productivity, soil fertility and soil erosion under cassava based agroforestry systems
TRANSCRIPT
Agroforestry Systems 8: 67-82, 1989. 67 © 1989 Kluwer Academic Publishers. Printed in the Netherlands.
Productivity, soil fertility and soil erosion under cassava based agroforestry systems
S.P. GHOSH, B.MOHAN KUMAR, S. KABEERATHUMMA and G.M. NAIR Central Tuber Crops Research Institute (1CAR), Trivandrum - - 695 017, Kerala, India
Key words: Cassava, soil fertility, soil erosion, agroforestry, multiple cropping
Abstract. The interactions between component species in three-tier agroforestry systems were studied on sloping laterite soils of South India for three years. The wood yield of Eucalyptus was found to increase in association with the intercrops, with cassava + groundnut resulting in the best growth of Eucalyptus. Green forage yield of Leucaena was adversely affected by cassava but was improved by inclusion of a short duration seasonal crop. Both spread and mean length of lateral roots of Eucalyptus and Leucaena were restricted by cassava intercrop- ping. Both the tree species were found to reduce the tuber yield of cassava and also the pod yield of both the seasonal crops when grown in association. Monocropping with cassava was found to improve the fertility and an increase in phosphorus and potassium contents of the soil was observed when grown in association with Eucalyptus and Leucaena. Soil fertility fell considerably after three years of cultivation of the tree species. The nutrient uptake by cassava was low when grown in association with perennial species. Both run off and soil loss were effectively reduced when cassava was grown in staggered mounds under Eucalyptus and Leucaena.
Introduction
Although cassava is a sun loving plant, it is widely grown in mixed cultures throughout the world [2, 8]. In India, cassava is grown extensively in Kerala, Tamil Nadu and North Eastern Region. Both in Kerala and the North Eastern hills, the crop is grown in sloping terrain receiving high rainfall and hence potential for soil erosion. In North Eastern Region, cassava is a standard crop component in mixed cropping under shifting cultivation. Since the crop is grown by the poor farmers in marginal lands, proper soil management is essential for maintaining sustained productivity. The in- clusion of trees and woody perennials on farm lands may bring improve- ment in the soil physical conditions and fertility, influence hydrological characteristics and help in erosion control from sloping sites [6]. It was, therefore, planned to study soil productivity and soil management in cass-
68
ava based agroforestry systems. Complementary interactions between the component tree species namely Eucalyptus and Leucaena with annuals cassava and the seasonals groundnut, French bean and cowpea in three-tier cropping systems have been studied.
Materials and methods
Field trials were laid out at the Central Tuber Crops Research Institute, Trivandrum (India) during June-July 1983. The site is a low level dissected laterite plateau with mounds and narrow valleys at 76 ° 57' E longitude and 8 ° 29' N latitude. General elevation of the area ranges from 50 to 65 metres above sea level and the slope of the experimental site ranges from 5-9%. The texture of the surface soil ranged from gravelly sandy clay to gravelly clay, the gravel percentage ranging from 73.0-80% in different layers.
: Climate
The climate is warm humid with a mean annual rainfall of 1700 mm of which 95% is received during March to December. The maximum mean tem- perature occurs in March and April (32.7-32.8 °C) but there is little vari- ation between seasons. The minimum mean temperature occurs in Decem- ber and January (21.0-21.3 °C). During January to March the average daily hours of sunshine is 9 but during June-July it is 4.8.
Treatment combinations
Three tier cropping systems comprising the perennials (Leucaena leucoce- phala and Eucalyptus tereticornis), the annual (cassava) and the seasonals (groundnut, French bean/vegetable cowpea) were as follows:
1. Leucaena + Cassava 2. Leucaena + Cassava + Groundnut 3. Leucaena + Cassava + French beans/Cowpea 4. Leucaena alone 5. Eucalyptus + Cassava 6. Eucalyptus + Cassava + Groundnut 7. Eucalyptus + Cassava + French beans/Cowpea 8. Eucalyptus alone
9. Cassava + Groundnut 10. Cassava + French beans/Cowpea 11. Cassava alone
69
The experiment was laid out in randomised block design replicated three times.
