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African Crop Science Journal
African Crop Science Society
ISSN: 1021-9730 EISSN: 2072-6589
Vol. 8, Num. 3, 2000, pp. 263-272
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African Crop Science Journal, Vol. 8. No. 3, pp. 263-272
African Crop Science Journal, Vol. 8. No. 3, pp. 263-272
TREE LEGUMES IN MEDIUM-TERM FALLOWS: NITROGEN FIXATION, NITRATE RECOVERY AND EFFECTS ON SUBSEQUENT CROPS
C.S. Wortmann* and C.K. Kaizzi1 *Formerly with CIAT; current address: Department of Agronomy and Horticulture, University of Nebraska-Lincoln, 279 Plant Science, NE 68583, USA 1Kawanda Agricultural Research Institute, P.O. Box 6247, Kampala, Uganda
(Received 15 December, 1999; accepted 14 August, 2000)
Code Number: CS00028
INTRODUCTION
Legumes tend to be successional species which are competitive
on low N soils, but become replaced by non-legume climax species once the soil
N deficiency is alleviated (Tothill, 1985). This feature of plant ecology is
applicable to farming systems using the legumes in improved fallows. Nitrogen
deficiencies may be alleviated by rotating legumes with crops through N2 fixation
and redistribution of N to the surface soil which is beyond the typical rooting
depth of the common crops. Nitrogen accumulation for densely planted legume
tree saplings ranges up to 500-600 kg N ha-1 yr-1 (Giller and Wilson, 1991).
The feasibility of fallows as a practice to manage soil fertility
is often questioned due to increasing land shortages. However, fallow is practiced
in some densely and moderately densely populated areas. In the highlands of
southwestern Uganda, two independent studies found over 20% of the arable land
was in fallow (excluding grazing, tree and non-cultivatable land) (Grisley and
Mwesiga, 1994; Linblade et al., 1998). In parts of eastern and central
Uganda, as well, approximately 20% of the arable land was found to be in fallow
(Wortmann and Kaizzi, 1998).
The probable advantage of legumes over grass cover crops is
their ability to fix atmospheric N through their symbiotic relationship with
Rhizobium spp., usually associated with the hosts root system. Often
information is inadequate to distinguish the proportions of plant N derived
from the atmosphere, from deep in the soil profile, and from surface soil layers.
Capacity for biological N2 fixation varies with species, environment
and management (Tothill, 1985). It is difficult to measure N2 fixation in the
field as tree root systems are likely to go very deep into the soil (Giller
and Wilson, 1991). They summarised a number of estimates of N derived from the
atmosphere which ranged from 0 100%; the median estimate was 60% nitrogen.
Cropping systems benefit from this accumulation of N if a major proportion is
derived from the atmosphere or from deep horizons of the soil profile, leached
beyond the typical rooting depths of other crops in the rotation.
Nitrate is adsorped on positively charged surfaces of widely
occurring acidic, low organic matter sub-soils high in kaolinite and Fe and
Al oxides. Legume tree (Sesbania sesban L.) fallows have been found to
be efficient in recovery of nitrates from the 1 to 2 m depth and deeper (Hartemink
et al., 1996; Mekonnen et al., 1997). Natural fallows can be as
efficient as the tree fallows in nitrate recovery in some situations (Mekonnen
et al., 1996) although Jama et al. (1998) found natural fallow
to be less effective in improving soil productivity than sesbania fallow.
Deep rooting fallows may contribute inadequate P on low P soils
to meet the needs of the subsequent crops; fallow plants acquire primarily crop
available P since little P is available in the sub-soil of most tropical soils
managed under low input conditions (Buresh and Tian, 1998). Nutrients released
by decomposing legume plant material lacks a N:P balance relative to the needs
of crops. On P deficient soils, P must be supplied from other sources to enable
a crop to efficiently use N supplied by the fallow (Jama et al., 1998).
