variation in soil fertility and capacity … · variation in soil fertility and capacity for...

Cey. J. Sci. (Bio. Sci.) 36 (1): 65-79, 2007

VARIATION IN SOIL FERTILITY AND CAPACITY FOR SUPPLYING SOIL NUTRIENTS IN A HEDGEROW

INTERCROPPING SYSTEM WITH DIFFERENT TREE SPECIES IN THE UPCOUNTRY WET ZONE OF SRI LANKA

H.M.S.P. Madawala Weerasinghe

Department of Botany, Faculty of Science, University of Peradeniya, Peradeniya, 20400, Sri Lanka Accepted 27 May 2007

ABSTRACT Sloping agricultural land technology (SALT), a form of hedgerow intercropping (HI) system, has been

accepted as an appropriate land management practice to overcome some soil related constraints faced by farmers in marginal lands in the wet zone of Sri Lanka. Despite its effectiveness in soil conservation and soil fertility improvement, farmers are reluctant to adopt this practice due to the lack of immediate monetary returns. The lack of expected nutrient inputs through hedgerow mulch and possible tree-crop competition for essential resources seems to be the main drawbacks. In order to test some of these aspects, a study was carried out to assess the soil fertility status after 4 years of HI on the slopes of the wet zone of Sri Lanka, with a special emphasis on the nutrient redistribution in alleyways due to the establishment of contour hedgerows. After 4 years of HI, some soil chemical parameters such as concentrations of organic C, total N, mineralizable N, and organic P were higher in the SALT-HI plot than that of farmer-managed vegetable plot and the abandoned tea land. However, 4 years of HI on the slopes at Doragalla, some chemical parameters showed a significant position effect down the slope compared to the control sites, with a tendency for accumulating nutrients immediately under the hedges compared to open alleyways, yielding a distinct ‘saw-tooth’ pattern down the slope. The organic C and total N concentrations were significantly higher under hedges than in the open alleyways, where the crops were grown. This nutrient patchiness was not present in the two control sites without hedgerows. This nutrient patchiness seems to be a direct consequence of establishing hedgerows along contours and hence will be a crucial factor determining the fate of this agroforestry system in Sri Lanka.

Key words: Hedgerow trees, soil fertility distribution, tree-crop interactions, within-alley soil erosion

INTRODUCTION

Over the past few decades agriculture in Sri Lanka has faced many challenges owing to the ever increasing human population, about 78% of which depends mainly on agriculture for their livelihood (NARESA, 1991). Agriculture in Sri Lanka is characterized by smallholdings with low levels of income and productivity. The use of compost, mulch and domestic waste to improve soil nutrient status was not common in Sri Lanka in the 1970’s. Since the lifting of fertilizer subsidies in 1988-90, farmers’ interest increased significantly towards organic material as a cheap substitute for high cost inorganic fertilizers.

Agroforestry, an alternative to high-input,

mechanized farming practices, has been used by farmers for decades and acts as a key contributor to ameliorating most of the prevailing soil constraints faced by the farmers. Hedgerow intercropping (HI) is said to be an accepted and appropriate agroforestry practice for smallholder farmers throughout the ecologically marginal

highlands of Sri Lanka, where soil erosion is considered a major problem. Widespread experimental findings have shown that contour HI reduce soil erosion effectively and suggest that contour HI is one of the best ways of controlling accelerated erosion on sloping agricultural fields (Young, 1989; Garrity, 1994). Studies have also revealed that HI could improve physical and chemical aspects of degraded tropical soils, hence ensuring sustainability of high annual crop yields (Kang et al., 1990).

In 1989, the Ceylon Tobacco Company

(CTC) introduced the Sloping Agricultural Land Technology (SALT), a hedgerow intercropping system, to tobacco lands in the hill country. Later it was extended to cultivation on degraded lands and on abandoned tea, coffee or cocoa lands in mid-elevations of Sri Lanka (Ranasinghe, 1996). In the late 1980’s the Upper Mahaweli Watershed Management Project, in collaboration with the German Agency for Technical Cooperation (GTZ), recommended and implemented the

Author’s e-mail: [email protected]

H. M. S. P. Madawala Weerasinghe

66

SALT system in a number of pilot areas through extension programmes.

Despite the large number of success stories of

HI in agriculture, it is difficult to make recommendations for a given environment due to poor documentation of site characteristics and research methodologies. Various aspects of contour HI systems have been studied in almost every agro-ecological zone of the south-east Asia (Garrity, 1996). In HI the response of conventional row-wise pattern on flat land has been contrasted with the patterns observed in contour hedgerow systems on slopes (Garrity, 1996). However, tree-crop interactions, an important aspect in HI research, have so far received little attention. These tree-crop interactions on sloping lands can dramatically alter the slope by developing terraces behind contour vegetation barriers. This makes the system more ‘patchy’ in terms of soil fertility and other micro-climatic parameters compared to that in a mono-crop agricultural system (Garrity, 1996). In many instances, crop yield reduction in the upper alley zone was attributed to rapid terrace development in contour HI systems (Agus, 1993; Solera, 1993; Garrity et al., 1995). Patchiness of soil fertility was suspected to be responsible for skewed grain yields on sloping lands with contour hedgerows. This led to evaluation of soil fertility gradients across the alleyways. In Sri Lanka, few studies have explored tree-crop interactions in HI systems (De Costa and Surenthran, 2005; De Costa et al., 2005). Consequently, this research was planned to address some aspects of HI on sloping lands by carrying out a detailed on-farm study using a 4-year old SALT-HI site in the up-country wet zone of Sri Lanka. The objectives of the present study were as follows; 1) to assess the sufficiency of nutrients added through hedgerow mulch for the sustenance of SALT-HI system through different hedgerow species by comparing the nutrient inputs with the dosages recommended by the Department of Agriculture, Sri Lanka ; 2) To quantify the soil fertility improvement after 4 years of contour HI by comparing the soil fertility status of the SALT-HI with another farmer-managed vegetable plot and an abandoned grassland, which represent the initial soil fertility status before establishing the SALT-HI plot; and 3) the extent of soil nutrient redistribution within alleyways caused by the inclusion of contour hedgerows.

