Farmer participatory research to minimize soil erosion on steepland
vegetable systems in the Philippines
D.D. Poudel
a,∗, D.J. Midmore
b, L.T. West
caDepartment of Agronomy and Range Science, University of California, Davis, CA 95616, USA
bSchool of Biological and Environmental Sciences, Central Queensland University, Rockhampton, QLD 4702, Australia cDepartment of Crop and Soil Sciences, University of Georgia, Athens, GA 30602, USA
Received 18 May 1999; received in revised form 21 September 1999; accepted 27 October 1999
Abstract
Soil erosion coupled with productivity decline is considered a major constraint to sustainable vegetable production in Southeast Asian steeplands, yet soil conservation technologies acceptable to vegetable growers have not been developed. Effectiveness of high-value contour hedgerows species [(Asparagus (Asparagus officinalis L.), pineapple (Ananas comosus (L.) Merr.), pigeon peas (Cajanus cajan (L.) Millsp.), lemon grass (Cymbopogon flexuosus (Nees ex Steud.) Wats.), and tea (Camellia sinensis (L.) O. Kuntze)] on control of steepland erosion was evaluated in a replicated researcher-managed field experiment, and 12 farmer-managed erosion-runoff plots from 1995 to 1998 across the landscape of the Manupali watershed in Mindanao, the Philippines. Annual soil loss from 42% slopes with superimposed researcher-managed high-value contour hedgerows treatment (45.4 Mg ha−1) was lower by 30% compared to the conventional practice of up-and-down cultivation
(65.3 Mg ha−1). Annual soil loss measured in farmers’ plots ranged from 1.4 Mg ha−1 to 52.5 Mg ha−1on slopes ranging
from 16 to 65%. Soil pH, organic C, total-N, and P downslope were greater by 7, 28, 13, and 10%, respectively, compared to upslope. Total-N, organic C, soil pH, Mg, and K measured at the end of the experiment in the researcher-managed contour hedgerows plots were lower by 45, 20, 30, 53, and 70%, respectively, compared to initial values. The Erosion-Productivity Impact Calculator (EPIC) model was used to assess the effects of annual cropping sequences under a contour hedgerow system on slopes ranging from 15 to 65%. The cabbage (Brassica oleracea var. capitata L.)-tomato (Lycopersicon
es-culentum Mill.)-cabbage sequence (the first crop planted in January) resulted in an average simulated annual soil loss of
28.1 Mg ha−1across slope ranging from 15 to 65%, whereas tomato-cabbage-tomato resulted in an annual simulated soil loss
of 98.3 Mg ha−1. The cropping sequence of tomato-cabbage-tomato lost 3.0 Mg ha−1more soil at 15% slope than did the
cabbage-tomato-cabbage sequence, while at 65% slope, the tomato-cabbage-tomato sequence lost 181.2 Mg ha−1more than
the other sequence. On average, two-thirds of the total soil loss occurred during September–December. In order to reduce soil loss and increase productivity in steep sloping lands, high-value contour hedgerows with sequential cropping sequences that include either corn (Zea mays L.) or cabbage rather than tomato during the most erosive period of the year and vari-able fertility management strategies along the slope positions are suggested. The coincidence of predicted to actual soil loss from farmer-managed plots, based largely upon model development from researcher-managed plots, and the acute awareness
∗Corresponding author. Tel.:+1-530-752-2023;
fax:+1-530-752-4361.
E-mail address: ddpoudel@ucdavis.edu (D.D. Poudel)
instilled amongst farmer cooperators of the magnitude of soil loss, strengthen the argument for farmer participatory research. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Soil erosion; Cropping sequences; Contour hedgerows; EPIC model; Steepland; Farmer participatory research
1. Introduction
Commercial vegetable production on steeplands of Southeast Asian countries has expanded considerably over the past decades, due to increasing demand for fresh vegetables in the lowland Asian cities (Pres-bitero et al., 1995; Midmore and Poudel, 1996; Cox-head, 1997). Population growth and bans and import restrictions imposed by governments on selected veg-etables in some Southeast Asian countries are largely responsible for increased domestic supply and demand for fresh vegetables, leading to the expansion of veg-etable crop production in ecologically fragile areas (Coxhead, 1997). Productivity decline induced by soil erosion is one of the major problems constraining the sustainability of agricultural crop production in the steeplands of Southeast Asia (Hashim et al., 1995; Presbitero et al., 1995; Midmore and Poudel, 1996). Without doubt, expansion of steepland vegetable pro-duction in Southeast Asian highlands has, besides re-sulting in increased soil erosion, also led to significant externalities such as sedimentation of waterways and reservoirs, nutrient losses in runoff water, and decline in down-stream water quality (Midmore et al., 1996, 1997).
