Agriculture Reference
In-Depth Information
BRS Agreste
120 kg N ha
-1
0
0 + rhizobium
50 + rhizobium
FIGURE 7.8
Growth of dry bean genotype BRS Agreste at different N treatments in the second year of
experimentation. Left to right 0 kg N ha
−1
, 0 kg N ha
−1
+
inoculation with rhizobium, inoculation with rhizo-
bium + 50 kg N ha
−1
, and 120 kg N ha
−1
.
work to be done in identifying the additional acid-tolerant legume and rhizobial germplasm, as well
as the deployment of acid-tolerant genes such as those that occur in
Rhizobium tropici
U M R1899
(Graham et al., 1994).
7.7.2 a
doptInG
a
pproprIate
s
oIl
and
C
rop
m
anaGement
p
raCtICes
Adopting appropriate soil and crop management practices can improve biological N fixation in
legumes. These practices include conservation tillage, crop rotation, planting higher N
2
-ixing crop
species or genotypes within species, pest, and disease control, plant nutrition and soil amelioration,
and plant breeding and selection. A brief discussion of these practices in relation to biological N
fixation is given in this section.
7.7.2.1 Conservation Tillage
Conservation tillage is widely adopted in many countries to save energy and conserve soil resources.
Convention tillage may increase the oxidation of organic matter nitrification, which may increase
nitrate N in the soil profile. The increase in nitrate level in the soil due to cultivation may be detri-
mental to biological N fixation as discussed earlier. Conservation tillage may create a favorable soil
structure and improvement in the soil moisture and a favorable soil temperature. All these changes
may bring favorable changes in the rhizosphere of legume crops for biological N fixation (Peoples
and Herridge, 1990). Peoples and Herridge (1990) reported that soybean cultivated in subtropical
Australia under conservation tillage improved nodulation and N
2
fixation compared with the culti-
vated system.
7.7.2.2 Crop Rotation
Peoples and Herridge (1990) reported that the quantity of soil nitrate available to a legume can be
influenced by crop rotation. In addition to the effects on soil nitrate, different crop rotations can
also influence biological N fixation through effects on legume growth (Peoples et al., 1992). Peoples
and Herridge (1990) gave an example that soybean grown immediately after an oat crop fixed
244 kg N ha
−1
compared with 143 kg N ha
−1
fixed by soybean grown in previously fallowed soil.
Similarly, Bergersen et al. (1989) reported that legumes can fix higher amounts of N and contributed