MATERIAL AND METHODS Experimental site

The experiment was conducted at the SALT demonstration plot at Doragalla established by the Upper Mahaweli Watershed Project (UMWP) for extension purposes since 1992. Doragalla is situated at 1300 m above sea level; it receives >2500 mm annual rainfall and has an annual average temperature of 28°C. It is located in the Upper Mahaweli catchment and also in the sub-catchment of the Kotmale Reservoir. It falls within the agroecological zone (WU2 – Up-Country wet Zone) that has steeply dissected mountains and a rolling terrain with 50-75% slopes (Agro-Ecological Zones Project 1978-1981). The great soil group in this region is Red-Yellow Podzolic soils (Panabokke, 1996), classified as Acrisols by FAO/UNESCO, the more widespread and dominant soil type in the humid tropics of Asia. The parent material is the meta-sediments of the Highland series of Precambrian rocks and therefore the soils are well-structured, less erodible and do not fall within the low activity clays (LACs) with respect to their cation exchange properties (Panabokke, 1996). Nevertheless, they are prone to soil degradation under poor management and high rainfall intensities. The A1 horizon has a sandy loam, sandy clay loam or loam, with a weak to moderate crumb or granular structure and friable consistence. Transition to Bt horizon is usually rather distinct, but rarely abrupt. In the Bt horizon the clay content is usually higher than in the A horizon and has either a sandy clay loam or clay texture. Base saturation of the subsoil decreases with increasing rainfall. Soil reaction is strongly acidic.

The SALT demonstration plot at Doragalla

was established on a tea land abandoned for more than 30 years. It remained as a low-strata grassland until it was cleared in early 1992. The contour hedges were established as double-hedgerows along the contour lines of this land that has slopes between 60-75%. Four hedgerow species were selected (i.e. Calliandra calothyrsus, Tithonia diversifolia, Gliricidia sepium and Pennisetum pupureum), some with nitrogen fixing ability, for the HI trial. The inter-hedgerow spacing was 6 m; each hedgerow width ranged from 40-70 cm depending on the habit of the species (Figure1). N-fixing (Calliandra calothyrsus and Gliricidia sepium) and non-N fixing (Tithonia diversifolia and Pennisetum purpureum, commonly known as Bana grass) hedgerow species were planted in alternate contours. Chemical fertilizers were not added, except NPK initially at the recommended dosage.

Variation in soil fertility in a hedgerow intercropping system

67

The hedges were pruned once every three months and the pruning was added to the adjacent alleyways as surface mulch. Vegetables such as cabbage, radish, potato and carrots were grown as mono-crops in each alleyway, twice per year. At harvest, only the edible parts were removed from the field leaving the plant residues to decay in the alleyways. Alleys were left fallow between the two cropping seasons.

The effects of HI (SALT-HI) on the soil

fertility status and also on the fertility gradient down the slope were tested and compared with two nearby sites viz., a farmer’s vegetable plot

(FARM) and an abandoned tea land (ABAN-TEA) under grass, with similar slope characteristics. The farmer had been cropping the land for nearly five years, established stone terraces to arrest soil erosion, grown mainly up-country vegetables such as carrot, potato, radish and cabbage and depended heavily on chemical fertilizers, pesticides and also organic manures from poultry and cattle for tuber crops. The soil nutrient status of a nearby abandoned tea land was sampled to demonstrate the soil fertility status (Table 1) before establishment of the SALT-HI system and the farmer’s vegetable plots.

Fig 1. Plot layout at the SALT-HI Model plot at Doragalla, Sri Lanka. Arrows marked along transect indicate the soil sampling positions down the alleyway from each contour hedgerow.

Calliandra

Tithonia

Gliricidia

Pennesetum

≈ 6m

1 m

3 m

1 m

2 m

slope

6 m

3 m

3 transects

1 m


68

Table 1. Means (standard error) of some physical and chemical soil parameters on the abandoned tea land at Doragalla, Sri Lanka, which represents the soil fertility status before establishing the SALT-HI plot.

Soil depth (cm) Soil character 0-30 cm 30-50 cm pH (1:1.5 CaCl2) 4.2 4.2 Texture (< 2 mm) S (%)

Silty clay 63%

Silty clay loam 63%Organic carbon (%) 2.8 (0.16) 2.5 (0.20)

Total N (%) 3.10 (0.14) 2.06 (0.20) Extractable phosphorus (mg/kg) 2.54 (0.33) 1.98 (0.15) Organic P (µg/g) 40.5 (3.76) 37.0 (7.42) Inorganic P (µg/g) 163.0 (17.02) 113.0 (9.77) Cation Exchange Capacity (CEC)

( l /k il)50.3 (26.6) 20.9 (7.99)

Exchangeable acidity 27.5 (1.03) 24.4 (1.33)

Standard error of the mean (SEM) given within the parentheses. Each mean is an average of five soil samples per transect, aligned down the slope. Slope was excluded in the calculations as was stone content in expressing the soil nutrient levels.

Plant sampling

In each hedgerow, three linear plots, each 5 m in length, were marked as permanent plots. In the marked plots the hedge height was recorded and then lopped at 0.5 m above the ground. Fresh biomass of loppings were recorded (including the woody parts) in the field using a hand-held scale. The respective dry weights were estimated by taking three sub-samples of 100 g from each species and oven-drying at 60°C, to constant weight.

Soil sampling

In the SALT-HI plot at Doragalla, soils were sampled along three transects, 2 m apart, down the entire slope covering four hedgerows and three alleyways. Soil was taken from (i) the hedge itself and (ii) 1 m and 3 m away from each hedgerow towards the alleyway, totaling 13 sampling positions along each transect (Figure 1), and at four depths down the soil profile (0-10, 10-30, 30-50 and over 50 cm). To reduce the inherent soil variability at each sampling point, three sub samples were collected and thoroughly mixed to make a composite soil sample for analysis.