Leucaena and Eucalyptus were spaced at 5.0m x 2.5m. Cassava was planted in the interrows at a spacing of 90 cm × 90 cm, The intercrops groundnut and French beans/vegetable cowpea were sown in between cass- ava rows either in double rows or single rows with an inter-row spacing of 20 cm (Fig. 1). The details on spacing, crop varieties, plant population and time of planting are presented in Table 1,
Observations
(a) Growth Plant height, canopy spread and stem diameter at breast height of Eu-
~x- Cassava
E u c a l y p t u s / L e u c a e n a
- .Intercrop
° : N N "
" m 0 , v 0 2 . 5 . , ,
Fig. I. Layout of experiments: Three-tier crop combinations of trees, cassava and seasonal intercrops.
Tab
le l
. E
xp
erim
enta
l de
tail
s
Cro
p S
paci
ng (
m)
Pop
ulat
ion/
V
arie
ty
Fer
tili
zer
Rem
arks
ha
sc
hedu
le
...O
Per
enni
al:
I T
ier
Cro
p
Leuc
aena
le
ucoc
epha
la
Euc
alyp
tus
tere
ticor
nis
Ann
ual-
H T
ier
Cro
p C
assa
va (
mix
ed)
Sole
Seas
onal
III
tie
r sp
ecie
s a.
Gro
undn
ut
(Per
enni
al +
C
assa
va)
Cas
sava
b. F
renc
h be
an
(Per
enni
al +
C
assa
va)
Cas
sava
5 x
2.5
800
Haw
aiia
n 10
0g P
2Os/
plan
t/
Hea
d ba
ck a
t on
e G
iant
yr
ye
ar s
tage
at
a he
ight
of
100c
m a
nd
subs
eque
nt c
utti
ngs
at 6
0 da
ys i
nter
vals
5 x
2.5
800
Hyb
rid
Nil
Is
t ha
rves
t at
33
mon
ths
stag
e.
Cop
pice
cro
p al
low
ed
0.9
x 0.
9 97
00
H-1
687
NP
K,
100k
g/ha
* P
lant
ed i
n st
agge
red
0.9
x 0.
9 12
345
each
m
ound
, ac
ross
the
sl
ope.
Har
vest
ing
at
10-1
1 m
onth
s
0.3
x 0.
2 86
000
TM
V-7
N
IoP2
0K20
kg/
ha
Har
vest
at
100-
105
0.3
x 0.
2 11
0000
da
ys
0.3
x 0.
2 86
000
Con
tend
er
N20
P30K
2o k
g/ha
H
arve
st a
t 70
-75
0.3
x 0.
2 11
0000
da
ys,
duri
ng 1
983
and
84 o
nly
c. V
eget
able
Cow
pea
(Per
enni
al +
C
assa
va)
0.9
x 0.
2 43
000
B-6
1 N
l0P
tsK
l0k
g/h
a C
assa
va
0.9
x 0.
2 55
000
Fro
m 1
985
onw
ards
in
pla
ce o
f F
renc
h be
ans,
60-
70 d
ays
crop
* F
ull
P an
d ha
lf N
and
K a
t pl
anti
ng,
rem
aini
ng h
alf
at 4
5-60
day
s fo
r th
e so
le c
rop.
Gro
undn
ut,
Fre
nch
bean
/Cow
pea
-- N
PK
app
lica
tion
one
mon
th a
fter
pla
ntin
g
71
calyptus and at 30 cm height in Leucaena were recorded periodically. (b) Rooting pattern: The number and lateral spread of first order primary roots of the peren-
nials were determined at 6 and 32 months after establishment by careful removal of soil and washing with water. Mean lateral spread was calculated from the length of 18 lateral roots of three plants sampled from three replicate plots.
(c) Yield: The first pruning of Leucaena was made at 12 months to establish a height
of I m and the subsequent loppings of twigs were made to maintain this height at intervals of 60 days. All side shoots of Eucalyptus upto a height of 1.5 m were removed. The yield of Eucalyptus was estimated from the wood weight after clear felling at 33 months stage.
Cassava tuber yields were recorded at 10-11 months after planting. The green pods of cowpea were harvested in five pickings starting from 50 days and groundnut was harvested after 105 days. The crop residues of ground- nut and cowpea were incorporated in the soil.
(d) Soil fertility and nutrient uptake: Fertility status of the surface soil (0-20 cm) was determined initially and
after harvesting of each cassava crop. The soil samples were analysed for their physicochemical characteristics. Soil pH was determined in 1:2.5 soil water suspension. Organic carbon, available nitrogen, phosphorus and potassium and exchangeable calcium and magnesium were determined by following standard procedure [3, 9].
