Senegal Report I. Manure and P and Ca Source Fertilizer

Senegal Report
I.
II.
III.
Manure and P and Ca Source Fertilizer
Phosphogypsum Efficiency to Correct Soil P
Deficiency and/or Soil Acidity
Impact of Piliostigma reticulatum on the Quality of
Soils and on Agricultural Output in the Central-South
Zone of Sénégal
Page
1
17
31
I. Manure and P and Ca Source Fertilizer
REPUBLIQUE DU SENEGAL
MINISTERE DE L'AGRICULTURE
INSTITUT SENEGALAIS
DE RECHERCHES AGRICOLES
-------
Effect of manure and P and Ca Source fertilizer on the optimization of soil
water and nutrient use in the Corn/Peanut rotation system in the Peanut Basin of
Senegal
M. SENE and A. NIANE-BADIANE
ISRA, Sénégal
Collaborating team
P.O. DIEYE, M. DIOP, S. FAYE, A.GUEYE, et P.S.SARR
May 2001
2
Abstract
A four-year nationwide programme has been underway since 1997 to boost the food
and cash crop production. P and Ca soil amendment using natural resources based
fertilizers is one of the strategies defined. A long term randomized complete bloc
design experiment has been installed at Nioro du Rip research station since 1997
within the corn/peanut cropping system in the Senegal Peanut Basin. The four
treatments under comparison are as follows : control with no P or Ca added (T1,
phosphogypsum and phosphate rock (PG + PR) mix at the rate of 1000 kg/ha (T2),
manure at the rate of 5000 t/ha once every 2 years (T3), and (PG + PR mix combined
with manure (T4). Different measurements have been made to determine over time
soil fertility change, crop water balance, and nitrogen use efficiency and yield
components. Soil fertility changes have been measured from soil samples collected
from each plot in 1997 and in 1999 after corn harvest. Crop water balance has been
determined using neutron probe and tensiometers while 15-N labeled fertilizer has
been used in 1999 to determine the corn N efficiency use.
For this experiment, the important Ca movement observed within the profile is
strongly related to the amount of Ca added for each treatment. Regardless the
treatment, deep water percolation is observed both in 1998 (relatively dry year) and in
1999 (very wet year). Corn yields in 1997 and 1999 obtained are low, as opposed to
relatively high peanut yields in 1998. There is no significant treatment effect during
the first 2 years.
In 1999, the laboratory analyses of soil samples collected after corn harvest have
confirmed the improvement of few soil chemical characteristics down to 0,6 m depth,
such as the Ca content and base saturation. Soil pH remained very low (pH < 5.5),
eventhough the manure application alone or combined with PG and PR mix tend to
increase the soil pH value. Corn yields (grain and stalk) remained low for the second
time (less than 2000 kg/ha, i.e., 50% of the corn variety potential yield value). This
confirms the still degraded soil fertility status. Compared with the control treatment,
the PG and PR mix has no significant effect on corn grain or stalk yield. But, the
manure alone or combined with the PG and PR mix has a significant effect on yield.
Despite the large amount of rainfall recorded this year, the crop water balance
indicated a low water use efficiency. The drainage water accounted for at least 40% of
the rainfall. Corn NUE efficiency is improved by manure alone or combined with PG
and PR mix, but no by this latter. NUE values are low (ranging from 16 % to 30%).
After 3 years of experimentation, the results obtained indicated the PG and PR mix as
applied does not show high efficiency in correcting P and Ca deficiencies.
3
INTRODUCTION
In the Senegal peanut basin, fallow practices have almost disappeared from the
farmers land use system. This situation is strongly related to the introduction of
peanut as a cash crop, but also results from an increased demand for food crops by an
increasing population. The high pressure on the naturally fragile soils combined with
the drought problem observed during the last 30 years is detrimental to the annual and
perennial vegetation cover. Therefore, through soil organic matter loss and
acidification due to continuous cropping and/or grazing, the food production system
has lost its resilience. In most farmer’s field situations, the degradation of soil water
characteristics leads to deep water percolation beyond the rooting depth, even under
moderate rainfall conditions. This also increases nutrient leaching risks. Manure
applications and plowing are very efficient in reducing the water and nutrients loss
through deep percolation by promoting a rapid crop root growth (Cissé, 1986).
Many studies have confirmed the efficiency of natural rock phosphate (RP)
amendment at an application rate of 400 to 500 kg/ha every 4 years to correct soil P
deficiencies. (Poulain et Mara, 1965). On soils with low pH, the agronomic efficiency
of the rock phosphate ranges from 82 to 91 % compared with the triple super
phosphate (Bationo et al, 1990). This value depends on the chemical characteristics of
the rock phosphate mines for which comparison results of the study are available
(Ndiaye, 1978 ; Cissé, 1980).
However, for phosphogypsum (PG) or the combination of RP and PG now being used
in Senegal in the national 4-year program, there is little information in terms of
agronomic value or soil P and Ca amendment efficiency. The on-going
experimentation comparing those two mineral compounds is designed to focus on that
aspect. Assuming a positive effect of the combination of RP and PG, the objective of
this study is to determine for the main cropping systems the efficiency of applying the
combined P source material and manure to a degraded soil to optimize water and
nutrient plant uptake in order to attain a sustainable crop production increase.
4
MATERIALS AND METHODS
Experimental design
The sites are selected according to the existing main cropping systems. Within the
Peanut Basin, the improvement of food security can be achieved in three cropping
systems. In the northern part of this agroecological zone, peanut followed by millet is
the predominant whereas in the southern part, peanut followed by corn is a common
practice. Two sites for long term experiment is selected in for peanut/corn rotation in
Nioro area (one at the ISRA Research Station and one on farm field near the station).
In this report, only data collected for the corn/peanut cropping system at Nioro du Rip
station will be presented. As a matter of fact, nutrient and water balance data during
the 1999 cropping season were only available for the on-station site. The initial soil
chemical characteristics are presented (Table 1).
Table 1 : Soil physical and chemical characteristics (0-10 cm). Nioro
Parameter
pHwater
pHkcl
Clay + Silt (%)
C (g/kg)
C/N
CEC (cMole/kg)
Base Saturation (%)
P total (mg/kg)
Available P (Olsen-Dabin)
Value
5.4
4.9
7.9
2.5
9.2
1.6
64.5
200
28.5
Five treatments compared are shown in table 2.
Plowing is performed when implementing the treatment, including the control. In fact,
this crop require deep tillage in order to express their potential.
Table 2 : Treatment description in the different cropping systems
Treatment
T1
Plowing (P) + Fertilizer N and K added at recommended rate for crops
5
(P + NK)
T2
P + NK + 50 %phosphogypsum (PG) and 50 % Taïba phosphate rock
(PR) mix at the rate of 1000 kg/ha: (P + NK + PG_PR)
T3
P + NK + manure at the rate of 5 t/ha added once every 2 years, in 1997
and in 1999 (P + NK + M)
T4
P + NK + PG_PR + M
N and K are applied annually as urea and KCl, respectively at the recommended rate :
•
for corn in 1997 and 1999: 12 kg N ha-1 at planting and two additional
applications of 46 kg N ha-1 each at 27 and 41 DAS, respectively and 41 kg K2O
ha-1 at planting ;
for peanut in 1998 and 2000: 9 kg N ha-1 at planting and 15 kg K2O ha-1 at planting.
The experimental design is a randomized complete bloc design with five treatments
repeated four times.
The varieties used for the different crops are as follows :
a) for peanut : variety 73-33 at Nioro sites for 105 days after sowing (DAS )
b) for corn : variety Synthetic C in 1997 (90 DAS), and variety Across 86 Pool DR
in 1999 (90 DAS).
Measurements and monitoring
Sites characterization
For this on-station experiment at Nioro installed in 1997, site chararacterisation was
done using the data obtained from the survey conducted throughout the station for soil
fertility rehabilitation purposes (Agetip, 1995).
Soil water balance
This monitoring is done for plant water uptake but also for the purpose of nutrient
balance. Soil water content in the soil profile is measured once a week at Nioro station
site. The methods combined the use of neutron probe, and the auger for calibration
purposes. Access tubes for neutron probe readings are installed at the depth of 4,3 m;
i.e., deeper than the maximum crop root depths. Three replicate for each treatment
6
were considered. Due to the limited number of tensiometers available, two replicates
per treatment among the 3 replicates selected for neutron probe readings were chosen.
For each plot selected, 2 tensiometers are installed at the depths of 1,4 and 1,6 m
respectively. The maximum corn rooting depth which is about 1,50 m in sandy soils
for peanut (Chopart, 1980). Daily tensiometer readings were available only for the
last two years (1999 and 2000).
