Grain Legumes and Green Manures for Soil Fertility in ... - cimmyt

to amount of biomass, quality of biomass in terms
of L+P: N ratio, litter fall, and preseason inorganic
N. These results were in agreement with those reported
by Mafongoya et al. 1999).
Based on these results we can safely conclude that
the main predictors of fallow performance are quantity
of biomass, quality of the biomass, preseason
inorganic N and texture on the soil. The relevance of
these predictors needs to be tested over a wide
range of conditions and with different fallov: species.
Soil Chemical Properties
The major soil chemical changes that take place under
tree fallows are increases in labile pools of SaM,
N stocks, exchangeable cations and extractable P
(Rao et al. 1998). Details of the mechanisms of soil
improvement by tree fallows were reviewed by
(Buresh and Tian, 1998). In theory, planted tree fallows
are expected to improve soils faster than natural
fallows since the land is completely covered by
fast growing leguminous trees for 2 to 3 years.
However the magnitude of these soil improvements
depends on tree species, length of fallow, soil and
climatic conditions. In this section, we will concentra
te on these changes as measured from experiments
in southern Africa.
Biological nitrogen fixation and N cycles
The contribution of leguminous trees through N2
fixation is well recognized, although not all legumes
fix N2. Nitrogen fixation in the humid and subhumid
zones of Africa has been reviewed by Sanginga
(1995). There has been little work on quantification
of N2 fixation by trees in southern Africa. This work
has proved to be difficult due to constraints in the
methodologies for measurins N2 fixed. A series of
multi-location trials have been set to measure the
amount of N2 fixed by different tree genera and
provenances (Table 3) using the 15N natural abundance
method. The data on percent Ndfa shows
high variability among provenances of the same
species for N derived from atmospheric N2 fixation.
Sanginga et al. (1990) found that percent Ndfa
ranged from 37 to 74% for provenances of Leucaena
leucocephala. The data shown in Table 3 falls within
the range reported by Sanginga et al. (1990). These
preliminary data show the huge potential of trees to
fix N2 and increase N inputs in N deficient soils.
Our future analysis will focus on factors responsible
for this variability in N2 fixation across s~tes and
how to optimize N2 fixation under field conditions.
Barrios et al (1997) m~asured availability of soil N
following 2- and 3-year fallows a N- deficient soils
in eastern Zambia. His results confirmed that tree
fallows increase N availability compared to continuous
cropping without fertilization. Subsequent N
measurements down to 200 cm in the soil profile
showed significant N inorganic accumulation at
depth during the cropping phase (Figure 2).
These results show that improved fallows can create
a very "leaky" N cycle after fallow clearance. Most
of the N is leached beyond the rooting depth of
maize and this N is released from organic inputs
before peak N demand by maize. Hence there will
be asynchrony between N release and N demand by
maize. Consequently there is need to design systems
which try to minimize N losses and increase N
use efficiency, and cycling.
Based on those results we designed mixed fallows
of coppicing species and noncoppicing species. The
hypothesis is that the coppicing species will act as a
permanent "safety net" for N when the noncoppicing
fallows are cut due to resprout growth and deep
root system in the soil. Results of gliricidia and sesbania
mixed fallows have shown higher maize prcr
ductivity and efficient N cycling compared to single
species fallows (Figure 3).
Soil acidification and cations
There are several reports on soil pH and improved
fallows. Topsoil pH decreased under fallows of
Acacia auriculiformis (Drechsel et al. 1996). However
Oonsson et al. 1996) found no changes in soil pH
after fallows. Our results over a 10-year period
showed significant decrease in topsoil soil pH, 0-60
cm and an increase in soil pH with depth (Figure 5).
This decrease in topsoil pH and increase in soil pH
is attributed to leaching of N03 and cations such as
magnesium as shown in Figure 6. The movements
of cations from the topsoil were also confirmed by
low CEC in 0-20 cm (6.25 compared with 9.50) in
the 20-100 cm soil profile. These pH changes, which
will take place after fallows, may have little effect
Table 3. Biological nitrogen fixation (%BNF) of coppicing species/
provenances across three sites in eastern Zambia after 1 year of
growth
Kalichero Kalunga Masumba
Treatment %BNF Nkg/ha %BNF Nkg/ha %BNF N kg/ha
A. angustisma 52.1 210.4 61.8 201.4 54.8 260.8
C. calothyrsus 48.4 81.4 44.1 214.4 48.7 193.
G. sepium 79.2 212.4 71.4 408.4 70.8 297.5
l. collinsii 74.7 303.2 57.2 236.7 102.1 475.9
l. diversif(Jlia 35/88 77.5 196.8 33.8 88.6 50.0 161.1
l. diversifolia 53/88 58.4 121.5 14.0 40.5 46.9 112.6
l. esculenta 52/87 70.9 99.3 . 46.6 110.1 46.7 274.5
l. esculenta·Machakos 84.7 223.6 35.2 120.2 69.0 538.0
l. pallida 58.6 87.8 33.7 125.2 44.7 168.1
146
Grain legumes and Green Manures for Soil Fertility in Southern Africa