In attempts to stem and manage soil erosion on steeplands, researchers have evaluated a number of erosion control technologies, such as alley cropping, contouring, strip cropping, and grass barriers (Tacio, 1993; Comia et al., 1994; Paningbatan et al., 1995; Presbitero et al., 1995; Sombatpanit et al., 1995). Alley cropping, which is a special form of an agroforestry system in which food crops are grown in alleys formed by hedgerows of trees or shrubs (Kang et al., 1986), has been effective in minimizing soil erosion on steep-lands (Tacio, 1993; Comia et al., 1994; Paningbatan, 1994). Considerable interest on the part of various research and extension agencies, especially in South-east Asia, exists in the planting of leguminous con-tour hedgerow trees on sloping lands as barriers for soil erosion and for the production of green manure
for field crops. However, there is no strong evidence of farmers’ adoption of these technologies (Fujisaka, 1989; Garrity, 1993; Comia et al., 1994; Poudel et al., 1998).
The practice of growing crops in sequence in the same field, which is also known as sequential crop-ping, is considered as one of the major tools of soil conservation (Troeh et al., 1991; Amir, 1996). Se-quential cropping provides a crop cover during most of the year. The crop cover during rainfall events re-duces kinetic energy of the raindrops and increases the infiltration rate, which minimizes runoff and soil loss. Presbitero et al. (1995) reported as much as 25 times more soil loss from bare plots than from plots with multiple cropping of corn and peanut (Arachis hypogaea) with Leucaena leucocephala con-tour hedgerows. Hashim et al. (1995) reported about 11 times more soil loss from a bare plot compared to a plot planted with cocoa (Theobroma cacao) in Malaysia. Several factors have been reported as major determinants of soil loss and runoff, especially with annual cropping systems. These include the number of days required for the development of a full canopy cover, planting distances, time of planting, fertility level, and soil management practices (Hudson, 1957; Aina et al., 1976; Lal, 1977; Paningbatan, 1994; Mid-more et al., 1996). Steepland vegetable production systems in the highlands of Southeast Asia are char-acterized by their broad range of cropping patterns, such as sole cropping (e.g. potato (Solanum tuberosum L.)-fallow), monoculture (e.g. cabbage-cabbage), and sequential cropping (e.g. tomato-corn-cabbage) (Tau-tho and Kumori, 1991; Jansen et al., 1995; Poudel, 1995; Midmore et al., 1996). Because of the multiplic-ity of possible cropping sequences, field experiments to evaluate their effects on soil loss and runoff water may be constrained by time and resources.
Soil Erosion-Soil Productivity Research Planning Committee, 1981). While the well-tested empirical Universal Soil Loss Equation (USLE) can be modi-fied to cope with steepland situations in the tropics (Lo, 1994), process-based models are generally pre-ferred (Paningbatan, 1994; Hashim et al., 1995). The Erosion-Productivity Impact Calculator (EPIC) model (Williams et al., 1984) is a comprehensive field-scale model that operates on a daily time step (King et al., 1996). It is process-based and has been tested exten-sively in a number of local, regional, and national studies in continental USA and Hawaii (Steiner et al., 1987; Williams, 1991; Phillips et al., 1993; Richard-son and King, 1995; King et al., 1996; Cavero et al., 1998). EPIC has the capability to simulate complex crop rotations for decades and centuries; it is designed to help the decision makers to evaluate alternative cropping systems, and predict their socio-economic and environmental sustainability (Cabelguenne et al., 1990; Jones et al., 1991).
Farmer participatory research (FPR) has been proposed as an approach in developing appropriate agricultural systems that are indisputably acceptable to farmers yet contribute to the improvement and mainte-nance of agricultural sustainability and environmental quality (Fujisaka, 1989; NRC, 1991; Edwards et al., 1993; Cox et al., 1996; Rhoades, 1997). Rhoades and Booth (1982) developed the Farmer-Back-to-Farmer model which was a forerunner to the participatory ap-proach. This model begins and ends with farmer, and it involves four activities: (1) farmer-scientist diagnosis (2) interdisciplinary team research (3) on-farm testing and adaptation, and (4) farmer evaluation/adaptation. Thus, the farmer is considered as an ‘expert’ member of the interdisciplinary team and is integrally engaged in problem identification, definition, and solution design.