The soil water balance equation stated below is used to determine the soil plant
evapotranspiration for specified time increment during the cropping season.
R – D – r ± ∆S = ETR
Where R = rainfall, D = drainage, r = runoff, ∆S = variation of stock, and ETR =
evapotranspiration. All these components are in mm of water.
Nitrogen use efficiency by corn in 1999
For this study, all the experimental plots were considered for 15-N applications. For
each replicate, a micro-plot (2.4 m x 1.5 m) was marked before corn planting for 15-N
applications. The first application at planting consisted of 12 kg N ha-1 as urea (1% 15N atom excess ). Then two additional applications at 27 and 41 days after sowing
(DAS), respectively were made. The amount applied were 100 kg N ha-1 as urea (1%
15-
N atom excess). At harvest, grain and stover yields were determined. Grain and
straw samples, but also soil samples at 0-20 and 20-40 cm were taken for 15-N
analysis.
The time schedule for agronomic measures Nioro du Rip Research Station are
presented (table 3). Due to rainfall distribution and timing, sowing date was earlier in
1998 and 1999 than in 1997. Every year, oxen driven plowing is done in optimum
soil moisture conditions, which tend then to delay the sowing operation.
Table 3 : Time schedule for agronomic measures from 1997 to 2000 at Nioro du Rip
station
Date of implementation
Operation
Corn
(1997)
Peanut (1998)
Corn
Peanut
(1999)
(2000)
7
RP+ PG or Manure 06/30/1997
07/02/1999
-
application
Plowing
06/30/1997
06/23/1998
06/16/1999
07/1/2000
Sowing
08/05/1997
07/20/1998
07/11/1999
7/15/2000
07/27/1998
07/22/1999
7/15/2000
N-K application
Thinning
08/05/1997
-
07/27/1999
Pre-emergence
-
-
-
7/16/2000
weeding
1st weeding
08/22/1997
07/27/1998
07/19/1999
st
09/05/1997
-
08/06/1999
nd
09/20/1997
-
08/23/1999
2nd weeding
09/10/1997
08/22/1998
08/23/1999
Bedding
09/10/1997
Harvest
11/3/1997
1 urea application
2 urea
8/01/2000
application
08/29/2000
11/5/1998
10/11/1999
10/25/2000
Rainfall conditions
Rainfall patterns are different for the 4 years (1997, 1998, 1999 and 2000) as reported
(SENE and BADIANE, 2000a).
In fact, we have experienced one early rainy season in 1997 as opposed to a late rainy
season in 1998. In Nioro, the total annual rainfall is about the same for the 2 cropping
seasons (600 mm). However, while the rainy season started early June in 1997, the
first important rain was recorded late July 1998. Although characterized by a rather
short duration of the rainy season, the 1998 cropping season has a much better rainfall
distribution. A long drought period (over 30 days) occurred early during the 1997
cropping season, causing a severe plant water stress, while in 1998 there were no
major water stress problem, except at the crop maturity phase. Unlike 1997 and 1998,
annual rainfall in 1999 and 2000 (total amount of 980 mm) was important. In this
well drained soil, deep percolation is therefore favored.
Data interpretation
8
This concerns the yield data, and the nutrients and water data. For most data, ANOVA
methods will be implemented to compare treatment effects, whenever it is possible.
Otherwise, comparison of means will be used
RESULTS AND DISCUSSION
Change in soil water content in the profile during the 1999 growing season in
relation with the treatments
The wetting front
The 1999 rainy season is characterized by its early start, the large amount and the
even distribution of the rainfall water. As a result, the field operations such as the
oxen plowing, fertilizer and organic matter applications, and corn sowing have been
conducted in optimum conditions (table 2). At the sowing date on 11/07/1999, that is
34 days after the first rainfall event, a total rainfall of 189 mm was recorded. Soil
water monitoring using neutron probe and tensiometers indicates a rapid downward
movement of the wetting front regardless the treatments (figure 1). The wetting front
was located at 1,30 m at the sowing date, at the estimated maximum corn rooting
depth of 1,5 m where drainage losses are determined. Around 09/23/1999, the wetting
front has gone below the 4,3 m which the depth of access tube installation. This has
led to an important drainage later accounted for in the soil water balance components.
The soil profile water content
The water content for 12 access tubes (3 replicates and 4 treatments) was determined
from weekly neutron probe readings started from the sowing date, after a first reading
was made one month earlier in fairly dry conditions. For the different treatments,
changes in soil water content within the profile are indicated (figure 2a, b, c and d). In
dry soil condition, water content values increased from 2 % on the top layer (0-0.10
m) to 9-10 % at the depth of 1.0 m. This trend follows the clay content in the profile.
During the cropping season, changes in water content are closely related to the rainfall
distribution and to crop development. The largest water content variation is observed
within the 0 - 0.5 m top layer where a maximum value of 22% obtained on the
07/14/1999 corresponded with a heavy rain few hours before the measurement.
9
Towards the end of august, water content from 0.5 m down to access tube depth
varied from 15 to 20 %. This confirmed the high amount of infiltrated water in the
soil profile. But no treatment effect was observed on this trend.
From the daily tensiometer measurements at 1,4 m and 1,6 m depth respectively), the
total hydraulic head changes for different treatments are presented (figure 3). The
value of the head gradient during from early august to early October confirmed the
downward water movement, thus the importance of the drainage component.
Soil water balance components
These components deal with the rainfall (R), the water stock variation (∆S) at 1,5 m
depth at different dates (S), the drainage water (D), and the evapotranspiration water
(ETR). The mean values determined at 10, 30, 45, 73 and 90 days after sowing for the
4 treatments are presented (Table 4). Drainage started early in the growing season.
Thirty days after sowing, drainage water represented about 15% of the total rainfall.
This fraction increased to 45% for T1, T2 and T3, and to 36 % for T4.
Corn water use was higher for the most intensive treatment (T4). But for all
treatments, water requirements were met during the different phenological phases.
Table 4 : Water balance components at different date after sowing (DAS). Nioro du
Rip Research Station. 1999
treatment
R, mm
D (mm)
ETR (mm)
T1
T2
T3
T4
T1
T2
T3
T4
10 DAS
188.7
14.3
12.1
19.5
12.9
23.0
17.9
17.9
22.6
30 DAS
374.6
62.3
70.5
77.5
54.6
98.7
88.9
89.5
98.3
45 DAS
649.3
205.8
221.3
224.7
191.3
162.2
138.5
147.8
171.1
73 DAS
666.5
298.5
340.8
318.9
253.6
326.2
274.4
308.7
357.1
90 DAS
711.3
299.5
347.0
324.2
254.4
397.2
348.2
377.1
435.1
Changes in soil chemical characteristics 3 years after amendment applications :
Soil analyses results obtained from samples collected right after corn harvest in 1997
and 1999 are presented (Table 5). There is no treatment effect on the pH values
measured. The severe soil acidity problem still held. However due to an increase in
10
exchangeable Ca content in the soil profile as a result of the PR and PG mix, there is
an increase of the base saturation values, especially for the top layer. From the initial
mean value of 64.5%, the base saturation value increased up to 90% in 1997 and to
78% in 1999. Despite the second manure application in 1999, no effect on soil organic
matter was observed.
Table 5 : Soil chemical analysis after harvest in 1997 and 1999. Nioro du Rip
Agricultural Research Station.
PH water
T1
T2
T3
T4
0-10
10-20
20-40
40-60
0-10
10-20
20-40
40-60
0-10
10-20
20-40
40-60
0-10
10-20
20-40
40-60
5.2
4.9
4.9
4.6
4.3
4.2
0.7
0.6
0.8
Base
saturation
(%)
1997 1999
73.3
69
76.5
63
78.3
72
92
90.0
84.5
82.7
5.7
5.3
5.0
5.0
4.3
4.2
0.6
0.5
0.7
93.7
76.5
67.0
1997
5.0
5.2
5.0
5.3
5.1
5.0
1999
5.4
5.4
5.5
5.7
5.5
5.5
5.2
5.6
PH kcl
1997
4.4
4.3
4.2
4.9
4.5
4.3
1999
4.5
4.4
4.5
4.8
4.6
4.5
4.3
4.7
Ca exch.
(mg/kg)
1997
0.5
0.5
0.7
0.8
0.6
0.8
1999
0.2
0.3
0.5
1.1
0.3
0.4
0.6
-
84.3
74.0
73.3
-
76
80
75
89
Effect of P and Ca amendment on crop yields
The effect of P and Ca amendments on corn yields in 1997 and 1999, and peanut in
1998 is presented (Table 6). There is a large yield variation among plot within
treatment as indicated by large CV values. Low corn yield values obtained in both
years in 1997 and 1999 are indicative of the low soil fertility status. Treatment effect
is significant on crop yield in 1999, but not in 197 and 1999.
