Notes on the Chapter: Amount of Organic Matter in Soils

Natural Factors

• The amount of soil organic matter (SOM) is the result of additions and losses over many years and is influenced by a wide range of environmental, soil, and agronomic factors.

• Living, dead, and very dead organic matter all play critical roles; their relative amounts may be affected differently by natural factors and practices.

• Additions (inputs) of organic residues increase SOM; losses (decomposition, erosion) decrease SOM.

• When inputs > losses, SOM accumulates; when inputs < losses, SOM depletes; in a balanced system, SOM remains relatively constant over time (inputs ≈ losses).

Temperature

• In the United States, higher average temperatures (north→south) generally lead to lower SOM, given sufficient rainfall.

• Warmer climates produce more vegetation (longer growing season) and accelerate decomposition by soil organisms, with faster decomposition often dominating and reducing SOM.

• In Arctic/alpine regions, input is limited by short growing seasons, but decomposition is very slow at cold temperatures, so SOM can be high.

• Warming Arctic/alpine regions accelerates SOM loss as per microbial respiration; CO₂ is released, and methane (CH₄) can also be lost from these soils.

Rainfall

• In arid climates, SOM tends to be low due to limited vegetation growth and low microbial activity.

• More rainfall generally supports more plant residues returned to soil, increasing SOM; however, excessive rainfall can reduce aeration and slow decomposition in poorly aerated soils, affecting the rate of SOM turnover.

• Very wet or swampy conditions promote accumulation of organic matter as peat or muck due to slow decomposition in anaerobic environments; artificial drainage can cause rapid decomposition and surface subsidence.

Storage of Organic Matter in Soil

• Organic matter is stored and protected by several mechanisms:

  • Formation of strong chemical bonds between organic matter and clay particles (and fine silt).

  • Physical protection inside small aggregates.

  • Conversion to stable humic substances resistant to decomposition.

  • Restricted drainage reducing activity of oxygen-dependent decomposers.

  • Stable char chemistry resulting from incomplete burning.

• Large aggregates consist of many smaller aggregates; they are held together by sticky substances from roots, bacterial colonies, and fungal hyphae.

• Organic matter inside large aggregates but outside small aggregates, and freely occurring particulate organic matter (the “dead”), remain available to soil organisms.

• Poor aeration from dense subsurface layers, compaction, or wetland/waterlogged conditions can reduce the rate of organic matter use by organisms.

• For SOM to be useful to soil life, it must be in a favorable chemical form and physical location with adequate moisture and aeration.

Storage of Organic Matter (continued)

• When it finally rains, decomposition can occur very rapidly, illustrating the dynamic balance of additions and losses.

Soil Texture

• Fine-textured soils (high clay and/or silt) tend to maintain higher SOM than coarse-textured sands or sandy loams.

• Typical natural ranges for OM by texture:

  • Sands: < 1%

  • Loams: 2–3%

  • Clays: 4% to >5%

• Strong chemical bonds between OM and clay/fine silt, along with small aggregate formation, protect OM from decomposition.

• Fine-textured soils often have smaller pores and less oxygen, which slows decomposition and supports higher SOM.

Soil Drainage and Landscape Position

• Decomposition is slower in poorly aerated soils; lignin and some plant compounds decompose slowly or not at all in anaerobic environments.

• Wet soils (peat/muck) accumulate OM; artificial drainage leads to rapid OM decomposition and surface subsidence.

• Soils in depressions or floodplains tend to accumulate more OM due to runoff and deposition from upslope.

• Soils on steep slopes or knolls tend to have lower OM due to continual erosion of topsoil.

Type of Vegetation

• The vegetation forming soil during formation influences natural SOM levels.

• Soils formed under grasses generally contain more and deeper OM than soils formed under forests, largely due to deep, extensive root systems and rapid root turnover.

• Grassland roots die, decompose, and continually contribute OM (including biochar from slow-burning fires) to soil; this supports high productivity in former grassland soils.

• Forest soils typically accumulate litter on the surface; mineral layers below may have relatively low OM.

• Grasslands historically contribute to high OM and soil fertility in productive agricultural soils; forests contribute high surface OM but lower subsoil OM.

Acidic Soil Conditions

• SOM decomposition is typically slower under acidic conditions than under neutral pH.

• Acidic conditions can inhibit earthworm activity, causing OM to accumulate at the soil surface rather than distributing through the profile.

Human Influences

• Erosion removes topsoil rich in OM and reduces total SOM; crop production compounds this loss, affecting yields.

• Erosion is natural but accelerated by agricultural practices (water/wind erosion, tillage).

• In the United States, erosion is associated with economic losses (estimates around $1 billion annually in nutrient losses, with higher total nutrient losses).

• Moderate erosion may reduce yields by 5–10%; severe erosion can reduce yields by 10–20% or more.

