Appendices - Soil Essentials: Managing Your Farm's Primary Asset - page 151

Appendix 7. Organic carbon (%) and total nitrogen (%)

Low levels of organic carbon indicate low organic matter contents in soils, thus
providing a guide to low fertility. 'Normal' levels of organic carbon in soils vary
according to the pasture or crop type involved. However, rough guidelines for
organic matter (%) calculated from organic carbon are as follows:

Crops Pastures Crops Pastures

Low rainfall Low rainfall High rainfall High rainfall

Low <1.5 <3.0 <2.5 <5.0

Normal 1.5-2.5 3.0-4.5 2.5-5.0 5.0-10.0

High >2.5 >4.5 >5.0 >10.0

The ratio of carbon to nitrogen lies between 10 and 15 for most soils. Peaty soils
have high C/N rations; often as high as 50 due to the high plant component. Soils
with C/N ratios of greater than 20 tend to be poorly nitrified.
Note . Organic matter % can be derived from organic carbon using the
equation: OM = organic carbon × 1.72.