In 1999, the PG and PR mix at the rate of 1000 kg/ha did not significantly improve
corn straw or grain yield over the control treatment. The positive effect on corn yield
11
is only associated with the manure application. Manure alone or associated with PG
and PR mix gave a 100 % yield increase.
In 2000, the PG and PR mix had no positive significant effect on pod or hay yields.
Compared to the control, this treatment tended to have a negative effect on pod or hay
yields. Manure alone or combined with PR and PR mix increased significantly pod
and yields. Compared to the PG and PR mix, manure gave an 100 % in pod yield
increase. The lowest hay yield is observed with the PG and PR mix. These results
indicate the low performance of the residual effect of the PG and PR mix on peanut
yield. As shown (table 5), PG and PR mix has not improved soil fertility, specially
soil pH which remained very low. The aluminum toxicity favored by Ca leaching with
time could explain the poor hay and pod yields obtained.
Table 6: yield components at Nioro Station in 1997 and 1998
Corn
Peanut
Corn
Peanut
(1997)
(1998)
(1999)
(2000)
Treatment
T1
Stalk
1800
Grain
710
Hay
2580
Pod
1420
Stalk
860 a
Grain
810 a
Hay
2950 b
Pod
936 a
T2
3030
1740
2120
1640
990 a
1040 ab
1750 a
773 a
T3
3460
1820
2780
1810
1590 b
1700 b
2600 b
1550 b
T4
3140
1880
2540
1940
1670 b
1830 b
3134 b
1450 b
Mean
2860
1540
2500
1700
1270
1340
2600
1780
Sign. Level
NS
NS
NS
NS
**
*
*
*
CV (%)
35
45
18.3
9.0
24.5
25.5
14.8
14.2
NS : no significant effect ; **, * : treatment effect significant at 1% and 5 % level,
respectively.
Corn water use efficiency during the 1999 rainy season
Based on corn water use estimated (Table 5) and on yield values (Table 6), mean
water use efficiency (WUE) values are determined for each of the treatment under
study (Table 7). Differences in corn WUE between treatments are the same than those
obtained for corn yield. Compared to control treatment, manure application at the rate
12
of 5 t/ha every two years, alone or combined with PG and PR mix application at the
rate of 1 t/ha once every 4 years increased by two fold the grain or straw yields. WUE
equal to 2 kg ha-1 mm-1 and 2.15 kg ha-1 mm-1 for grain and straw respectively reached
a value of 4 kg ha-1 mm-1 when manure was applied.
Tableau 7: Corn water use efficiency during 1999 growing season at Nioro du Rip
Station.
Corn Water use Efficiency
Treatment
Corn water Dry matter yield
(kg ha-1 mm-1)
use
(kg ha-1 yr-1)
-1
(mm yr )
Grain
Straw
Grain
Straw
T1
806
857
2.03 a
2.15 a
397.7±8.4
T2
1042
986
3.09 ab
2.93 a
336.9±6.6
T3
1698
1588
4.50
b
4.21 b
377.1±11.6
T4
1665
4.20 b
3.83 b
435.1±17.3 1829
Corn Nitrogen Use efficiency during the 1999 season
Yield data obtained in 15-N microplots were utilized for the computation of NUE. The
rate of application rate was 212 kg N ha-1. For grain and straw, large yield variations
were obtained (annex 1). No significant treatment effect at 5 % level was detected on
any of 4 parameters considered in Table 8, either on grain nor on straw. However,
manure alone or combined with PG and PR mix gave a 100 % increase in mean straw
or grain yield over the control treatment. This trend is about the same than for corn
N-Urea uptake and NUE ; the PG and PR mix effect being comparable with the
control effect. While total grain N is not affected by the treatment (1.75 % as a mean
value), total n in straw tended to be lower in more intensive treatment ( 0.78 % for
T4) than for the control treatment ( 1.03 % for T1).
The N-urea uptake for grain increased from 20 kg/ha for T1 to 39 kg/ha for T4 for
grain, and U-urea uptake for straw from 16 kg/ha for T1 to 24 %. As a result, low
NUE values were observed. These are ranging from 9.4 % to 18.5 for grain and from
7.5 % to 11.8 % for straw. NUE value for total dry matter yield ranged from 16.9%
for T1 to 30 % for T3 and T4. These results confirm those obtained by Ganry (1990)
13
and Badiane et al. (1995) for the same crop in the Peanut Basin and the Casamance
region of Senegal.
Table 8 : Effect of P and Ca-source and manure amendment on corn nitrogen use
efficiency at Nioro du Rip Research Station . Mean and standard deviation values are
in normal and italic characters, respectively.
Grain (G)
Straw (S)
Yield
Tot. N
N fert.
NUE
Yield
Tot. N
kg/ha
(%)
Uptake
(%)
Kg/ha (%)
kg/ha
T1
T2
T3
T4
G+S
N fert.
NUE
NUE
Uptake
(%)
(%)
kg/ha
1610
1.80
19.9
9.4
2230
1.03
15.9
7.5
16.9
880*
0,18
9.8
4.6
930
0.05
5.9
2.8
7.0
2330
1,75
25.8
12.2
3350
0.87
19.2
9.1
21.2
1720
0,31
12.2
5.7
1740
0.12
7.0
3.3
9.1
3300
1.75
39.3
18.5
5220
0.87
25.0
11.8
30.3
700
0.07
12.2
5.7
1430
0.07
2.2
1.0
4.7
3560
1.68
38.9
18.4
5120
0.78
24.3
11.5
29.8
1470
0.14
16.2
7.6
2120
0.07
8.3
3.9
11.6
CONCLUSION
For the corn/peanut rotation in the Senegal Peanut Basin, PG and PG mix alone or in
combination with manure amendments are applied in order to enhance soil fertility for
degraded soils. During the last 3 years, changes in soil fertility status indicated some
improvement only for exchangeable Ca content in the top soil layer. The positive
effect of PG and PR mix after 1997 harvest corresponded to base saturation increase
of 15 % as compared with the control treatment. Beside a wide range of corn yield
variation, low corn yield were obtained in 1997 and 1999, suggesting a still low status
soil fertility given the satisfactory rainfall conditions. in 1999. PG and PR mix has no
significant positive on crop yields (corn in 1997 and 1999, and peanut in 1998). But
Manure alone or in combination with PG and PR mix significantly increased the corn
grain and straw yield in 1999. Low WUE and NUE values are obtained. As compared
14
to the control treatment, the PG and PR mix had no of significant effect of on those
parameters in 1999, as opposed to the manure amendment at the rate of 5000 kg/ha
applied once every 2 years which provided a significant improvement.
ACKNOWLEDGMENTS
Financial support was provided by the United States Agency for International
Development (USAID) and the Joint FAO/IAEA Division. Technical assistance
CIRAD/Soltrop Laboratory is greatly appreciated.
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Cissé L. 1986. Etude des effets d’apports de matière organique sur les bilans
hydriques et minéraux et la production du mil et de l’arachide sur un sol sableux
dégradé du Centre-Nord du Sénégal. Thèse de doctorat en sciences agronomiques.
Institut National Polytechnique de lorraine, Nancy, 279 p.
Ganry, F.1990. Application de la méthode à l’étude des bilans azotés en zone
tropicale sèche. Thèse de doctorat d’état, Université de Nancy I, France, 255 p.
Piéri, C., 1976. L’acidification des terres de cultures exondés au Sénégal. L’agron.
Trop., 31(1): 329-351
15
Poulain et Mara, 1965. Comparaison de l’action de différents engrais phosphatés
utilisables au Sénégal. Colloque sur la conservation et l’amélioration de la fertilité des
sols. Khartoum, 1965.
