What Is Humus Soil? Soil Humus Vs. Loam

What is humus, and how does it compare to loam? This important distinction is often confused by gardeners, so let’s clear things up…

Healthy soil is made up of four macro-ingredients including organic matter, rock particles, air and water.

The word humus is used to describe several soil components or additions including ready-to-use homemade or store-bought compost, naturally occurring compost obtained from a forest, vermicompost (organic matter processed by earthworms) or even topsoil that contains a large percentage of organic matter. But technically humus is organic matter that can no longer be broken down, theoretically able to maintain its makeup over centuries if not millenia.

Humus is also darker than other organic soil material due to its high levels of organic carbon. The darker the humus, the more organic carbon it contains.

While humus is as good as it gets when it comes to soil aeration, nutrient retention and water retention and drainage, it can’t grow plants on its own. It should make up only a certain percentage of the ideal soil for growing, also called loam.

Loam is soil that has all of the ideal soil qualities required for plant growth, containing just the right amount of sand, silt, clay and organic material. There are also different types of loam that have different levels of sand, silt or clay content such as medium loam, sandy loam, silty loam, clay loam and silty clay loam.

No “perfect” soil combination exists. Instead, different combinations of silt, sand, clay and organic material are appropriate for different plants (this resource will help you figure out what percentage of each your soil has).

Soil Testing

There are two ways to test your soil: home soil testing kits and professional testing by your local Cooperative Extension office.

In general, healthy organic garden soil (loam) should…

  • Feel crumbly and full rather than gritty (sandy), smooth (silty) or slippery/sticky (clay
  • Have a pleasant earthy odor or very little odor at all
  • Appear rich dark brown or black in color
  • Have a garden soil pH conducive to plant growth, usually between 6 and 7
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