Uganda has phosphate rock deposits of low reactivity. The Busumbu
P rock deposit lies at 0° 50-55 N and 34° 14-18 E and contains deeply weathered carbonatite of a calcite-magnetite-apatite-phlogopite assemblage (Davies, 1956).
It consists of soft rock which is composed of fine, soft, earth material (5-7%
P), and hard rock ore (13% P) where secondary apatite has cemented particles
together (Van Kauwenbergh, 1991). In earlier on-farm research in Uganda, unprocessed
Busumbu soft P rock was not reactive in a maize-bean rotation over four seasons,
although agronomic efficiency was comparable to P fertiliser on an acid soil
(pH 4.5) deficient in P and Ca in Rwanda (Nyabuhunga, pers. comm., 1999). Reactivity
may be improved in the active rhizosphere of vigorously growing legumes where
pH may be reduced due to N fixation (Loomis and Connor, 1992, pp. 209). Organic
acids and CO2 produced during the decomposition of green manure may induce increased
acidity. Legumes, however, typically have higher ash alkalinity than grass crops
(Pierre and Banwart, 1973) due to the cation/anion imbalance caused by biological
N fixation; if so, decomposition of legume plant materials may reduce acidity.
Negatively charged organic compounds, such as citrates and oxalates, produced
by hydrolysis of organic material, may chelate Ca2+ ions and thus lower Ca2+
ions in the soil solution providing a driving force for the dissolution of P
rock (Hammond et al., 1986). Reactivity of the P rock may be improved
by mixing it with triple superphosphate (TSP). Upon application, acidification
occurs at the surface of granules of TSP which may improve the reactivity of
P rock adhering to the granules. TSP is also expected to stimulate rapid root
growth of the legumes, producing more rhizosphere to interact with the P rock.
The objectives of this research were to determine: legume
species efficiency in the use of P from Busumbu P rock; the amount of atmospheric
N2 fixed by the legumes; and the effects of legumes on the subsequent maize
and bean intercrops.
MATERIALS AND METHODS
Field experiments were conducted in Uganda at Senge Farm near
Kawanda Agricultural Research Institute over four cropping seasons in 1997 and
1998. The site has a bi-modal rainfall distribution of about 1200 mm yr-1 with
wet seasons occurring from March to June, with a peak in April, and from August
to December, with a peak in November. Samples of the deep, well-drained, red
sandy clay loam (Rhodic Kandhapludalf) soil taken to 20 cm depth revealed a
pH of 5.0, organic matter concentration of 3.3%, and 7 mg kg-1 available P and
exchangeable K 0.39 cmolc kg-1.
Two factors were evaluated including legume species and P
application. The legumes included sesbania (Sesbania sesban), tephrosia
(Tephrosia vogelii), mucuna (Mucuna pruriens), and pigeonpea (Cajanus
cajana). The P treatments included: 21 kg P ha-1 applied as triple super
phosphate (TSP) each season; 86 kg P ha-1 applied as Busumbu P rock (8.6% P)
in a single application at the beginning of the first season; 86 kg P ha-1 applied
as 875 kg of Busumbu rock phosphate and 54 kg of TSP per hectare in a single
application; and no P applied. Phosphate was broadcast and incorporated. The
Busumbu P rock was obtained from a quarry at the southern part of the deposit
and was of the soft rock type (van Straaten, 1997).
A split plot design was used with the legumes as main plots
and P treatments as sub-plots. There were four replications. Sub-plot size was
4.8 m x 5 m with 1.5 m between sub-plots, and the harvested sub-plot area was
3.5 m x 4 m.
The dates of various operations are presented in Table 1. Legumes
were sown in rows spaced at 0.60 m with 3 plants m-1 for pigeonpea,
sesbania and tephrosia, and two plants m-1 for mucuna. Maize-bean
intercrop was produced in the third and fourth seasons. Maize was sown in rows
spaced at 0.75 m with two plants per hill and 0.60 m between hills. Bean was
sown in single rows between the rows of maize with 0.10 m between plants. No
rhizobium inoculum was applied to any of the legumes.