Ndiaye, J.P.,1978. (to be completed)
Roche et al., 1978. La carence en phosphore des sols intertropicaux et ses méthodes
d’appréciation. Sciencedu Sol, Bulletin de l’Association Française de l’Etude du Sol
4 :251-268
16
Annex 2 : Corn nitrogen use efficiency as affected by soil P, Ca or manure
amendment for experimental plots. Nioro du Rip Research station. 1999
Replicate Treatment
Grain
N total
Yield
%
kg / ha
613
1,96
1212
1,68
2637
1,61
1975
1,96
N yield
N / kg
12,01
20,36
42,45
38,71
% excess % Ndff N fert. NUE
uptake Grain (G)
plante
Kg/ha
%
4,2
0,731
73,9
8,9
6,9
0,711
71,9
14,6
0,692
70,0
29,7
14,0
12,5
0,675
68,3
26,4
Straw
yield
kg / ha
1199
2175
3462
2062
R1
R2
R3
R4
T1
T1
T1
T1
R1
R2
R3
R4
T2
T2
T2
T2
1262
1412
4325
3850
1,89
1,96
1,4
1,89
23,85
27,67
60,55
72,76
0,694
0,751
0,647
0,625
70,2
75,9
65,4
63,2
16,7
21,0
39,6
46,0
7,9
9,9
18,7
21,7
2288
2400
5349
6100
R1
R2
R3
R4
T3
T3
T3
T3
4100
3000
2787
3185
1,96
1,82
1,89
1,89
80,36
54,6
52,67
60,19
0,649
0,656
0,545
0,666
65,6
66,3
55,1
67,3
52,7
36,2
29,0
40,5
24,9
17,1
13,7
19,1
4587
4212
6850
3900
R1
R2
R3
R4
T4
T4
T4
T4
2962
2475
5237
3450
1,54
1,68
1,82
1,89
45,61
41,58
95,31
65,2
0,672
0,672
0,597
0,684
67,9
67,9
60,4
69,2
31,0
28,3
57,5
45,1
14,6
13,3
27,1
21,3
4650
3362
7500
3762
17
II. Phosphogypsum Efficiency to Correct Soil P
Deficiency and/or Soil Acidity
REPUBLIQUE DU SENEGAL
MINISTERE DE L'AGRICULTURE
INSTITUT SENEGALAIS
DE RECHERCHES AGRICOLES
-------
Phosphogypsum efficiency to correct soil P deficiency and/or soil acidity
M. SENE and A. NIANE-BADIANE
ISRA, Sénégal
Collaborating team
P.O. DIEYE, M. DIOP, S. FAYE, A.GUEYE, et P.S.SARR
May 2001
18
Abstract
To boost the agricultural production, the nationwide four-year program started in
1997 utilizes phosphogypsum (PG) and Taïba phosphate rock (PR) mix as soil P
amendment source to correct soil P deficiency and/or acidity. However, there is no
previous study proving the efficiency of PG correct this soil deficiencies. A long
term experiment has been installed in 1997 at Nioro agricultural research station to
study the efficiency of PG as compared with PR and lime to increase crop yields for
the corn/peanut rotation and improve soil P and pH status. For the degraded soil
fertility site selected, a randomized complete bloc design composed of 8 treatments
and 4 replications is used. The amendments applied once every 4 years were based on
recommended rate for Taïba phosphate rock (PR), which is 400 kg/ha. For the
phosphogypsum (PG) amendment, the application rate was derived from the P content
obtained after chemical analysis of the fertilizer material. The treatments were :
control with no P or Ca added (T0), P added as TSP at the rate of 30 kg/ha P2O5 (T1),
0% PR + 100 % PG (T2), 25% PR + 75% PG (T3), 50% PR + 50% PG (T4), 75% PR
+ 25% PG (T5), 100% PR + 0%PG (T6) and 400 kg/ha lime or 100% lime (T7)
For soil fertility monitoring purposes, samples were collected from each experimental
plot after corn harvest in 1997 and 1999. The analyses performed on those samples
included pH, total carbon, total nitrogen, base elements, CEC and aluminum. The
treatment effect on corn or peanut yields is analyses on a yearly basis. In 1999, after 2
years of experimentation, N use efficiency be corn using 15-N labeled fertilize has
been evaluated in 4 treatments.
In 1997, no significant direct effect was observed on corn yield components. Corn
grain yield ranging from 750 to 1100 kg/ha was low as compared to 4000 kg/ha which
is the potential yield of the used variety. This was partially explained by rainfall
shortage. Nevertheless, the analysis of soil samples taken after harvest showed a low
soil fertility status, especially a low pH, but also an important Ca movement within
the soil profile. In 1998, residual effect of treatments is significant only on pod yield.
The positive effect is more important for the lime treatment. As compared to the
control, all the treatments (except T5) improved significantly the pod yield. This
confirm the positive effect of Ca application on the pod filling. The foliar analysis
has not shown any significant treatment effect, despite the plant chorosis observed. In
1999, in a context of favorable rain conditions, low corn yields comparable with those
obtained in1997 were recorded. No significant treatment effect is obtained on corn
yields. The analyses of soil samples have still shown evidence of poor soil fertility
conditions for most treatments under study, especially the soil pH which remained
low (pH<5.5). Accordingly, regardless the treatment, the corn N use efficiency was
kept low.
19
INTRODUCTION
The problem of soil deficiencies in the peanut basin of Senegal is well documented
(Bouyer, 1971 ; Roche et al., 1978). In order to correct this constraint to food crop
production, many research activities have been conducted using the local phosphate
rock (PR) mines. This allowed to determine the optimum PR soil amendment rates for
the rainfed zone (Poulain et Mara, 1965). Once every four years, the recommended
PR rate initially set at 500 kg/ha was reduced to 400 kg/ha for stimulate farmers
adoption during the first national agricultural programme which ended in 1980
(source). Acidity is also a main constraint for this soils with low buffer capacity. A
survey conducted in the mid-seventies indicated that 20% of cultivated land (or 400
000 ha) had reached a very low soil pH that needed lime amendment (Piéri, 1976).
Lime applications were recommended for this programme at a rate of 400 kg/ha.
For a number of reasons, among which the severe drought that occurred, this
programme did not reach the assigned objective, i.e., to achieve food production self
sufficiency based on a sound soil fertility management.
As a second attempt to boost the agricultural production, an other national agricultural
programme was initiated in 1997. To enhance or regenerate soil fertility, soil P
amendment was one of the main recommendations. However, for this purpose,. the
newly processed P-source amendment material bags distributed to farmers consists of
combination by weight of 50 % rock phosphate (RP) and 50 % phosphogypsum
(PG).The latter is a by-product of the phosphoric acid produced from the PR mines.
But, there is no previous study that prove the efficiency of PG to correct soil P
deficiency for crop uses, or to reduce eventually soil acidity given the large Ca
content.
The general objective of this experiment was to study the efficiency of PG as
compared with phosphate rock and lime.
The specific objectives are :
• Evaluate the effect of PG or PG and PR mix on crops Ca uptake ;
• Evaluate the effect of PG or PG and PR mix on nitrogen use efficiency by corn.
20
MATERIALS AND METHODS
Site selection
This long term experiment started in 1997 is installed in Nioro at the ISRA agronomic
research station in a ferruginous leached soil. For the purpose of this study, a highly
chemically degraded soil site (pH < 5.5 and available P < 30 ppm) was selected. This
site happened to be one of the most degraded sites selected from the soil fertility
assessment undertaken throughout the research station five years ago (Agetip,1995).
The 20 m by 20 m grid sampling used allowed a spatial variability analysis of the
various plots within the station.
Cropping system
The Nioro area is still a reliable zone, as far as the rainfall is concerned. Peanut
(variety 73-33 with a cycle length of 105 days ) as a cash crop and corn (variety
Synthetic C in 1997 and Across Pool in 1999 ; 90 days for cycle length) which is very
sensitive to soil fertility are the chosen crops. The corn/peanut rotation used has
started in 1997 with the corn. The potential yield is 4 t/ha for corn ,and 2,5 t/ha for
peanut.
Experimental design
The experiment was conducted as a randomized complete block design with 8
treatments and 4 repetitions. The size for each of the 32 plots is 84 m2 (15 m x 5.6 m).
The number of rows to be sown each year depend on the crop : 7 rows for corn sown
at a spacing of 0,8 m, and 11 rows for peanut sown at a spacing of 0.50 m.
The treatments under comparison consist of combined use of phosphogypsum and
rock phosphate as indicated (Table 1) ; the lime treatment acting as a reference for
soil acidity control.
21
Table1 : Treatments under comparison for the phosphogypsum efficiency experiment
at Nioro du Rip Research Station.
Treatments
T0
T1.
T2.
T3
T4.
T5.
T6
T7
Specifications
Check : No P added, only N and K
0 % P from rock phosphate (RP) + 0 % P from phosphogypsum
(PG) + 30 kg/ha P2O5 from triple superphosphate (TSP)
0 % P from PR + 100 % P from PG
25 % P from PR + 75 % P from PG
50 % P from PR + 50 % P from PG
75 % P from PR + 25 % P from PG
100 % P from PR + 0 % P from PG
100 % lime
Rate of application is different for the two P sources : 100 % P from rock phosphate
corresponds with an application rate of 400 kg/ha of RP, and 100 % P from PG
defines an application rate of 1000 kg/ha of PG. For treatment 8, 100 % Ca from CaO
refers to a
lime application of 400 kg/ha. The chemical analysis data of these
fertilizers are presented (Table 2). These rates will be applied once every 4 years after
2 complete rotation.
