Agriculture Reference
In-Depth Information
The selection of varieties that have a high degree of resistance to locally signifi cant
pests and diseases is an important element in any crop protection strategy. This is
especially important for diseases such as late blight in potatoes (
Phytophthora infestans
).
Intercropping with two or more crop types (e.g. brassicas with an understorey of clover)
has also been shown to reduce foliar disease and pest attack, but this practice is rare in
conventional cropping systems.
Fertilizer strategy can also be altered to optimize the crop yield/crop health balance.
As a general rule, excessive nitrogen (N) fertilization tends to result in higher levels of
foliar disease (van Bruggen, 1995; Walters & Bingham, 2007). Organic crops tend to
have a reduced N supply compared to conventional crops, and consequently a higher dry
matter content and lower N content. This may make them less susceptible to air-borne or
foliar disease. Cooke (1993) has shown that foliar and stem-base cereal disease levels are
generally lower in long-term organic fi elds than in recently converted fi elds. Soil-borne
pathogens and root diseases are also generally lower in organic than in conventional
systems (van Bruggen, 1995; van Bruggen & Termorshuizen, 2003).
Pesticide legislation varies between different countries and between different US states,
but conventional farmers in most temperate countries have access to several hundred
approved synthetic and natural pesticides for controlling pests, diseases and weeds. A
number of naturally occurring fungicides, including sulphur, copper and some plant
extract-based fungicides are permitted for use in organic farming under specifi c circum-
stances. Biological control agents of fungal diseases are used to a very limited extent,
mainly in protected and high value crops.
5.6.2
Soil health
Soil health is central to any sustainable farming system where reliance on synthetic
fertilizers and pesticides is minimized, but its potential has not yet been fully explored.
Soil health has physical, chemical and biological components and is concerned with the
idea that soil is a living dynamic organism that functions in a holistic way depending upon
its condition or state. The biological component of soil health depends on the numbers,
diversity and health of the macro, meso and microfauna and microfl ora present. It has
been formally defi ned as 'the capacity of a soil to function within ecosystem boundar-
ies, to sustain biological productivity, maintain environmental quality, and promote plant
and animal health'. (Doran
et al.
, 1996). However it is now generally accepted that this
defi nition should be reserved for soil quality, which is a broader concept. Soil health can
be considered a part of ecosystem health and is associated with biological diversity and
stability (van Bruggen & Semenov, 2000; van Bruggen & Termorshuizen, 2003). It is
therefore thought likely that there are links between soil health, the ability of the biologi-
cal community to suppress plant pathogens, populations of soil-borne plant pathogens
and also disease incidence and severity (Abawi & Widmer, 2000).
Soil health monitoring is rarely practiced in Europe, but in some parts of the United
States, farmers are using test kits to determine chemical, physical and biological
components of soil health (www.attra.ncat.org; www.solvita.com). These have proved
useful in demonstrating effects of management on soil health (Ditzler & Tugel, 2002).
The biological indicators used are earthworm count and soil respiration. It is recognized
that soil health depends on more than just these two parameters and that the presence
