Agroforestry Systems 21: 147-- 158, 1993. �9 1993 KluwerAcademic Publishers. Printed in the Netherlands.

Alley cropping rice (Oryza sativa var. Barafita) with cassia (Cassia siamea): soil fertility and crop production

A. A. D A N S O 1 and P. M O R G A N * College of Forestry, Wildlife, and Range Sciences, University of Idaho, Moscow, ID 83843, USA; 1 Present address: Forestry Department, No. 5 Marina Parade, Banjul, The Gambia; * Author for correspondence

Key words: crop yield, hedgerow proximity, The Gambia, agroforestry, N, P, K, organic matter

Abstract. Rice grain weight and quality (weight of larger size fractions), soil pH, and N, P, K and organic matter content were studied where rice was alley cropped with cassia on a semi- arid site in The Gambia. The four treatments, control (no cassia prunings or inorganic fertilizer added), only prunings added, prunings plus half the recommended fertilizer rate and prunings plus full recommended rate of fertilizer, were applied in a Latin square design with 10 X 8 m plots, each sub-divided into two 4-m wide alleys. Fertilizer was applied twice; full rates were 93.7 kg/ha NPK (8:24:24) plus 32.4 kg/ha urea followed two weeks later by 100 kg/ha urea applied as side dressing. Soil samples collected before and after cropping at 0--10 cm and 10--15 cm depths and cassia pruning samples were analyzed for pH, N, P, K and organic matter content.

Our results do not show significant benefit of cassia prunings applied as mulch to grain weight or quality in alley cropping rice with cassia. The addition of inorganic fertilizer plus cassia prunings did not increase rice grain and straw weights (p = 0.3447 and p = 0.0691, respectively) compared to the control and prunings only treatments. In all treatments, the outer rows, those within 80 cm of hedgerows, produced significantly less grain (p = 0.0002) and straw than inner rows. Neither the larger grade A nor the smaller grade B grain weights were significantly different (p = 0.6017 and p = 0.0629, respectively) between treatments. Weight of grain, straw, and larger grade A and smaller grade B quality grain did not differ significantly for inner and outer rows (p = 0.6329, p = 0.7148, p = 0.7171 and p = 1.000, respectively).

Introduction

Al ley c ropp ing is p romis ing in Af r i ca [3, 6, 9] as an a l te rna t ive to shifting cul t iva t ion for m o r e divers i f ied and m o r e sus ta inable f o o d - c r o p p r o d u c t i o n [9]. A l l e y c ropping , a re la t ively new agrofores t ry system, is def ined as the p lant ing of c rops in alleys be tween hedge rows of t rees or shrubs which are pe r iod ica l ly p r u n e d to r educe shading and supp ly nut r ients to soil [2, 11, 22]. Al l ey c r o p p i n g has wide app l i ca t ion in bo th h u m i d and sub-humid t ropics [2, 9]. A l l ey c ropp ing has the bes t po ten t i a l whe re use of inorganic fer t i l izer and pes t ic ides is l imi ted and soils a re acid [9, 10]. The sys tem is also useful whe re weeds are pa r t i cu la r ly t r o u b l e s o m e and cost of weed con t ro l is high [22].

In the semi -a r id t ropics , t rees c o m p e t e with c rops for mois tu re and nut r ients [28]. The re la t ive success of di f ferent combina t ions of t rees and c rops g rown with l imited or no inorgan ic fer t i l izer and in di f ferent c l imatic

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conditions is either not well documented or poorly understood for the arid and semi-arid regions of western Africa [1, 6, 28].

This study was designed to investigate the relative contribution of cassia (Cassia siamea) prunings and inorganic fertilizer to yield and quality of rice (Oryza sativa var. Barafita) and to soil N, P, K and organic matter content. Rice yield was measured in terms of weight of grain and straw, and rice quality was measured in terms of weight of the 2 largest size classes of grain. A second objective was to assess the effect of the proximity of hegerows on crop yield and quality. The study was conducted in 1990 in the sub-humid tropics on a semi-arid site in The Gambia.

The Gambia

The Gambia is located in western Africa. Senegal borders the country on three sides with the Atlantic Ocean to the west. The climate is Sudano-sub- Sahelian, characterized by a wet season from June to October with annual rainfall ranging from 760--1400 mm [25, 27] and mean annual temperatures of 16 ~ to 43 ~ The dry season extends from November to May.