In both FARM and ABAN-TEA plots, three

20 m transects, 2 m part, were sampled at 5 m intervals down the slope giving five sampling positions in each transect. At the FARM plot four soil depths were sampled similar to that of the SALT-HI plot, but due to shallowness of the soils at the ABAN-TEA land, only two depths (0-30 cm and 30-50 cm) were sampled. As

stoniness is high in Doragalla soils (60-70%), field samples were air dried and crushed before passing through a 2 mm sieve. Unless otherwise stated all the soil nutrient measurements at the Doragalla site were expressed without taking stone content into account.

Soil physical and chemical properties

The soil was sampled at the end of 1996, 4 years after establishment of the SALT-HI plot. The present study investigated the spatial variability of soil chemical properties rather than the average value for a particular soil depth. To achieve this, samples were analyzed separately rather than bulking them together to obtain a ‘representative’ value for each depth. The dried and sieved soil samples were analyzed at the laboratories of the Department of Geography, University of Cambridge, UK. The measurements made were texture using the Laser Particle Size Analyzer (MELVERN), soil organic carbon content by wet oxidation using the modified Mebius method (Bremner and Malvaney, 1982), organic matter content by multiplying the organic carbon by 1.74, total soil nitrogen by a modified micro-Kjeldahl procedure, available phosphorus colorimetrically after extracting soils with Olsen’s solution (sodium bicarbonate solution at pH 8.5), mineralizable nitrogen after incubating soils anaerobically for 7 days at 40°C (Waring and Bremner, 1964), calcium, potassium and magnesium by barium chloride extraction (Bache, 1976), and cation exchange capacity by ‘compulsive exchange’ by


69

magnesium sulfate. The extracts were analyzed for Ca and Mg using atomic absorption spectrophotometry and K by corning flame photometry. Exchangeable acidity was determined by titration and soil pH after mixing with 4 mM CaCl2 in a ratio of 1:1.5.

Biomass production of hedgerows

Biomass production of each hedgerow species was monitored after establishment of the plot in 1992. The species were first pruned 6-7 months after establishment, considering their differential growth. The hedges were pruned on average at 3-4 month intervals (4 times per year), avoiding the drier months because pruning then could further reduce their coppicing ability and stunt their growth. The biomass data obtained during 1994 to 1996 were used in the analysis and presented in this study.

Plant nutrient contents

The total amounts of N, P, K, Ca and Mg added as prunings were calculated as the product of the respective tissue nutrient concentrations and total biomass of prunings (average of 3 years). The dried and ground plant samples were processed using the dry combustion method for laboratory analysis and were analyzed for phosphorus colorimetrically using the flow injection analyzer, for potassium using the flame photometer and for Ca and Mg using the atomic absorption spectrophotometer, for N content by a modified Kjeldhal method; the carbon content was calculated by loss on ignition using the conversion factor of 0.45. The nutrient inputs through poultry and cattle manure were also calculated assuming the rate of application as 2.5 t/ha per cropping season.

Data analysis

One-way ANOVA was carried out to compare the foliar nutrient concentrations of different hedgerow species and other organic inputs and followed by a Tuckey’s t-test for mean separation. To analyze the position effect (hedge, 1m and 3 m away from the hedge) down the slope in SALT-HI, FARM and ABAN-TEA plots, a split-plot ANOVA was carried out for each three sites separately. The nutrient concentrations under hedges and in open alleyways (1 m and 3 m away samples were pooled) were compared using a t-test. The results were analyzed using GENSTAT 5 Release 3.2 (Lane and Payne, 1996; GENSTAT 5 Committee, 1996) and MINITAB (Release 13.2) statistical packages.

RESULTS Performance of hedgerow species

The total above-ground dry biomass production (t/ha) was calculated taking into account the whole area under HI, across pruning dates from 1994 to end of 1996 (Table 2). The mean annual biomass produced, averaged over the years, for Calliandra calothyrsus was 4.5 t/ha and its variation between years was relatively consistent; Pennisetum purpureum showed the highest production of 7.7 t/ha and was highly variable between years; Tithonia diversifolia produced 2.7 t/ha while Gliricidia sepium produced the lowest biomass of all (1.2 t/ha). All 4 hedgerow species showed a relatively high variability in their biomass production during the year 1994 and then gradually decreased in subsequent years.

Nutrient additions through organic inputs

Foliar and non woody stem nutrient concentrations for each hedgerow species showed that T. diversifolia had the highest values for all the nutrients tested (Table 3). Among the three remaining species, P, K and Mg were highest in P. purpureum followed by G. sepium and least in C. calothyrsus. Among the four species, K levels varied the most followed by Ca, N, P and least in Mg. Poultry manure significantly had higher N, P and K concentrations to that of cattle manure.

Tissue nutrient concentrations and biomass

production among the hedgerow species differed significantly. Using these data, the contribution of nutrients through different hedgerow species was calculated (Table 4). Calliandra produced the highest N yield of 152 kg ha-1 annually. Despite the fact that Tithonia had the highest tissue N concentration, Calliandra and Pennisetum both supplied a higher N yield to the system than Tithonia because of the higher biomass production of the latter two species.

Although N-fixing, the annual N production

of G. sepium was only 43 kg ha-1 due to its consistently poor growth. Comparatively, Calliandra and Pennisetum produced higher N yields. Bana grass also provided the highest P, K and Mg yields to the system while Calliandra supplied significantly lower K than the rest despite its comparatively high biomass production. A correlation analysis showed that total amounts of all the nutrients added to the soil had strong positive correlations with the total biomass production of prunings (r2 values of 0.99, 0.87, 0.85, 0.81 and 0.62 for Mg, P, N, K and Ca respectively), indicating the importance of their ability to produce sufficient mulch material.


70

Table 2. Dry biomass production (t/ha) during 1994, 1995 and 1996 of four hedgerow species established in late 1992 at the SALT-HI plot at Doragalla.

