What is a realistic decay constant?

For temperate climates with conventional tillage, 0.04 (4%) is typical. Warmer climates or intensive tillage may have k = 0.06-0.08. No-till systems can reduce k to 0.02-0.03. The decay constant is the single most important factor in SOM trajectory.

How much carbon do cover crops contribute?

A productive cover crop like cereal rye can produce 4,000-8,000 lb dry matter/acre, of which about 40% is carbon (1,600-3,200 lb C/acre). A modest cover crop like a winter mix might contribute 1,000-2,000 lb C/acre. Use conservative estimates for planning.

Can I reach any target SOM level?

No. The equilibrium SOM is determined by your annual carbon inputs and decay rate. If your target exceeds the equilibrium, you would need to increase carbon inputs (more cover crops, compost) or reduce decomposition (no-till). The calculator shows the equilibrium to help set realistic targets.

Soil Organic Matter Build Plan Calculator: Model Year-by-Year SOM Trajectory

Model how soil organic matter changes over time with cover crops and compost inputs. This calculator uses first-order decay kinetics to project year-by-year SOM trajectory, shows the equilibrium level your inputs can sustain, and helps set realistic SOM-building goals.

Inputs Explained

Current SOM %: Your starting soil organic matter percentage from a recent soil test. Typical agricultural soils range from 1-6% SOM.
Target SOM %: Your goal SOM level. The calculator will show whether this target is achievable with your planned inputs and how many years it takes.
Annual Carbon Inputs: Pounds of carbon per acre per year from cover crops and compost combined. A productive cover crop contributes 1,500-3,000 lb C/acre; compost adds additional carbon.
Decay Constant (k): The fraction of SOM that decomposes each year. Typical values: 0.02-0.03 for no-till, 0.04 for conventional tillage in temperate climates, 0.06-0.08 for warm climates with intensive tillage.
Projection Years: How many years to model. SOM changes are slow — use 10-30 years to see meaningful trajectory differences.

How This Calculator Works

Based on: First-order decay kinetics: SOM(next) = SOM(current) + additions% - SOM(current) × k, with Van Bemmelen factor (1.724) for C-to-OM conversion

Best for: Long-term planning of soil health practices and setting realistic SOM improvement targets based on carbon input levels

Check locally: Calibrate the decay constant to your climate and tillage system — local extension services or NRCS can help estimate k for your region

Units supported: Metric (kg C/ha), Imperial (lb C/acre)

Worked Example

Starting at 2.5% SOM, targeting 4.0%, with 2,500 lb C/acre/year inputs, k = 0.04, projected over 20 years.

1. Enter current and target SOM
Set current SOM to 2.5% and target to 4.0%.
2. Enter carbon inputs
2,500 lb C/acre/year from cover crops (1,800 lb C) plus compost (700 lb C).
3. Set decay constant
Use k = 0.04 for conventional tillage in a temperate climate.
4. Calculate equilibrium
Equilibrium SOM = (2,500 × 1.724) / (0.04 × 2,000,000) × 100 = 5.39%. Target of 4.0% is below equilibrium, so it is achievable.

SOM reaches 4.0% in approximately 15 years, with an ultimate equilibrium of 5.39% SOM.

How to Interpret Your Results

Condition	What It Means
Target below equilibrium	Your target is achievable with current inputs — the chart shows when you will reach it.
Target above equilibrium	Your target exceeds the maximum sustainable SOM for your inputs. Increase carbon inputs or reduce tillage to raise the equilibrium.
SOM declining year-over-year	Carbon losses exceed additions. Current practices are depleting soil organic matter. Increase cover crop biomass or add compost.
SOM leveling off (plateau)	Approaching equilibrium where annual additions equal annual losses. This is the steady state for your current management.

Common Mistakes to Avoid

Using an overly optimistic decay constant

A lower k means slower decomposition and higher equilibrium SOM. Use k = 0.04 for conventional tillage as a starting point; only use k = 0.02 if you have documented no-till for many years.

Overestimating cover crop carbon inputs

Be conservative — a typical cover crop mix produces 2,000-4,000 lb dry matter/acre, of which ~40% is carbon (800-1,600 lb C). Use the lower end unless you have biomass data.

Expecting rapid SOM increases

Building 1% SOM requires adding ~20,000 lb of organic matter per acre, and most decomposes. Expect 5-20+ years for meaningful SOM increases. The trajectory chart helps set realistic timelines.

Related Calculators

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Lime Requirement Calculator

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Gypsum Requirement Calculator

Calculate gypsum amendment rate for sodic soil reclamation from ESP or SAR, CEC, and soil properties

Related Guides

Building Soil Organic Matter

Learn how cover crops, compost, and reduced tillage work together to build soil organic matter over time.