The Gambia faces a rapidly increasing human population and a rapid decline in forests and woodlands which currently cover 20% of the land [26]. There are about 60 persons per km 2 of arable land [26]; 85% live in rural areas [25] and about 75% are engaged in agriculture [26].

The dominant farming system is characterized by a single cash crop, groundnuts, and subsistence-oriented food and livestock production on a small scale, all usually produced through shifting cultivation and primitive technology. Most farm soils are acidic and of low fertility [14]. The traditional extensive farming systems provide food for only a small population. Presently, the traditional 3--4 years fallow periods are reduced to 1--2 years [14]. As soils are depleted, farmers clear more forests and woodlands [24] or use commercial inorganic fertilizer, but most farmers cannot afford to sustain the recommended fertilizer application rates [14]. Alley cropping is a potentially viable alternative to inorganic fertilizer for improving soil fertility and increasing crop production.

Cassia siarnea

Cassia is a deep-rooted, leguminous, evergreen tree species which does not fix nitrogen [12, 16, 23]. It is native to southeastern Asia from Indonesia to Sri Lanka and grows in humid, sub-humid, dry and arid climates [16]. In drier areas with 500--700 mm annual rainfall, the tree grows well in the second or third year after planting and is promising for alley cropping on acid soils [9, 16, 18]. The tree coppices vigorously, producing 2--5 shoots per stock [17], and on average 70% of the biomass consists of leaves as compared to 33--60% for gliricidia (Gliricidia sepium) [71. Cassia can produce as much as 10 tons/ha of dry matter per year [29]. It grows well

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with annual crops [10]. Cassia prunings decompose at the rate of 85% of dry biomass in 120 days as compared to 100% for gliricidia [29]. The leaves can provide nutrients for growing crops [29, 30], but prunings applied as mulch may lose nitrogen through volatilization or through leaching in sandy soils (Dr M. Raw, pers. comm.). Nutrients are released 3--6 weeks after pruning application (Dr M. Raw, pers. comm.). Because cassia may also be used for fuelwood, economic returns from alley cropping with cassia are almost twice those of crops grown without trees [21].

Materials and methods

This study was conducted in 1990 in The Gambia on the Agricultural Experiment Station at Old Yundum Tandako station, 29 km west of Banjul, located in the Kombo north of the western division. The mean annual rainfall in the western division is 640 rnm. The cassia hedges were established in the east-west direction in 1987 on a Nyambai sandy loam (70, 14 and 16% sand, silt and clay, respectively) by direct seeding at 10 seed/m within rows and 4 m between rows. The hedgerows were established from different local sources. Rice was alley cropped with cassia, and prunings and fertilizer treatments were applied each year from 1987 to 1990. Our data were collected in 1990; no data on crop yield or soil fertility are available from previous years.

The pH in the surface soil (0--10 cm) in the experimental plots ranged from 5.6--6.2 in the dry season (two months prior to start of rains) and 5.0--5.9 at the end of the cropping season. The electrical conductivity was 0.08 mmhos/cm; soils contained 1.13% organic matter [LOI (%)], 0.06% total N (micro-Kjeldahl), 3.7 ppm available P (Bray-1 p-Avl.), 32.5 ppm exchangeable K (NH4OAc), 44.6 ppm exchangeable Na (NH4OAc) and 373.5 ppm exchangeable Ca (NH4OAc) within 0--15 cm depth.

The experimental design was a Latin square with four replications of four treatments. The plots were 10 • 8 m, each subdivided into two alleys of 4 m width. The plots were plowed 10 cm deep with a single mold plow using animal traction. The seeds were treated with super homai (5 g/kg of seeds) before seeding to protect them from insects in the ground before germination. In each alley, the rice was row-seeded at the rate of 100 kg/ha. Spacing was 30 cm between rows, and there were eleven rows per alley. The outermost rows were 50 cm from the hedge rows. Later, rice seedlings were transplanted by hand from more dense rows to fill gaps where establishment was poor.