Pruning Frequency Species Year 1 2 3 4 5 Total % CV

T. diversifolia 1994

0.53

0.74

0.76

0.46

0.32

2.81

33

1995

0.60

0.60

0.67

0.74

NH 2.61

10

1996

0.60

0.73

0.81

0.65

NH 2.79

13

C. calothyrsus* 1994

0.99

1.66

1.16

0.69

NH 4.48

36

1995

0.98

1.52

1.43

1.06

NH 5.00

21

1996

1.06

1.02

1.13

1.02

NH 4.24

5

P. purpureum 1994

1.51

2.02

2.41

1.63

1.18

8.75

27

1995

1.32

1.74

1.76

1.42

1.54

9.36

12

1996

1.57

1.76

1.77

1.42

NH 6.52

10

G. sepium* 1994

0.32

0.46

0.33

NH NH 1.11

21

1995

0.33

0.42

0.43

NH NH 1.17

14

1996

0.30

0.29

0.31

0.30

NH 1.20

5

NH = Not Harvested. * N-fixing trees.

Table 3. Mean tissue nutrient concentration (leaf and non-woody stem parts) of four periodically pruned tree species at the SALT-HI plot at Doragalla and the nutrient concentrations of poultry and cattle manure used by the farmers in the region.

Tissue Nutrient Concentration (%)* Species N P K Ca Mg T. diversifolia 3.79a 0.66b 5.73 a 4.07 a 3.27 a C. calothyrsus 3.32b 0.25a 0.83 b 3.20 b 3.00 b P. purpureum 2.00c 0.38c 4.33 c 1.53 c 3.13 a G. sepium 3.72a 0.32d 2.03 d 2.67 b 3.13 a Poultry manure 2.13d 0.91e 2.15 d 0.60 c d 3.20 a Cattle manure 1.19e 0.26a 0.45 e 0.20 d 3.10 a

Different letters indicate significant differences between means. *Average of 3 replicates per pruning.


71

Table 4. Nutrient inputs (kg/ha/yr) through hedgerow prunings at the SALT-HI plot at Doragalla, Sri Lanka.

Species Average hedgerow

Biomass (kg ha-1 yr-1) N

P

K

Ca

Mg

T. diversifolia 2781 103.7 18.0 156.8 111.4 89.5 C. calothyrsus 4647 151.8 11.4 37.9 146.4 137.2 P. purpureum 8342 153.7 29.2 332.7 117.5 240.5 G. sepium 1179 43.3 3.7 23.6 31.0 36.4

Nutrient additions through different organic

manures were compared with the fertilizer dosage recommended by the Department of Agriculture, Sri Lanka for vegetables in the upcountry of Sri Lanka (Table 5). Phosphorus addition through poultry manure (23 kg/ha per application) was remarkably higher than through hedgerow biomass per cropping season due to its high P content (0.91%). However, Ca application through poultry and cattle manure was little compared to Ca additions through hedgerow prunings. A comparison of the major nutrients added through hedgerow prunings with the recommended dosages of inorganic fertilizers revealed that none of the hedgerow species alone could provide the total requirement of N and P for any crop. However, the K requirements of crops were nearly met by Tithonia and Pennisetum prunings.

Effect of hedgerow intercropping on the soil fertility status

Both SALT-HI and the FARM plots maintained a higher soil pH at 5.8 in the upper soil layers compared to the 4.2 in the nearby ABAN-TEA plot (Table 6). But in the FARM plot, the sub-soils remained comparatively acidic (4.7 – 5.1). Soil pH distribution down the slope in the SALT-HI plot did not show any consistent pattern within alleyways, even though the position effect is highly significant down the slope and at all 3 depths in all three land-use types (results not given). However, the lowest pH values were recorded in the mid-slope position within the Tithonia-Gliricidia hedge in the SALT-HI plot.

The positive effect of HI on the soil fertility

status was more pronounced in terms of soil organic carbon and total N contents in the SALT-HI plot at Doragalla. Frequent additions of organic residues seem to have a marked cumulative effect on the soil organic carbon and nitrogen concentrations at the SALT-HI plot. The SALT-HI plot had considerably higher soil organic C and total N concentrations compared to the other two plots (Figure 2). However the ABAN-TEA plot (now covered with

Cymbopogon nardus) showed a slightly higher organic C and total N contents compared to the FARM plot despite the fact that farmers depend highly on inorganic fertilizers and other organic manures.

Incorporation of rich organic amendments

may have increased the soil nitrogen concentrations while inorganic fertilizer application seems not to have increased the total soil nitrogen concentrations at the FARM plot. In terms of total nitrogen poorer sub soils were also found in the FARM plot. The ABAN-TEA plot contained higher total nitrogen levels than in the FARM plot, indicating poor ability to retain N added through chemical fertilizers and other organic manures in the farmer-managed vegetable plot. The correlation between organic carbon and total soil nitrogen was highest in the abandoned tea plot (R2 = 0.98 and 0.95 in 0-30 and 30-50 cm soil depths respectively) where soil disturbance was minimal compared to R2=077 and R2=0.47 in the upper soil layers (0-10 cm) of the SALT-HI and FARM plots respectively.

As reflected by the total nitrogen levels,

mineralizable nitrogen level was also higher in the SALT-HI plot indicating its greater availability than in the two control plots. Though the farmer’s plot received N fertilizers, it retained less nitrogen compared to the SALT-HI plot. The FARM plot had lower total N but its mineralizable N levels were nearly double than that in the ABAN-TEA land (data not shown).

