Agriculture Reference
In-Depth Information
emphasise a variety of pre-crop effects including soil physical, chemical and biological aspects,
as well as ecological services such as crop protection and weed control. Green manuring in a
rotation contributes to a higher level of biodiversity in time and space.
Depending on the period and the purpose of cultivation, there is a distinction between
green manure as
main crop
,
intermediate crop
and
companion crop
. Terms such as catch crop or
cover crop are synonymous with
intermediate crop
. The first term clearly indicates the expecta-
tions for N-conserving activities in the crops used, mainly fast-growing crucifers and grass
species. The second term is similar, but also emphasises the soil conservation action of covering
the soil with vegetation. Undersowing or, more recently introduced, living mulch systems
increase the complexity of a growing system by combining different partners at the same time
(Paine and Harrison 1993). Often, such an approach in planting design is also named the
intercropping system, a concept that was commonly applied in traditional farming systems
throughout the world. It now covers a wide range of techniques such as undersowing, dual
cropping, strip cropping and even agroforestry, hedging, shelterbelts and windbreaks. All of
these methods involve increased plant diversification within a farm and can result in increased
opportunities for agroecological processes to occur (Altieri and Letourneau 1982). The poten-
tial benefits of intercropping include improvements in soil fertility, resource capture, pest and
disease control, weed management and risk management. However, intercropping requires
good farm management skills to satisfy the full agronomic needs of more than one crop. Useful
introductions to the intercropping concept and assessment are Francis (1986), Vandermeer
(1989) and more recently, Theunissen (1997).
Intercropping systems using
in situ
mulch crops have been widely tested, particularly in the
United States of America (USA) (Abdul-Baki and Teasdale 1993, Creamer
et al.
1996, Hutchin-
son and McGiffen 2000). Using leguminous cover crops in cabbage, Brandsæter and Riley
(2002) found soil fertility and weed control benefits but also noted that interspecific competi-
tion was a major problem. Careful matching of secondary crops with the primary crop and the
prevailing growing conditions is needed based on relative vigour, timing of growth phases and
complementarity of resource use. Intercropping may be especially suitable for row crops with
poor competitiveness against weeds.
Perennial farming systems with tree, bush or vine crops also provide niches for short and
longer term intercropping. In addition to the use of cover crops on the orchard f floor of many
crops including apples (Hartley
et al.
2000) and wine grapes (Bugg
et al.
1996), plantings of
commercially and environmentally valuable species within or around the orchard have also
been used for pollination, disease and pest control (Finckh and Wolfe 1998, Altieri 1999) in
crops such as such as leeks (Baumann
et al.
2001) and carrots (Brandsæter and Riley 2002).
Within the European Union (EU), specific programs facilitate the use of one-year set-aside
crop mixtures. This is often managed within a biennial or longer grass clover crop, out of
which one year is financed by subsidies and not fed to animals. Similar effects, although on a
lower level, can be expected from a one-year forage crop by excluding the last cut as roughage
and providing additional biomass as C and organic N supplied to the soil (Heß 1993). The one-
year green manuring is most valuable as a humus source, whereas the quickly degraded biomass
of green manure provides more nutrients and energy sources for soil organisms and improves
the fertility for the subsequent crop (Schonbeck
et al.
1993).
Forage legumes were found to be more efficient in supplying N to subsequent wheat
(
Triticum aestivum
) crops than grain legumes such as pea (
Pisum
spp.) or vetch (
Vicia
spp.)
(Evans
et al
. 2003). Similar N conserving effects of different cover crops have been described
by Fowler
et al.
(2004) and Haas (2004) in regard to organic farms in New Zealand and
Germany respectively. The improvement in soil structure by the root residues of the preceding
forage legumes was more important for the performance of a succeeding lettuce (
Lactuca
sativa
) crop than the impact of the N supply alone (Wivstad
et al
. 2003).
