Species Interactions in Crop Communities - Agroecology: The Ecology of Sustainable Food Systems - page 213

FIGURE 15.5 A border of corn
spurry (Spergula arvensis)
around a cauliflower crop. The
weed's flowers attract beneficial
insects.

INTERCROPPING

practices (Amador and Gliessman, 1990). There was
significant yield reduction for the two associated crop
species, but the total yields for the three crops together
were higher than what would have been obtained in an
equivalent area planted to monocultures of the three
crops. As shown in Table 15.3, this comparison is made
using the concept of land equivalent ratio , explained
in greater detail in Chapter 16. A land equivalent ratio-
greater than 1 indicates that an intercropping system is
overyielding in relation to monocultures of its compo-
nent crops.
Additional research has identified some of the
ecological mechanisms of these yield increases:

Whenever two or more crops are planted together in the
same cropping system, the resulting interactions can have
mutually beneficial effects and effectively reduce the need
for external inputs. The body of information documenting
these interactions has grown considerably in recent years
(Francis, 1986; van Noordwijk et al., 2004), and several
authors have discussed how an ecological approach to
multiple cropping research can provide an understanding
of how the benefits of intercropping come about (Hart,
1984, 1986; Vandermeer, 1989; Ong et al., 2004).
The most successful intercropping systems are
known from the tropics, where a high percentage of
agricultural production still is grown in mixtures.
Because smaller-scale farmers in the tropics have limited
access to purchased inputs, they have developed inter-
cropping combinations that are adapted to low external-
input management (Gliessman et al., 1981; Innis, 1997;
Joshi et al., 2004).
The traditional corn, bean, and squash polyculture
cropping system of Central America and Mexico, with
roots in the pre-Hispanic period, has been studied in some
detail. Both removal and addition interferences occur in
this system, leading to habitat modifications and
mutualistic relationships of benefit to all three crops
(Figure 15.6).
In a series of studies of the corn-bean-squash poly-
culture, done in Tabasco, Mexico, it was shown that
corn yields could be stimulated as much as 50% beyond
monoculture yields when planted with beans and squash
using the techniques of local farmers and planting on
land that had only been managed using local traditional

• In a polyculture with corn, beans nodulate more

and are potentially more active in biological
fixation of nitrogen (Boucher and Espinosa,
1982; Santalla et al., 2001).

• Fixed nitrogen is made directly available to the

corn through mycorrhizal fungi connections
between root systems (Bethlenfalvay, et al.,
1991; Hauggaard-Nielsen and Jensen, 2005).

• Net gains of nitrogen in the soil have been

observed when the crops are associated, despite
its removal, with the harvest (Gliessman, 1982;
Maingi et al., 2001).

• The squash helps control weeds; the thick,

broad, horizontal leaves block sunlight, pre-
venting weed germination and growth, while
leachates in rains washing the leaves contain
allelopathic compounds that can inhibit weeds
(Gliessman, 1983; Fujiyoshi et al., 2002).