The treatments were 1) no cassia prunings or fertilizer added (control), 2) prunings added but no fertilizer, 3) prunings added with half the recom- mended fertilizer rate, and 4) prunings plus full recommended fertilizer rate. The full recommended rate for the first fertilizer application was 93.7 kg/ha of NPK (8:24:24) plus 32.4 kg/ha urea. The fertilizer was broadcast after plowing but before sowing. In the second fertilizer application, a side

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dressing of urea was applied three weeks after sowing; the full recommended rate for urea in the second fertilization was 100 kg/ha. Prunings were obtained by cutting the cassia hedges 50 cm above the soil surface when the rice had germinated, nine days after sowing. Total biomass of cassia prunings was recorded by plot by weighing the tender tissues (leaves and twigs) before spreading them evenly within the alleys. The amount of prunings appfied per plot varied and no prunings were applied to the controls. Sub-samples of prunlngs applied on each plot were oven-dried at 60 ~ for 24 hours to determine the dry weight. The same sub-samples were ground for nutrient analysis.

Nutrient analysis of cassia tissue and soils was done using the University of Wisconsin-Madison procedure [19]. Soil samples were collected at two depths, 0--10 and 10--15 cm, during the dry season before sowing and again after harvesting. A sub-sample for nutrient analysis was drawn from the composite of six soil samples taken within each plot at each depth. Cassia nutrient yields were expressed as amounts in the dry biomass actually applied. Plant samples were analyzed for N, P and K at the University of Wisconsin-Madison, USA. P and K were analyzed simultaneously with an inductively coupled plasma emission (ICP) spectrophotometer (model 34000). Because nitrogen is added as HNO~ in the wet ashing process of preparing samples, total nitrogen was determined on a separate sample using a micro-Kjeldahl procedure. The soil samples were analyzed at Yundum soil laboratory in The Gambia for total N, available P, exchangeable K, pH and organic matter content.

During the growing period, the rice was weeded twice using short-handled hoes. The first weeding coincided with the second fertilizer application two weeks after germination. In the second weeding, only the bigger weeds were pulled. Pests such as grasshoppers (locusts), birds and squirrels were very common. Scarecrows and a crop guard were used to keep the birds away. Pesticides were not used for fear of the residual effects on humans when the grain was consumed.

A machine was used to thresh the rice, thus separating the grain (paddy) from the straw (residue). The grain and straw were weighed shortly after harvest. The grain was air-dried by spreading the samples on a concrete floor until a constant weight was obtained. For the determination of any significant difference in quality of the grain produced in each treatment, the grains were screened to separate the two largest grain size classes based on their weights in a sub-sample of 1000 grains from each plot. To quantify the effects of proximity of hedgerows on crop production, the 2 outer rows of rice (within 80 cm of each hedgerow) in each alley were harvested separately from the remaining 7 inner rows; all comparisons are made on an area basis.

Analysis of variance was used to test for significant differences between treatments (F test, p < 0.05). Where there were significant differences, muRiple comparisons were done using Fisher's protected least significant difference 0LSD, p < 0.05).

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Results

All results are f rom a single year, the 1990 growing season. This was the third growing season that pruning and fertilizer t reatments were applied to the plots, but no data on soil fertility or crop yield in previous years are available.

Soil nutrients

N, P and K content in the surface 10 cm of soil did not differ significantly between treatments either before rice was planted (p = 0.1614, p = 0 .3135 and p = 0.4986, respectively) or after harvest (p = 0.2862, p = 0.4547, p = 0 .7834, respectively). There was no significant difference (p -~ 0 .9824) among treatments in K content at the 10 - -15 cm soil depths after c rop harvest. The surface 0 - - 1 0 cm soil depth contained m o r e P post-harvest (p = 0 .0095) than the deeper soil (Table 1).

Table 1. N a, P, K and organic matter content of soils at 0--10 and 10--15 cm depths after harvest of rice grown between cassia hedgerows. Means and standard deviations (in paren- theses) are calculated separately for 0-- 10 and 10-- 15 cm soil depths.

TRT N P K Organic (%) (ppm) (ppm) matter (%)

0--10 ~ 0--10 10--15 0--10 10--15 0--10 10--15

1 0.06 4.50 3.25 21.75 20.75 1.67 1.46 (0.01) (2.38) (3.20) (1.71) (1.71) (0.17) (0.26)

2 0.08 3.25 3.00 21.50 20.50 1.69 1.09 (0.01) (1.50) (0.82) (1.73) (4.12) (0.33) (0.19)

3 0.06 4.50 3.00 23.50 20.50 1.77 1.27 (0.01) (1.91) (2.71) (6.40) (2.65) (0.78) (0.08)

4 0.06 4.75 4.00 23.00 20.00 1.62 1.38 (0.01) (0.50) (0.96) (4.97) (5.23) (0.15) (0.22)

a N analyzed only at 0--10 cm depth.

