What Is Humus and How Is It Formed?

Think you know humus? You may be surprised what we traditionally call "humus" is actually something different.

If you’ve been gardening for any length of time, you probably know what humus is. You’ve seen bags of soil labeled “humus” at garden centers, and may have used this rich, black soil to enrich your garden.

It might surprise you, then, to learn humus is not actually soil. Scientists and home gardeners define what humus is quite differently. Let’s dig into the details.

The Scientific Definition of Humus

Scientific journal articles about humus do not define it as a rich type of soil you can make or buy. Rather, they describe it as a soft, spongy essence comprised almost entirely of carbon and devoid of any form of life or nutrients. It’s created through natural decay processes that take decades to complete.

Humus is what remains after decaying plants, animals and insects are broken down into the smallest possible molecules. It’s been forming on earth for millions of years. The circle of life, from the first sprout of a seed to the end of a plant’s life cycle to its decay into compost and finally into humus, is a continual process. From humus-rich soil, more seeds will sprout.

Across the world, humus is only found in the top 12 inches of the earth’s crust. You’re most likely to find it in wooded areas left undisturbed for decades. Because it takes so long for organic materials to break down into humus, it’s not something that can be commercially manufactured. So what you see labeled at garden centers as “humus” isn’t actually humus at all — it’s compost.

A Gardener’s Definition of Humus

Most gardeners think of humus as a soil amendment filled with beneficial nutrients, minerals and microbes that improve the health and texture of poor soils. As we mentioned above, it’s really compost — nutrient-rich, living soil continually breaking down into smaller elements thanks to tiny organisms, including microbes, bacteria and algae.

As it decays, its nutrients feed plants and help them thrive. Humus is what’s left after compost, manure, leaf litter, wood mulch or any other carbon-based life form breaks down as far as it can go.

Benefits of Humus

If humus is technically a lifeless essence made of carbon, how does it benefit your garden? These three are the most important to gardeners.

  1. Humus is like a sponge — it can hold up to 90 percent of its weight in water. Humus near your plants’ roots help them stay moist longer than they would in soil devoid of humus. That means you won’t need to water your garden as often.
  2. The chunky texture of humus improves soil texture by making it looser and more friable (easily crumbled). Plants’ roots spread easily in humus-rich soil, allowing easier access to water and oxygen as they grow.
  3. Scientifically speaking, humus has a negative charge. That helps nutrients like magnesium, phosphorus, nitrogen, calcium and iron stick to it instead of being washed through the soil. Roots that come in contact with humus can then absorb these beneficial nutrients. So, while humus itself does not contain minerals, it carries and delivers them.

Feeding Your Soil

With or without human intervention, humus will continue to form perpetually. You can help the cycle along by adding organic materials to your garden each year. Composted food scraps, manure, shredded leaves, wood mulch and any other carbon-based materials spread on top of your garden beds will slowly decompose and turn into humus over time.

The process of forming humus is beneficial to all insects and microorganisms that call your garden home. When you add organic materials to your garden, you are essentially “feeding” your soil. That, in turn, feeds your plants by improving your soil’s fertility, texture and oxygen level.

Whether the package labeling on bags of “humus” is arguably accurate or not, you can be sure that what’s inside will benefit your plants.

Susan Martin
Susan Martin is a lifelong gardener who enjoys sharing her passion for plants, gardening and the business of horticulture with fellow plant enthusiasts across North America. She has spent over two decades working in the horticulture industry on new plant development, garden design, sales, marketing and consulting. Susan has received visitors from around the world in her home garden which has been featured in numerous gardening publications. Her goal is to inspire and educate people about how to garden every day.