Agriculture Reference
In-Depth Information
Soil organic matter
The environmental relevance of organic matter content is based on its capacity to improve
nutrient availability as well as biological activity and to reduce the vulnerability to physical
damage. Soil organic matter strongly inf luences many soil properties including bulk density,
water-holding capacity, infiltration rate, hydraulic conductivity and aggregate stability (Alföldi
et
al
. 2000, Shepherd
et
al
. 2003). Research on organic matter concentrates on soil organic
carbon (C) content as a key parameter. The level of organic matter content correlates with site-
specific conditions such as soil type, texture and precipitation as well as farming system (Stolze
et
al
. 2000).
Soil organic matter (SOM) and humus are important components in the organic farming
philosophy. Fertilisation is based on organic substances such as farmyard manure from animal
husbandry, compost, green manure, plant residues and commercial organic nitrogen fertilis-
ers. Consequently, there is an extensive supply of organic matter passing through aerobic
decomposition processes. Mineralisation and decomposition processes are inf luenced by
humidity, temperature and oxygen. This means that under tropical conditions these processes
run faster and all year long, whereas under temperate conditions they are slower and come to a
halt during the colder months (Stolze
et
al
. 2000, Alföldi
et
al
. 2002).
Several long-term trials that compare organic farming to conventional farming have been
performed in various European countries. The research shows that soil organic carbon content
is higher in organic systems than in conventional farming (e.g. Goldstein and Young 1987,
Garcia
et
al
. 1989, Mäder
et
al
. 1995, Petersen
et
al
. 1997, Clark
et
al
. 1998, Stolze
et
al
. 2000). As
for pastures, the differences are less pronounced (Shepherd
et
al
. 2003). However, some authors
could not report an increase of SOM in organically managed soils. Gosling and Shepherd
(2004) found that soils in England under mixed organic arable rotations maintained concen-
trations of SOM at similar levels to those under typical conventional systems.
The most important farm practices for organic matter supply vary in different regions as
European organic farm characteristics differ considerably between climatic zones. Organic
farming in the Northern countries is characterised by a high percentage of leys in crop rota-
tions because animal husbandry is the dominant farm type. SOM content and composition on
organic farms in the Mediterranean countries is based more on plant residues and green
manure (Persson 1994, Pomares
et
al
. 1994, Vizioli 1998, Stolze
et
al
. 2000).
Overgrazing and the need for fuel (both wood and dung) in the countries of Africa, Asia,
Latin America and the Caribbean can both exert extreme pressures on local bioproductivity,
leading to nutrient losses and reducing the amount of available compostable and recyclable
organic materials. According to Parrot and Marsden (2002), organic and agroecological
systems do not provide panaceas for areas with depleted and declining nutrient status. However,
their case studies in Burkina Faso and Tigray (Ethiopia) show that organic and agroecological
systems can significantly help address problems of declining soil fertility by building up local
productive capacity (both ecological and social), rather than relying upon external inputs.
Acidity and pH leels
The pH of the soil is an important parameter since it can affect the plant's ability to take up
nutrients and the microbial activity in the soil that inf luences the processes required for plant
nutrition. Changes in soil pH occur by the displacement of cations or by additions of sources
of acidity such as hydrogen and aluminium ions (Tisdale
et
al
. 1993).
Chemical fertilisers are highly reactive and can cause extreme pH f fluctuations in localised
areas such as those near the fertiliser band (Cooke 1967). In contrast, organic manure can
increase the buffering capacity of soils, preventing swings in pH, because of additional organic
matter. Comparative studies of conventional and organic farming systems revealed that organic