To estimate the nutrient removal by cassava, plant samples (leaf, stem and tuber) were collected at harvest from each replicate, processed and dried in the oven. The dried and powdered samples were analysed as follows. Ni- trogen was estimated using the Kjeltec system II and phosphorus, potass- ium, calcium and magnesium in the triacid extract. Phosphorus was deter- mined by Vanado molybdophosphoric yellow method, and potassium and calcium by flame photometry. Magnesium was measured by atomic absorp- tion spectroscopy [3].
(e) Runoff studies: Runoff studies were conducted during 1984-86. Six runoff plots of stan-
dard size (25 m x 5 m) on 8-9% slope fitted with 'multi-slot devisors' were constructed and runoff measurements were made in the following systems (a) bare fallow (b) pure cassava in staggered mounds (c) Eucalyptus + cass- ava (d) pure Eucalyptus (e) Leucaena + cassava (f) pure Leucaena. Runoff
and soil loss data were measured after each rain during July-November, 1985 and June-December in 1986.
72
Results
Growth of perennials
Growth of Leucaena was adversely affected by cassava during the first 12 months, whereas a marginal increase in the height (12.7%) of Eucalyptus was noticed after 6 months when intercropped with cassava. Cassava inter- cropping actually promoted growth of Eucalyptus and the trend was main- tained till 18 months.
Data recorded on the trunk girth of Eucalyptus at 33 months stage are presented under different thickness groups in Table 2. The data show that while about 53.1%0 plants in the sole stand attained a girth above 30 cm by 33 months, 76.2% plants in association with cassava + groundnut and 71.9% of the plants in association with cassava + cowpea attained similar girth during the same period. Eucalyptus grown with cassava also had bigger girth (63.1%).
Root growth in perennials
Observations recorded on spread of lateral roots (skeleton/primary roots originating from the main trunk of the plants) at 6 and 32 months after
Table 2. Trunk girth (B.H.) of Eucalyptus at 33 months stage (Frequency distribution of thickness groups)
Girth cm Number of plants (thickness groups) Sole stand With With cassava + With cassava +
cassava groundnut cowpea
5-10 2 - - 1 1 10-15 3 1 3 2 15-20 3 3 4 3 20-25 10 5 2 3 25-30 12 15 5 9 30-35 22 20 13 15 35-40 8 12 21 18 40-45 4 9 12 9 45-50 - - - - 2 4
% Population
Girth less than 46.9 36.9 23.8 28.1 30 cm Girth more 53.1 63.1 76.2 71.9 than 30 cm
Tab
le 3
. R
oo
t g
row
th o
f E
ucal
yptu
s an
d L
euca
ena
in p
ure
an
d m
ixed
sta
nd
s
Ro
ot
Euc
alyp
tus
char
acte
rs
Leu
caen
a
6 m
on
ths
32 m
on
ths
6 m
on
ths
32 m
on
ths
Sol
e +
C
assa
va
Sol
e +
C
assa
va
Sol
e +
C
assa
va
Sol
e +
C
assa
va
Max
imu
m
late
ral
spre
ad c
m
49.5
46
.0
485.
1 26
8.3
Av.
ro
ot
spre
ad c
m
47.0
43
,3
320.
2 20
4.5
No.
of
1st
ord
er r
oo
ts
8 7
21
20
168.
7 18
0.0
133.
0 15
7.0
6 4
346.
1 19
4.7
195.
6 11
3.2
28
30
t.a.a
74
planting of the perennials are given in Table 3. At 6 months, spread of lateral roots in Leucaena was higher having fewer 1st order skeleton roots when grown in association with cassava as compared to the sole stand. Root growth of Eucalyptus showed only marginal reduction in both lateral spread and no. of 1 st order skeleton roots in mixed stand with cassava at 6 months.
Cassava intercropping restricted the spread of lateral roots of both Eu-
calyptus and Leucaena at 32 months after planting. In Eucalyptus, against the maximum lateral spread of 4.85 m in sole stand, such spread was only 2.68 m when grown in association with cassava. Similar trend was recorded in mean root length also (Table 3). In Leucaena, root growth both in terms of maximum lateral spread (3.46m) and average length of laterals was significantly superior in the sole stands.
