Agriculture Reference
In-Depth Information
5.11.3 Edaphic factors
Here much emphasis has been on low pH and the high levels of aluminium, often
found in acid soils. A number of legumes occur naturally in such areas, for example
species of
Lupinus
in the Mediterranean and many genera and species in some of the
Cerrado regions of Brazil (Chapter 2). Both plant and rhizobial genotypes have been
selected from wild material and, again especially in the case of lupins, have produced
many commercial cultivars. There are other legumes such as white clover (
Trifolium
repens
) which are very intolerant of aluminium. Thus, as Australian agronomists have
been doing for many years, collecting plant material from areas that naturally have
the adverse conditions is the best procedure (see also Chapter 2). High pH is also a
problem in many soils, often coupled with high levels of various minerals, but again,
there are legumes adapted to such conditions (Plate 2.4).
Hundreds of papers have been written on the need of nodulated legumes for high
levels of available soil phosphorus. Without disputing any of these, it must be em-
phasised that there are numerous nodulated legumes that are native to soils with very
low available phosphorus and for some, 'normal' soil levels of phosphorus are toxic
(Sprent, 1999). The latter include
Stylosanthes humilis
, where 1000
Mof phosphorus in-
hibited nodulation, which was normal at 62.5
M (Date & Ratcliffe, 1989). Recent work
suggests that the toxicity may be because such species are unable to down-regulate
their high-efficiency phosphate uptake systems (Lambers et al., 2007). Nevertheless,
when introducing crops such as soybeans into new areas (particularly in the tropics),
additional phosphorus may be one of the keys to success. The whole topic of nitrogen
fixation in tropical cropping systems has been well covered by Giller (2001) and will
not be covered in detail here. It remains one of the iniquities of the world that the
poorest countries are the most in need of nitrogen-fixing crops and yet comparatively
little research is being conducted on most of the crops that are adapted to their local
conditions. However, the tide is beginning to turn and a later sectionwill consider some
of the crops that have potential for such areas. Apart from the obvious humanitarian
needs, with the onset of global climate change a better understanding of how legumes
function in adverse conditions is a sensible strategy for general food security.
5.12 Legumes, pests and pathogens
Legumes do not grow alone and their interactions with other species - plant, animal
and microbial - need further study. Because of their high nitrogen content, legumes are
a prime target for many pests and pathogens, many of which, such as beetles attacking
stored grain legumes, have been extensively studied. The production of toxins by
legumes todiscourage herbivoryhas also beenwell studied, for example the production
of fluoroacetate by some Australian legumes (see Section 1.3.5 in Chapter 1). However,
the situationmay be complex. In the case of
Oxytropis sericea
(white locoweed), the toxic
alkaloid swainsonine is synthesised inside the legume by a fungal endophyte
Embellisia
sp. Its production is enhanced by the presence of effective nodulation by rhizobia
(Valdez Barillas et al., 2007). Some weevil larvae specifically attack the pink centre of
