Agriculture Reference
In-Depth Information
system. In addition, Gibson and Jordan (1983) reported that this technique has been of great benefit
to N fixation research, especially under laboratory conditions. However, it is essential that great care
be exercised in applying and interpreting field assays.
7.5.2 n
odule
e
valuatIon
Dinitrogen fixation depends on the formation of nodules on root system. Hence, their number, size,
and distribution on the root system have been used to measure the symbiotic activity in legumes.
The counting of the nodule number and size of the root system is simple, quick, and inexpensive.
The degree of nodulation can be related to the degree of N
2
fixation. Poor nodulation indicates that
the N
2
fixation process is low in a determined legume or legumes. Peoples and Herridge (1990)
reported that the nodule evaluation technique can at best provide an indirect indication of a legume's
potential to fix N
2
and cannot be used to quantify the amount of N
2
fixed.
7.5.3 d
etermInInG
the
n
ItroGen
B
alanCe
The N
2
measured by this technique requires the determination of the quantity of N change in the
soil under legume growth. It requires that all the N inputs in the soil such as rain water, dust, animal
droppings, and weathering be measured. Similarly, all the N output sources such as denitrification,
volatilization, leaching, erosion, and removal of crop or animal products within a given soil-plant
system should be measured. The net increase in N under a legume is attributed to N
2
fixation. This
technique requires many N input and output measurements and the inputs should be large enough
to detect any difference in N accumulation or fixation. Experimental data should be repeated over
several years to get meaningful results.
7.5.4 n
ItroGen
d
IfferenCe
m
ethod
In this method, N
2
fixation by a legume is compared with a nonfixing control. The difference in total
N accumulated by the legume (N
2
) and nonfixing control (N
1
) legume is regarded as the contribu-
tion of N
2
fixation to legume growth. The fixation of N
2
can be calculated by using the following
equation:
Quantity of N
2
fixed = N
2
- N
1
This method is simple and can be used when total N analysis facilities are available. In this
method, the choice of control crop is very important. Ideally, the two plant types should explore the
same rooting volume, have the same ability to extract and utilize soil mineral N, and accumulate
soil N over the same period of time (Peoples and Herridge, 1990). A non-N
2
-ixing control may be
a nonlegume, an uninoculated legume, or a nonnodulation legume, preferably an isoline of the test
legume (Peoples and Herridge, 1990).
7.5.5 n
ItroGen
e
quIvalenCe
In the N equivalence technique, the N
2
fixation is assessed by growing N-fertilized non-N
2
-ixing
plants in plots alongside the unfertilized N-fixing test legume. The N fertilizer levels at which the
yields of the nonfixing plants match those of the legume are equivalent to the amount of N
2
fixed
(Peoples and Herridge, 1990). The value obtained is usually expressed as fertilizer N equivalence.
Table 7.3 provides data of NFE of legume cover crops to succeeding nonlegume crops. The NFE
values varied from 12 to 182 kg ha
−1
. Smith et al. (1987) reported that NFE values range from
40 to 200 kg ha
−1
, but more typically are between 75 and 100 kg ha
−1
. Interseeding red clover