TABLE 1. Dates on which trial operations were performed |
|
1997 |
1998 Season A |
1998 Season B |
Legumes sowing |
2 April |
|
|
15N application |
9 May |
|
|
Harvest of 15N-enriched sub-plots |
16 September |
|
|
Legumes harvest |
|
27-30 January |
|
NO3 samples collected |
|
14 March |
17 September |
Maize-bean sowing |
|
13 March |
1 September |
Bean harvest |
|
9 June |
December |
Maize harvest |
|
15 July |
12 January |
Sesbania and tephrosia grew for two seasons. Mucuna matured
at the end of the first season, but the biomass, including seed, was left in
place and it reseeded and voluntarily established a full stand the second season.
Pigeonpea also matured at the end of the first season, but the seed was stolen
and a natural fallow established for the second season. Prior to planting for
the third (1998A) season, all plants were cut near the soil surface. The aboveground
standing biomass and woody (> 15 mm diameter) material were weighed in the
field, and sub-samples were taken. The sub-samples were sun-dried and, later
oven-dried at 60o C for two days. The dry weight of whole plant and woody biomass
produced were determined. Biomass dry weight was also determined at 150 days
after sowing from 1 m2 plots.
Woody material of tephrosia and sesbania was removed from the
plots. The remaining plant material was surface applied but some mixing of plant
matter with surface soil occurred during tillage. Non-woody and woody plant
materials were analysed for P content (Murphy and Riley, 1962).
Soil nitrate. Soil samples were collected from sub-plots
on 14 March, 1998 and fractionated to depth increments of 0-15, 15-30, 30-50,
50-70 cm. Samples were combined for main plots and dried at approximately 40oC,
crushed, and sieved. Nitrate plus nitrite was determined by CD reduction (Dorich
and Nelson, 1984), with subsequent colorimetric determination of nitrite. As
nitrite was probably small relative to nitrate, the values were simply reported
as nitrate.
Nitrogen fixation. Ammonium sulfate was applied to micro-plots
of 2.25 m2 to supply 20 and 100 kg N ha to the legumes and reference crop (sorghum
"cv Seredo"), respectively. The 15N atom excess in the N fertiliser
was 5 and 1% for the legumes and sorghum, respectively. The fertiliser was dissolved
in water and applied as a solution to ensure uniform distribution. The labeled
fertiliser was applied on 9 May, 1997. The micro-plots were harvested on 16
September, 1997 by cutting the plants 10 cm above the ground. Fresh biomass
weight was recorded. Sub-samples were oven dried at 60oC for dry weight determination
and for subsequent N-15 analysis. The N-15 dilution method was used to calculate
the extent of biological nitrogen fixation in the legumes.
Analyses of variance were conducted using Statistix V.2.0 (Analytical
Software, 1998).
RESULTS AND DISCUSSION
Legume growth and nutrient acquisition. The aboveground
standing biomass harvested for tephrosia and sesbania (26 and 31 Mg ha-1) was
the product of ten months of growth with more than normal rainfall during the
last 5 months due to the El Nino effect (Fig. 1). In comparison, where soil
P was lower and tree plant density less than in this study, Jama et al.
(1998) harvested 22 and 28 Mg ha-1 for sesbania at two locations in Kenya following
3 or 4 seasons of growth.
Tephrosia and sesbania had more aboveground biomass at the
end of the second season than the mucuna and the pigeonpea-weedy fallows (Table
2). The standing biomass and plant nutrients for sesbania and tephrosia were
similar in quantity, however, a greater proportion of the sesbania biomass was
woody material. Jama et al. (1998) reported a much higher percentage
of woody material than measured here, but they considered only the leaves and
pods to be non-woody and the sesbania in their study was 18 months old at the
time of cutting. Plant biomass and nutrients at the end of the second season
were similar for the mucuna and pigeonpea-weedy fallows. Substantial amounts
of nutrients, especially potassium, were removed in the woody biomass of sesbania
and tephrosia. The amounts of nutrients returned to the soil by the shrub fallows,
however, greatly exceeded that of the herbaceous fallows.