In the upper slopes of the Manupali watershed in northern Mindanao in the Philippines, soil erosion on commercial vegetable farms was reportedly largely responsible for a declining crop productivity. As part of a larger project on Sustainable Agriculture and Natural Resources Management (SANREM-CRSP) (NRC, 1991), farmer participatory research on soil erosion management was started in 1994. The ob-jectives of this research were: (1) to measure soil erosion losses on farmers’ fields using Farmer Partic-ipatory Research approach, (2) to assess the effects of
cropping sequence on soil erosion in steepland veg-etable systems, and (3) to provide recommendations for choice of cropping sequences to farmers, credit agencies and agricultural technicians for enhanced production system sustainability.
2. Methods
2.1. Description of the study area
The study area is located in the Manupali
wa-tershed (124◦47′ to 125◦08′E and 7◦57′ to 8◦08′N)
(Kanemasu et al., 1997) in northern Mindanao, the Philippines. The soil parent materials were thick de-posits of siliceous volcanic ejecta, either deposited in place (volcanic cone) or transported from ups-lope as colluvial or alluvial materials. Elevations in the watershed range from 320 m above sea level (masl) to 2938 masl. This watershed has four broad geomorphic units: the Mountains (1400–1900 masl), the Upper Footslopes (700–1400 masl), the Lower Footslopes (370–700 masl), and the Alluvial Terraces (320–370 masl) (West et al., 1997). According to FAO classification system, soils in the Mountains were Silic Andosols, Haplic Acrisols, and Dystric Cambisols, while those in the Upper and the Lower Footslopes were Lixic Ferralsols and Haplic Ferralsols as were those in the Alluvial Terraces.
Mean annual precipitation (1994–1996) measured at a weather station in the watershed (1500 masl) was 2825 mm (Table 1). Rainfall is not equally distributed throughout the year, but there is normally no month
with<100 mm of rainfall. Mean monthly minimum
and maximum air temperatures were 15.5 and 26.4◦C,
respectively. Major crops grown are corn, sugarcane (Saccharum officinarum L.) and rice (Oryza sativa L.) at the lower elevations, whereas tomato, potato, sweet pea (Pisum sativum L.), cauliflower (Brassica
oler-aceavar. botrytis L.), cabbage and other leafy
vegeta-bles are dominant field crops in the upper elevations.
2.2. Formal survey and farmer-scientist diagnosis
Table 1
Mean monthly rainfall, temperature, and relative humidity for the research site in the Manupali watershed, Mindanao, the Philippines (1994–1996)a
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Annual
Precipitation mm 150 124 97 164 363 327 194 412 242 421 118 213 2825
Number of days with precipitation 21 16 17 16 24 27 22 23 21 27 20 21 256 Maximum air temperature◦C 23.2 23.1 23.8 24.5 24.5 24.1 23.6 23.7 23.4 23.4 26.4 25.4 24.1
Minimum air temperature◦C 15.5 15.2 15.5 16 16.8 16.7 16.6 16.4 16.7 16.5 17.4 17.5 16.4
Relative humidity % 88 87 85 85 86 89 86 87 86 85 88 87 86.5
aWeather data recorded in SANREM-CRSP automatic weather station at Bulogan, Lantapan.
inputs-outputs, market infrastructure, profitability, etc., in the Manupali watershed (Poudel et al., 1998, 1999a). Seven preliminary transects within vegetable production zones were identified for diagnostic sur-vey. These transects were discussed with and the interviewing schedule was reviewed by, leading veg-etable farmers and political figures of the area. Eight
barangays or villages (Capitan Juan, Kaatuan, Alanib,
Sungco, Cawayan, Victory, Kibangay, and Basac) were identified as survey sites representing vegetable growing zones. The total number of vegetable grow-ers in each barangay was obtained from data recorded by the Municipal Agricultural Office (MAO), Lanta-pan, and the sample size was determined to represent 15% of that population. A composite random soil sample (0–15 cm) was collected from the main parcel of vegetable land and nearby uncultivated (i.e. from under hedges or the field perimeters) land for each of the respondents surveyed. Each of these soil samples were analysed to determine the soil texture (Day,
1965), soil pH in both 1 : 1 H2O, and 1 : 2 CaCl2
so-lution, organic C (Nelson and Sommers, 1982), and
exchangeable Ca extracted with NH4OAc at pH 7
and measured by atomic absorption spectrophotome-try (Blakemore et al., 1987). For the same parcel the representative natural slope and slope of the superim-posed rows was measured with a Sunnto clinometer.
