Organic Matter and Sustainability - Sustainable Management of Soil Organic Matter - page 18

significantly to the aggregate stability of Hamble, Lawford and Denchworth
soils in southern England, but not to that of the calcareous Andover soil
(a Rendzina). The latter is not surprising as the stabilizing effect of high
calcium values on soil structure is well known.
In an attempt to derive a general classification of the structural stability
of soils in England and Wales, Greenland et al . (1975) examined 180 soil
samples from a very wide range of soils. They summarized their work as:

Soils < 2% organic carbon: unstable

•

Soils 2-2.5% organic carbon: moderately stable •

Soils > 2.5% organic carbon: stable •

This classification illustrates a little considered point in all such studies,
in that it represents a relatively narrow range of values for the critical limits,
but the methods of determination of SOC in common use (mostly based on
acid dichromate oxidation of the carbon) are relatively imprecise (a varia-
tion of 0.1% in a determination of 2% SOC is not uncommon).
The description of soil structural relationships by power functions,
as in Kemper and Koch (1966), is not common. Much more typical is that
found by Stengel et al . (1984) for a wide range of British soils (Fig. 1.1.2),
in texture classes ranging from sandy to clayey, and with and without free
calcium carbonate present.
A study of some New Zealand soils by Haynes and Swift (1990) illus-
trated a point originally made by Emerson (1954), that the physical behav-
iour of soils is strongly influenced by the initial treatment of, for example,
the aggregates. Figure 1.1.3 shows that the behaviour of the field moist
aggregates is best described by a linear relationship, whereas the behaviour
of the air-dried aggregates is best described by a power function. This
again highlights the contrast between different studies; most find that the
behaviour of air-dried aggregates is best described by a linear relationship

Fig. 1.1.2. Relationship between soil organic carbon (%) and water-stable aggregates
(WSA mass/mass) in samples from Ashley, Hamble, Wotherstone, Fincham, Hanslope, Wicken,
Denchworth, Lawford, Newchurch, Newport and Andover soil (redrawn from Stengel et al., 1984).