TABLE 2. Aboveground standing biomass production
(Mg ha-1) and nutrient acquisition (kg ha-1) by the
legumes |
Legume |
Biomass dry weight |
N |
P |
K |
Non-wood |
Wood |
Non-wood |
Wood |
Non-wood |
Wood |
Non-wood |
Wood |
Mucuna |
2.68 |
-b |
64.0 |
- |
6.5 |
- |
53.3 |
- |
Pigeonpea/weedya |
2.85 |
-c |
68.1 |
- |
6.9 |
- |
56.5 |
- |
Sesbania |
18.2 |
12.7 516.9 |
89.0 |
40.9 |
7.5 |
376.5 |
163.6 |
|
Tephrosia |
16.2 |
9.7 387.0 |
77.9 |
39.3 |
5.7 |
321.5 |
150.0 |
|
LSD (0.05) |
2.36 |
2.18 |
63 |
NS |
5.5 |
1.1 |
48.1 |
NS |
a One season of pigeonpea followed by weedy fallow
b Mucuna is a herbaceous plant, lacking woody tissue
c All plant tissue was treated as herbaceous |
Maize and bean performance. Maize yield was greatest
following sesbania and tephrosia in both seasons but the beneficial rotation
effect of these fallow species was much reduced in the second successive season
(Table 3). Maize performance was better following mucuna than the pigeonpea-weedy
fallow rotation, but this advantage was also reduced in the second cropping
season. Crop performance and response to the different fallow treatments in
the 1998B season may have been constrained by soil water deficits in November
and December (Fig. 1) which was the time of flowering and early grainfill. Bean
yield in 1998A was similar following all legumes, but higher following sesbania
and tephrosia fallows in 1998B. Bean yield in the 1998A season was presumably
constrained by competition with the associated maize.
TABLE 3. The rotational effect of four legumes on the yield
of the maize bean intercrop |
Legume species |
Crop yield |
First season |
Second season |
Maize |
Bean |
Maize |
Bean |
Mg ha-1 |
Mucuna |
3.76 |
0.38 |
2.11 |
0.28 |
Pigeonpea/weedya |
2.42 |
0.44 |
1.77 |
0.20 |
Sesbania |
4.16 |
0.37 |
2.23 |
0.38 |
Tephrosia |
4.47 |
0.35 |
2.36 |
0.35 |
LSD (0.05) |
0.52 |
NS |
0.26 |
0.06 |
a One season of pigeonpea followed by weedy fallow |
P uptake from the Busumbu P rock. P uptake by the legumes
was not increased by P application, neither as TSP nor as P rock (data not presented).
Mean bean yield across the two seasons was higher with TSP applied, but it was
not affected by application of P rock. Maize did not respond to applied P, presumably
due to adequacy of P released from the accumulated fallow biomass and to mineralisation
of soil phosphorus.
Nitrogen fixation. Approximately 50% of plant N was
estimated to be derived from the atmosphere for sesbania and tephrosia (Table
4). If this rate of fixation persisted over the 10 month period of growth, these
two species fixed 200 to 300 kg N ha-1. Pigeonpea had much less N2
fixation and obtained most of its N from the soil. Species effect was not significant
for the proportion of applied N recovered because of a significant species by
P interaction effect. Pigeonpea recovered more N from fertiliser with TSP applied,
while N recovery and N2 fixation by tephrosia and sesbania was not
affected by P application.