Organic P in the SALT-HI plot was more than

double the levels found in the farmer’s plot, and ten times more than in the abandoned tea land (Figure 3). In contrast to the Po levels, Pi levels in the farmer’s plot were comparatively higher than in the SALT-HI and ABAN-TEA plots. This difference in the organic/inorganic distribution may be due to the use of phosphorus-rich organic manures in the FARM plot. The Olsen-available P showed a pattern similar to the Pi levels, ie. highest in the FARM plot. In the SALT-HI plot, a significant positive correlation (R2 = 0.86) was observed between available P


72

and inorganic P levels; no such correlation was found in the control plots. The extremely poor available P status in the ABAN-TEA land suggests high adsorption capacity at low pH levels. The concentrations of exchangeable calcium, magnesium and potassium were highest in the FARM plot and lowest in the ABAN-TEA plot (data not shown). The exchangeable acidity (Al3+ + H+) was considerably lower in SALT-HI plot compared to the ABAN-TEA land and to a lesser extent the FARM plot. These results are consistent with the soil pH values. In SALT-HI, the exchangeable acidity increases down the soil profile, but in the FARM plot the increase is much greater in magnitude than in the SALT-HI plot.

Soil nutrient re-distribution within alleyways

After 4 years of HI on the slopes at Doragalla, some chemical parameters showed a significant

position effect down the slope compared to the control sites (Table 7). The organic C and total N concentrations were significantly higher under hedges than in the open alleyways (Figure 4). This distinct soil fertility gradient in the SALT-HI plot was found in all soil depths. The distribution of mineralizable nitrogen within alleyways also showed a highly significant position effect down the slope in the SALT-HI plot, with higher nitrogen contents under the hedges and lower levels in the open alleyways (Figure 5). Neither the FARM nor the ABAN-TEA plot showed any similar position effect down the slope. In contrast to this soil fertility gradient observed in the SALT-HI plot, in the control plots the organic C and total N levels remained more or less similar down the landscape (Figure 6). Statistical analysis also revealed no significant position effect down the slope in the control plots (Table 7).

Table 5. Nutrients recommended for different crops in the region compared to the nutrients added through various organic inputs.

A. Nutrients added through organic inputs per cropping season (2 prunings per cropping season). Nutrient inputs through cattle and poultry manure were calculated at a rate of 2.5 t/ha of manure per crop.

Nutrients (kg ha-1 ) Species N P K Ca Mg

T. diversifolia 48 8.4 72 52 42 C. calothyrsus 76 5.6 19 72 68 P. purpureum 66 12.4 142 50 102 G. sepium 26 2.2 14.4 18 22 Poultry manure 53 22.7 53.7 15 80 Cattle manure 30 6.5 11.2 5 77

B. Fertilizer dosages (in two splits) recommended by the Department of Agriculture, Sri Lanka.

Crop Nutrients (kg ha-1 per crop) N P K Potato 210 32.5 75 Leeks 149.5 39 75 Cabbage 216.2 39 75 Carrot 210 35 80

Table 6. Mean and standard error of soil pH in the SALT-HI, FARM and ABAN-TEA plots at Doragalla, Sri Lanka.

Depth intervals (cm)

SALT-HI (n=39)

FARM-PLOT (n=15)

ABAN-TEA (n=15)

0-10 5.8 (0.46) 5.8 (0.27) 10-30 5.9 (0.37) 5.9 (0.39)

4.2 (0.08)

30-50 5.7 (0.29) 5.1 (0.59) Over 50 5.6 (0.38) 4.7 (0.18)

4.2 (0.02)


73

Figure 2. Soil organic carbon (%) and total N (%) levels in each soil depth in the three experimental plots at Doragalla, Sri Lanka. Vertical error bars indicate the standard error of the mean (SEM).

Figure 3. Soil organic P , inorganic P and extractable P fractions in the three experimental plots at Doragalla, Sri Lanka. Vertical error bars indicate the standard error of the mean (SEM).

The distribution of Po levels in the SALT-HI

plot also followed a ‘saw-tooth’ pattern similar to the distribution of organic C and N, but the position effect is more pronounced in the lower soil layers than in the upper-most soil layers. The levels of Pi and Olsen-available phosphorus in the upper soil layers followed a pattern similar to each other but do not follow this so-called ‘saw-tooth’ pattern. In the SALT-HI plot, the upper and lower alleyways contained higher available phosphorus and Pi levels than in the mid-slope

level. The available P and Pi levels in the upper soil layers of the SALT-HI plot showed a significant position effect but this effect was not so pronounced with the increasing depth down the soil profile. The exchangeable acidity levels in the SALT-HI plot showed a gradual increase down the slope and this pattern is more conspicuous in the subsoil levels than in the upper soil layers. The highest levels of exchangeable acidity in the subsoil were observed under the Gliricidia hedges.

0

200

400

600

800

1000

1200

1400

SALT-HI FARM ABAN-TEA

Inor

gani

c/or

gani

c P

(ug/

g)

Org.P

Inorg.P

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

0-10

cmS

ALT

-HI

10-3

0cm

30-5

0cm

over

50c

m

0-10

cmFA

RM

-PLO

T

10-3

0cm

30-5

0cm

over

50c

m

0-30

cm

AB

AN

-TE

A

over

50c

m

Soi

l Org

anic

Car

bon

(%)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0-10

cm

10-3

0cm

30-5

0cm

over

50c

m

0-10

cm

10-3

0cm

30-5

0cm

over

50c

m

0-30

cm

over

50c

m

SALT-HI FARM-PLOT ABAN-TEA

Tota

l N (%

)

0

10

20

30

40

50

60

70

80

SALT-HI FARM ABAN-TEA

Extra

ctab

le P

(mg/

kg)


74

Table 7. Position effect down the slope of different soil chemical parameters (only up to 30 cm in depth) for each land-use type are presented with their mean values and F values (from split-plot analyses of variance).

