What Is Humus in Soil?
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What is humus in soil? Humus is the part of soil organic matter that has broken down far enough to become stable. It is not the same as fresh leaves, manure or compost. The stable portion breaks down slowly and stays in the soil for a long time, and it changes how the soil holds water and nutrients. This article explains the idea using three university and extension sources on soil organic matter, and it separates what those sources say about humus from what they say about other materials.
Quick answer
- Humus is the stable fraction. New Mexico State University says “The stable organic matter (humus) is relatively unavailable; the other pools of organic matter are in various stages of being decomposed.”
- It lasts a long time. The Vermont source calls humus “passive soil organic matter” and says it “is relatively stable over time.”
- It holds water and nutrients. New Mexico State University says that “a unit weight of humus fraction can hold up to 20 times its weight in water.”
- Bagged humus is often something else. The same university says bags of “humus” in garden centers “are really more like compost or peat.”
Where humus comes from
Soil organic matter starts as living and once-living material. The Mississippi State University extension page gives a plain definition: “Soil organic matter is the fraction of the soil composed of anything that once lived.” It then lists what that includes: “It includes plant and animal remains in various states of decomposition, cells and tissues of soil organisms, and substances from plant roots and soil microbes.” That range matters because not all of it is equally stable. Fresh residue, living organisms and older material sit side by side in the soil.
The New Mexico State University circular breaks the pool into parts. It says “Soil organic matter contains a mix of decomposing organic matter, stabilized organic matter, living organisms, and fresh materials like manures and compost.” It then splits the pool into an active fraction and a passive fraction. The active fraction “contains living organisms, organisms that have died, and fresh material that can serve as food for living organisms.” Humus belongs to the other group.
The University of Vermont source uses the same logic. It describes the passive pool as “cells and tissues that are only gradually decomposing” and says “Passive soil organic matter, also known as humus,” is the portion that persists. In the Vermont page’s wording, the passive fraction is the slow part of the cycle, and humus is what remains in that slow part.
What humus is made of
New Mexico State University names the compounds that make up the passive fraction. It says “The passive fraction is composed of carbon compounds that are recalcitrant (hard to decompose).” It lists “humic substances and fulvic acids, glomalin, and other organic compounds” as members of that group. The source then uses the word humus as a collective term: “collectively, they can be referred to as humus (stabilized soil organic matter).”
The circular also says that humus is the part of soil organic matter that matters most for long-term soil quality. It states that “It is humus that has the longer-lasting effects on the quality of garden and farm soils.” This is the reason the source focuses on humus rather than on the fresh material added to a bed each season. Fresh residue feeds the soil life, but the stable fraction stays in place for longer.
The Vermont page describes the same point in different words. It says the stable material “is relatively stable over time” and that “Usually, humus represents the majority of total soil organic matter.” Those two statements together describe humus as the large, slow-moving part of the pool, rather than a small extra.
Why humus holds water
The clearest physical effect of humus is its water capacity. New Mexico State University says that the humus fraction “is colloidal in nature (very small particles that will stay suspended when dispersed in water) and is capable of absorbing a tremendous amount of water.” It gives a number for this: “In fact, a unit weight of humus fraction can hold up to 20 times its weight in water (Stevenson, 1982).” The source then draws the conclusion for gardeners: “That is why improving the organic matter of the soil makes it more resilient to drought conditions.”
The Vermont page links humus to the same capacity. It says “Humus has a lot to do with the ability of a soil to retain nutrients and water.” The two sources agree that humus affects how much water soil can keep, and the New Mexico circular gives the most specific figure. The figure is for the humus fraction by weight, so it should not be read as the water a whole garden bed will hold.
The Mississippi page gives a useful comparison for scale. It says “Mississippi soils generally are less than 2 percent organic matter.” That statement describes the typical organic matter content of soils in that state. It does not give a target for any particular soil.
How humus holds nutrients
Humus also holds nutrients through charge. New Mexico State University says “The humus fraction is also negatively charged, just like clay particles.” It explains the consequence: “This is important because the humus fraction can hold on to positively charged ions in the soil.” The circular names some of those ions and states that “Nutrients such as potassium (K + ), magnesium (Mg ++), and calcium (Ca ++) can be held by the humic substances, which prevents them from being lost through leaching.”