TABLE 4. Nitrogen derived from the soil, fertiliser, and atmosphere
at 167 days after sowing, and nitrogen fixed, and percent recovery of applied
nitrogen |
Treatment |
N derived from |
N fixedb kg ha-1 |
Fertiliser N recovery % |
Soil % |
Fertiliser % |
Atmosphere % |
Pigeonpea/weeda |
85.0 |
4.2 |
10.8 |
9.2 |
19.1 |
Sesbania |
49.7 |
2.0 |
48.3 |
201.9 |
11.0 |
Tephrosia |
50.4 |
2.0 |
47.6 |
178.1 |
10.4 |
LSD (0.05) |
18.5** |
1.0*** |
19.4** |
92.1** |
NS |
Sorghum |
82.2 |
17.8 |
NA |
NA |
38.3 |
aOne season of pigeonpea followed by weedy fallow
bThe estimates of amount fixed are for above ground plant N,
and based on 2 seasons of growth for sesbania and tephrosia, but on 167
days after sowing for pigeonpea
NS, ** and *** indicate non-significant at P = 0.05 and significant at P
=0.01 and P = 0.001 levels, respectively |
N2 fixation was not measured for mucuna in this trial, but
in a nearby trial, N derived from the atmosphere, averaged over two seasons,
was 67% of plant N. Higher levels of fixation might have been achieved for pigeonpea
with inoculation: the numbers of effective rhizobia may have been low because
pigeonpea had not been produced on this land for at least 30 years. However,
pigeonpea belongs to the promiscuously nodulating "cowpea" rhizobia
cross inoculation group, although some evidence suggests it is slightly specific
among that group (Giller and Wilson, 1991).
The problem of using sorghum, with its relatively shallow
root system, as a reference crop for estimating N2 fixation by young trees was
recognised. However, the trees were only 5.5 months of age when sampled for
15N content and a major proportion of their N was probably acquired from the
surface horizon. If the trees did acquire significantly more N from lower soil
profiles than did sorghum, the probable implication is that the estimates of
N derived from the atmosphere are low.
Soil nitrate. Surface soil nitrate levels were high
at 43 days following the cutting and surface application of fallow biomass (Table
5). Soil nitrate levels were highest with tephrosia and lowest with mucuna in
the surface soil. Presumably, all legumes were efficient in acquisition of nitrate
in the surface soil, but more nitrate may have been released from senescent
nodules and leaves of tephrosia than for other species. There was more nitrate
following mucuna at the 50-70 cm depth than after tephrosia and sesbania, indicating
that mucuna was less efficient than sesbania and tephrosia in acquisition of
deep nitrate, possibly due to fewer deep roots with mucuna.
TABLE 5. Nitrate concentrations and amounts in the soil after
cutting and surface application of the top growth of three legumes, and
prior to establishment of the subsequent maize bean intercrop |
Legume species |
Nitrate levels at 0-15 cm |
Nitrate levels at 50-70 cm |
mg kg-1 |
kg ha-1 |
mg kg-1 |
kg ha-1 |
Tephrosia |
43.0 |
83.8 |
5.0 |
21.0 |
Sesbania |
31.0 |
60.4 |
5.2 |
21.8 |
Mucuna |
23.6 |
46.0 |
14.8 |
62.2 |
LSD (0.05) |
9.21 |
18.0 |
2.69 |
11.3 |
The high levels of nitrate in the surface soil was unexpected
as the growing vegetation should have extracted most of the nitrate. However,
over 40 days elapsed between the time of cutting the vegetation and the sampling
for nitrates. During this period, 125 mm of rain fell with about 60 mm as small
rainfall events of less than 10 mm each and one event of over 40 mm. With this
periodic wetting and drying, significant amounts of nitrate may have been released
through the mineralisation of surface soil organic matter, as well as the decomposition
of the lower C:N components of the fallow plant material with subsequent nitrafication.
The rainfall events were not sufficient to cause significant leaching, implying
that most of the newly formed nitrate remained in the surface soil.
Nutrient release from legume biomass. Nutrient mineralisation
and immobilisation were not measured for the legume biomass. As the plant materials
were surface applied, the soil:plant material interface was much less than if
the material were incorporated probably resulting in slower rates of decomposition
and mineralisation of nutrients.
