ROOTS - The Soil - page 54

F IG. 18
Rhizobium root nodules on broad bean. (Photograph J.Day.)
R OOT NODULES
Mycorrhizas are not the only association between roots and soil microbes. That
between legumes and soil bacteria that can fix nitrogen from the atmosphere has been
recognized for much longer and is of enormous agricultural significance. Most mem-
bers of the family Leguminosae (peas, beans, clovers) have nodules on their roots
which contain the bacterium Rhizobium ( Fig. 18 ). In the nodules, this can convert the
abundant gas nitrogen (four-fifths of the air) into forms that plants, and ultimately all
other organisms, can use. As a result such 'nitrogen-fixing' crops have been main-
stays of most agricultural systems for thousands of years, and, on a global scale, more
nitrogen is still added to farmed systems by symbiotic nitrogen-fixation than by arti-
ficial fertilizer.
Rhizobium bacteria are common in soil but they only fix atmospheric nitrogen
if they can colonize legume roots. Once inside the root they change form and induce
the plant to produce the nodule. There are many strains of Rhizobium and some are
more effective than others at fixing N, partly because the strains are specific to partic-
ular types of legume, so that a strain that fixes N well in bean roots may not do so in
clover. The biochemistry of the fixation process is remarkable. Chemists turn N 2 gas
into ammonia (NH 3 ) at high temperatures and pressures. The bacteria have no need