Forage and wood yields o f perennials
(a) Leucaena: A total number of six cuttings could be taken from Leucaena over a period
of 33 months, the first cutting being given at 12 months after planting (Table 4). The highest forage yield of Leucaena (7.50t/ha) was obtained when grown in association with cassava and groundnut and the lowest (4.83 t/ha) in association with cassava.
Table 4. Fresh forage yield of Leucaena (t/ha)
Crop Months after planting combination
12-18 19-24 25-33 Total
Leucaena 2.400 0.500 3.633 6.533 Leucaena + Cassava 0.744 0.250 3.833 4.827 Leucaena + Cassava + 2.733 0.380 4.389 7.502 Groundnut Leucaena + Cassava + 1.573 0.493 3.440 5.506 Cowpea
Table 5. Wood yield of Eucalyptus (t/ha)
Month a f t e r Euca lyp tus Eucalyptus + Eucalyptus + Eucalyptus + planting Cassava Cassava + Cassava +
Groundnut Cowpea
Side shoots 0.520 0.356 0.400 0.325 removed at 18 months Wood recovery 30.11 28.50 40.40 43.53 at 33 months
75
Table 6. Tuber yield of cassava (t/ha) under multiple cropping systems
Treatments 1983-84 1984-85 1985-86 Average
Cassava (Sole) 29.4 17.7 28.1 25.07 Eucalyptus + Cassava 20.9 3.6 9.3 11.27 Eucalyptus + Cassava + 18.6 5.6 7.1 10.43
Groundnut Eucalyptus + Cassava + 20.6 7.0 6.1 11.23 Cowpea Leucaena + Cassava 23.8 5.4 11.1 13.43 Leucaena + Cassava + 34.0 8.4 6.4 16.27 Groundnut Leucaena + Cassava + 32.3 7.8 9.1 16.40 Cowpea
(b) Eucalyptus: The wood yield of Eucalyptus at 33 months growth is given in Table 5. The
air dry wood yield of Eucalyptus at 33 months was maximum (43.5 t/ha) when grown in association with cassava + cowpea whereas the sole stand had only 30.1 t/ha wood yield (Table 5).
Tuber yield of cassava
There was significant difference in the fresh tuber yield of cassava among the treatments and seasons (Table 6). The yield of cassava under Leucaena was
Fig. 2. Three-tier cropping with Eucalyptus + cassava + cowpea.
76
Fig. 3. Three-tier cropping with Leucaena + cassava + groundnut.
23.8 t/ha which was on par with sole stand of cassava (29.4 t/ha) during the first year. However, a significant reduction in the yield of cassava was observed when grown in association with Eucalyptus (20.9 t/ha).
During the second and third year cropping, the tuber yield of cassava was significantly higher under sole stand as compared to other treatment com- binations. The yield of cassava under Eucalyptus and Leucaena was poor during the 2nd and 3rd year. The yield reduction due to Leucaena and Eucalyptus when compared to sole stand of cassava was 60.6 to 69.6% and 67.0 to 79.8% respectively during 2nd and 3rd year cropping.
YieM of seasonal intercrops
The performance of French bean was very poor during the first two years
Table 7. Yield of seasonal intercrops (t/ha)
Crops 1985 1986
Cassava Leucaena + Eucalyptus + Cassava Leucaena + Eucalyptus -4- Cassava Cassava Cassava Cassava
Groundnut 0.92 0.36 0.24 1.21 0.64 0.58 Cowpea 3.12 2.18 1.45 6.50 3.15 2.43
Tab
le 8
. In
itia
l so
il f
erti
lity
and
fer
tili
ty s
tatu
s af
ter
thre
e ye
ars
of c
ropp
ing
Per
iod
Tre
atm
ent
pH
Org
anic
ca
rbo
n
(%)
Ava
ilab
le n
utri
ents
(kg
/ha)
E
xcha
ngea
ble
cati
on (
mg/
100
g)
N
P K
C
a M
g
Init
ial
Aft
er
3 ye
ars
Cas
sava
4.
92
0.80
26
5.15
14
.84
119.
09
7.77
4.
05
Euc
alyp
tus
5.00
0.
83
288.
41
20.3
5 85
.87
9.30
5.
45
Leu
caen
a 5.
03
0.57
24
4.21
16
.80
128.
43
10.1
0 3.
84
Cas
sava
4.
80
0.97
27
4.40
58
.24
161.
28
13.0
7 2.
12
Euc
alyp
tus
5.20
0.
52
222.