The legume biomass generally had sufficient N concentration
and low C:N ratios (<= 20) for net N mineralisation to occur shortly after
cutting (Loomis and Connor, 1992; Yadvinder-Singh et al., 1992; Jarvis
et al., 1996). Only sesbania had a C:N value which may be near the critical
level for significant net immobilisation to occur.
Mineralisation of P is often more important than N supply on
tropical soils (Sanchez et al., 1989). Generally, P concentrations and
C:P ratios were within the ranges where temporary net immobilisation of soil
P at the soil:plant residue interface is expected to occur (Table 6). The critical
level for net P mineralisation is estimated to be between 0.2 and 0.3% P (Yadvinder-Singh
et al., 1992) and all species in the current study fell within this range.
Assuming a biomass C content of 40%, the C:P ratio for most samples ranged from
200-400 while the critical C:P ratio for P mineralisation to occur may be as
low as 55 to as high as 300. The moderately high C:P ratio may have reduced
the effectiveness of decomposing legume plant material in improving the solubility
of the P rock (Zaharah and Bah, 1991).
TABLE 6. Nutrient concentrations and carbon:nutrient ratiosa
of the non-woody biomass at the time of cutting the improved fallows |
Legume species |
Nutrient concentration |
C:nutrient ratio |
N |
P |
K |
N |
P |
K |
Tephrosia |
3.0 |
0.29 |
2.3 |
13.3 |
138 |
17.4 |
Sesbania |
2.0 |
0.23 |
2.0 |
20.0 |
174 |
20.0 |
Mucuna |
2.8 |
0.23 |
2.1 |
14.3 |
174 |
19.0 |
Pigeonpea/weedyb |
2.2 |
0.21 |
1.6 |
18.2 |
190 |
25.0 |
LSD (0.05) |
0.48 |
0.063 |
0.39 |
|
|
|
a Assuming 40% C in biomass dry matter.
b One season of pigeonpea followed by weedy fallow |
Plant biomass is composed of components including leaves, branches
and stems which vary in nutrient contents and C:nutrient ratios (Fischler et
al., 1999). The higher quality components are likely to mineralise quickly
with a positive mineralisation-immobilisation turnover and an increase in nutrient
availability. These rapidly released nutrients probably contributed to the nutrient
supply for the subsequent maize-bean intercrop and nourished micro-organisms
feeding on the lower quality plant components. The effect essentially follows
a two component model for nutrient mineralisation described by Yadvinder-Singh
et al. (1992) using two simultaneous first-order reactions.
Nutrient recovery. Crop yields were highest following
tephrosia and sesbania fallows. The additional yield accounted for an added
uptake of 18 kg N and <1 kg P per hectare over the two seasons as compared
to the pigeonpea-weedy fallow. This assumes that concentrations of N for the
mature maize and bean plants were 0.75 and 2.58%, respectively. However, the
average N content of the tephrosia and sesbania fallows exceeded that of the
pigeonpea-weedy fallow by approximately 380 kg ha-1. Thus, recovery of N was
low following the tephrosia and sesbania fallows. The fate of the remaining
N was not determined, but most may have been lost. Giller and Cadisch (1995)
reviewed several studies relevant to tropical agricultural systems and estimated
N recovery in the first season to range from 6-28% and 9-12% of N in green manures
and leguminous tree prunings, respectively. They estimated that recoveries of
legume N in second and subsequent seasons were generally small (2-15%), and
information on long-term recovery is lacking. Nitrogen losses, however, may
well exceed the gains from N2 fixation resulting in a negative N balance.
Much N loss may have occurred due to volatilisation as the
biomass was initially surface applied, especially for tephrosia and mucuna which
had relatively high N concentrations. Some leaf material was incorporated when
a light tillage operation was performed to remove weeds before planting the
maize-bean intercrop, but significant N losses, as much as 30%, may have occurred
from the high quality leaves in the interim (Yadvinder-Singh et al.,
1992; Giller and Cadisch, 1995). Termites fed on the small branches which were
not removed as woody material; the fate of the N in this material is not known.