Leaching is the loss of dissolved materials as water moves down through the soil. By holding nutrients in place, humus slows that loss. The source says the humus holds on to nutrients “which prevents them from being lost through leaching that takes place when water flows through the soil.”
The Vermont page says the same thing from a different angle. It states that humus “also supplies organic chemicals to the soil solution.” The soil solution is the water and dissolved materials in the soil pores. Humus therefore affects what the soil holds.
Humus and compost are not the same
People often use the words compost, peat and humus as if they mean the same thing. The New Mexico circular makes a direct point about the bagged product sold as humus. It says “However, bags of “humus” found in garden centers are really more like compost or peat, and don’t meet our definition of soil humus.” The statement is about whether the bagged product meets the definition of soil humus.
The same circular lists compost among the inputs. It states that “Soil organic matter contains a mix of decomposing organic matter, stabilized organic matter, living organisms, and fresh materials like manures and compost.” In that sentence, compost is a fresh input that feeds the active fraction. Over time, some of that material moves into the stable fraction. The circular describes humus as the end result of the process, and it says that “As that fresh organic material is converted to humus, there is positive improvement in the water-holding capacity of the soil.”
Our guide on how to make garden compost quickly covers the fresh-material side of that cycle. The distinction in this article is about the end product rather than the method.
What humus does in a garden
The sources describe humus as part of what makes soil good for growing plants. The New Mexico circular says that the compounds in the humus fraction “help improve the qualities of soil that we equate with a good soil for a farm or garden.” That is a broad statement. It means humus contributes to several soil qualities at once, including water and nutrient holding.
The Mississippi page describes the visible form of well-decomposed material. It says “Well- decomposed organic matter forms humus, a dark brown, porous, spongy” material. The description is about appearance and texture. The page does not link that appearance to a measured amount of humus.
In practice, the sources support a simple idea. Adding fresh organic material feeds the soil life and eventually builds the stable fraction. Maintaining that stable fraction is what gives long-term benefits. The New Mexico circular makes the time point clear: humus is the portion with “the longer-lasting effects.” Our guide on how to improve clay soil over time covers a related topic.
How we chose
We chose three non-retail sources: a University of Vermont page on soil organic matter, a Mississippi State University Extension page on soil organic matter, and a New Mexico State University circular on soil organic matter and humus. Each was fetched directly with curl and checked for the quoted statements in this article. The New Mexico circular is the most detailed of the three, so it supplies most of the mechanism. The other two pages confirm the basic definitions and the link between humus and water holding.
What we could not verify
The sources define humus and describe its effects, but they do not give a single number for how much humus a particular garden soil contains. They also do not give a rate at which fresh material becomes humus. The New Mexico circular notes that the stable fraction is “relatively unavailable,” which is a qualitative statement, and the Vermont page says humus is “relatively stable over time” without a time span.
What the sources do not say
None of the three pages gives a target percentage of humus for a vegetable bed or a lawn. The New Mexico circular gives the water-holding figure for the humus fraction by weight, but it does not give the water a whole soil will hold. The Mississippi page gives an organic matter figure for that state’s soils, not an amount for any single garden. Where a number is not in the quoted text, this article does not supply one.
Frequently asked questions
Is humus the same as compost?
No, according to the New Mexico State University circular. It says that bags of “humus” in garden centers “are really more like compost or peat, and don’t meet our definition of soil humus.” Compost is a fresh input that can become part of the stable fraction over time.
Why is humus important for soil?
Humus holds water and nutrients. The New Mexico circular says that it “is capable of absorbing a tremendous amount of water,” and that the humus fraction “can hold on to positively charged ions in the soil.”
How long does humus last in soil?
The University of Vermont page says humus “is relatively stable over time.” None of the three sources gives a time span in years, so this article does not state one.
Does humus feed plants directly?
The sources do not say that humus feeds plants in a direct way. The Vermont page says humus “also supplies organic chemicals to the soil solution,” and the New Mexico circular describes nutrients being held by humic substances. The sources describe holding and supplying, rather than a direct food source.
Can I buy humus at a garden center?
The New Mexico circular cautions that bagged products labeled humus “are really more like compost or peat.” A buyer who wants the soil science meaning of humus should read the label and the definition, since the sources do not say which product matches it.