Erosion Effects (Table 3.1 example data)

• Corwin: Erosion levels — slight, moderate, severe; Organic Matter (%) = 3.03, 2.51, 1.86; Available Water Capacity (%) = 12.9, 9.8, 6.6

• Miami: Organic Matter (%) = 1.89, 1.64, 1.51; Available Water Capacity (%) = 16.6, 11.5, 4.8

• Morley: Organic Matter (%) = 1.91, 1.76, 1.60; Available Water Capacity (%) = 7.4, 6.2, 3.6

Tillage Practices

• Conventional plowing and disking provide seedbed benefits and may enhance nutrient release by increasing OM decomposition, but break down soil aggregates and increase erosion susceptibility.

• Disturbing soil increases the rate at which OM within aggregates becomes available to decomposers, accelerating OM loss and CO₂ release.

• Moldboard plowing and residue incorporation speed decomposition and disrupt soil structure; the analogy used is opening the air intake of a stove to ignite a hotter fire.

• Historical context: In Vermont, 20% OM decrease after five years of growing silage corn on clay soil previously in sod for decades.

• In the Midwest, rapid OM loss occurred (up to ~50%) within ~40 years after conversion from native prairies to cropland; this motivated fertilizer and soil amendment use to maintain productivity.

• Conservation tillage (no-till, reduced tillage) tends to leave more residues on the surface and reduces soil disturbance, often increasing surface OM and earthworm populations, while a full profile OM response may take longer to develop.

• No-till often increases surface OM quickly; deeper changes are slower; long-term no-till effects on total profile OM vary by climate and soil type.

Crop Rotations and Cover Crops

• OM levels can fluctuate during different crop rotations; perennial grasses and legumes generally increase SOM due to high root production and reduced soil disturbance.

• Grass/legume hay rotations build SOM through persistent residue input and extensive root systems.

• Corn grain harvest leaves more residues in the field than silage harvest or legume crops; this affects residue return to soil.

• Continuous year-after-year row crops with high harvest of residues and intensive tillage can deplete SOM.

• Cover crops protect soils from erosion between cash crops and can contribute OM, depending on biomass and how much is retained after planting the next crop.

Effect of Rotation Type on OM (New York study example, Table 3.3)

• Conventional vegetable systems: 2.7% OM; Aggregate stability 27%

• Annual grain systems: 2.9% OM; Aggregate stability 30%

• Dairy systems: 3.4% OM; Aggregate stability 36%

• Mixed vegetables (mostly organic): 3.9% OM; Aggregate stability 44%

• Pasture (no-till, perennial): 4.5% OM; Aggregate stability 70%

• Higher OM generally correlates with greater aggregate stability when soils are not heavily tilled.

Cover Crops

• Cover crops protect soils from erosion during the non-cash crop period and can add organic matter through biomass accumulation, with the amount depending on crop type and biomass produced before planting the next cash crop.

Use of Synthetic Nitrogen Fertilizer

• Nitrogen fertilization can increase crop yields and biomass (hence residues), but N is often applied in excess (sometimes up to ~50% more than crop needs), which increases costs and environmental risk.

• Proper N management (often with soil testing) is necessary to balance crop needs and residue production; discussed in Chapter 19.

Use of Organic Amendments

• Manures and other organic residues recycled from livestock or off-field sources can maintain or increase SOM.

• Vermont study: to maintain SOM when silage corn is grown annually, about 20–30 tons (wet weight, including bedding) of dairy manure per acre are needed (roughly one to one-and-a-half times the annual manure production of a large Holstein cow).

• Types of manure differ (bedded, liquid, digested) and affect OM and nutrient availability; storage and field handling (surface application vs. incorporation) also matter (discussed in Chapter 12).

Organic Matter Distribution Within Soil

• With Depth

  • Generally, SOM declines with depth; topsoil is usually more OM-rich than subsoil, contributing to higher productivity.

  • A wide distribution of OM is often observed near the surface in forests and prairie-derived soils; deeper OM is more limited depending on soil texture and disturbance history.
    -a) Forest soils: litter layer may contain 30% or more OM; OM distribution changes with depth as litter is mixed into mineral soil when plowed or cultivated.

• b) Agricultural soils derived from forest: deeper distribution of OM after management and cultivation shifts.

• c) Prairie soils: deeper distribution of OM due to long-term prairie vegetation; after cultivation for ~50 years, OM is greatly diminished.

• d) Agricultural soils originally prairie: OM distribution altered after cultivation; traditional prairies had deeper OM profiles that declined with cropping.

• Inside and Outside Aggregates

  • OM occurs outside aggregates as living roots and larger organisms or as residues; some OM is associated with mineral surfaces (clay/silt) and within small aggregates.

  • Very small aggregates can protect inside them from decomposition; larger aggregates are held together by fungal hyphae and root exudates.

