose, you need to replenish humus every year. Simply adding fertilizer to the soil is not a viable solution because humus does more than add nutrients; it also gives the soil structure and texture. Fertilizer also does not do anything to support the activity of microorganisms, which are crucial to turn organic matter into usable plant nutrients.
How Humus Is Made
Humus can be made in two different ways: naturally and by humans collecting organic material—composting it—or leaving it in place and letting it decay to add to soil afterwards. Regardless of whether it’s Mother Nature or gardeners, the process of making humus is the same.
Piled up plant and animal materials decompose in a wide range of environmental conditions. In nature, humus is made over a longer period of time. For man-made humus, aeration, the right amount of moisture, and heat in a compost pile or bin speed up the decomposition process.
In the early stages of decomposition, some of the carbon, hydrogen, oxygen, and nitrogen in the decaying materials are released as water, carbon dioxide, methane, and ammonia, which creates the typical rotten compost smell. As the decomposition progresses, the organic materials are broken down further into more stable forms of nutrients that plants can use as food.
Humus vs. Compost
Compost is one source of humus—the man-made kind. Humus, on the other hand, is the general term for fully decayed plant material that is naturally present in the top layer of soil or had been added by humans.
The organic materials in a compost bin are much more diverse from those that naturally accumulate in any given location from trees dropping leaves and other plant and animal waste. A compost bin is composed of an accumulation of food waste that you wouldn’t find in nature, that’s why balancing “brown” and green” materials, adding moisture, and turning the compost are important for successful composting.
How to Use Humus in the Garden
Humus is indispensable to plant growth but adding too much organic material or in the wrong form does more harm than good. Humus makes up only 3 to 6 percent of productive soil used for crops. It is unlikely that with a yearly addition of humus to your garden, you risk of overloading the soil with more nutrients than the plants are able to use. The only reliable way to determine the nutrient content in your garden soil is a soil test, ideally two years in a row. That will give you a baseline to determine how the addition of humus is affecting the nutrient content.
Adding a lot of organic material that is not fully broken down, such as fresh wood chips, can lead to a temporary nitrogen deficiency. As the number of bacteria in the soil increase and they work to decompose the organic material, they tie up nitrogen. Once the organic material is decomposed, the nitrogen is released back to the soil.
What exactly is humus?
Humus is what remains after organic litter from plants and animals has decomposed.
Is compost and humus the same?
Humus is the general term for naturally decayed organic material. Compost consists of organic materials such as food waste and other plant residue that humans have accumulated for decomposition.
What is the difference between humus and manure?
Humus is fully decomposed plant and animal material, whereas manure consists of animal feces at varying stage of decomposition.
“Soil Organic Matter.” Cornell University Cooperative Extension, 2008.
“Mulch Basics.” University of Connecticut Home and Garden Education Center, 2016,