ORGANIC MATTER matters. IN FACT, THERE MAY 

BE NO OTHER COMPONENT THAT’S MORE IMPORTANT TO A HEALTHY SOIL THAN ORGANIC MATTER. 

The tiny fraction of soil composed of anything and everything that once 

lived—organic matter—is more than an indicator of healthy soils. 

The carbon in organic matter is the main source of energy for the allimportant soil microbes and is also the key for making nutrients available to 

plants. The list of positive influences high levels of organic matter have on 

healthy soils includes:

1. Provides a carbon and energy source for soil microbes

2. Stabilizes and holds soil particles together

3. Supplies, stores, and retains such nutrients as nitrogen, 

phosphorusand sulfur

4. Improves the soil’s ability to store and move air and water

5. Contributes to lower soil bulk density and less compaction

6. Makes soil more friable, less sticky, and easier to work

7. Retains carbon from the atmosphere and other sources

8. Reduces the negative environmental effects of pesticides, 

heavy metals and other pollutants

9. Improves soil tilth in surface horizons

10. Increases water infiltration rates

11. Reduces crusting

12. Reduces water runoff

13. Encourages plant root development and penetration

14. Reduces soil erosion

high in organic matter.

Uni

Dep

Agr

USDA is an equal opportunity provider and employer.

HEALTHY SOILS ARE:

Natural Resources Conservation Service

Losing Organic Matter

Organic matter is vital to healthy 

soils, yet most modern agricultural 

operations are not managed in 

ways to retain high levels. Only 

half the original organic matter 

remains in most modern cultivated 

soils. In general, organic matter 

levels have fallen from 5-6 

percent of the soil to less than 3 

percent on most cropland soils.

Using tillage depletes organic 

matter. Each time the soil is 

tilled, oxygen is stirred into it, 

stimulating microbial action to 

decompose organic matter at an 

accelerated rate. As a matter of 

fact, when a woodland is cleared 

and planted or a prairie is plowed, 

most of the organic matter that 

was built over hundreds of years 

is lost within 10 years of tillage.

Combining frequent tillage with 

farming practices that leave little 

plant residue for soil microbes to 

eat (such as burning or removing 

crop residues) will lead to the 

depletion of organic matter.
Considering the long list of benefits organic matter has on soil 

health and crop production, increasing organic matter may well be the 

most important management step a producer can take to improve a 

farm’s profitability and sustainability. In general, there are three ways 

to do that: 

1. Increase the amount of plant and root production; 

2. apply carbon-rich materials to the soil; and 

3. use practices that slow rather than speed decomposition.

Cover crops, green manure crops, and perennial forage crops add 

organic matter, as do compost and manure. Growing crops and roots 

add biomass above and below the soil surface. However, not all that 

biomass is converted to soil organic matter—much of it is released as 

carbon dioxide and water. It can take 20,000 pounds of organic inputs 

such as crop residue to increase the actual soil organic matter from 

4 percent to 5 percent.

Compost in particular breaks down more slowly and improves soil 

structure more quickly than other organic materials. Manure breaks 

down quickly to add nutrients for crops, but takes longer to improve the 

soil than compost.

HEALTHY SOILS ARE: high in organic matter.

Active and Stabilized 

Organic Matter

Organic matter can be divided into two 

categories: active and stabilized. The portion 

made of fresh organic material and living 

organisms, as well as partially decomposed 

material that is slowly decomposing, 

is called “active organic matter.”

Active organic matter and the microbes 

that feed on it are central to nutrient 

cycles in the soil. Nutrients, especially 

nitrogen, phosphorus, and sulfur, are 

held in this active organic matter until soil 

organisms release them for plant use. 

This accounts for there being much more 

nutrient volume in the soil than is available 

for plant use at any one time. For example, 

a soil with 3 percent organic matter contains 

about 3,000 pounds per acre of nitrogen, 

but only a small part of that (30-100 pounds) 

may become available to plants in any one 

year, depending on decomposition rates.

While active organic matter may decompose 

over a few decades, the stabilized portion 

of organic matter is made of larger, more 

complex compounds that are much more 

difficult for microbes to degrade. Much of 

the stabilized organic matter in the soil 

is highly decomposed plant and animal 

tissues that grew more than a century, 

and possibly several centuries, ago. This 

organic matter becomes carbon-rich 

humus that’s resistant to further decay. 

“Stabilized organic matter” or humus, acts 

like a sponge and can absorb six times 

its weight in water. It’s also a reservoir 

for nutrient storage, sequestering carbon 

from the atmosphere and other sources.

Healthy soils need both active and 

stabilized organic matter to function well. 

COMPARING ACTIVE AND STABILIZED ORGANIC MATTER

PORTION OF ALL 

ORGANIC MATTER

DECOMPOSITION 

TIME

FUNCTIONAL 

IMPORTANCE

ACTIVE One-half to two-thirds Up to several 

decades

Decomposes 

organic material 

to produce plant 

nutrients

STABILIZED One-third to one-half A century or more Exceptional water 

holding capacity, 

soil structure 

benefits; reservoir 

for nutrients, 

including carbon

WANT TO LEARN MORE? VISIT WWW.NRCS.USDA.GOV