  • There is a relationship between clay/silt content and the amount of OM required to form stable aggregates: more clay/silt means more OM is needed to stabilize aggregates.

  • A soil with 50% clay may need roughly twice as much OM as a soil with 10% clay to reach >50% water-stable aggregates.

Active vs Passive Organic Matter

• Focus on balancing living, active (dead, freshly decomposing), and passive (humus) OM.

• Some OM is protected from decomposition by chemical bonding or by being inside small aggregates; however, a portion remains as active (“dead”) or as free particulate OM that feeds soil organisms.

• Forest/grassland soils typically lose a large amount of the biologically active portion when converted to cropland unless managed with residues, cover crops, and organic amendments.

• Active organic matter increases fastest with soil-building practices (reduced tillage, better rotations, cover crops, manures/composts) because these practices increase the supply of fresh residues.

Amounts of Living Organic Matter

• In soils, fungi and bacteria vary by cropping system:

  • Forest soils: fungi-weight > bacteria-weight.

  • Grasslands: roughly equal weights of fungi and bacteria.

  • Agricultural soils with routine tillage: bacteria > fungi due to reduced surface residues and increased compaction.

• Soil pores are critical for living organisms; large soil pores (~1 mm) support earthworms and beetles; compaction reduces these pores and the living biota.

• Root tips are very small (~0.1 mm) and sensitive to compaction; loss of larger pores reduces habitats for soil fauna.

How Much Organic Matter Is Enough?

• No universal target for all soils, but some general guidelines exist:

  • 2% OM in sandy soils is very good but may indicate depletion in clay soils where more OM is needed to build stable aggregates and protect against erosion/compaction.

  • Soils with more silt/clay generally require more OM to stabilize aggregates and maintain soil structure.

• SOM testing is complex; total SOM is not a definitive indicator of soil health; some soils may be saturated with OM in one fraction and not others.

• A soil test that isolates the active OM fraction can be a sensitive indicator of soil health and management impact (see Chapter 23).

Organic Matter and Cropping Systems

• Virgin (natural) soils generally have higher SOM than agricultural soils, but differences exist across crop systems.

• General comparisons (New York study, Table 3.3):

  • Conventional vegetable: 2.7% OM; aggregate stability 27%

  • Annual grain: 2.9% OM; aggregate stability 30%

  • Dairy: 3.4% OM; aggregate stability 36%

  • Mixed vegetables: 3.9% OM; aggregate stability 44%

  • Pasture: 4.5% OM; aggregate stability 70%

• The data show higher SOM and much greater aggregate stability in pastures, reflecting continuous biomass return and limited disturbance.

The Dynamics of Raising and Maintaining Soil Organic Matter Levels

• It is difficult to dramatically increase SOM and to maintain high SOM levels; sustained, integrated practices are required.

• Soils that are highly aerated (coarse texture) are harder to build OM in because aggregation is limited and OM protection is reduced; conversely, high-clay soils with restricted aeration can retain OM more readily with lower input levels.

• Increasing SOM is easier in soils that are depleted than in soils already high in OM given their texture and drainage.

• The question of how much OM to aim for varies by soil type, slope, erodibility, and prior history; strongly erodible soils and slopes require more input.

• On the carbon-farming front, continuously adding residues to soils (manures, composts, cover crops) helps maintain and build SOM over time.

• The idea of restoring SOM is not simply to achieve a target percentage but to maintain a healthy turnover of organic matter to feed soil biota and improve aggregates.

Starting Point

• A three-glass analogy illustrates that severely degraded soils (case 1) have empty SOM; soils cycling SOM for a long time (case 3) are near saturation; case 2 is intermediate and can be improved.

• If farms near each other have different baselines (case 1 vs case 3), there can be opportunities to transfer organic residues (e.g., manure) from livestock farms to soil-depleted croplands.

• The amount stored in soil depends on soil type, especially clay content; finer textures generally allow for larger storage glasses.

Adding Organic Matter

• When changing practices on depleted soils (or soils that have low OM), initial gains occur by forming mineral-OM bonds, small aggregates, and larger aggregates held by mycorrhizal fungi, roots, and microbial films.

• Eventually, OM accumulates mainly as free particulate OM within larger aggregates when sites for protection are filled.

• Over years of building practices, the soil comes to an equilibrium where gains and losses balance; further changes in management may move the system toward a new equilibrium.

Equilibrium Levels of Organic Matter

• A simple equilibrium model can estimate steady-state SOM when gains balance losses.

• Definitions used in the model:

  • Gains = (F) × (A), where F is the fraction of fresh residues that remains after one year and A is annual residue additions.

  • Losses = K × SOM, where K is the annual rate of organic matter mineralization (decomposition).