The chemical characteristics of the different P- and Ca-Source amendment used for
this experiment are presented (Table 2).
Table 2 : Chemical characteristics of phosphogypsum (PG), Taïba phosphate rock
(PR), and PG + PR mix
Composition
Ca (%)
Mg (%)
K (%)
Na (%)
Fe (%)
Al (%)
P (%)
S (%)
Mn (mg/kg)
Cu (mg/kg)
Zn (mg/kg)
PG
18
< 0.01
< 0.03
0.05
0.10
0.1
0.4
14.3
2.3
3.5
<1
PR
27
0.04
0.02
0.07
1.1
0.5
16.2
404
72
522
50% PG + 50% PN
22.5
0.02
0.4
0.06
0.3
0.4
8.4
6.8
108
27.7
290
22
It shows that very little P is added (<1%) when PG is used as an amendment, as
compared with the phosphate rock which contains 16.2% P.
Mixing equal amount of the two compounds leads a half dilution of P content. But
regardless the source, a fair amount of Ca is added upon amendment. However, one of
the hypotheses would be that Ca contained in PG would be more readily available to
crop than that contained in PR due to acidic attack when processing phosphoric acid
from PR.
The time schedule for agronomic measures during the years 1997 to 1999 is presented
(Table 3). When applying the phosphocalcic amendment, plowing was used to mix
the fertilizer in the top 20 cm soil layer.
Table 3 : Field operations and dates of implementation for corn and peanut during the
3 last growing seasons at Nioro du Rip station.
Operations
RP, PG or Lime application
Oxen driven plowing
Sowing
N-K application
Thinning
Pre-emergence weeding
1st weeding
1st urea pplication
2nd urea application
2nd weeding
Bedding
Harvest
Dates of implementation
Corn (1997) Peanut
(1998)
07/02/1997 07/03/1997 07/10/1997 06/21/1998
07/02/1997 07/27/1998
08/05/1997 07/27/1998
08/05/1997 08/10/1998
08/05/1997 08/25/1997 08/25/1997 08/21/1998
09/01/1997 10/31/1997 11/06/1998
Corn (1999) Peanut 2000
07/02/1999
06/16/1999
07/11/1999
07/22/1999
07/27/1999
07/19/1999
08/06/1999
08/23/1999
08/23/1999
10/11/1999
07/01/2000
07/15/2000
07/15/2000
07/16/2000
08/01/2000
08/29
10/20
Nitrogen and Potassium were applied on each plot at the following rates :
•
12 kg/ha of N at sowing, 22 kg/ha Nat the first and at the second urea application
for corn ; 12 kg/ha of N at sowing for peanut
•
40 kg/ha of K at sowing for both peanut and corn.
Soil fertility monitoring
23
After the initial soil physical and chemical characterization mentioned above, soil
samples have been taken after corn harvest in 1997 and 1999. The parameters of
interest down to 0,6 m depth were mostly soil pH, P and Ca contents, and base
saturation. For the first time in 1997, 108 samples have taken according the following
scheme :
•
For treatment T0, T2 and T6, all the plot s are sampled at 4 depths (0-10, 10-20,
20-40, and 40-60 cm. The treatments have been chosen to allow a analysis of Ca
movement in the profile ;
•
For the treatment, all the plots have been sampled at only 3 depths (0-10, 10-20,
20-40 cm)
For the second time in 1999, 64 samples at 4 depths (0-10, 10-20, 20-40 and 40-60
cm) were taken from four treatments (T0, T2, T4 and T7)
Nitrogen use efficiency by corn for selected treatments in 1999
For this study, four (4) treatments (T0, T2, T4, T7) and 4 replicates were selected for
15-
N applications. For each replicate, a micro-plot (2.4 m x 1.5 m) was marked before
corn planting for 15-N applications. The first application at planting consisted of 12 kg
N ha-1 as urea (1%
15-
N atom excess ). Then two additional applications at 27 and 41
days after sowing (DAS), respectively were made. The amount applied were 100 kg N
ha-1
as urea (1%
15-
N atom excess). At harvest, grain and stover yields were
determined. Grain and straw samples, but also soil samples at 0-20 and 20-40 cm
were taken for 15-N analysis.
Rainfall conditions
Rainfall characteristics for the last three year are presented (Table 4 ). Annual rainfall
in 1999 and 2000 were similar and were 51% greater than that of the 2 previous
years. During the 1999 and 2000 rainy season, more than 10 rainfall events exceeded
30 mm. Drought constraint was important during 1997 rainy season only. Water
requirements were obtained in 1998 and 2000 for peanut and in 1999 for corn. For the
latter rainy season, water losses due to drainage, and to a lesser to runoff are
important.
Table 4: Rainfall characteristics during the past three years at Nioro du Rip Station
1997
1998
1999
2000
Annual rainfall (mm)
617,1
682.1
978.8
977,8
24
Number of events
Beginning date of rainy season
Ending date of rainy season
59
5/06/97
12/10/1997
46
21/07/98
12/10/1998
75
27/06/1999
17/10/1999
56
13/06/2001
18/10/2001
RESULT AND DISCUSSION
Changes in soil chemical characteristics
1/ Soil acidity
After four years, The site acidity problem has not improved for any treatment; except
after lime application where a 0.2 to 0.3 soil pH increase is observed (Table 5). For
most cases, soil pH values are below the threshold below which aluminum toxicity is
observed.
Compared to the control, the base saturation increase observed for the other
treatments. This can be related to the Ca content increase in the profile
Table 5: Soil acidity changes three years after P and Ca amendments. Nioro du Rip
Station.
pH water
T0
T2
T4
T7
0-10
10-20
20-40
40-60
0-10
10-20
20-40
40-60
0-10
10-20
20-40
40-60
0-10
10-20
20-40
40-60
1997
5.4
5.3
5.1
5.4
5.4
5.3
5.2
5.1
5.1
5.6
5.8
5.2
5.9
1999
5.2
5.2
5.2
5.6
5.4
5.4
5.5
5.9
5.4
5.4
5.3
5.5
5.5
5.4
5.2
5.6
pH kcl
1997
4,5
4,3
4,3
4,3
4,7
4,3
4,2
4,4
4,5
4,4
4,2
4,7
4,9
4,3
Ca exch.
(mg/kg)
1999
4,3
4,2
4,2
4,6
4,5
4,4
4,4
4,5
4,4
4,4
4,3
4,5
4,5
4,4
4,2
4,6
1997
0,2
0,3
0,7
1,1
0,5
0,3
0,6
0,9
0.4
0.6
0.4
0.5
0.5
Base saturation
(%)
1999
0,2
0,3
0,5
1,1
0,4
0,5
0,9
1,4
0,3
0,4
0,7
1
0,3
0,4
0,6
-
1997
53
47
66
80
84
84
71
81
70
74
73
100
97
76
-
1999
58
60
68
88
74
73
82
96
77
81
81
93
67
71
67
88
Over the 3 years period, while the organic matter content have not changed much over
the 3 years period, N content tended to decrease (Table 6).Mean available P content
value decreasing with depth (from 21 mg/kg P for 0-0.1 m depth to 3.0 mg/kg P for
25
0.4-0.6 m depth) is not affected by the soil P amendment three years after the
application. Soils are sampled in 2001 and soils analysis are undergoing.
Table 6: Changes in organic matter, Nitrogen and Phosphorus soil contents after 3
years of P and Ca amendments. Nioro du Rip Station.
Treatment
T0
T2
T4
T7
Horizon (cm)
0-10
10-20
20-40
40-60
0-10
10-20
20-40
40-60
0-10
10-20
20-40
40-60
0-10
10-20
20-40
40-60
Organic Matter
(g/kg)
1997
1999
2,9
3,6
3,1
3,3
3
3,1
2,7
2,4
2,9
3,9
2,7
3,6
2,6
3
2,3
2,6
2,8
3,5
2,7
3,4
2,1
2,9
2,5
2,3
3,4
2,6
3,4
2,3
2,8
2,6
1997
0,29
0,23
0,24
0,27
0,23
0,20
0,24
0,23
0,26
0,25
0,25
0,35
0,28
0,25
-
Ntotal
(g/kg)
1999
0,16
0,14
0,15
0,15
0,17
0,15
0,16
0,16
0,15
0,14
0,16
0,15
0,15
0,15
0,14
0,15
P Olsen-Dabin
(mg/kg)
1997
1999
21,4
7,3
6,9
3,7
20,2
9,6
6,6
2,4
23,4
9,4
6,9
2,4
23,8
8,2
2
2,4
Effect of P and Ca amendments on crop yields
The effect of the treatments under comparison on crop yields (corn in 1997 and 1999,
and peanut in 1998 and 2000) are presented in Table 7.