Soil P and K declined during cropping in all t reatments (Table 2), but declines did no t differ significantly among treatments (p ~ 0 .7810 and p -- 0 .6860, respectively). Percent changes in N content f rom before planting to after harvesting varied significantly (p = 0 .0407) among treatments. N content increased where prunings only were applied, but decreased with no prunings or with prunings and fertilizer relative to pre-planting soil nutrient status (Table 2).

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Table2. Change in soil nutrient status after crop harvest in 0--10 cm soil depth.

TRT Nitrogen (%) Phosphorus (ppm)

Pre Post Change Pre Post Change

1 0.07 0.06 --14% c 5.00 4.50 -10% a 2 0.06 0.08 +33% a 3.75 3.25 -13% a 3 0.08 0.06 -25% c 4.75 4.50 - 5% a 4 0.06 0.06 0% c 6.75 4.75 --30% a

TRT Potassium (ppm) Organic matter (%)

Pre Post Change Pre Post Change

1 40.50 21.75 -46% a 1.91 1.67 -12% a 2 36.00 21.50 -40% a 1.62 1.69 + 4% a 3 39.50 23.50 -40% a 1.75 1.77 § 4% a 4 36.25 23.00 -36% a 1.56 1.62 + 4% a

Soil organic mat ter content in the 0 - - 1 0 cm soil did no t change signifi- cantly f rom pre-plant ing to post-harvest (p = 0.8834), no r did it differ significantly among treatments post-harvest (p = 0 .9575) (Table 2). Organic mat ter content in the 1 0 - - 1 5 cm soil depth did not differ among treatments before c ropping (p = 0 .5420) but differed significantly after crop harvest (p = 0.0382). Maimo [13] repor ted a decrease in organic mat ter and ni trogen with depth in alley cropping. Soil p H did no t differ significantly among treatments at 0 - - 1 0 cm soil depth before or after c ropping (p = 0.1961 and p = 0.3950, respectively), no r was it significantly different among treatments at 10 - -15 cm soil depth after crop harvest (p = 0.3617).

Cassia biomass and nutrient yields

There were no significant differences between treatments in biomass of prunings p roduced nor in amount of total N, P and K in prunings applied in treatments (p = 0.3655, p = 0.8077, p = 0 .2323 and p = 0.0836, respec- tively) (Fig. 1). The average content of nutrients in dry cassia leaves was 1.73 ___ 0.12% N, 0.13 _+ 0.01% P and 0.59 _+ 0.39% K. Slightly different

nutrient content was repor ted for cassia leaves in the sahel zone in Burkina Faso, 1 .48 - -1 .60% N, 0.88% P and 1 .17- -1 .22% K [30].

Treatment effect on crop yield

Grain and straw weights did not differ significantly be tween treatments (p = 0 .3447 and p = 0.0691, respectively) (Figs. 2 and 3). Grain weight differed significantly within rows and columns (p = 0 .0042 and p = 0.0014, respec-

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450-

~- 400- E ---. 350-

--- 300-

m 250- t- O 200- 0

.~ 150- Q;

lO0-

z 50-

0

a a a

1

b

/

2 3 4 Treatment

I [~----"~ NITROGEN ~ PHOSPHOROUS ~ POTASSIUM

Fig. 1. Nutrient content of cassia prunings. The treatments were 1) no cassia prunings or fertilizer added (control), 2) prunings added but no fertilizer, 3) prunings added with half the recommended fertilizer rate, and 4) prunings plus full recommended fertilizer rate. Means with the same letters are not significantly different for a given nutrient.

E

v

E_

L~ s

0.05-

0.045-

0.04-

0.035-

0.03-

0.025-

0.02-

0.015-

0.01

0.005

6

a a

b

1 2 3 Treatment

4

[ ~ INNER ~ OUTER

Fig. 2. Grain weight did not differ among treatments (p = 0.3447) but differed significantly with proximity to hedgerows (p = 0.0002). Means with the same letters are not significantly different within treatment.

154

~ 0.25 0,1

E 0.211 1:;)1 ~v -(:: 0.15 ._~

0"11t E" D

0.05 O - . L - ~

1 2 3 TREATMENT

a

,&_.