• Equation 1: ext{SOM change} = ext{gains} - ext{losses}

• In steady-state, Equation 2: 0 = ext{gains} - (K imes ext{SOM})

• Therefore, Equation 3: ext{SOM} = rac{ ext{gains}}{K}

• Example (illustrative): If 5,000 pounds of residue are added annually, with 20% remaining after one year, then gains = 0.20 × 5,000 = 1,000 pounds per acre per year.

  • If decomp rate K = 3% (0.03 per year), then SOM at equilibrium ≈ 1,000 ÷ 0.03 ≈ 33,333 pounds per acre in the top 6 inches (assuming the top 6 inches weigh about 2,000,000 pounds).

  • This corresponds to roughly an OM percentage of about 1.7% in the top 6 inches (
    ext{OM%}
    oughly rac{33{,}333}{2{,}000{,}000} imes 100 \approx ext{1.7%}
    ).

• Other scenarios illustrate how OM builds more rapidly when soils are highly depleted or when higher residue inputs are maintained.

• Time to detect changes: changes in total SOM can take years to decades; the effects of management on the active fraction can be observed earlier.

Increasing Organic Matter vs Managing Organic Matter Turnover

• Increasing SOM is most effective when using a combination of:

  • Minimizing soil disturbance to preserve aggregates and protection of OM.

  • Keeping the soil surface covered (living cover crops or crop residues) to reduce erosion and inputs to soil OM.

  • Rotations that include perennials and cover crops with extensive root systems and high residue return.

  • Adding organic materials (manures, composts) from off the field when possible.

• The balance of gains and losses is dynamic; continuous inputs help maintain and increase SOM, but management should aim at sustaining turnover and preventing losses.

Appendix: Calculations for Table 3.3 and Figure 3.7 Using a Simple Equilibrium Model

• The SOM balance model:

  • ext{SOM change} = ext{gains} - ext{losses} ag{Eq.1}

  • Gains = F imes A ag{Eq.2} where F is the fraction of fresh residues that remains after one year and A is annual residue additions.

  • Losses = K imes ext{SOM} ag{Eq.3} where K is the annual mineralization rate.

• Steady-state (Eq.4):

  • 0 = ( ext{Gains}) - (K imes ext{SOM})

  • Thus, ext{SOM} = rac{ ext{Gains}}{K} ag{Eq.5}

• Example from the Appendix:

  • If 5,000 pounds of residue are added annually and 20% remains after one year, gains = 1,000 pounds/year.

  • With K = 3% (0.03) per year, SOM ≈ rac{1{,}000}{0.03} = 33{,}333 ext{ pounds of OM per acre in the top 6 inches} (~1.7% SOM, given top 6 inches ≈ 2,000,000 pounds soil).

• The model helps explain why soils with very different textures and drainage conditions reach different equilibrium OM levels under similar management practices.

• NOTE: The appendix also provides data for Table 3.3 and Figure 3.7, illustrating how equilibrium OM changes with different combinations of decomposition rates (K) and residue additions (A).

Quick References from the Chapter

• Figure 3.1: Additions and losses of organic matter from soils.

• Figure 3.2: Root systems of annual wheat and wheatgrass; root turnover contributes significantly to soil OM.

• Figure 3.3: Organic carbon changes when growing corn silage or alfalfa (illustrates how different crops affect OM inputs).

• Figure 3.4: Soil surface after harvest of corn silage vs corn grain (illustrates residue distribution).

• Figure 3.5: OM content with depth in forest, agricultural, and prairie soils.

• Table 3.1: Effects of erosion on soil OM and available water capacity (examples for three soils: Corwin, Miami, Morley).

• Table 3.2: Location and type of soil organic matter (Active vs Passive; Living organisms occupy spaces between aggregates; details on where organic matter is located).

• Table 3.3: Organic matter levels and aggregate stability across cropping systems in New York (conventional vegetable, annual grain, dairy, mixed vegetable, pasture).

• Table 3.4: Estimated equilibrium OM after many years for different K (decomposition) rates and residue additions, showing how texture and aeration influence equilibrium OM.

• Appendix: Provides the calculation framework and sample numbers to illustrate how to estimate equilibrium SOM from inputs and losses.

Poignant Takeaways

• SOM is controlled by a balance of inputs (residues, manures, cover crops) and losses (mineralization, erosion, decomposition).

• Soil texture, drainage, and vegetation history strongly shape how much SOM can be stored and where it is stored within the soil profile.

• Conservation practices that reduce disturbance and protect surface residues tend to sustain and build SOM, particularly in finer-textured soils with higher potential for aggregate stabilization.

• Understanding the active vs passive fractions of SOM helps explain both immediate soil health improvements and long-term carbon storage potential.

• While there is no universal target for SOM, practical management should aim to maximize SOM in a way that supports soil structure, nutrient cycling, and resilience to erosion.