As for the 1997 rainy season, a large variability of plot yields of corn has been
obtained in 1999, as indicated par the coefficient of variation values (Table 7).
Furthermore, yield values which are not affected by the amendments have remained
low regardless the treatment and despite the good rainy conditions. As a matter of
fact, the yield observed (< 2000 kg/ha) are well below the potential yield (about 4000
kg/ha) of corn variety used.
In 1998 and 2000, the peanut yield variability was low compared to corn yield
variability observed in 1997 and 1999. Furthermore, the treatment effect was
26
significant on pod yield in 1998 and on hay yield in 2000. The overall peanut yield
obtained in 1998 is higher compared to the 2000 experiment. The 2000 pod yield is
very low (the mean average is 1005 kg/ha compared to 1530 kg/ha, the mean average
of 1998 study). The positive effected of treatment observed in Pod and Hay may be
closely related to exchangeable Ca content of the soil. This confirmed the importance
of Ca in peanut pod filling and growth increase at plant maturation.
Observations, especially for corn yield performances in 1997 and 1999, indicated the
still low soil fertility status of the experimental site.
Table 7 : Effects of P and Ca amendment on corn yields in 1997 and 1999 and peanut
in 1998 and 2000 at Nioro du Rip Station.
T0
Corn 1997
Straw
Grain
(kg/ha) (kg/ha)
930
737
Peanut 1998
hay
Pod
(kg/ha) (kg/ha)
2580
1354 d
Corn 1999
Straw
Grain
(kg/ha) (kg/ha)
1501
1640
Peanut 2000
Hay
Pod
(kg/ha)
(kg/ha)
3197 a
1022
T1
1554
1072
2670
1480 c
1600
1390
3010 ab
850
T2
1310
1110
2410
1590 b
1850
1820
2583 abcd
1048
T3
1240
970
2670
1590 b
1590
1560
2126 cd
883
T4
1000
740
2900
1630 ab 1670
1500
1980 d
849
T5
910
780
2370
1390 d
1740
1680
2512 bcd
1044
T6
990
750
2800
1540 bc 1580
1540
2640 abc
1100
T7
984
750
2720
1660 a
1330
1370
2417 bcd
1249
Mean
1150
880
2640
1530
1610
1560
2558
1005
Signification
level.
CV %
NS
NS
S
NS
NS
S
NS
7.8
26,0
25,6
17,2
26,9
Treatment.
III.
S
25,2
44,2
16.6
Corn Nitrogen use efficiency in 1999
For the selected treatments, results obtained for nitrogen use efficiency are presented
for corn grain, straw and total dry matter yields. (Table 8). The yield difference
between Table 7 and Table 8 can be related to larger N-urea rate applied in the
15-
N
microplots. There is no significant treatment effect on grain, straw or whole plant
nitrogen use efficiency. Corn NUE values are low and vary between 6 and 12.5 % for
grain, 4.6 and 8.2% for straw. The maximum corn total dry matter NUE value
27
reached is about 20%. The low value observed for a degraded soil fertility site
corroborate results obtained in different studies (Ganry, 1990, Badiane et al., 1995).
Table 8: Corn Nitrogen use efficiency as affected by P and Ca source amendments.
Nioro du Rip Research Station. 1999
Straw (S)
T0
T2
T4
T7
Grain (G)
G+S
Yield
Tot. N N fert.
NUE
Yield
Tot. N N fert.
NUE
Total
kg/ha
uptake
Uptake
(%)
kg/ha
uptake
Uptake
(%)
NUE
(%)
kg/ha
(%)
kg/ha
1880
1.75
24.2
11.4
2300
1.05
17.8
8.4
19.8
1320*
0.22
14.6
6.9
820
0.18
3.7
1.8
8.6
2130
1,70
26.5
12.5
2720
0.75
15.3
7.2
19.7
1360
0.18
15.7
7.4
1410
0.03
8.5
4.0
11.4
1760
1.70
22.8
10.7
2260
0.84
14.0
6.6
17.3
990
0.09
12.7
6.0
750
0.13
4.7
2.2
8.1
1000
1.70
12.8
6.0
1580
0.85
9.8
4.6
10.7
500
0.04
5.8
2.8
440
0.11
3.5
1.6
3.7
(%)
* Numbers in italic character designate the standard deviation
Conclusion
Four years after this experiment has been conducted, soil P and Ca amendments
applied, particularly the 50 % PG and 50 %PR and lime treatments at recommended
rate have not noticeably improved soil fertility status over the control treatment,
except for Ca content which increased. As the result, low corn yields were obtained in
1997 and 1999 and low yield on peanut in 2000. Significant treatment observed on
peanut pod yield in 1998 but not in 2000 ; only hay yield have significant treatment.
This response may be attributed to the positive effect of Ca uptake which resulted
from improved Ca content of the top layer. In 1999, corn NUE values obtained were
low and were not at all positively affected by P and Ca source amendment. The NUE
mean value for 50% PG and 50%PR treatment for total dry matter was in particular
28
low (17.3 %). We must stress also that for 50% PG and 50% PR treatment peanut
yield was too low compared to others treatments in 2000. More striking is the mean
NUE value for the lime treatment, which was the lowest (only 10.7%). It is suspected
that the P and Ca source amendment were added at low rate. Further amendment rates
will be made the years to come.
IMPACTS AND SUCCESSES OF THIS PROGRAM
Over the fours years, the Intercrsp programme has strengthened the links between the
West and East group and within these groups.
•
Important exchanges knowledge between various NARS “mainly south-south
collaboration “;
•
•
Use of important simulation models for and soil water and nutrient management:
•
Kineros model for soil and water management,
•
NUMASS for nutrient management,
•
DSSAT for decision tool for nutrient management.
Technology transfer between countries (Use of phosphogypsum, courbe de
niveaux);
•
Technical and financial support of scientists working with NRM pole headed by
INSAH.
•
The Intercrsp project has allowed the contribution of the Joint FAO/IAEA
Division to provide nutrients probes, N15 fertilizers and isotope analysis.
ACKNOWLEDGMENTS
Financial support was provided by the INTERCRSP/United States Agency for
International Development (USAID) and the Joint FAO/IAEA Division in Vienna,
Austria. Technical assistance CIRAD/Soltrop Laboratory is greatly appreciated.
REFERENCES
Bouyer, 1971 : Etudes sur la fertilisation phosphatée des sols en Afrique Tropicale et
Madagascar. Phosphore et Agriculture 57 :1-12
Roche et al., 1978. La carence en phosphore des sols intertropicaux et ses méthodes
d’appréciation. Sciencedu Sol, Bulletin de l’Association Française de l’Etude du Sol
4 :251-268
29
Poulain et Mara, 1965. Comparaison de l’action de différents engrais phosphatés
utilisables au Sénégal. Colloque sur la conservation et l’amélioration de la fertilité des
sols. Khartoum, 1965.
Ganry, F.1990. Application de la méthode à l’étude des bilans azotés en zone
tropicale sèche. Thèse de doctorat d’état, Université de Nancy I, France, 255 p.
Aminata N. Badiane, Modou SENE, Saliou Faye, Jean C. MANGA. 1999. Increasing
food security in sub-saharan Africa monitoring of nutrients in two valleys of lower
Casamance Senegal. 1995-1998 Report. Regional Project RAF/5/036. ISRA-CNRA,
9 p + annexe.
Piéri, C., 1976. L’acidification des terres de cultures exondés au Sénégal. L’agron.