Results from the above survey were presented to the respondents in a 2-day farmers’ workshop organized in April in the following year (Poudel, 1995). Objectives of this workshop were to validate the survey results, to identify indigenous technology systems that would be useful to minimize soil and nutrient losses from vegetable fields, to determine future research activities on soil erosion control, and to identify farmer cooper-ators for on-farm soil conservation research. Based on this highly interactive process, two issues on
steep-land vegetable production systems were identified: declining land productivity due to soil erosion, and increasing incidence of diseases and pests. In order to minimize soil and nutrient losses and improve farm productivity and income, a set of alternative technolo-gies to the traditional farmer up-and-down the slope plantings, such as high-value contour hedgerows, contouring, and tree-vegetable intercropping were identified as practices favoured by farmers for exper-imentation. Planting contour hedgerows across the slope was identified as the most acceptable conserva-tion practice. However, most farmer participants did not like the conventional hedgerow species because of the following reasons: (1) reduction in arable land area, (2) shading of vegetable crops due to lateral spread over the field by e.g. Flemingia macrophylla (a leguminous tree), (3) requiring regular mainte-nance, (4) not providing immediate economic return, and (5) unavailability of planting material. Napier grass (Pennisetum purpureum (K.) Schum.) was not popular because its roots spread laterally. However, one farmer suggested making a ditch around the rows of napier so that roots cannot spread out in the field. Farmers wanted to use high-value crops as hedgerows in their vegetable fields. The following were the crops vegetable growers were interested to test as hedgerows: asparagus, pineapple, pigeon peas, lemon grass, and tea. Results from tree-vegetable intercrops are considered elsewhere (Nissen et al., 1999).
2.3. Design and implementation of participatory field experiments
Fig. 1. Methodological framework for farmer participatory soil conservation research in the Manupali watershed, Mindanao, the Philippines.
Data were collected from both for seven cropping seasons.
2.3.1. Researcher-managed field experiment
A researcher-managed field experiment was set up in a site selected by the farmers and researchers.
The researcher-managed site had 24 erosion-runoff
plots (19 m×8 m each) at on average 42%
These soil-collecting buffers were covered by tents to avoid mixing of eroded soil and direct rainfall falling in the buffer zone, and each erosion-runoff plot had a pair of runoff-collecting barrels at the bottom. The first barrel had 10 equal-sized holes, one linked to the next barrel with a connecting hose to collect one-tenth of run-off water from the first barrel. Eroded soils were collected after every rain event and weighed. This experiment tested 12 treatments (four erosion conservation practices (i.e. contour hedgerows spaced between up-and-down cultivation of vegetables, contour planting of vegeta-bles, strip cropping of vegetables and beans
(Phase-olus vulgaris L.), and the farmers’ usual practice,
up-and-down cultivation of vegetables) by three crops (i.e. tomato, corn, and cabbage combinations) in a replicated randomized block design. There were three annual cropping sequences (first crop planted in Jan-uary): tomato-corn-cabbage, cabbage-corn-tomato, and corn-cabbage-tomato. A wide range of data were collected from the researcher-managed site including, rainfall amount, soil and runoff losses, tillage prac-tices, crop yields, crop cover, nutrient losses, and soil scouring. Composite soil samples (0–15 cm) were col-lected from original soil surface and at the end of the experiment for all erosion-runoff plots. Results from this experiment are presented elsewhere (Poudel et al., 1999b).
Table 2
Site characteristics and selected soil chemical properties of the original soil surface (0–15 cm) on farmer-managed erosion-runoff plots in the Manupali watershed, Mindanao, the Philippines
Plot Location Elevation Natural Total-N Organic C pH-H2O P Ca Mg K
ID (masla) slope (%) (g kg−1) (g kg−1) (mg kg−1) (cmol
ckg−1) (cmolckg−1) (cmolckg−1)
1 Sungco 1180 31 4 46 5.3 2.9 1.8 1.1 0.6
2 Mapawa 1305 23 4 53 5.1 1.8 1.6 1.3 0.5
3 Mapawa 1315 40 4 64 5.0 2.1 2.3 1.8 0.6
4 Mapawa 1345 33 4 78 5.0 2.1 1.2 1.1 0.4
5 Cawayan 1185 20 2 38 6.3 4.6 3.8 1.0 0.5
6 Cawayan 1200 16 2 43 4.8 2.1 2.7 0.9 0.3
7 Cawayan 1205 20 3 57 5.2 3.9 2.7 1.8 0.8
8 Victory 1210 37 5 73 5.9 8.1 7.4 2.0 0.9
9 Victory 1310 36 1 51 4.7 4.2 2.3 1.1 0.3
10 Kibangay 1280 44 3 46 4.7 4.9 0.9 1.1 0.5
11 Kibangay 1470 65 4 50 5.1 1.4 1.8 0.7 0.4
12 Basac 1000 62 4 43 5.0 3.2 1.7 2.1 0.9
aMeters above sea level.
