Humus comes from the death and decay of organic materials that have fallen from leaf litter or other raw organic matter that get incorporated into the soil. You could say plants grow their own soil. Also, the contributions of beneficial soil microbes and fungus make soil more productive.

When plants die, it may appear they have fallen to the ground to rot away as waste. Mother Nature never wastes anything. The beauty of plant design is it only looks this way. A plethora of different varieties of microbes and fungi eat dead plants. As they do, the microbes and fungi release nutrients back into the soil for future plant generations.

In addition to returning to the soil all the elements that plants once borrowed from it, microbes contain the nutrient energy plants captured from the sun as they grew and thrived. The microbes and fungi pass on their accumulated nutrients to the soil as they actively decompose the plant remains. These remains are in turn decomposed by other microorganisms and consumed over and over, life to death, death feeds life, life to death and so on…consumed several times by many different organisms. This cycle creates stable humus and helps form rich, healthy soil.

Gardeners have known for centuries that to grow vegetables when they have nutrient poor, rocky soil they must crush the rocks to tiny particles, the smaller the better, and mix them with decaying organic materials. The organic matter added to plant materials, animal waste and the bodies of dead animals enriches a soil's physical and chemical properties. Remember "ashes to ashes, earth to earth." Everything that goes back in to the soil helps to build it by providing the essential nutrients right where plants need them, at the root zone.

Humus is usually dark brown, spongy and gooey. Some types of humus contain highly soluble molecules that readily break down further, known as active humus, the kind gardeners value most for feeding soil microbes. Others resist breaking down, because their larger, insoluble molecules bind tightly to clay molecules. Stable humus is crucial to fertile soil's physical qualities. A nice balance between both types is best and helps maintain exceptional soil health and tilth. For this reason, you should add both raw organic matter and composted organic matter to your garden. This keeps the biology going strong and adds the physical properties to create beautiful, healthy soil.

Many plant fibers and animal tissues are recalcitrant, meaning they resist decomposition. The tough fibers of dead plants like leaf veins and other woody materials are connected together to form long chains of complex organic molecules that do not easily break down.

Because organisms vary in how easily they break down, we need a highly diversified soil food web. All organisms in garden soil serve different functions. One might be great at digesting cellulose, another at carbohydrates, fats or lignins. Many macro-organisms such as worms and small insects prepare recalcitrant organic materials by passing them through their digestive processes, which prepares them for other microscopic digesters to fully break them down into a simpler molecular structure. When nutrients break down into simpler forms, plant roots can absorb them to grow. This is the essence creating humus.

Organic matter gets consumed a number of times. As microbes and fungus continue to break down the remains of various organisms into simpler compounds more available to plants, they also add stable organic matter to the soil. This organic matter is vital as the primary source of nutrients for plants in the form of carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium and trace elements or micronutrients. (Click here for more on essential micronutrients.) Humus also acts as an important reserve between the fresh organic matter from which it comes and the simpler forms of carbon dioxide, water, and minerals that returns to the soil, sometimes several years later.

Humus formation varies with air temperature. When the temperature rises above 80°F, organic matter decays faster than it is generated. Conversely, when temperatures drop below 40° F, biological activity in the soil slows down, building humus reserves for plants to draw on later.

Eventually, this digested organic matter transforms into tiny particles of dark humus. Humus is delicious for plants, loaded with nutrients and full of structural integrity. Plants love humus and you will love what it does for your garden.

Among the environmental benefits of humus in your backyard garden (and in large farming operations that use organic practices): It holds carbon dioxide, sequestered in the humus soil particles, until microbes digest them and liberate the carbon dioxide gas back into the atmosphere. Carbon dioxide is also held in the soil while complex chains of organic compounds wait for microbes to digest them in preparation for plant root absorption.

Carbon dioxide plays a role in the troubling phenomenon of global warming, a reality of our time. Organic gardening works organic materials back into the soil until microorganisms use it as an energy source. The energy then passes to actively growing plants in different compounds for their use. Animals and humans consume these plants, further sequestering carbon dioxide that otherwise would release into the atmosphere. In this way, even your little backyard makes a difference on a global level.

Milo Lou Shammas
Founder and Formulator