Trop., 31(1): 329-351
Agetip,1995. Programme de rehabilitation des stations ISRA. Informations
pédologiques et étude cartographique des sols de la station de Nioro
T0
T0
T0
T0
T2
T2
T2
T2
T4
T4
T4
T4
T7
T7
T7
T7
R1
R2
R3
R4
R1
R2
R3
R4
R1
R2
R3
R4
R1
R2
R3
R4
487
1675
850
1000
3050
637
1637
1725
600
1625
3800
2500
725
2065
3662
1062
Treat. yield
kg / ha
Rep
1,75
1,68
1,68
1,68
1,61
1,68
1,68
1,82
1,96
1,61
1,54
1,68
N total
content
%
1,96
1,68
1,47
1,88
8,52
28,14
14,28
16,8
49,1
10,7
27,5
31,39
11,76
26,16
58,52
42
Total N
Uptake
kg/ha
14,21
34,69
53,83
19,96
GRAIN
0,793
0,729
0,753
0,745
0,789
0,789
0,714
0,739
0,826
0,73
0,783
0,716
%N
Excess
%
0,802
0,757
0,806
0,733
80,2
73,7
76,1
75,3
79,8
79,8
72,2
74,7
83,5
73,8
79,2
72,4
6,8
20,7
10,9
12,7
39,2
8,5
19,9
23,5
9,8
19,3
46,3
30,4
% Ndff N fert.
uptake
kg/ha
81,1
11,5
76,5
26,6
81,5
43,9
74,1
14,8
3,2
9,8
5,1
6,0
18,5
4,0
9,4
11,1
4,6
9,1
21,9
14,3
5,4
12,5
20,7
7,0
NUE
%
1700
2149
1212
1275
3324
1588
2163
1975
1274
1912
4424
3262
1499
2610
3300
1792
yield
kg / ha
1,01
0,8
0,8
0,77
0,77
0,77
1,03
0,8
0,7
0,77
0,77
0,77
1,29
1,01
0,87
1,01
N total
%
17,17
17,19
9,7
9,82
25,59
12,22
22,28
15,8
8,92
14,72
34,06
25,11
Total N
Uptake
kg/ha
19,33
26,36
28,71
18,09
STRAW
0,746
0,728
0,718
0,654
0,769
0,743
0,69
0,709
0,72
0,746
0,757
0,685
0,804
0,717
0,774
0,769
%N
Excess
75,4
73,6
72,6
66,1
77,8
75,1
69,8
71,7
72,8
75,4
76,5
69,3
81,3
72,5
78,3
77,8
13,0
12,7
7,0
6,5
19,9
9,2
15,5
11,3
6,5
11,1
26,1
17,4
15,7
19,1
22,5
14,1
% Ndff N fert.
uptake
Annex: Corn Nitrogen use efficiency as influenced by P, and Ca soil amendment at Nioro du Rip Research Station in 1999.
6,1
6,0
3,3
3,1
9,4
4,3
7,3
5,3
3,1
5,2
12,3
8,2
7,4
9,0
10,6
6,6
NUE
%
9,3
15,8
8,4
9,1
27,9
8,3
16,7
16,4
7,7
14,3
34,2
22,5
Total
NUE
%
12,8
21,5
31,3
13,6
III. Impact of Piliostigma reticulatum on the Quality of
Soils and on Agricultural Output in the CentralSouth Zone of Sénégal
REPUBLIQUE DU SENEGAL
MINISTERE DE L'AGRICULTURE
INSTITUT SENEGALAIS
DE RECHERCHES AGRICOLES
-------
Impact du Piliostigma reticulatum sur la qualité des sols et
sur les rendements des cultures de la zone Centre-Sud du
Sénégal
Impact of Piliostigma reticulatum on soil quality and yields of crops in the Central-South zone of
Senegal
M. SENE and A. NIANE-BADIANE
ISRA, Sénégal
Collaborating team
P.O. DIEYE, M. DIOP, S. FAYE, A.GUEYE, et P.S.SARR
May 2001
32
English Synopsis
Drought and cultural practices have combined to reduced parkland agriculture and increase
soil mining in the region. The consequent loss of soil fertility is a major factor in the
continuing decline of agricultural productivity and farmer incomes. Lack of available crop
residues and manure to incorporate into soils suggests that better management of parklands
may be a solution to the problem of nutrient recycling. This reports the results of continuing
multi-year research into the use of Piliostigma reticulatum to enrich parkland soils and
improve crop yields. Only P. reticulatum treatments used in combination with chemical
fertilizers produced significant yield results in peanut the first year. A chemical fertilizer
control treatment will be added to the design to determine the initial impact of only P.
reticulatum. Higher doses of P. reticulatum (>1t/ha) may provide greater initial impacts and
the impact of lower doses may become more apparent in the longer term.
Introduction
Le Bassin Arachidier occupe 1/3 du territoire national et assure 75% de la production du
Sénégal en arachide et mil. Toutefois, l’utilisation intensive et continue des terres dont le
corollaire est la disparition progressive de la jachère a entraîné une dépréciation de leur
fertilité.
La dégradation générale des sols traduit l’action combinée de la sécheresse et des pratiques
culturales inappropriées qui ont notamment favorisé la diminution de la densité des parcs
agroforestiers traditionnels. En effet, la mise en valeur des terres ont révélé des exportations
continues non compensées d’éléments tels que l’azote et le phosphore du sol, mais aussi une
baisse graduelle du taux de matière organique. L’appauvrissement provoqué des sols est un
facteur majeur qui explique la baisse tendancielle des productions agricoles et donc des
revenus des paysans.
En conséquence, il y a lieu de reconfigurer les systèmes de production en vue d’inverser le
flux d’éléments nutritifs dans les sols de culture. Compte tenu de la non disponibilité des
résidus de cultures et des faibles quantités de fumier produites au niveau des exploitations, le
recours à une meilleure gestion du parc agroforestier permet d’assurer un meilleur recyclage
des éléments nutritifs pour une amélioration de la qualité des sols. Dans le sud du bassin
arachidier, le parc arbre/arbuste constitué par Cordyla pinnata et Piliostigma reticulatum peut
ajouter un rôle majeur dans cette perspective.
P. reticulatum, appartenant à la famille des Césalpiniacées, est une légumineuse endophytique
non fixatrice d’azote. Dans la zone, et notamment sur les sols sableux, cette espèce se
retrouve très souvent à l’état d’arbuste et accidentellement à l’état d’arbre. En fin de saison
sèche, l’arbuste atteint une hauteur de 0,9 m avec un diamètre de houppier variant entre 1 et
1,75 m. La densité moyenne P. reticulatum est estimée à 317 arbustes par ha, chaque arbuste
donnant une biomasse aérienne de 1,3 kg (Diack, et al., 1998). La biomasse produite est
fortement influencée par le précédent cultural. En effet, sur le précédent arachide, P.
reticulatum semble mieux se développer que sur le précédent mil (Diack, 1998). La
variabilité de la densité est étroitement liée au type de sols.
Les résultats d’enquête (Diack, et al 1998) indiquent qu’après C. pinnata et A. albida, P.
reticulatum est l’espèce arbustive que les paysans préfèrent conserver dans leur champs de
culture en raison de son rôle dans l’amélioration de la fertilité du sol et la lutte contre
l’érosion hydrique. La gestion de ce parc par les paysans est étroitement liée aux opérations
culturales dont la mécanisation a pour effet général de réduire la densité de peuplement.
33
Après une première coupe et un brûlage de la biomasse de l’arbuste en mai-juin, les rejets
successifs au cours de la saison des pluies sont coupés 2 ou 3 fois (en fonction du nombre de
sarclages réalisés et donc de la culture en place). Sous l’angle de l’amélioration de la qualité
des sols, les axes à investir en collaboration avec les paysans pour une bonne gestion à
l’échelle du champ du parc à P. reticulatum sont les suivants :
• augmentation de la densité de peuplement de l’espèce à l’échelle du champ ou du terroir.
Il s’agit d’une part d’augmenter le rendement biomasse pour arriver à des doses d’apport
de matière sèche plus importante, et d’améliorer l’efficacité du recyclage des éléments,
notamment une augmentation de la remontée en surface par les racines des bases
entraînées en profondeur par les pertes d’eau par percolation d’autre part. En fonction du
type de sols, il serait bon de retenir une densité optimale à atteindre ;
• meilleure valorisation de l’eau pluviale stockée dans le profil : L’amélioration de
l’infiltration de l’eau pluviale qui traduit en partie le rôle positif que joue P. reticulatum
sur la lutte anti-érosive est nécessaire en vue satisfaire les besoins en eau de la culture en
début de cycle. De plus les stocks d’eau résiduels après la récolte sont utiles pour
l’alimentation en eau de l’arbuste au cours de la saison sèche. Le rôle de P. reticulatum sur
le fonctionnement hydrique du profil doit être préciser.
Par ailleurs, l’étude de la décomposition de la biomasse de P. reticulatum a montré une perte
de masse plus importante au champ (in situ) qu’en conditions contrôlées de laboratoire (Diack
et al, 1998). Ce résultat suggère l’importance de la microflore dans les conditions naturelles
de décomposition.
L’objectif de l’essai est de déterminer l’influence des apports de P. reticulatum sur les
propriétés physiques, chimiques et biologiques des sols et sur les rendements des cultures.
Matériels et Méthodes
L’essai est implanté en milieu paysan dans le terroir villageois de Diamaguène (près de la
station agronomique de Nioro du Rip). Sur le plan pédologique, le site est dans la classe des
sols ferrugineux tropicaux lessivés bien drainés appelé localement Deck-Dior compte tenu de
la teneur superficielle de surface. Le pH eau moyen du sol de 5,7 indique que l’acidité n’est
pas une contrainte majeure pour ce site par opposition à ce qui est observé au niveau d’un
grand nombre de parcelles dans la station expérimentale. Par contre les faibles taux de matière
organique et d’azote, respectivement de 0,47% et 0.045%, sont indicateurs d’un niveau de
dégradation de la fertilité du sol.
