Soil Carbon Dynamics and Nutrient Cycling - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 102

entities, especially polysaccharides derived
either from roots or as products of microbial
decomposition.
It has been shown that, under appropriate
conditions, resistance to a penetrometer being
pushed into soil is a good predictor of resist-
ance to penetration by a growing root, and that
this is influenced strongly by soil C content
(Bengough and McKenzie, 1997; Whalley et al .,
2005). Organic C content also strongly influ-
ences soil bulk density and soil strength, and
these have major impacts on root growth
(Whalley et al ., 2007). The influence of soil
physical structure on root exploration in soil,
and the resulting uptake of P, a relatively im-
mobile nutrient in soil, is illustrated by the re-
sults in Table 7.2. Three crops (spring barley,
potato and sugarbeet) were grown in a range of
treatments in a field experiment in the UK
where organic C and crop-available P (termed
Olsen P) differed due to manure and P add-
itions during the previous 12 years. Because a
wide range of Olsen P levels were present, it
was possible to determine the P level required
to achieve maximum yield when N and other
nutrients were not limiting growth. Table 7.2
shows that, for the three crops, the maximum
yield that could be reached was similar in the
low C and the high C soil. However, in the soil
with lower organic C content (0.87%), the re-
quired Olsen P level to achieve this yield was
2-3 times greater than in the soil containing

1.40% C. This was suspected to be due to the
less extensive root growth in the low C soil be-
cause of its visibly poorer soil structure; this
soil was particularly difficult to plough. This
conclusion was confirmed when samples of the
soils were brought to the laboratory, air-dried
and ground to pass through a 2 mm sieve, thus
eliminating the differences in structure be-
tween the low C and the high C soils. The soils
were then used in a pot experiment in which
ryegrass was grown, and in this situation the
level of Olsen P required to attain the max-
imum yield was equal in both soils - a strong
indication that it was differences in soil struc-
ture that were influencing P uptake and crop
growth under field conditions. Thus, main-
taining soil structure that is conducive to root
proliferation is important for the efficient use
of nutrients; adopting management practices
that maintain or increase the concentration of
soil C is generally the most effective means of
achieving this.

Organic C and the Abundance,
Diversity and Activity of Soil Biota

It is very well established that soils having a
higher organic matter content, or those receiv-
ing larger inputs of organic matter, have lar-
ger microbial biomass content and exhibit
greater activities of the different groups that

Table 7.2. Effect of soil organic carbon (C) concentration on crop yield responses to available phosphorus
(Olsen P) in soils having differing C and Olsen P concentrations due to past treatments. (From Johnston
et al ., 2009.)

Maximum
crop yield a
(t ha -1 )

Olsen P associated
with maximum yield
(mg kg -1 )

Variance
accounted
for (%)

Soil organic
C (%) Crop product

Field experiments

Spring barley grain 1.40 5.00 16 83

0.87 4.45 45 46

Potato tubers 1.40 44.7 17 89

0.87 44.1 61 72

Sugarbeet sugar 1.40 6.58 18 87

0.87 6.56 32 61

Pot experiment

Ryegrass dry matter 1.40 6.46 23 96

0.87 6.52 25 82

a Yield shown is slightly less than the absolute maximum. It is yield at 95% of the maximum assessed from curves of
yields plotted against Olsen P concentration.