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Compost Interpretation

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.

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%.