Soil

parameter SALT-HI

FARM ABAN-TEA

0-10 cm 10-30 cm 0-10 cm 10-30 cm 0-30 cm pH 5.8 5.9 5.8 5.93 4.23

383*** 847*** 313*** 676*** 8.11**

Organic C 5.68 5.03 2.39 2.37 2.81

(%) 2.97** 1.68 ns 0.61 ns 0.94 ns 1.84 ns

Total N 0.45 0.39 0.26 0.25 0.31

(%) 2.98** 2.21* 1.11 ns 2.70 ns 1.46 ns

Mineral. N 0.104 0.083 0.064 0.058 0.035

(mg/g) 23.2*** 4.27** 0.99 ns 2.70 ns 4.91*

Organic P 520 505 186 190 40.5 (µg/g) 0.94 ns 0.88 ns 0.27 ns 4.42* 0.36 ns

Inorganic P 666 596 1011 873 163

(µg/g) 3.25** 4.15*** 1.43 ns 4.94* 3.01 ns

Available P (mg/kg)

24.5 7.89***

19.5 6.63***

63.9 2.73 ns

64.4 3.52*

2.54 2.31 ns

CEC 94.9 104.2 90 76 50.3

(mmol(-)/kg) 0.87 ns 0.93 ns 2.71 ns 6.81** 2.05 ns

The degree of significance indicated as follows: *** P < 0.001; **P < 0.01; * P < 0.05; ns not significant.

Figure 4. Soil organic C and total N under hedges and in open alleyways. 1m and 3m away sampling points were pooled together (n=27) and compared with hedgerow sampling points (n=12) for each soil depth separately, using a t-test. Vertical bars indicate the standard error of the mean. Significance differences indicate by different letters along with the degree of significance (*** P < 0.001; **P < 0.01; * P < 0.05) between hedges and alleyways in each soil depth separately.

0

1

2

3

4

5

6

7

0-10 cm 10-30cm 30-50 cm over 50 cm

Hedges

alleyw ays

Org

anic

C c

onte

nt (%

)

Soil depths (cm)

a*

b a*

ba***

b a**

b

0

0.1

0.2

0.3

0.4

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0-10 cm 10-30 cm 30-50 cm over 50 cm

Soil depth (cm)

Soil T

otal

N (%

)

hedges

alleyw aysa**

a***a**

a*

b

bb

b


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Figure 5. Mineralizable N contents under hedges and alleyways. In this graph, only 3m away sampling positions (N=9) were compared with hedgerow sampling points (n=12) for each sampling depth separately, using a t-test. Error bars indicate the standard error of the mean. Significance differences indicate by different letters along with the degree of significance (*** P < 0.001; **P < 0.01) between hedges and alleyways in each soil depth separately.

Figure 6. Graphs (a) and (b) showed the so-called “saw-tooth” pattern of distribution of soil organic carbon (%) and total N (%) levels down the landscape in the SALT-HI plot; graphs (c) and (d) indicate the fluctuations of organic carbon (%) down the landscape of the FARM and in the ABAN-TEA plots at Doragalla, Sri Lanka.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0-10 cm 10-30 cm

Min

eral

izab

le N

(mg

N p

er g

of s

oil)

Hedge

Alleyway

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b

a**

b

0.0

1.0

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5.0

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liand

ra 3m

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onia 3m

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icid

ia 3m

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il

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anic

Car

bon

(%)

(a)

0.0

1.0

2.0

3.0

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8.0C

allia

ndra 3m

Tith

onia 3m

Glir

icid

ia 3m

Hem

il

Tota

l N (%

)0-10cm10-30cm30-50cmover 50cm

(b)

FARM-PLOT

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0m 5m 10m

15m

20m

Org

anic

C (%

)

0-10cm10-30cm30-50cmover 50cm

ABAN-TEA

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0m 5m 10m

15m

20m

Org

anic

C (%

)

0-30cm30-50cm


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DISCUSSION AND CONCLUSIONS Results of the present study indicate that the

hedgerow species Calliandra and Pennisetum produced higher hedgerow biomass and consequently a higher contribution of nutrients to the SALT-HI system than Gliricidia and Tithonia. Despite the higher foliar nutrient concentrations in Tithonia, Calliandra and the forage grass produced more pruning biomass and therefore served as a better source of nutrients for crops, showing the importance of combination of factors. Calliandra also produced relatively a consistent hedgerow biomass over the years indicating its ability to withstand continuous pruning. In contrast to the performance of the other hedgerow species at the site, N fixing Gliricidia showed a poor growth with a decreased biomass production over time. The relatively poor performance of Gliricidia may be due to its inability to tolerate unfavourable soil/climatic conditions. Samsuzzaman et al. (1999) also concluded that non-N fixing species fared well in terms of pruning biomass and nutrient contributions compared to N fixing hedgerow trees, under acidic soil conditions.

The nutrients provided in the leaves of

hedgerow trees and other organic inputs were compared with the fertilizer dosages recommended by the Department of Agriculture for vegetables in the region. The nutrient additions, especially N and P, to the system through hedgerow prunings seem to be insufficient. Potassium demands were met only through Tithonia and Pennisetum. Phosphorus was not provided in sufficient quantities to meet crop requirements by any of the hedgerow species, despite the fact that P is the most limiting nutrient in acid soils in the tropics. A similar inadequate supply of P through hedgerow prunings has also been established in other studies conducted elsewhere in the tropics (Palm et al., 1991, Salazari et al., 1993, Samsuzzaman et al. 1999). The nutrient additions through hedgerow mulch were compared with those from poultry and cattle manure. Results indicate that poultry manure can provide more nutrients to vegetables than through mulch material per pruning. Poultry manure seems to have provided a sufficient amount of P and K for crops and also sufficient N to meet the basal dosage recommended by the Department of Agriculture. Present results have also indicated that the ability of hedgerow trees to produce more biomass seems to be the decisive factor in adding more nutrients to the system rather than their ability to fix atmospheric nitrogen.

The results of this study have also indicated

that the incorporation of hedgerow prunings over a long time may have a positive effect on some soil physical and chemical parameters. The maintenance of pH around 5.8 in the upper soil layers in both SALT-HI and FARM plots over the value of 4.2 in the nearby ABAN-TEA plot indicates that addition of organic amendments may have contributed to this higher pH level. Bache and Heathcote (1969) showed that farmyard manure reduces soil acidity, probably because of its reaction with Al3+ and this could account for the results from the FARM plot, in spite of the addition of chemical fertilizers over a long period of time.