4

INNER ~ OUTER I

Fig. 3. Straw, the residue remaining after rice was harvested, weight did not differ significantly (p = 0.0691 and p = 0.7148 among treatments or with proximity to the hedgerows within treatment, respectively).

tively). Rice responded to application of 30 kg/ha N fertilizer but not to inorganic fertilizer when rice was inter-cropped with leucaena (Leucaena leucocephala [9]. Rice yield increased by 89% with application of only 8 ton/ha of leucaena leaves which contained the equivalent of 69 kg/ha N [4 as cited in 8].

Grain quality

The proport ion of the larger grade A and B grains did not differ between treatments (p -- 0.6017 and p = 0.0629, Fig. 4). However, there were significant differences between rows in weight of grade B quality (p = 0.0236) and between columns for weight of both grades A and B (p = 0.0363 and p ~ 0.0279, respectively).

Hedgerow effect on crop yield

When agricultural crops and trees are intercropped, they compete for light, moisture, space and nutrients. Those effects were not studied here, but the effect of hedgerows proximity was considered.

Grain weight was significantly less adjacent to hedgerows (p -- 0.0002, Fig. 2), but was not significantly different among treatments for inner or

155

0.02-

0.018-

0.016-

~- 0.014-

"--- 0.012-

.c_ 0.01

0.008

n 0.006-

0.004-

0.002-

a

1 2 3 Treatment

4

[ ~ GRADE A 1 GRADE B

Fig. 4. Grain quality did not differ significantly with treatment (p = 0.6017 and p = 0.0629 for grades A and B, respectively).

outer rows (p = 0.6329). The dry grain weight of rice for outer rows, those within 80 cm of cassia hedgerows, was 0.02 _+ 0.01 kg/m 2 for all treatments, and yields in inner rows, those in the middle of the alleys, were 0.03 _+ 0.02, 0.04 _+ 0.02, 0.37 _+ 0.02 and 0.05 + 0.02 kg/m 2 respectively for treat- ments 1 to 4. Straw weight was significantly different (p -- 0.0117) within treatment, however, straw weight was not significantly different among treat- ments for inner and outer rows (p = 0.7148, Fig. 3). Weight of the larger grade A grain produced in inner rows and outer rows differed significantly within (p = 0.0117) but not among treatments (p = 0.7171), while the weight of the smaller grade B in inner and outer rows did not differ within (p = 0.1089) or among treatments (p = 1.000).

Discussion

Soil fertility was not improved by application of cassia prunings or inorganic fertilizer; N, P, K and organic matter content did not differ significantly among treatments before or after harvest. Although N content increased significantly during cropping where prunings were applied, P and K content in the surface 10 cm of soil declined by 5 to 30% and 36 to 40%, respec-

156

tively, in all treatments (Table 2). Change in organic matter content was variable (Table 2). Nutrients were lost through violatilization, leaching and or surface runoff when leucaena prunings were applied as mulch [5]; the slow decomposition rate of the cassia leaves also contributes to nutrient loss [29].

Application of inorganic fertilizer plus cassia prunings did not increase grain weight significantly over the application of prunings alone. However, the inner rows where only prunings were applied produced 79% more straw than control plots, and application of half and full rates of inorganic fertilizer produced 64% and 58% more straw relative to where neither fertilizer nor prunings were applied respectively. Less grain was produced adjacent to cassia hedgerows, although the weight of straw did not differ with proximity to the hedgerows.

Grain quality did not differ significantly among treatments. The propor- tion of larger grain was similar for all treatments (Fig. 4) for rice plants growing in the center of alleys and within 80 cm of cassia hedgerows.

In a similar experiment nearby, yield of maize that was alley cropped with cassia was not increased by the addition of prunings alone, and grain quality and yield were higher where inorganic fertilizer was added [5]. As in this study, yield of maize plants growing adjacent to the hedgerows was less than plants growing in the central parts of the alleys [5].

Our one-year observations from the third year of alley cropping rice with cassia suggest that on this semi-arid site in The Gambia there is no benefit in terms of rice grain yield and grain quality from addition of prunings or prunings and inorganic fertilizer. Crop production did not increase with addition of prunings even though cassia prunings were high in N (Fig. 1).

Acknowledgements

Funding was provided by the Gambia Agricultural Research and Diversifi- cation Project (GARD), USAID project No. 635--0219 and by Government of The Gambia, including both Departments of Forestry and Agriculture Research. We appreciate reviewer comments. Contribution No. 659 of the Idaho Forest, Wildlife and Range Experiment Station.

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