Growing Food: A Guide to Food Production

crop. This ability to enrich soils by fixing nitrogen in them is of great value to food
producers and deserves to be better understood and fostered by all concerned.

Figure 11. Field pea root nodule

Rhizobia
Cereal crops need a lot of nitrogen to grow well so a mixed cropping system of cereals
and legumes is often a good idea. The benefits of this type of cropping have been known
for thousands of years, and have been exploited in South America with maize/bean

since earliest times.
Many different kinds of bacteria are capable of fixing nitrogen from the air and
making it available to plants, both legumes and non legumes, but it is the Rhizobia kind
which are the most relevant for food production.
There are many different strains, or species, of Rhizobia present in leguminous
crops. The Rhizobia enter the plant roots, which are stimulated to produce lumps of
various shapes and sizes ranging from 1 to 10 mm in diameter where the bacteria form
a symbiotic relationship with legumes.
In some soils, Rhizobia bacteria are naturally present; in others they have to be
artificially introduced, or inoculated, into the soil by the gardener or farmer.
Rhizobia have two main features that are relevant to food producers:

• They are specific to certain Genera or species of legumes. In other words, one
  strain of Rhizobium that is effective for soybeans may not also be effective for lima
  beans, for example.


• They can only survive for a short time when they are not in either seed or soil.

Inoculum
Inoculum (or “inoculant”) is a live, biological product—a bacteria—which should
always be stored in a cool, dry and shaded place—about 4°C is optimum.
Because soils are often specific to certain groups of legumes, they frequently have to
be inoculated to get the best crops. The Rhizobium culture is contained in certain inert
substances such as peat and is inoculated into the soil directly or, more commonly, via
the seed itself.

GROWING FOOD – THE FOOD PRODUCTION HANDBOOK 55

farming systems and in Ethiopia with cowpea/sorghum and cowpea/millet systems