Organic Matter and Sustainability - Sustainable Management of Soil Organic Matter - page 22

Table 1.1.1. Contribution of different factors to volumetric soil water content (V%) of A horizons
of mineral soils at different tensions (in kPa).

%Variance explained

Factor Clay (< 2 µ m) + Silt (2-60 µ m) + Organic carbon No. of samples

V% ( − 5 kPa) 44.8 52.4 65.7 652

V% ( − 10 kPa) 51.4 60.5 69.3 597

V% ( − 40 kPa) 60.8 70.2 73.8 597

V% ( 200 kPa) 64.6 70.8 72.4 516

−

V% ( − 1500 kPa) 72.0 74.3 75.3 652

Table 1.1.2. Relationship between soil volumetric water content at − 5 kPa tension (V%) and
other soil properties (soil organic carbon = SOC%; clay content = C%; silt content = Z%) in A
horizons of mineral soils in England and Wales stratified by clay content and land use (SSLRC
data, unpublished).

Equation explaining greatest
proportion of the variance in V%

% Variance
explained

No. of
observations Land use Clay content

Arable > 35% V% = 32.46 + 0.30C 39 45

Between 18 and 35% V% = 23.43 + 0.26C + 0.11Z +

1.28SOC

36 109

< 18% V% = 18.58 + 0.35C + 0.12Z +

2.28SOC

56 101

Ley
grassland

> 35% V% = 9.20 + 0.51C + 0.32Z 59 14

Between 18 and 35% V% = 33.34 + 2.32SOC 25 44

< 18% V% = 20.71 + 0.21Z + 3.12SOC 56 48

Permanent
grass

> 35% V% = 45.8 + 0.77SOC 20 55

Between 18 and 35% V% = 32.12 + 0.10Z + 2.13SOC 31 125

< 18% V% = 25.68 + 0.08Z + 0.49C +

1.30SOC

27 58

total SOM, some or much of which could be 'inert'. In summary, they
concluded that:

•

only part of SOM stabilizes aggregates;

•

above a certain amount of SOM, there is no further stabilizing effect;

•

SOM itself is not a major binding agent;

•

disposition of SOM can be more important than amount; and

•

particle arrangement is more important in some soils than the absolute
amount of SOM.