Most avid gardeners know that there is more to soil than just “dirt”. Healthy soils are comprised of a combination of materials that serve to allow proper water filtration, retention, or drainage, as well as provide a suitable home for plant roots, support untold millions of helpful microbes, and serve as the conduit for all of the necessary macro and micronutrients needed for survival. Humus is a critical player in the healthy soil matrix.

What is Humus?

Humus, also referred to as organic matter, is the portion of a soil that is comprised of formerly living matter. Plant debris, dead animals, and other previously alive organic matter is decomposed through micro-biotic organisms and earthworms to create the nutrient dense substance, humus.

What Does Humus Do?

Humus serves to add structure to soils by increasing its moisture-holding capacity and by supporting root development in plants. It performs the latter by converting nutrients in the soil into more usable forms for plant to absorb. In essence, the humus is a plant’s digestive system, taking raw material and producing a product that is more conducive for roots to uptake.

Does Your Garden Soil Have Enough Humus?

Great, so now that we know what humus is, how do we know if we have enough, and if it is good enough for our garden? The first step of any such assessment, is to obtain a soil analysis. For many such soil health metrics, there a number of reliable, do-it-yourself kits on the market. For a valid measurement of your soils’ organic matter content, sending it off to a lab is the best bet.

"Crops that leaves behind roots, such as lettuces, greens, and grains are an excellent source of humus-making material."

Most county cooperative extension agencies either perform this service for a nominal fee or can recommend a private lab that will perform that service. The amount of humus needed in any garden is unique to that particular site and dependent upon the crops that are to be grown.

A good rule of thumb, though, is that a healthy soil will have between five and 10 per cent organic matter. If you have an organic matter content in this range, you are good to go! If not, let’s look at some of the things we can do to get there.

Sourcing More Humus for Your Garden Soil

Any humus is good humus. All sources of humus however are not created equal. Good humus can be added by introducing things to the garden such as composted animal manures (don’t add “fresh” manure as it can burn plant roots, not to mention how, let’s say “unpleasant” that would be) or composted food and plant scraps.

Growing certain crops can also help increase humus. Crops that leaves behind roots, such as lettuces, greens, and grains are an excellent source of humus-making material. Crops that leave behind a lot of surface residue, such as brassicas and corn, will also help to build up good humus as it decomposes.

There are commercially available blends of both bulk and bagged humus as well. These are perfectly acceptable to add to your garden as a good humus, but it is also the most expensive way to add humus to your garden. There are numerous free and low cost sources of good humus worth looking at first.

Maintain a Good Amount of Humus in Your Garden

Once you have a good level of humus in your garden, it is important to try and maintain it. Humus levels are not constant. As crops and plants use nutrients, the amount of organic matter present in a soil is likely going to be reduced. Worse yet are events that disturb the soil such as erosion, compaction, and even heavy tillage.

To properly manage your levels of good humus, plan on adding a couple of inches per year of composted material at the end of the season. By doing it at this time of year, the material will be incorporated into your garden through the freezing and thawing action in colder climates or by rain and animal activity in warmer climates.

It can be applied in the spring as well, but the benefits of it may not be fully realized until later in the season.

At the end of the day, our best advice is to test your soil, add good compost or other beneficial amendments, and repeat annually.