In the laboratory, organic matter is measured as total carbon by high-temperature combustion analysers which raise the soil sample to a temperature of about 1,300°C and the total carbon is measured as carbon dioxide.
Total carbon is expressed as a percentage of the soil weight. The measurement includes both organic and inorganic (carbonates) carbon. However, since most New Zealand soils contain very little inorganic carbon, total carbon is a good measure of organic content.
To obtain the percentage of organic matter, total carbon is multiplied by a conversion factor of 1.72. This assumes that organic matter is 58% carbon. In reality, the carbon content of organic matter varies and so it is better not to convert and just report the measured total carbon as is.
Ratings of total carbon for New Zealand soils are as follows:
Depending on soil type but regardless of land use, desirable provisional target values of total carbon for New Zealand mineral soils set by Landcare Research are:
• above 2% Semi-arid Pumice and Recent soils
• above 3% Allophanic soils
• above 2.5% all other soils.
The quality of soil organic matter can be described by the so-called carbon to nitrogen (C/N) ratio which is the total carbon (C) content of the soil divided by its total nitrogen (N) content.
A C/N ratio of less than 10 means the organic matter is easily mineralised which releases soluble inorganic forms of N (ammonium and nitrate) into the soil.
On the other hand, a C/N ratio greater than 10 means that organic matter is less easily mineralised. A high C/N ratio may indicate possible N deficiency but different land uses will produce varied responses. In pastures, the optimum C/N ratio ranges from 8 to 12.
Organic matter levels in soil
Soil temperature influences the activity of soil micro-organisms decomposing organic matter. The optimum soil temperature range for bacterial activity is between 21°C to 38°C, but some activity may occur at temperatures as low as 5°C although at greatly reduced rates.
Most soil micro-organisms are aerobic and require oxygen and water for their respiration. When the soil is saturated with water or gets compacted, microbial respiration slows down significantly which then reduces the decomposition of soil organic matter. Partially or continuously saturated soil conditions has protected organic (peat) soils from losing organic matter through oxidation.
Rainfall and irrigation influence soil moisture content which in turn determines plant growth and the resulting input of plant biomass into the soil. Rainfall also influences soil moisture content by affecting soil microbial activity. Organic matter decomposition is faster in moist soil than in dry soil. As the soil becomes wet or saturated, microbial activity declines, as does decomposition.
Soil reaction or pH.
Under acidic soil conditions, bacterial activity which is responsible for most of the decomposition of organic matter is greatly reduced. However, the activity of soil fungi is less affected by low pH.
Source: Bay of Plenty Regional Council: Land management – Soil organic matter