SOIL ORGANIC MATTER (SOM)
The target we are aiming for.
SOM is sometimes used synonymously with “soil health” or “soil quality”. This is not necessarily wrong but
the later terms are broader in their meaning and application.
Many soil properties are directly tied to SOM and benefit from its presence. They are:
Bulk Density –
The density of soil, or Bulk Density, decreases with increasing SOM. This indicates more pore
space or porosity. Porosity positively influences what we call “available water holding
capacity” or AWC. This of course means the soil holds more of the water it receives by
precipitation or irrigation. Since more water is held in the soil profile, there is less nutrient
and salt loss (called leaching) into our ground water, rivers, and lakes. Increased AWC buffers
the soil (and crops) against drought! SOM itself, depending on its decomposition state and
amount, can hold 10-1000 times more water than the mineral fraction of the soil.
Soil Structure –
Also known as aggregation, the strength of soil increases with increasing SOM. This strength
can be visually observed as the structure of the soil. Structure is also related to Bulk Density,
Stronger structures have lower bulk densities than their weaker counter parts.
Strong Soil Structure helps to keep compaction to a minimum and thus keeps the porosity
and AWC at higher levels and allows abundant root growth. It also buffers the soil from
occasional abuses such as over grazing or heavy vehicle traffic. Structure can also be
influenced by other soil properties such as calcium carbonate, iron oxides, and clay
percentage (texture).
Friability –
Essentially this is how “soft” the soil is, or how easily it crumbles in your hands. Soft soil has
good friability. Friable soils are easy to cultivate and specifically are easier to plant into with
no-till drills and seeders and generally have better results with broadcast or aerial seeding.
Surface Infiltration –
Increasing SOM increases porosity, and structure. If the surface of the soil is minimally
disturbed these properties, in turn, increase water infiltration from the surface down into the
soil profile. Water that does not infiltrate into the soil profile is called runoff. Decreasing
runoff decreases erosion and movement of sediment, fertilizer, pesticide, and herbicide into
our river and lake systems.
Nutrient cycle/CEC –
Nutrient status is increased in 2 ways: 1) less leaching and runoff as explained above. 2) SOM
itself can be a large source, or pool, of nutrients available for use by plants. Nutrients in the
form of SOM are especially beneficial because they are not susceptible to transport and they
are “time released”. The releasing of nutrients from SOM is called mineralization. Cation
Exchange Capacity (CEC) is a soils ability to hold nutrients and make them available to plants.
SOM greatly increases CEC and may be the dominant source of CEC in sandy soils.
Toxin Retention –
SOM may bind, or immobilize, heavy metal and organic contaminants in soils where they may
be held for some time and/or degraded over time. But this chemistry can be quite complex
and in some instances SOM may increase metal solubility/mobility.
Key effects of higher levels of SOM on land manager activity:
-
Less inputs of fertilizer, pesticide, and herbicide (short term inputs may be higher, esp. herbicides, as soil is
regenerated)
- Fewer field passes = less fuel usage, less equipment “wear and tear”, less soil compaction
- Likely higher yields and nutrient dense crops (and critters)
- More “brain work” in planning and management but less over all labor and input cost
*These points typically equate to at least similar profits as the status quo conventional system. The bottom line
(for everyone), in the near future, will be better as the soil health increases.
How do we build or increase SOM?
Some principles to consider:
 Keep the soil covered as long as possible through the year, preferably with living plants that actively
contribute more organic matter directly into the soil via their roots. Compost and mulch are good alternatives
or co-contributors to organic matter at the soil surface. (Mulch residues, in any form, on the soil surface
further reduce erosion.)
 Increase the activity and more importantly the diversity of the soil biology, otherwise known as the Soil Food
Web (bacteria, fungus, micro & macro invertebrates). Feed your underground herd with the roots of living
plants, composts, compost teas, etc. Increasing diversity in the Soil Food Web is a huge step toward
decreasing pest pressures.
 Till the soil as little as possible. When we till we greatly disturb the habitat of the underground herd. We are
destroying structure/strength and injecting too much oxygen into that environment thus allowing the food
web balance to be skewed greatly towards bacteria which consume SOM, negating efforts to increase it.
Tillage also reduces surface infiltration. Initially tillage may increase infiltration on a crusted soil but that
condition is very short lived. An exception to soil disturbance would be the action of grazing animal’s hooves
for short durations.
Practical application:
 Practice no-till or at least conservation tillage. Do what you have to, but go with as little tillage and soil
disturbance as you possibly can. NO recreational tillage! (No brainer considering fuel prices.)
 Try to plant diverse or multi-species cover crops after and/or before your cash crop; practice companion
or “shadow cropping”. We are still learning a lot in this arena and specifics are highly dependent on local
environmental conditions.
 Apply composts, compost teas, and mulches however it is practical in your situation. This may be
especially beneficial in the transition or “healing” period after a change to your system. Eventually these
inputs would be reduced or ideally eliminated. But there are many variables that determine continued
inputs.
 These same principles apply to grazing and rangeland situations even though the soil itself is less
intensively managed – graze “properly” with a plan and allow for diverse plant communities. A more
intensively managed and planned grazing system is essential. It may entail more work initially but it will
pay off down the road in increased range and soil health. Ideally animals would also be incorporated
into a cropping system and “pasture cropping” could also be considered where feasible.
“External” benefits of building SOM
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Improved air, water, and food quality
Carbon dioxide sequestration from the atmosphere
Improved human & animal health
How do we measure & monitor SOM to know if our management system is working?
Analysis:
- Lab tests for Soil Organic Matter or Soil Organic Carbon (SOC)
- Lab tests for Active Carbon
(Use same laboratory and methods over the monitoring period for these tests)
- Indirectly through soil “respiration” field test (microbial activity) – highly dependent on field conditions
(moisture and temperature) and can be difficult to make consistent measurements and interpretations.
Biological Analysis:
Following are analysis focusing on soil biology populations and diversity. They do not measure SOM.
Soil Food Web analysis
- PLFA (phospholipid fatty acid) test.
Comparison methods:
1) Monitor a single area and management system over time or,
2) Compare areas under differing management concurrently. May also be monitored over time.
Chuck Peacock, Soil Scientist, USDA-NRCS, Owner Soilhealth.net
Soil Health (Soil Quality) is ”the capacity of a specific kind of soil to function, within natural or
managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance
water and air quality, and support human health and habitation” (Karlen et al., 1997). Soil
Science Society of America’s Ad Hoc Committee on Soil Quality