Similar losses may have occurred with incorporation, however. Jones et al.
(1997) reported that the residual effects of surface applied high quality tree
legume leaves exceeded that of incorporated leaves, suggesting greater losses
with incorporation. Hagedorn et al. (1997) found that 25% of the N in
tephrosia plant material (2.85% N) was mineralised within the first five days
following rainfall, and that topsoil mineral N decreased by 50-70% during a
two week period of high rainfall in Rwanda. They also found that N leaching
at a depth of 20 cm was increased by 50% by incorporation of green and farmyard
manure versus no application, suggesting that losses by denitrification and
possibly leaching were exacerbated by incorporation.
Nitrogen use efficiency might have been improved by applying
the tree biomass more extensively and at lower rates. The soil nitrate levels
at the time of sowing the first maize-bean intercrop (Table 5) were sufficient
to support a good crop without further mineralisation, suggesting that remaining
legume biomass could have been better used on nearby land where legumes had
not been produced. Removal of more of the plant material as wood, however, would
have resulted in a greater N deficit.
CONCLUSIONS
Sesbania and tephrosia were most effective in improving soil
productivity, probably due to high biomass production, high N2 fixation and
recovery of leached nitrates. These legume fallows contributed more than mucuna
and pigeonpea fallows to the soil N balance, deriving about 50% of plant N from
the atmosphere. Recovery of N was low, however, and losses may have been high,
possibly in excess of that gained from the atmosphere. Busumbu P rock was of
low reactivity in the rhizosphere of the legumes and it was not improved by
mixing Busumbu P rock with TSP. Processing, but possibly only crude beneficiation,
is required to utilise the P rock more efficiently. Alternatives for the use
of the tree biomass need to be investigated to improve N recovery. Medium-term
improved fallow might be produced in small blocks, or in bands across the field,
with transfer of a major part of the leafy biomass to adjacent land which is
continuously cropped.
ACKNOWLEDGEMENTS
The authors are grateful to: National Agricultural Research
Organisation and the Director of Research for Kawanda Agricultural Research
Institute for providing land, facilities and other assistance for this research;
Ms. G. Nalukenge for providing technical assistance; Dr. P. Smithson of ICRAF
for assistance with analyses of samples; and the Canadian International Development
Agency and Swiss Development Cooperation for funds provided.
REFERENCES
- Buresh, R.J. and Tian, G. 1998. Soil improvement by trees in sub-Saharan
Africa. Agroforestry Systems 38:51-76.
- Davies, K.A., 1956. The Geology of Part of Southeast Uganda with Special
Reference to the Alkaline Complexes. Geological Survey of Uganda, Memoir No.
III. Government Survey Unit, Entebbe, Uganda.
- Dorich, R.A. and Nelson, D.W. 1984. Evaluation of manual cadium reduction
methods for determination of nitrate in potassium chloride extracts of soil.
Soil Science Society of America Journal 48:72-75.
- Fischler, M., Wortmann, C.S. and Feil, B. 1999. Crotalaria (C. ochroleuca
G. Don) as a green manure in maize-bean cropping systems in Uganda. Field
Crops Research 61:97-107.
- Giller, K.E. and Wilson, K.J. 1991. Nitrogen Fixation in Tropical Cropping
Systems. CAB International, Oxon, UK. 171 pp.
- Giller, K.E. and Cadisch, G. 1995. Future benefits from biological nitrogen
fixation: an ecological approach to agriculture. Plant and Soil 174:255-277.
- Grisley, W. and Mwesigwa, D. 1994. Socio-economic determinants of seasonal
cropland fallow decisions: smallholders in southwestern Uganda. Journal
of Environmental Mana-gement 42:81-89.
- Hagedorn, F., Steiner, K.G., Sekayange, L. and Zech, W. 1997. Effect of
rainfall pattern on nitrogen mineralisation and leaching in a green manure
experiment in South Rwanda. Plant and Soil 195:365-375.