2.3.2. Farmer-managed field experiment
There were 12 farmer-managed erosion-runoff plots across the landscape of the Manupali watershed (Table 2). As in the researcher-managed erosion-runoff plots, the high-value species contour hedgerows in the farmer-managed erosion-runoff plots included (from top to bottom): asparagus, pineapple, pigeon peas, and lemon grass which replaced tea after the first season. Slopes for the farmer-managed research plots ranged between 16 and 65% (Table 2). As in the researcher-managed erosion-runoff plots, each erosion-runoff plot was demarcated with galvanized iron sheets set at 22.5 cm above and 20 cm below the ground surface. Each erosion-runoff plot had a leveled soil collecting buffer at its base.
The number of contour hedgerows on farmer-managed erosion-runoff plots varied according to their natural slope, as vegetable fields were placed
into one of three categories: <25% slope, 25–40%
slope, and >40% slope (Poudel, 1995). The first category represented relatively gentle sloping areas and were mostly plowed by draft animals. Vegetable fields under the second and the third categories were cultivated without the use of draft animals. The ac-ceptable distances between contour hedgerows for
these three slope categories were: 7 m for <25%
Table 3
Measured and simulated annual soil loss and simulated annual runoff on farmer-managed erosion-runoff plots in the Manupali watershed, Mindanao, the Philippinesb
Plot ID Cropping sequence Annual soil loss Simulated annual runoff (mm)
Measured (Mg ha−1) Simulated (Mg ha−1)
1 Cabbage-fallow-tomatoa 16.5 11.0 56
2 Fallow-corn-cabbage 18.2 14.5 37
3 Cabbage-corn-potato 26.6 33.9 77
4 Fallow-cabbage-potato 34.0 45.3 81
5 Fallow-fallow-fallow 23.7 22.9 40
6 Cabbage-fallow-tomato 8.4 6.0 23
7 Sweet pepper-fallow-cabbage 13.4 11.8 33
8 Cabbage-potato-cauliflower 1.4 Nab Nab
9 Fallow-fallow-potato 23.1 18.1 87
10 Potato-cauliflower-sweet pea 52.5 57.7 58
11 Fallow-fallow-potato 17.1 27.3 123
12 Fallow-fallow-potato 19.1 23.6 118
aThe first, second and the third crops represent January, May and September plantings, respectively. bNot available.
the four hedgerow species planted at 4 m intervals, those with 25–40% slope had asparagus, pineap-ple and pigeon pea planted at 5 m intervals, and
those with<25% slope had asparagus and pineapple
planted at 7 m intervals. Cropping sequences (first crop planted in January–February) in farmer-managed erosion-runoff plots included: cabbage-fallow-tomato, corn-cabbage, cabbage-corn-potato, fallow-cabbage-potato, fallow-fallow-fallow, sweet pepper (Capsicum annuum L. var. annuum)-fallow-cabbage, cabbage-potato-cauliflower, fallow-fallow-potato, and potato-cauliflower-sweet pea (Table 3).
Farmer-managed erosion-runoff plots were visited fortnightly by researchers to make sure that eroded soils had been collected, weighed and recorded prop-erly in the data sheets provided to each farmer coopera-tors. Eroded soils were collected after every rain event. Researchers made their visual observation on pest and disease infestation, crop growth, erosion and weeds regularly. To ensure a better interaction between farm-ers and researchfarm-ers, visits were scheduled on those days when the farmers were available on their farms. Data collection in farmer-managed research con-centrated mainly on soil erosion, tillage practices, crop management, and inputs and output. For the first crop-ping season, eroded soils were collected, air-dried, weighed, and recorded. However, farmers complained of the time and space needed for air-drying.