13
12.6
9 66
.31
4.27
4.
39
Euc
alyp
tus
+
5,00
0.
95
207.
76
22.0
3 15
3.44
6.
64
4.52
C
assa
va
Leu
caen
a 5.
30
0.43
20
3.84
11
.20
63.0
9 6.
40
3.65
L
euca
ena
+
4.90
0.
77
216.
91
35.4
7 11
3.48
7.
47
1.82
C
assa
va
",-.,.I
78
as a result of severe pest infestation and inadequate rainfall. However, groundnut and vegetable cowpea were successful during the third and fourth year (Table 7). The pod yield of groundnut and cowpea was higher when intercropped with cassava and lower when grown in association with peren- nial + cassava.
Soil fertility and nutrient uptake
The initial soil fertility and fertility status after three years of cropping were evaluated (Table 8). The soil reaction at the initial stage was found to be acidic and the organic carbon content was medium to high. The available nutrients were low to medium in all the plots. After three years of cropping the pH did not show any variation and it remained in the acidic range. The organic carbon content of the soil was improved when cassava was grown as pure crop or along with perennials, and showed a decline when the perennials were grown alone. The available nutrient status was higher in sole cassava plots as compared to the cassava-perennial combinations. Mono- cropping of cassava resulted in the build up of available phosphorus and potassium. Sole cropping of Eucalyptus and Leueaena considerably reduced the available nutrient status of the soil after three years of cultivation.
Fig. 4. Multislot devisor with collection tank.
Tab
le 9
. N
utri
ent
upta
ke b
y ca
ssav
a (k
g/ha
) un
der
diff
eren
t cr
op c
ombi
nati
ons
Tre
atm
ent
Nit
roge
n P
hosp
horu
s P
otas
sium
1984
19
85
1986
M
ean
19
84
1985
19
86
Mea
n
1984
19
85
1986
M
ean
Cas
sava
18
3.6
123.
0 28
2.1
196.
2 16
.2
9.2
14.2
13
.2
120.
8 10
6.8
106.
8 10
6.5
Euc
alyp
tus
+
135.
4 48
.9
119.
5 10
1.3
9.7
4.2
5.6
6.5
60.7
24
.1
49.0
44
.6
Cas
sava
L
euca
ena
+
171.
8 74
.9
129.
3 12
5.3
13.9
5.
9 6.
6 8.
3 10
6.5
40.9
54
.9
67.4
C
assa
va
80
However, a build up of phosphorus and potassium was observed in Leu- caena + cassava and Eucalyptus + cassava combinations respectively. In all the treatment combinations the exchangeable bases (Ca and Mg) were found to decline; excepting in the case of sole cassava where the exchange- able calcium content showed an increase.
The nutrient uptake by cassava under various treatment combinations for three years is presented in Table 9. There was considerable variation in the uptake pattern for the different seasons. In general, the nutrient uptake was more in 1984 season and less in 1985. During all the years the nutrient removal by cassava was more when it was grown alone as compared to the cassava-perennial combinations. Among the crop combinations, cassava in association with Eucalyptus recorded the lowest nutrient removal.
Run off and soil loss
The run off and soil loss data recorded during 1985 and 1986 are presented in Table 10.
Maximum water and soil loss were recorded from the bare fallow plot (19-25% and 4.7 to 11.94 t/ha respectively) during both the seasons. Run off and soil loss were also high in sole stands of cassava (10.5-18.9% in water loss and 1.78-7.30t/ha in soil loss). Sole crops Eucalyptus and Leucaena were found to be almost on par with cassava in checking soil erosion. However, two tier cropping of perennial + cassava could bring down run off and soil loss substantially (70-80%).