- Hammond, L.L., Chien, S.H. and Mokwunye, A.U. 1986. Agronomic value of unacidulated
and partially acidulated phosphate rocks indigenous to the tropics. Advances
in Agronomy 40:89-140.
- Hartemink, A.E., Buresh, R.J., Jama, B. and Janssen, B.H. 1996. Soil nitrate
and water dynamics in sesbania fallows, weed fallows and maize. Soil Science
Society of America Journal 60:568-574.
- Jama, B., Buresh, R.J. and Place, F.M. 1998. Sesbania tree fallows on phosphorus-deficient
sites: maize yield and financial benefit. Agronomy Journal 90:717-726.
- Jarvis, S.C., Stockdale, E.A., Shepherd, M.A. and Powlson, D.S. 1996. Nitrogen
mineralisation in temperate agricultural soils: processes and measurements.
Advances in Agronomy 57:187-235.
- Jones, R.B., Snapp, S.S. and Phommbeya, H.S.K. 1997. Management of leguminous
leaf residues to improve nutrient use efficiency in the sub-humid tropics.
In: Driven by Nature: Plant Litter Quality and Decomposition. Cadisch,
G. and Giller, K.E. (Eds.), pp. 239-250. CAB International, Oxon, UK.
- Linblade, K.A., Carswell, G. and Tumuhairwe, J.K. 1998. Mitigating the relationship
between population growth and land degradation: land-use change and farm management
in southwestern Uganda. Ambio 27:565-571.
- Loomis, R.S. and Connor, D.J. 1992. Crop Ecology: Productivity and Management
in Agricultural Systems. Cambridge University Press, Cambridge, U.K..
- Mekonnen, K., Buresh, R.J. and Jama, B. 1997. Root and inorganic nitrogen
distributions in sesbania fallow, natural fallow and maize fields. Plant
and Soil 188:319-327.
- Murphy, J. and Riley, J.P. 1962. A modified single solution method for the
determination of phosphate in natural waters. Analytical Chemistry Acta
27:31-36.
- Pierre, W.H. and Banwart, W.L.1973. Excess base and excess base/nitrogen
ratios of various crop species and plant parts. Agronomy Journal 65:91-96.
- Sanchez, P.A., Palm, C.A., Szott, L.T., Cuevas, E. and Lal, R. 1989. Organic
input management in tropical agroecosystems. In: Dynamics of Soil Organic
Matter in Tropical Ecosytems : Coleman, D.C., Oades, J. M. and Uehara,
G. (Eds.), pp. 125-152. University of Hawaii Press, Honolulu, Hawaii.
- Statistix for Windows 1998. Analytical Software, Tallahassee, FL,
USA.
- Tothill, J.C. 1985. The role of legumes in farming systems of sub-Saharan
Africa. In: Potentials of forage legumes in farming systems of sub-Saharan
Africa. Haque, I., Jutsi, S. and Neate, P.J.H. (Eds.), pp. 162-185. ILCA,
Addis Ababa, Ethiopia.
- van Straaten, P. 1997. Geological Phosphate Resources in Central East
Africa. Trip Report, 23 June to 21 July, 1997. University of Guelph.
- van Kauwenbergh, S.J. 1991. Overview of phosphate deposits in East and Southeast
Africa. Fertilizer Research 30:127-150.
- Wortmann, C.S. and Kaizzi, C.K. 1998. Nutrient balances and expected effects
of alternative practices in farming systems of Uganda. Agriculture Ecosystems
and Environment 71:115-130.
- Yadvinder-Singh, Bijay-Singh and Khind, C.S. 1992. Nutrient transformations
in soils amended with green manures. Advances in Soil Science 20:237-309.
- Zaharah, A.R. and Bah, A.R. 1991. Effect of green manures on P solubilization
and uptake from phosphate rock. Agroforestry Systems 48:247-255.
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