After 4 years of HI, some soil chemical

parameters clearly indicate a favourable effect on the soil fertility status. Concentrations of organic C, total N, mineralizable N, and organic P, were higher in the SALT-HI plot than that in FARM and ABAN-TEA plots. The soil organic C concentration in the SALT-HI plot was considerably higher, sometimes more than double the concentrations in the FARM and ABAN-TEA plots. The maintenance of a high organic C concentration in the SALT-HI plot may reflect the continuous addition of large quantities of organic matter as mulch. Similar increases in a comparatively short period of HI were also observed by other workers else where in the tropics (Chirwa et al., 2006; Matthews et al., 1992; Jones et al., 1996, Kang et al., 1981). ABAN-TEA plot showed a slightly higher soil organic C compared to the FARM plot indicating continuous cropping might have decrease the soil organic C pool. Incorporation of N-rich organic amendments may also have increased the total soil N and mineralizable N concentrations at the SALT-HI plot compared to other two plots. However, these results further indicate that the addition of inorganic fertilizers for arable crops has little or no effect on building up organic C or organic N levels in the soil. Wild (1988) also stressed that nitrogen reserves cannot be built up by the use of N fertilizers.

The distributions between organic (Po) and

inorganic phosphorus (Pi) levels in different plots have also provided interesting data. The SALT-HI plot contains about 45% of the total P in organic forms with only 15% in the farmer’s plot and 20% in the abandoned tea plot. Sanchez (1976) concluded that 60-80% of the P in tropical soils is usually organic, but can vary out side this range especially in managed tropical soils. High Pi contribution to the total P at the FARM plot is


77

most likely due to the use of P fertilizers in terms of chemical and other P-rich organic amendments such as poultry manure. Furthermore, in SALT-HI plot the available P and Pi showed a significant correlation (R2 = 0.86) while no such correlation was observed in other two plots. The extremely poor available P status in the ABAN-TEA plot indicates the high adsorption capacity at lower pH levels.

Although soil erosion was not measured in the

present experiment, we have observed the formation of natural terraces due to within-alley soil erosion. After 4 years of establishing the hedgerows, approximately 1 m drops were observed in some places from the upper to the lower side of the alleyway. In keeping with the distinct development of forward-sloping terraces at the SALT-HI plot at Doragalla, some chemical parameters also showed a distinct ‘saw-tooth’ pattern of distribution down the landscape, with the drops coinciding with the drops in soil levels between alleys. In contrast, both in FARM and ABAN-TEA plots, no such nutrient distribution pattern were observed. A similar ‘saw-tooth’ pattern was also observed by Turkelboom et al. (1993) who monitored soil organic carbon on an entire slope transect under HI. Agus (1993) observed a linear soil fertility gradient in an experiment where he sampled five points across alleyways between Gliricidia contour hedgerows. A similar linear nutrient gradient was also observed by Samzussaman (1994) in a contour HI system with Senna and upland rice. This heterogeneous nutrient distribution down the landscape under HI has also associated with crop yield reductions in upper alleyways (Agus, 1993; Solera, 1993; Garrity et al., 1995, Basri et al., 1990).

The present study has revealed that nutrients

accumulate under the hedges to a greater extent compared to the open alleyways where the crops are established. This nutrient accumulation under the hedges than in the open alleyways may have been caused by the within-alley soil erosion (scouring effect) and/or due to the effect of hedgerow trees on their immediate environment. This nutrient accumulation has also yielded a distinct ‘saw-tooth’ pattern in both organic C and total N levels down the entire slope with the drops coinciding with the drops in soil level between the hedgerows (Figure 5). This tendency of accumulating soil nutrients under the hedges was observed even in the sub-soil (30-50 cm) levels after 4 years of contour HI. This nutrient accumulation immediately under the hedges may have a negative impact on the crop yield and

consequently will question the suitability of this HI system to improve the crop productivity on slopes.

Overall, the results imply that the addition of

nutrients through hedgerow mulch was not sufficient to meet the requirements recommended by the Department of Agriculture for vegetables in this area. Even though HI has improved the soil nutrient status due to the long-term addition of hedgerow prunings, the present findings support evidence of skewed yield responses observed by many other workers that may have been caused by this patchy nutrient distribution within alleyways. The contour HI has been designed especially for farmers in upland areas to reduce soil erosion and to improve crop productivity through enhanced soil nutrient status, but other considerations such as tree-crop interactions and heterogeneous soil nutrient distribution could make this system unattractive to farmers.

Nutrient accumulation in the immediate

vicinity of the hedges, where crops could not be grown, and relatively poorer soils in the open alleyways where cropping will occur is a potentially a serious consequence of contour HI. The “saw-tooth” pattern observed with some soil nutrient parameters in the present study was a clear indication of within-alley soil erosion. Consequently the eroded soils from the upper slopes get trapped in the lower slopes near the hedges, forming natural terraces. This is precisely what was intended by the erosion control measures. A similar tendency towards rapid terrace development was frequently observed in contour HI trials on a range of soil types (Sajjapongse, 1992). One can argue that this scouring effect can get dissipated with time but it is not known how long this process would take under different management conditions and also under different agro-ecological zones. It would be interesting to address this issue by sampling after another 10-20 years time. However, this patchiness of soil fertility may reduce the value of such contour HI systems at least when it occurs on steep slopes.

ACKNOWLEDGEMENTS The author wishes to thank to Cambridge

Commonwealth Trust, UK for providing funds to carryout this research and also the Department of Geography, University of Cambridge, UK for laboratory facilities to carryout soil and plant chemical analyses and gratitude to Dr. B. W.


78

Bache for his kind guidance and assistance during this study.