Table 10. Surface run off and soil loss under various multiple cropping system
Treatment Run off rainfall (%) Soil loss (t/ha)
1985 1986 1985 1986
Bare fallow 19.52 25.30 4.667 11.94 Cassava 10.52 18.92 1.783 7.30 (Staggered mound) Eucalyptus 8.29 10.72 2.150 6.13 Leucaena 9.12 9.87 1.970 5.15 Eucalyptus + Cassava 4.80 7.58 0.779 2.89 Leucaena + Cassava 5.49 5.23 0.834 3.34
Total rainfall (mm) 1357.79 1152.19
No. of rains 85 67
Discussion
The trunk girth of Eucalyptus recorded prior to its harvest indicated that the
81
plants were benefitted by the association of cassava (second tier crop) and seasonals like groundnut and cowpea (third tier crops) even though the root spread of both Eucalyptus and Leucaena were restricted in the mixed stands. This clearly demonstrates the capability of Eucalyptus to yield better in the mixed stands exploiting the moisture and nutrients rather than the compan- ion crops. However, this system provides an enhanced nutrient economy because of differential nutrient absorbing zones of the root system which facilitates balanced removal of nutrients. It was also observed that the oil palm production was not affected when it was intercropped for 12 years with food crops like yams, cassava and maize [4]. However, the yield of both cassava and the seasonal intercrops were reduced when grown in association with the perennials. With regard to nutrient utilization, cassava was found to perform better in monocropping system and a considerable reduction in the nutrient removal was observed when grown with Eucalyptus. This further establishes the aggressive habit of the fast growing perennial species in effectively utilizing the available nutrients and moisture at the expense of other companion crops. Considerable reduction in soil fertility was noticed when the perennials are grown continuously for three years. Deterioration of soil physical and chemical conditions are often associated with monocul- tural plantation forests of fast growing species [6]. The decline in soil fertility in the sole stands of perennials observed in this study is also partly attributed to the low rate of fertilization adopted for the perennials. However, when cassava was grown as an intercrop with the perennials the fertility status of the soil could be maintained without much deterioration. Studies conducted in Southern Nigeria also revealed that interplanting young forest plan- tations with food crops did not adversely affect the soil fertility [7].
Both run off and soil loss were effectively reduced when cassava was grown on staggered soil mounds along with Eucalyptus and Leucaena as compared to the respective sole stands. This may probably be due to better canopy coverage on the soil surface which protects the soil from erosion. It is believed that in many man made tropical forests increased soil erosion hazards and nutrient depletion are experienced [6]. Even in 'Taungya' system erosion hazards have been experienced [1].
In general, the run off and soil loss were higher during 1986 as a result of higher percentage of erosive rains (68.7%) against 57.6% in 1985. More- over, 90% of the rains received during 1986 were of high intensity and short duration (duration was 15-45 min) whereas during 1985 such rains of high intensity were received only in October-November months when cassava crop was fully developed.
During 1985, the canopy development of Eucalyptus could provide such protective cover that brought down the surface runoff and soil loss consider-
82
ably. C a n o p y coverage o f Leucaena was restricted due to periodical lopping
and ground coverage was only partial. However , during 1986just before the onset o f rains, harvest ing o f Eucalyptus was done and the coppice growth
was just at the initial stage when erosive rains were experienced which might have resulted in higher loss in the sole stands. In te rcropping o f cassava could however bring down the soil loss substantially. Thus, soil erosion which is
normal ly accelerated by deforestat ion o f tropical rain forests can be success- fully minimised by a p roper combina t ion o f agricultural crops with forest
species.
References
1. Alexander TG, Sobhana K, Balagopalan M and Mary MV (1980) Taungya in relation to soil properties, soil degradation and soil management. Res Rep 4. Kerala (India): Kerala Forest Research Institute, Peechi
2. Diaz RO and Pinstrep-Anderson P (1977) Description agro-economic del production de yuca en Colombia. CIAT Bull. Edicion Preliminar pp 4
3. Jackson ML (1967) Soil Chemical Analysis. Prentice Hall of India Pvt Ltd New Delhi. pp 488
4. Koval JML and Tinher PBH (1959) Soil changes under a plantation established from high secondary forests. J West Afr Oil Palm Res 2:376-389
5. Lundgren B (1978) Soil Conditions and nutrient cycling under natural and planatation forests in Tanzanian High Lands. Reports in Forestry Ecology and Forest Soils No 3: Swedish Univ Agric Sci Uppsala, Sweden pp 426
6. Nair PKR (1984) Soil productivity aspects of agroforestry. International Council of Research in Agroforestry (ICRA), Nairobi, pp 85
7. Ojeniyi SO, Agbede OO and Fagbenro JA (1980) Increasing food production in Nigeria. I. Effects of agrisilviculture on soil chemical properties. Soil Science 130:76-81
8. Okigbo BN and Greenland DJ (1976) Intercropping systems in Tropical Africa. In: Stelly M eds Multiple Cropping, ASA Spec Pub No 27:63-101
9. Piper CS (1966) Soil and Plant analysis. Hans Publishers, Bombay, pp 368