There-fore, they were provided a bucket to collect moist soil and record volumetrically. The wet soils were cali-brated into dry weight based on researcher managed site. Air-dried weight was 40% of the wet weight. This minimized farmers’ time and risk of loosing eroded soils while air drying. Except for plot number 10, 11 and 12 whose natural slope exceeded 40% (Table 2), the first cultivation of these erosion-runoff plots was done by a draft-animal drawn plow while all other cul-tivation and tillage practices were done by hand. All cultivation and tillage practices were done by hand for plot number 10, 11 and 12.
Composite soil samples (0–15 cm depth) were col-lected prior to planting the first crop (July, 1995) and the end of the experiment (August, 1997). Se-lected chemical properties were determined for soil samples collected from both farmer-managed and researcher-managed erosion-runoff plots. Soil pH was
measured in 1 : 2 H2O. Organic C was determined by
modified Walkley–Black method (Nelson and Som-mers, 1982), while total N was determined by the modified Kjeldahl method (Black, 1965).
Exchange-able K, Ca, and Mg were extracted with NH4OAc at
(0–15 cm) on farmer-managed erosion-runoff plots are presented in Table 2.
2.4. EPIC modeling
The EPIC model was calibrated using data for the tomato-corn-cabbage cropping sequence from the researcher-managed erosion-runoff plots. The tomato-corn-cabbage cropping sequence was selected
because its annual soil loss (44.1 Mg ha−1) was similar
to the average annual soil loss (45.4 Mg ha−1) for the
three cropping sequences tested: tomato-corn-cabbage, corn-cabbage-tomato and cabbage-tomato-corn in the researcher-managed site. The annual soil loss values for corn-cabbage-tomato and cabbage-tomato-corn
were 53.5 and 38.9 Mg ha−1, respectively, suggesting
that corn-cabbage-tomato sequence was the most ero-sive of the three (Poudel et al., 1999b). Higher soil loss in the corn-cabbage-tomato cropping sequence is attributed to less canopy cover during the erosive months of August through October. EPIC model calibration was also done for a fallow-fallow-fallow cropping sequence on farmer-managed plot 5. The fallow crop (best fit given by a substituted sorghum (Sorghum bicolor (L.) Moench.) hay crop to simulate (Imperata cylindrica (L.) Beauv.) was established in October in 1995, and the calibration was undertaken for fallow starting then.
The Green-Ampt infiltration equation available in the EPIC model was used to estimate runoff. Soil loss was estimated using the small watershed version of the Modified Universal Soil Loss Equation. Weather files were developed based on the 3 years’ weather data col-lected at the weather station in the watershed, which was approximately 2 km from the researcher-managed experimental site. The weather file included daily records on precipitation, temperature, solar radia-tion, and relative humidity. Since wind velocities were lacking in this dataset, the Priestley–Taylor method that requires only radiation and temperature was used to estimate the potential evapotranspiration. Soil information for each of the nine horizons from the surface to 1.9 m depth was obtained from pro-file sampling and analyses (Poudel and West, 1999). Crop parameters used were from the USDA crop file provided in the model. The model was initialized as close as possible with the measured annual soil loss values of tomato-corn-cabbage cropping sequence.
The model was validated with independent data sets from replicated cropping sequences: cabbage-tomato-corn, corn-cabbage-tomato, fallow-fallow-potato, and cabbage-fallow-tomato from farmer-managed and researcher-managed experiments, by comparing pre-dicted values to the measured values. The effective-ness of the model for soil loss and runoff prediction under steepland vegetable systems was evaluated using statistical measures including mean, standard deviations, and the root mean square error (RMSE). The RMSE values for each cropping sequence was calculated as follows:
where, Oi are observed values and Si are simulated
values, and N is the number of observations. This method is commonly used to evaluate model perfor-mance (Smith et al., 1996). The smaller the RMSE, the closer the agreement between simulated and observed values.
Three-year simulation runs predicting annual soil loss and runoff with 15, 25, 35, 45, 55, and 65% slopes were made for each of the selected cropping se-quences: tomato-fallow-fallow, cabbage-fallow-fallow, cabbage-fallow, tomato-corn-fallow, fallow-corn-cabbage, tomato-fallow-corn-cabbage, cabbage-tomato-corn, cabbage-corn-tomato, tomato-cabbage-tomato, cabbage-tomato-cabbage and corn-cabbage-tomato. These cropping sequences represent those commonly used for commercial vegetable production in the wa-tershed (Poudel, 1995; Poudel et al., 1998). Simulated plantings took place during the first week of January, May and September, as is common in the region.