La biomasse de P. reticulatum récoltée sur le site au débroussaillage, lors des sarclages et à la
récolte est utilisée pour l’apport de matière organique verte sous forme de paillage légèrement
enfoui à la houe.
L’essai initié en 1998 en rotation arachide/mil, n’a finalement fait l’objet d’une mise en
oeuvre complète qu’en année 2000. L’arachide (variété 73-33) a été semée.
Traitements :
La dose apportée de 1 t/ha de biomasse anhydre de P. reticulatum est calée sur la quantité
pouvant être mise à disposition sur le site compte tenu de la densité de peuplement
déterminée. Six traitements sont mis en comparaison.
34
•
•
•
•
•
•
T0 : témoin (sans apport de fumure minérale ou organique)
T1 : épandage en surface entre les ligne de repousses de P. reticulatum ; dose : 1 t/ha de
biomasse anhydre ;
T2 : T1 + fumure recommandée selon la culture
T3 : T2 + épandage de en surface de P. reticulatum recépé à la récolte (1 t/ha biomasse
anhydre)
T4 : épandage en surface après débroussaillage de P. reticulatum (1 t/ha de biomasse
anhydre)
T5 : épandage en surface après débroussaillage de P. reticulatum (1 t/ha de biomasse
anhydre (T4) + fumure minérale recommandée selon la culture
Dispositif en blocs complètement randomisés : (6 traitements, 4 répétitions, parcelle
élémentaire de 45 m2)
Les observations et mesures telles que initialement retenues portent sur le sol, les plantes
(diagnostic foliaire sur mil et arachide):
Sur le sol :
- suivi dynamique du C du sol , pH , Ca, etc. Les prélèvements d’échantillons de
sols sont faits en début et en fin de saison des pluies ;
- biomasse microbienne et activité biologique ;
- comportement hydrique, densité apparente et stabilité structurale
Plante :
-
Analyse foliaire (N, P, Ca, Mg, S)
Rendement et composantes
Au cours de la campagne, les prélèvements d’échantillons de sols ont été réalisés en
août pour l’analyse de l’activité biologique et en fin de saison sèche en mai 2001 lors
du débroussaillage. Par contre, il n’y a pas eu de d’analyse foliaire. Les données
disponibles portent sur la conduite de la culture de l’arachide (densité à la levée et à la
récolte, rendement en gousses et en paille).
L’analyse de ces données agronomique est fait par la méthode de l’analyse de la
variance.
Calendrier cultural
Le tableau 1 présente le calendrier cultural mis en oeuvre au cours de la saison des
pluies 2000.
35
Tableau 1 : Calendrier des opérations effectuées sur l’essai Piliostigma à Diamaguène
Opération réalisée
Date de l’opération
09/06/00
10/06/00
13/7/00
13/7/00
28/07/00
28/07/00
20/08/00
20/10/00
03/11/00
Piquetage / Nettoyage
Epandage biomasse Piliostigma reticulatum T4,T5
Semis
Epandage Engrais minéral et Radou
Epandage biomasse Piliostigma reticulatum T1, T2 et T3
1er sarclage
2ème sarclage
Récolte
Epandage de P. reticulatum de fin de cycle (T3)
Pluviométrie de la campagne 2000 à Nioro
L’hivernage 2000 est, tout comme la campagne 1999, caractérisée par une bonne
pluviométrie (977,2 mm en 56 épisodes pluvieux). La bonne distribution de la pluie
enregistrée a non seulement permis de satisfaire les besoins en eau de culture, de
l’arachide notamment, mais a favorisé un enherbement important qui a rendu ardus
les travaux de sarclages. En particulier, les importantes pluies de fin de cycle (161,2
mm en 10 pluies en octobre) sont en outre favorable à la croissance et au
développement d’adventices et d’arbustes post-récolte, dont P. reticulatum.
Résultats
Les résultats portant sur la densité de population à la récolte, les rendements en
gousse et en fane sont présentés au tableau ci-dessous.
Tableau 2 : Effet des apports de P reticulatum sur le rendement de l’arachide
Rendement en Rendement en Rapport
Traitement
Densité à la
gousse/fane
fane
gousse
récolte
(kg/ha)
(kg/ha)
(pied/ha)
T0
54380
2800
1020 a
0,36
T1
53020
2460
940 a
0.38
T2
52290
3230
1270 bc
0.39
T3
53650
3540
1280 bc
0.36
T4
53230
2760
1000 a
0.36
T5
56350
3690
1430 c
0 .39
36
A la récolte, la densité observée avec une moyenne générale de 54000 plants/ha est
faible comparée à la densité préconisée. En fait, la densité de l’arachide a été
généralement faible en milieu paysan, en dépit des bonnes conditions pluviométriques
de semis. Ceci repose le problème de la qualité des semences et/ou des équipements
de semis. Il n’a pas été observé d’effet significatif des traitements mis en comparaison
sur ce paramètre.
En ce qui concerne les rendements, il apparaît que le rendement en gousse peut être
considéré comme faible à moyen dans la mesure où il représente environ 1/3 du
rendement en fanes.
en gousses
En moyenne, le rendement de 1150 kg/ha atteint sur le site d’essai est moyen à bon
compte tenu de la densité observé. Les traitements ont un effet significatif positif au
seuil de 5% sur les gousses. Toutefois par rapport au témoin (sans apport) pour lequel
le rendement atteint est de l’ordre de 1000 kg/ha, l’apport d’émondes de P.
reticulatum à la dose de 1 t/ha en l’absence d’engrais (T1 : apport de P. reticulatum au
sarclage et T4 : apport de P. reticulatum au débroussaillage) est sans effet. Seuls les
apports de P. reticulatum combinés avec l’apport de fumure minérale ont eu un effet
positif significatif se traduisant par un surplus de rendement variant entre 25 et 40%.
Pour une première année d’application donc, il semble que l’effet positif observé est
le fait exclusif de l’engrais minéral apporté. Le rendement moyen des fanes est de
1925 kg/ha. Il n’y a pas d’effet significatif des traitements sur les fanes.
Conclusion et perspectives
Pour cette première année d’application, seuls les effets combinés du P. réticulatum et la
fumure minérale vulgarisée ont eu un effet positif sur les rendements. En vue d’isoler les
différents individuels, il serait judicieux d’inclure un traitement fumure minérale seule au
dispositif. Par ailleurs, les doses de P. reticulatum apportées de 1 t/ha sont pour l’essentiel
faibles pour espérer un effet positif sur la qualité du sol. Il est possible de porter cette dose à 2
voire 3 t/ha en augmenter le nombre d’apports. Il est ainsi envisagé de faire un apport
séquentiel, étalé sur l’année. Cela se justifie par le mode de gestion de l’arbuste par les
paysans. Ainsi les apports sont prévus successivement au débroussaillage, au sarclage et en
fin de cycle. Dans ce nouveau schéma, le plan de prélèvement d’échantillon de sol et/ou de
plante sera réactualisé en vue de suivre l’évolution de la fertilité du sol. En perspective, les
traitements sont modifiés de la manière suivante :
T0 : témoin absolu (pas de fumure apportée)
T1 : fumure minérale à dose recommandée P reticulatum sur mil et arachide ;
T2 : P. reticulatum apporté à la dose de 2,5 t/ha au débroussaillage (1,5 t/ha) et au sarclage
(1 t/ha) :
T3 : T1 + T2
T4 : P. reticulatum apporté à la dose de 2,5 t/ha de matière sèche au sarclage (1 t/ha) et au
sarclage (1,5 t/ha) ;
T5 : T1 + T4
37
En outre, compte tenu de l’effet observé du précédent cultural sur la production de
biomasse de P. reticulatum (Diack, 1998) ou de G . senegalensis (Traoré, 1999), une
analyse plus approfondie du parc doit être menée. Dans cette perspective, il s’agira de
bien étudier le fonctionnement hydrique du profil et l’effet éventuel de la fixation
biologique sur l’alimentation en N de l’arbuste.
Pour toute ces raisons, il apparaît que le parc ainsi identifié constitue un axe privilégié
de recherche à investir.
Références :
Diack M., Badiane A. N., Sène M., Diatta M. Dick R. 1998. Cordyla pinnata en association
avec Piliostigma reticulatum : impact sur la régénération des sols dégradés au Sénégal.
Rapport final ISRA/NRBAR. LIR/02. USAID, 25p.
Dack M. 1998. Piliostigma reticulatum dans un parc à Cordyla pinnata : effet sur la
régénération des sols dégradés au Sénégal. ISRA Kaolack, 48p.
Traoré I. 1999. (à compléter)