Agriculture Reference
In-Depth Information
Although microbes have been shown to improve various soil quality parameters in differ-
ent soil types and environments, yield improvements are not always observed and generally
speaking the addition of significant volumes of organic matter is essential to increase the size
or activity of the soil biological community (Edmeades 2003, see also Chapter 2). Soil organic
matter content is especially difficult to increase in some environments; for example dryland,
semi-arid environments with high temperatures and low precipitation, threatening the
sustainability of organic farms in such areas (see
Chapter
2
). Increasing soil organic matter in
organic agriculture is restricted by:
(a) slower accumulation of organic matter inputs compared with conventional farms as a
result of lower crop yields and less intensive animal production systems;
(b) organic leys decomposing more quickly because of lower carbon to nitrogen ratios; and
(c) tillage required for weed control (Gosling and Shepherd 2005).
The application of microbial ecology (e.g. soil food webs) to agricultural production
systems has provided a useful framework for understanding a range of soil processes including
nutrient mineralisation and pathogen control (Ingham
et
al
. 1985, Drinkwater
et
al
. 1995,
Wardle
et
al
. 1999, Manici
et
al
. 2004). Several specific strategies for increasing microbial
activity have been developed such as effective microorganisms, compost teas, biodynamic
preparations and so on (Sena
et
al
. 2002, Chen
et
al
. 2003, Compost Tea Task Force 2004, see
also
Special
topic
2
); however, many have not been rigorously evaluated and the mechanisms of
action remain unclear (see
Chapter
2
).
In addition to maintaining soil health, microbes are also important in converting manures,
crop residues and other organic materials into composts, humus and plant available nutrients
(Welbaum
et
al
. 2004, see also
Chapter
2
and
Special
topic
2
), providing biological control of
certain pests and diseases (Trejo-Estrada
et
al
. 1998, Brimner and Boland 2003) and decom-
posing crop residues in the paddock (Vazquez
et
al
. 2003).
A general objective in biodynamic soil management is to produce a compensating, stabilis-
ing inf luence on growth in which extremes are avoided (Goldstein and Barber 2005, see also
Special
topic
2
). Biodynamic farmers utilise microbial activity in a unique way through the use
of manure-based preparations (Raupp 1999) and positive agronomic and economic outcomes
have been reported many times for biodynamic systems in different circumstances (e.g.
Reganold 1995, Mäder
et
al
. 2002). Attempts to measure the effect of biodynamic preparations
are often limited by factors such as insufficient time under biodynamic management and con-
founding with whole-system effects (Carpenter-Boggs
et
al
. 2000a, see also
Special
topic
2
).
Conseration tillage
Tillage has been a necessary part of agriculture for centuries and it remains a key tool in
organic farming. Important functions performed by tillage are seedbed preparation, incorpo-
ration of soil amendments, increasing soil mineralisation, crop protection and, most
commonly, weed control. However, over-reliance on tillage, rather than tillage itself, is prob-
lematic in several situations, such as erosive, compacted, dry or wet soils and on sloping
ground. The consistent lack of negative effects by organic farming on soil structure and micro-
bial activity suggest that organic management practices are contributing to a resilience in the
soil (see
Special
topic
4
).
Many of the positive soil and water conservation benefits from conservation tillage prac-
tices can be lost in organic farming systems as a result of the reliance on multiple inversion
tillage operations for seedbed preparation, incorporation of cover crops and weed manage-
ment that can degrade soil quality (see
Chapter
2
). High-residue reduced-till systems using
commonly accepted grass-legume mixtures, permanent soil cover and very limited strategic