REFERENCES Agus, F. (1993). Soil Processes and Crop Production under Contour Hedgerow Systems on Sloping Oxisols. Ph. D dissertation, North Carolina Sate University, Raleigh, NC Agro-Ecological Zones Project (1978-1981). Vol. 1-4. World Resources Report No. 48, FAO, Rome Bache, B.W. (1976). Determination of cation exchange capacity of soils. Journal of Agricultural Food Science 27: 273-280. Bache, B.W. and Heathcote, R.G. (1969). Long-term effects of fertilizer and manure on soil and leaves of cotton in Nigeria. Experimental Agriculture 5: 241-247. Basri, I.H., Mercado, Jr. A.R. and Garrity D.P. (1990). Upland rice cultivation using leguminous tree hedgerows on strongly acid soils. Agronomy, Plant Physiology and Agroecology Division, International Rice Research Institute, Los Baños, Philippines. Bremner, J.M. and Mulvaney, C.S. (1982). Nitrogen- Total. In: Page, A.L., Miller, R.H. and Keeney, D.R. (eds) Methods of Soil Analysis 2. Chemical and Microbiological Properties Agronomy Monograph No. 9, American Society of Agronomy, USA. De Costa, W.A.J.M. and Surenthran, P. (2005). Tree-crop interactions in hedgerow intercropping with different tree species and tea in Sri Lanka: 1. Production and resource competition. Agroforestry Systems 63: 211-218. De Costa, W.A.J.M., Surenthran, P. and Attanayake, K.B. (2005). Tree-crop interactions in hedgerow intercropping with different tree species and tea in Sri Lanka: 2. Soil and plant nutrients. Agroforestry Systems 63: 199-209. Garrity, D.P. (1994). Sustainable land-sue systems for sloping uplands in Southeast Asia. In: Ragland J and Lal, R (eds) Technologies for sustainable Agriculture in the Tropics. American Society of Agronomy, 41-66 pp.

Garrity, D.P. (1996). Tree-Soil-Crop Interactions on slopes. In: Ong CK and Huxley PA (eds) Tree-Crop Interactions. A Physiological Approach. CAB International in association with ICRAF, Kenya, 299-318 pp. Garrity, D.P., Mercado, Jr, A.R. and Solera, C. (1995). The nature of species interference and soil changes in contour hedgerow intercropping systems on sloping acidic lands. In: Kang BT (ed) Alley Farming. International Institute of Tropical Agriculture, Ibadan, Nigeria Genstat 5 Committee (1996). Reference Supplement. NAG, Oxford Jones, R.B., Wendt, J.W., Bunderson, W.T. and Itimu, O.A. (1996). Leucaena + maize alley cropping in Malawi 1. Effects of N, P and leaf application on maize yields and soil properties. Agroforestry Systems 33: 281-294 Kang, B.T., Wilson, G.F. and Sipkens, L. (1981). Alley cropping maize (Zeamays L.) and Leucaena (Leucaena leucocephala LAM) in Southern Nigeria. Plant and Soil 63: 165-179 pp. Kang, B.T., Reynolds, L. and Atta-Krah, A.N. (1990). Alley cropping. Advances in Agronomy 43: 315-359. Lane, P.W. and Payne, R.W. (1996). Genstat for Windows: An Introductory Course, Lawes Agricultural Trust, UK Matthews, R.B., Holden, S.T., Volk, J. and Lung, S. (1992). The potential of alley cropping in improvement of cultivation systems in the high rainfall areas of Zambia. Agroforestry Systems 17: 219-240. NARESA (1991). Natural Resources of Sri Lanka. Conditions and Trends. A report published by the Natural Resources, Energy and Scientific Authority of Sri Lanka. Palm, C.A., McKerrow, A.J., Glasener, K.M. and Szott, L.T. (1991). Agroforestry systems in lowland tropics: Is phosphorus important?. In: Tiessen H., Lopez-Hernandez H. and Sakedo I.H. (eds) Phosphorus Cycles in Terrestrial and Aquatic Systems. Regional Workshop 3, Saskatchewan Institute of Pedology, Saskatoon, Canada, 131-141 pp.


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Panabokke, C.R. (1996). Soil and Agro-ecological Environments of Sri Lanka. Natural Resources, Energy and Scientific Authority of Sri Lanka. Ranasinghe, D.M.S.H.K. (1996). Agroforestry in Sri Lanka: An overview. In: Huxley PA and Ranasinghe DMSHK (eds), Agroforestry for Sustainable Development in Sri Lanka. Proceedings of the participatory training course, September 1994, Colombo, Sri Lanka. Sajjapongse, A. (1992). Management of sloping lands for sustainable agriculture in Asia: An Overview. In: Technical report on the Management of Sloping Lands for Sustainable Agriculture in Asia, Phase 1 (1988-1991). Network Document No. 2, International Board for Soil Research and Management, Bangkok, Thailand. Samsuzaman, S. (1994). Effectiveness of alternative management practices in different hedgerow-based alley cropping systems. PhD dissertation, University of the Philippines at Los Baños. Samsuzaman, S., Garrity, D.P. and Quintana (1999). Soil property changes in contour hedgerow intercropping systems on sloping land in the Philippines. Agroforestry Systems 46: 251-272.

Sanchez, P.A. (1976). Properties and Management of soils in the Tropics. John Wiley and Sons, New York, USA. Salazari, A., Szott, L.T. and Palm, C.A. (1993). Crop-tree interactions in alley cropping systems on alluvial soils of the Upper Amazon basin. Agroforestry Systems 22: 67-82. Solera, C.R. (1993). Determinants of competition between hedgerow and alley species in a contour hedgerow intercropping systems. PhD thesis, University of the Philippines. Waring, S.A. and Bremner, J.M. (1964). Ammonium production in soil under waterlogged conditions as an index of nitrogen availability. Nature 201: 951-952. Wild, A. (1988). Plant nutrients in soil: Nitrogen. In: Wild A (ed) Russell’s Soil Conditions and Plant Growth. Longmans, UK. Young, A. (1989). Agroforestry for Soil Conservation. International Council for Research and Agroforestry and CAB International, Wallingford, UK.

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