Agriculture Reference
In-Depth Information
while the cultivated plant is still at the green vegetative stage, or just after
the flowering stage. Green manure can also be crushed or rolled before
no-till seeding (Figure 1).
A cover crop is any crop grown to provide soil cover, regardless of
whether it is later incorporated into the soil. Cover crops are grown pri-
marily to prevent soil erosion by wind and water. Cover crops and green
manures can be annual, biennial, or perennial herbaceous plants grown
in a pure or mixed stand during all or part of the year (Figure 1). In addi-
tion to providing ground cover and, in the case of a symbiotic N
2
-fi xing
legume, they provide substantial amounts of N. They also help suppress
weeds [70] via allelopathic legume cover and mulching species [71] and
reduce insect pests and diseases [72-74]. When cover crops are planted to
reduce nutrient leaching (N in particular) following a main crop, they are
often termed “catch crops.” [75,76]. Moreover, growing green manures
on site is a way to prevent the often inhibitive handling and transportation
costs of other organic inputs [69]. There are a large variety of cover crop
species that are appropriate for a farmer and a particular region. Details
on the use of catch crops to prevent N leaching losses during the winter
period and of N fertilization using green manures (including N fi xing le-
gumes), can be found in the review by Thorup-Kristensen et al. [77] and
in the handbook:
Managing Cover Crop Profi tability
[78].
Legumes are widely used as cover crops since there is a large choice of
different species suited to a particular environment (Figure 1). Legumes
are defi ned by their unique fl ower structure, their pod, and the ability of
88% of the species examined so far to form atmospheric N
2
fi xing nod-
ules [79,80]. Legumes are only of second importance after grasses to
humans, by contributing signifi cantly to grain, pasture and forage, and
forestry production [33,81]. Since legumes are able to fi x symbiotically
atmospheric N
2
, they require minimal or even no inputs of N fertilizers.
If part of this “free” N is made available to a following cultivated crop,
the use of legumes in a rotation can allow a signifi cant reduction in the
use of N fertilizers. Additionally, legumes can also enhance both the colo-
nization of crop roots by mycorrhizae [82] and the tripartite symbiosis
between the host plant AM fungi and N-fi xing bacteria thus fi nally affect-
ing N uptake by the host plant [83]. The legumes used as cover crops or
green manure can be classifi ed into two categories: tropical and temperate.
