What Is Humus in Soil?

Humus is what remains after organic litter from plants and animals has decomposed, and it is a rich material that no matter how big or small your yard and what ornamentals or edibles you grow in it, you need for optimal growing conditions. As dead plants, insects, animals, leaves, and other organic matter decompose, they break down into chemical elements leave behind a rich brown or black substance. This substance is humus, and it is full of nutrients that are necessary for plants.

Humus is what transforms sterile dirt into fertile soil so plants can grow, and it's essential to soil so that plants and crops have the nutrients they need to grow. Yet, while it's found in abundance in wooded areas with lots of plant debris to decay, the soil in yards and gardens tends to have little humus—luckily there are ways to add more humus into your yard.

Here is a basic guide on how humus is made, why it is so important to plant growth, and how you can both increase the humus in your yard and use it effectively.

How Is Humus Made?

Humus is the substance that is left over after plants and animals have undergone a long process of 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 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. In deciduous forests and land that hasn’t been used or developed, the fallen leaves will create humus each year. For man-made humus, aeration, the right amount of moisture, and heat in a compost pile or bin speed up the decomposition process. As the decomposition progresses, the organic materials are broken down into more stable forms of nutrients that plants can use as food. 

Humus contains the elements necessary for plant growth: nitrogen, phosphorus, sulfur, calcium, magnesium, and potassium. High-quality farmland soil will have 3-6% humus, giving it the nutrients plants and crops need to thrive. Besides the humus, there is typically 70% minerals , 8% organisms (of the microscopic and visible variety!), 15% air and water, and 5% yet-to-be-decomposed plant and animal remains—that will one day be humus.

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 provides them in a form that is highly usable for plants.

Humus also improves your soil and gives it a crumbly and loose texture so oxygen can get in and reach the plant roots. Humus also 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, many people will remove 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. 

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. You can add compost to your soil as a soil amendment if you do not have plant debris naturally occurring (or it's been removed).

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 is to test your soil 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.