2.5. Farmer cooperators survey
the future improvement of FPR on soil erosion? The survey information was analyzed and reported.
3. Results and discussion
3.1. Soil loss from high-value contour hedgerows erosion-runoff plots
In the researcher-managed site, annual soil loss measured from the high-value contour hedgerows
treatment (45.4 Mg ha−1) was lower by 30% compared
to the conventional practice of up-and-down
culti-vation (65.3 Mg ha−1). This suggests that high-value
contour hedgerows are effective measures to minimize soil erosion on steepland vegetable systems. Poudel et al. (1999b) discussed in detail the effectiveness of high-value contour hedgerows in minimizing soil movement down the slope, and the management of natural terraces formed in between contour hedgerows. Annual soil loss measured on farmers’ plots ranged
from 1.4 Mg ha−1to 52.5 Mg ha−1with an average of
21.2 Mg ha−1(Table 3). There was a good agreement
between measured and simulated soil losses. One of the farmer-managed erosion-runoff plots (plot 5) rep-resented an unplanted plot (fallow-fallow-fallow) and
lost 23.7 Mg ha−1year−1, equivalent to establishment
Table 4
Selected chemical properties of original soil surface (0–15 cm), eroded sediment, and final soil surface (0–15 cm) on contour hedgerows treatment in researcher-managed erosion-runoff plots in the Manupali watershed, Mindanao, the Philippines (n=6)
Total-N Organic C pH-H2O P Ca Mg K
(g kg−1) (g kg−1) (mg kg−1) (cmol
ckg−1) (cmolckg−1) (cmolckg−1)
1. Original 4.2 (0.5)a 54 (5.5) 5.3 (0.1) 3.9 (0.9) 2.6 (0.6) 1.5 (0.2) 1.0 (0.1) 2. Eroded soil
First cropping season 2.6 (0.9) 68 (4.6) 5.3 (0.1) 9.5 (1.5) 6.3 (0.8) 1.7 (0.2) 0.7 (0.1) Second cropping season 4.9 (0.5) 66 (6.4) 4.9 (0.2) 6.1 (1.6) NAb NA 1.1 (0.1)
Third cropping season 4.3 (0.2) 76 (2.6) 4.8 (0.1) 11.7 (1.2) 5.3 (0.8) 2.0 (0.3) 0.9 (0.1) Fourth cropping season 4.1 (0.3) 57 (4.5) 4.7 (0.1) 10.9 (1.1) NA NA 0.6 (0.1) Fifth cropping season 3.4 (0.7) 67 (5.1) 4.3 (0.1) 15.7 (1.8) 5.0 (1.4) 0.8 (0.2) 0.7 (0.0)
Sixth cropping season 1.5c 63 4.3 15.7 2.3 0.1 0.63
Seventh cropping season 2.8 (0.2) 56 (2.8) 4.0 (0.1) 22.7 (4.8) 2.8 (0.5) 0.8 (0.1) 0.3 (0.0) 3. Final soild,e 2.3 (0.2)∗∗ 43 (3.1)ns 3.7 (0.1)∗∗∗ 8.4 (1.2)∗ 2.5 (0.4)ns 0.7 (0.1)∗∗ 0.3 (0.0)∗∗∗
aFigures in paranthese are standard error of mean. bNot available.
cSample size (n)=1.
dStudent-t test for the similarity of the means between the initial properties versus final properties.
e∗,∗∗,∗∗∗Significant at 0.05, 0.01, and 0.001 probability level, respectively, with student t-test. ns indicate not significantly different at
0.05 probability level by student t-test.
of a natural fallow. Most soil loss was during the first unplanted season when rainfall was high. This farmer cooperator could not plant a crop in his erosion plot due to ill health. The farmer cooperator managing Plot 8 established additional contour hedgerows in the plot and planted trees along the hedgerows which resulted in extremely low amounts of annual soil loss
(1.4 Mg ha−1).
