Compost Interpretations
| Component | Abbreviation | Range | Notes | |
| Unfinished | Total Nitrogen | TN or N | variable | decreases as a compost matures |
| Ammonium | NH4+-N | variable, less than TN | decreases as a compost matures | |
| Carbon | TC or C | variable | will be mostly in organic form if pH <8.3 | |
| Carbon:Nitrogen Ratio | C:N | 25-30:1 | decreases as a compost matures | |
| Phosphorus and Potassium | P&K | variable | ||
| Organic Matter | OM | variable | ||
| Soluble Salts, Electrical Conductivity | EC | variable | ||
| pH | 5.0-8.5 | |||
| Moisture | 45-65% | decreases as a compost matures | ||
| Finished | Total Nitrogen | TN or N | 0.5-2.5% | based on dry material |
| Ammonium | NH4+-N | less than TN | ||
| Carbon | TC or C | 54% | will be mostly in organic form | |
| Carbon:Nitrogen Ratio | C:N | 10-15:1 | ||
| Phosphorus and Potassium | P&K | 0.3-2% each | plant availability is undetermined | |
| Organic Matter | OM | 50-60% | ||
| Soluble Salts, Electrical Conductivity | EC | 1-10 mmhos/cm | high values can be toxic to salt-sensitive plants | |
| pH | 5.0 - 8.5 | neutral pH of 7 is suitable for most applications | ||
| Moisture | 40-50% |
Nitrogen
Total nitrogen includes organic, ammonium, and nitrate nitrogen. The normal range for total nitrogen in finished composts is 0.5 to 2.5 on a dry basis. Ammonium levels decrease as the maturity of the compost increases. Ammonium levels may be high during initial stages of the composting process, but will decrease as maturity increases. Organic nitrogen is determined by subtracting the ammonium and the nitrate nitrogen (an optional test) from the total nitrogen. However, since nitrate nitrogen levels are generally very low, total nitrogen minus ammonium nitrogen will give a good estimate of organic nitrogen in most composts.
Summary Table
Carbon
Carbon is a measurement of the organic and inorganic carbon in the compost. Carbon will be in the organic form unless the sample has a pH > 8.3 or is known to contain carbonates. Compost organic matter typically contains around 54% organic carbon.
Carbon:Nitrogen Ratio
This is the ratio of total carbon to total nitrogen (C:N) in the sample. The C:N ratio typically decreases during composting if the starting C:N ratio is > 25, but may increase if the starting C:N ratio is <15 and nitrogen is lost during the composting process. Composts with C:N > 30 will likely immobilize nitrogen if applied to soil, while those with C:N ratio < 20 will mineralize organic nitrogen to inorganic (plant-available) nitrogen.
Phosphorus & Potassium
Values reported are for total amounts given in the oxide forms (P2O5 and K2O). These results provide an indication of the nutrient value of the compost. However, plant availability of total phosphorus and potassium in compost has not yet been established.
Organic Matter
An organic matter content (dry basis) of 50 – 60 % is desirable for most compost uses.
EC (Soluble Salts)
Composts typically range from 1 to 10 mmhos/cm. High salinity may be toxic to plants.
pH
Most finished composts will have a pH value in the range of 5.0 to 8.5. A lower pH is preferred for certain ornamental plants while a neutral pH (pH 7) is suitable for most applications.
Moisture
A typical starting compost mix will have ideal moisture of 45% - 65%. Finished compost should have a moisture of 40%- 50%.
