What is it?
Carbon is one of the basic building blocks of organic matter. The total content of organic matter in the soil is not easily measured accurately, but soil carbon can be measured accurately. Consequently carbon is usually measured and reported as an estimate of the soil organic matter content of a soil. We assume that all the carbon in the soil is from organic matter, and that contributions to total carbon from carbonates is very small. This assumption is valid for most New Zealand soils.Soil organic matter is an important component of the soil because it is relevant to all biological, chemical and physical systems in the soil. Biologically, soil organic matter is the source of energy for soil micro-organisms (or microbes). Microbes are the 'engine' that drives the cycling of nutrients within the soil. Chemically, soil organic matter is a major reservoir of plant nutrients. It is the major source of plant nitrogen, sulphur and phosphorous, which are cycled through accumulation and decomposition of soil organic matter. Other nutrients, such as calcium, magnesium, potassium and sodium, are loosely attached as positively charged cations to the negatively charged bonding sites on organic matter. Physically, soil organic matter stabilises soil structure and soil pores and therefore has a marked effect in enhancing structural stability, aeration, water storage capacity and rainfall infiltration.
How to interpret it
Total carbon is used as an indicator of soil quality in mineral soils but not in organic Soils. Organic matter can be estimated as a percentage of the soil by multiplying parentage total carbon by a factor of 1.7.Levels of total carbon need to be interpreted with reference to the soil type.For all land-use types:
Ample - Levels are close to optimal for the soil type.
Normal - Levels are normal for the soil type. This normal level will be relatively
low in Semiarid, Pallic and Recent Soils, relatively high in Allophanic and Oxidic
Soils, and intermediate in other soils.
Low - The level signals concern about soil quality. Soil ecosystem functions are
probably not significantly affected, but there is either an increase in the likelihood
of deleterious effects, the loss of intrinsic resources, or loss of a measure of
resistance or resilience in the soil system.
Very low - Soil quality is likely to be very poor and critical soil ecosystem functions
(including functions that affect productivity) are likely to be severely affected.
A large proportion of the soil organic resource has been lost.
How to improve it
Adequate soil C is important for most soil chemical, biological and physical processes. Soil C can often be low in soils that have been cultivated for long periods of time. Tillage breaks up soil aggregates allowing more organic matter to be lost by microbial activity and erosion (particularly on bare fields) or transported deeper into the soil. Minimum tillage can help to minimise organic matter losses.
Organic matter levels can be directly improved by ploughing in crops grown specifically for the purpose, or by adding waste organic material such as manure. Addition of organic material is usually impractical for large areas, in which case, natural processes can be encouraged to accumulate soil organic matter as a by-product of plant growth. Natural rates of accumulation can be very slow but may be accelerated by increasing plant productivity and improving conditions that will favour the activity of living organisms in the soil. Vegetation productivity and incorporation of organic matter are encouraged by adequate fertilisation, the encouraging of good plant cover and litter accumulation, and by irrigation (where plant productivity is limited by droughtiness).
Technical details
Topsoil carbon is measured from a subsample of 15–20 bulked soil cores taken with a ‘standard soil corer’ to depths of 0–100 mm.The method of determination that should be used is the induction-furnace method, in which the carbon is converted to carbon dioxide and is measured volumetrically. This method is rapid, convenient for most soils, and accurate. It measures total-soil-carbon content, which includes that of any carbonate or live root material present. The laboratory results are reported as percent total carbon.
Reference material
Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987: Methods for chemical analysis of
soil. DSIR, New Zealand Soil Bureau Scientific Report 80.
Hewitt, A.E.; McIntosh, P.D. 1996: Soil organic matter in the South Island high
country. Landcare Research Science Series No. 18.
McLaren, R.G.; Cameron, K.C. 1990: Soil Science: an introduction to the properties
and management of New Zealand Soils. Oxford University Press, Auckland.
Tate, K.R.; Giltrap, D.J.; Claydon, J.J.; Newsome, P.F.; Atkinson, I.A.E.; Taylor,
M.D.; Lee, R. 1997: Organic carbon stocks in New Zealand’s terrestrial ecosystems.
Journal of the Royal Society of New Zealand 27 :315–335.