A Strategy for Taking Soil Carbon into the Policy Arena - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 68

replaced by technical solutions, leading to
fertigation in hydroponic horticultures in the
most extreme case. However, these technical
solutions require investments that are only
profitable in the case of high-value crops in
greenhouses, such as salad vegetables, flowers
and pot plants. The long-term sustainability
of these high-input agricultural systems may
be questioned. The majority of agricultural
production still relies on most of the func-
tions of SOM mentioned above that guaran-
tee sustainable soil management for future
generations.
The supply of nutrients such as react-
ive N and P contained within the SOM can
be estimated using the C:N and C:P ratios.
In the literature, the average rates of SOC
increase by modified agricultural practices
are around 0.4 Mg C ha - 1 year - 1 (Freibauer
et al ., 2004). At US fertilizer prices in 2012,
this would result in a gain of US$40 for N
and US$8 for P per hectare. In the case of nu-
trient mining of soils rich in C, these would
be regarded as positive externalities. In
poor soils, the increase in nutrients will be
an investment in the soil capital, and will
therefore require opportunity costs. Diels
et al . (2002) concluded that for a case study
in West Africa, SOC increases resulted in
an enhanced buffer capacity that would
lead to higher efficiency of mineral fertil-
izers. However, care should be taken for
negative externalities. There are many
examples that such an increase in SOC and
fertility has been achieved by concentrat-
ing organic residues from forests/heathlands
on fields close to the farm. For instance,
Bationo et al . (2007) state that up to 30 ha
of dry-season grazing may be required to
produce the manure necessary to maintain
the fertility of 1 ha of cropland in West
Africa: promoting the use of manure may
therefore increase grazing pressure and the
degradation of SOC stocks on the grazing
land, and may further reduce the amount
of crop residues that might be applied dir-
ectly to the cropland. This will lead to the
fertility gradients mentioned in Chapter 10,
this volume. Diels et al . (2002) have demon-
strated that increasing the SOC content
from 0.8 to 1.3% results in an increase in
water-holding capacity of 1 mm for a study

area in Kenya. Even though the increase
in SOC is substantial, the effect is limited
at 50 mm.

National scale

The multiple benefits derived from SOC
interact at scales beyond the individual farm,
and therefore should be addressed and
remunerated through public incentives at
scales ranging from the catchment to the
nation. Natural capital is thus the stock of nat-
ural ecosystems that yields a flow of valuable
ecosystem goods or services into the future
(Chapter 22, this volume). Pascual et al .
argue that:

Valuing soil carbon from an economic point
of view, however, is not straightforward,
since most of the benefits derived from its
conservation and sustainable use are not
reflected in the markets. This is mostly
because current markets only reveal
sufficient information about the scarcity of
a small subset of goods and services from
nature. Most natural resources, ecosystem
processes and functional components are not
incorporated in transactions as commodities
or services, and their economic value is not
reflected in any market prices.
(Chapter 15, this volume)

A more immediate way of considering
the benefits of SOC is its value that is attributed
by people through their willingness to pay
for the goods and services that flow from it
(Pascual et al ., 2010). Costanza and Daly
(1992) were the first to define the concept of
ecosystem services. Soil, and in particular
SOC, are considered to be an intermediate
for final goods and services such as climate-
regulation and water-regulation services.
Noellemeyer and Six (Chapter 22, this vol-
ume) discuss the range of services that SOC
can provide and detail the role of SOC in
soil structure maintenance and improve-
ment, erosion control, climate regulation,
pollutant attenuation and degradation, pest
and disease control and biodiversity conser-
vation. Abson et al . (Chapter 18, this volume)
argue that these ecosystem services should be
estimated jointly for case studies, as in many
cases the positive effects on one service can
have negative impacts on another.