Organic Matter and Sustainability - Sustainable Management of Soil Organic Matter - page 7

Oxisols) and 'smectitic' (e.g. Vertisols) soils. A critical threshold of soil
organic matter, based on a linear equation utilizing soil silt and clay
content, was useful in predicting the sustained fertility and productivity
of a collection of tropical kaolinitic soils (Feller and Beare, 1997), while a
critical value of organic matter could be established that was associated
with decreased runoff water turbidimetry for a Vertisol (Feller et al ., 1996).
Generally, critical values for total soil organic matter would be soil
specific, developed using a range rather than a set value, and probably
based on the most limiting soil function and process.
Standardization of soil organic matter measurements
Standardization deals with the development and applications of technical
rules, specification and protocols with regard to a measuring method
(Nortcliff, 1997). At present, there is a need to develop acceptable standard
sampling and measurement protocols to monitor and evaluate change
in soil organic matter. In many cases, estimates of organic matter status
and change are still based on carbon concentration rather than mass,
and comparisons between treatments derived from unequal soil depth,
densities or soil mass. Ellert and Bettany (1995) emphasize the importance
of assessing management-induced changes in soil organic matter on an
'equivalent mass' or a per unit area basis. Such procedures provide some
degree of standardization for soil sampling among soil management
comparisons, but their interpretation may be problematic in situations
where soil erosion has been dominant, or in ecological studies comparing
differences in vegetation types.
In addition to sampling protocols, there is no well-accepted operational
definition of soil organic matter (Agricultural Soils Working Group, 1999).
Does a measure of soil organic matter include plant litter, crop residues or
root material? For a wide range of study sites in eastern Canada in mainly
Luvisols, Cambisols and Podzols, estimated annual straw and root carbon
inputs of cereal crops (Bolinder et al ., 1997) ranged from 2 to 5% of the
total (0-60 cm depth) soil carbon (Carter et al ., 1998). This range would
probably be higher for grasses and legumes. Thus, accommodating
crop-derived carbon in soil organic matter estimates has implications for
soil handling protocols after sampling, such as sieve size.
Regulation of Organic Matter Functioning in Soil
In agroecosystems, factors that regulate organic matter functioning in soil
are related to organic matter additions or inputs that influence particulate
or macroorganic matter, and the relationships between organic matter and