Agriculture Reference
In-Depth Information
Fig. 1.1.1.
The relationship between aggregate stability and soil organic matter content in 519
soil samples from the western USA and Canada (redrawn from Kemper and Koch, 1966).
soils are split by land use and depth. The authors also investigated the
influence of 'free' Fe oxides and exchangeable Na on aggregate stability.
The effects of these components was not large, as demonstrated in the
relationships found:
% water
49.7 + 13.7 log(OM%) + 0.61 (Clay%)
−
0.0045
stable
(Clay%)
2
+ 9.0 (Fe
2
O
3
%)
−
1.6 (Fe
2
O
3
%)
2
−
=
aggregates
0.28 (ES%)
−
0.06 (ES%)
2
[31% variance
(all soils)
explained]
% water
40.8 + 17.6 log(OM%) + 0.73 (Clay%)
−
stable aggregates
=
0.0045 (Clay%)
2
+ 3.2 (Fe
2
O
3
%) [44% of
(arable topsoils)
variance explained]
% water
45.1 + 22.6 log(OM%) + 0.28 (Clay%)
−
stable aggregates
=
0.0021 (Clay%)
2
+ 1.55 (Fe
2
O
3
%) [38% of
(grass topsoils)
variance explained]
It is interesting to note that the explanation of the variance in aggregate
stability was slightly greater for arable soils, i.e. those with smaller amounts
of SOC, than in the permanent grass soils. This is not something which has
been noted often. Kemper and Koch (1966) invoke a number of possible
mechanisms to explain the different behaviour of their three soil groups,
including qualitative differences in SOM/SOC related to land use history,
the kinds, amounts and distribution of clay particles, and whether the soils
had been 'disrupted', i.e. ploughed, or not. However, they were unable to
differentiate clearly between these effects.
Douglas and Goss (1982), however, found that the amounts of
organically bound Fe and Al (although generally < 0.2%) contributed
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