Organic Matter and Sustainability - Sustainable Management of Soil Organic Matter - page 19

and that of moist aggregates by a power function, whilst others find the
opposite. We have been unable to establish clearly why this should be so.
Not all work has been carried out solely on aggregate stability. An
equally important soil property is bulk density (Db). Williams (1971)
studied 189 British soils, and found the following relationships:
Db (arable topsoils) = 1.42-0.78(total N%) [38% variance explained]
Db (grass topsoils) = 1.37-0.76(organic carbon %) [69% variance
explained]
The identification of total nitrogen as the most important factor in the
arable topsoils is surprising, given that it is usually strongly related to total
SOC. Total N may be more of a reflection of fertilizer regimes in arable
soils than in grassland soils. There is no obvious mechanism by which
inorganic N should affect Db in this way, unless it is through stimulation
of mechanisms such as fungal growth, the hyphae of which often have been
implicated in soil structural development (Tisdall, 1991). More recently,
Cannell et al . (1994) found a very strong linear relationship between SOM
Fig. 1.1.3. Relationships between organic carbon (%) and aggregate stability expressed as
mean weight diameter (MWD) of air-dried (ad) and field-moist (fm) aggregates for some soils from
New Zealand (drawn from data given by Haynes and Swift, 1990).