Agriculture Reference
In-Depth Information
Also, sequential tests may prove more useful for predicting the availability of nutrients to
plants through a growing season in organic farming systems. The Organic Advisory Service in
the United Kingdom performs a sequential P extraction to determine soil reserves, plant avail-
able and water soluble P (Fortune
et al.
2001).
Combining soil analysis with nutrient budgets provides another way to monitor soil
chemical fertility in organic farming systems, particularly in view of the concern that organic
farming systems are mining soil nutrient reserves (Fortune
et al.
2001, Watson
et al.
2002b,
Öborn
et al.
2003, Oenema
et al.
2003). Nutrient budgets quantify nutrient inputs and outputs
from a defined area in a given time (Lampkin 1990, Watson
et al.
2002b). Surpluses indicate
the potential for nutrient losses to the environment and deficits suggest soil reserves of nutri-
ents are being exploited and question the sustainability of the system in terms of soil chemical
fertility (Fortune
et al.
2001). Nutrient budgets may also be used to determine nutrient use
efficiency. However, nutrient budgets may be inaccurate because it is difficult (i) to estimate
the nutrient contents and application rates for manures and composts and (ii) to directly
measure N fixation and leaching on commercial farms (Watson
et al.
2002b). Nitrogen fixation
is generally estimated from published studies (e.g. Drinkwater
et al.
1998) or modelling (e.g.
Korsæth and Eltun 2000), and N leaching is often excluded from nutrient budgets (e.g. Clarke
et al.
1998).
Soil biological fertility
Soil biological fertility refers to soil processes involving organisms that improve plant growth
both directly (e.g. symbioses with root nodule bacteria and mycorrhizal fungi) and indirectly,
through their effects on soil chemical fertility (e.g. organic matter mineralisation and mineral
dissolution) and physical fertility (e.g. soil aggregation) (Lee and Pankhurst 1992, Degens
1997). Soil biological fertility can be quantified by measuring the size, activity, diversity and
function of communities. However, there are no agreed standards (Abbott and Murphy 2003,
Gil-Sotres
et al.
2005) and the full implications of increased populations, activity and diversity
of soil organisms on soil function and plant growth are not known. Measures of soil biological
fertility may be useful as indicators of long-term changes in overall fertility because they
respond rapidly to changes in soil conditions (Nortcliff 2002).
Learning how to manage beneficial soil biological processes may be a key step towards
developing sustainable agricultural systems (Lee and Pankhurst 1992, Welbaum
et al.
2004).
Central to organic farming is the aim of optimising plant production by maintaining a rich
biological diversity in the soil (Le Guillou and Scharpé 2000, IFOAM 2002, Treadwell
et al.
2003). A soil's capacity to support biological fertility is determined by inherent physical and
chemical characteristics as well as management practices. The Sustainable Agriculture Farming
Systems Project (SAFS) trial (see Table 2.1) showed that the relative importance of several
environmental variables governing the composition of microbial communities were ranked in
the order: soil type > measurement time > specific management practice > management
system > spatial variation (Bossio
et al.
1998). Management practices that may be used to
maximise the benefits of soil organisms include organic matter additions, increasing plant
diversity, reduced tillage practices and certain soil amendments (Lee and Pankhurst 1992,
Ryan 1999, Welbaum
et al.
2004). Without increased inputs of organic matter, neither organic
nor conventional farming systems can expect to increase the size or activity of the soil biologi-
cal community (Ryan 1999, Stockdale
et al.
2001, Stockdale and Cookson 2003).
Debate surrounds the potential role of soil biological processes in maintaining soil chemical
fertility in organic farming systems. In theory, the chemical fertility and sustainability of
organic farming relies on soil biological fertility to a greater extent than in conventional farming
systems (Le Guillou and Scharpé 2000, IFOAM 2002). The fertilisers permitted in organic