3.2. Impacts of soil erosion on soil fertility
Impacts of soil erosion on soil fertility were ev-ident by the end of the experiment on both the researcher-managed experiment and farmer-managed erosion-runoff plots. In the researcher-managed con-tour hedgerows erosion-runoff plots, total-N, soil pH, Mg, and K were significantly lower at the end of the experiment compared to the original values (Table 4). Although statistically not significant,
or-ganic C values decreased from an average of 54 g kg−1
at the start of the experiment to 43 g kg−1 at the end
Table 5
Selected soil properties of the final soil surface (0–15 cm) at the upper, and the lower slope positions of 20 m long farmer-managed erosion-runoff plots in the Manupali watershed, Mindanao, the Philippines (n=12)
Total-N (g kg−1) Organic C (g kg−1) pH P (mg kg−1)
Upper 2.4 (0.2)a 49 (5.3) 4.5 (0.1) 11.8 (4.9)
Lower 2.7 (0.2) 63 (5.9) 4.8 (0.1) 13.0 (4.4)
Difference (lower–upper)b 0.3 (0.2)ns 14 (4.9)∗
0.3 (0.1)∗∗
1.2 (1.7)ns
aFigures in paranthese are standard error of mean.
b∗,∗∗,∗∗∗indicate significantly different at 0.05, 0.01, and 0.001 probability level by paired-comparison t-test. ns indicate not significantly
different at 0.05 probability level by paired-comparison t-test.
average value of 3.9 to 8.4 mg kg−1. However, eroded
soils showed consistently higher values for P com-pared to original soil surface (Table 4). Since P is one of the yield limiting nutrients in these soils (Poudel and West, 1999), unwarranted removal of P from the field requires immediate attention.
In farmer-managed erosion-runoff plots, notable differences in crop establishment and soil qualities between the upper and the lower portions of 20 m long erosion-runoff plots were observed during the study. Soil pH and organic C in the lower portions of the erosion-runoff plots were significantly higher compared to that of upper slope position (Table 5). The lower slope positions showed, on average, 7% greater soil pH and 28% greater organic C. Although statistically not significant, total-N and available P in the lower portions were higher compared to that of the upper portions. Similar results were found in
researcher-managed erosion-runoff plots (Poudel
et al., 1999b). These differences in soil fertility gra-dient across the slope positions suggest the need for a
Table 6
Means, standard deviations (SD), and root mean square errors (RMSE) of measured and simulated annual soil loss and runoff values for the selected cropping sequences in the Manupali watershed, Mindanao, the Philippines
Cropping Size Natural Annual Annual
sequence (n) slope soil loss runoff
Mean (%) SD (%) Measured Simulated RMSE Measured Simulated RMSE
Mean SD Mean SD Mean SD Mean SD
(Mg ha−1) (Mg ha−1) (Mg ha−1) (Mg ha−1) (mm) (mm) (mm) (mm)
Cabbage-tomato-corna 2 41.5 2.5 38.9 16.9 33.1 6.0 23.6 58 12 52 3 17
Corn-cabbage-tomato 2 41.5 1.5 53.5 12.3 53.2 6.1 6.3 51 2 48 2 3
Fallow-fallow-potato 3 54.3 13.1 19.7 2.5 23.0 3.7 7.0 – – 109 16 –
Cabbage-fallow-tomato 2 23.5 7.5 12.5 4.0 8.5 2.5 4.2 – – 40 17 –
aThe first, second and the third crops represent January, May and September plantings, respectively.
variable management strategy to improve soil fertility on these sloping lands.
3.3. Effects of cropping sequence on soil erosion
The simulated annual soil loss for the four repli-cated cropping sequences under high-value contour hedgerows were reasonably close to measured val-ues (Table 6). The cropping sequence of cabbage-tomato-corn showed a relatively larger RMSE value
due to a large difference in actual (55.8 Mg ha−1)
versus simulated annual soil loss (27.1 Mg ha−1)
for one of its two replicates. This replicate (plot 23 in researcher-managed site) had a soil loss of
66 Mg ha−1 during the first cropping season, while
its counterpart (Plot 1 in researcher-managed site)
lost only 2 Mg ha−1of eroded soil. This large
Table 7
Simulated average annual soil loss and runoff for 15, 25, 35, 45, 55, and 65% slope under different cropping sequences in the Manupali watershed, Mindanao, the Philippines
Cropping sequence Soil loss Runoff (Mg ha−1) (mm)
I. Single cropping
Tomato-fallow-fallowa 3.1 59.5 Cabbage-fallow-fallow 6.8 59.5 Fallow-cabbage-fallow 96.6 47.3
II. Double cropping
Tomato-corn-fallow 53.5 49.1
Fallow-corn-cabbage 61.3 47.2
III. Triple cropping
Tomato-corn-cabbage 52.8 47.3
Cabbage-tomato-corn 44.9 49.0
Cabbage-corn-tomato 61.3 48.3
Tomato-cabbage-tomato 98.3 46.2 Cabbage-tomato-cabbage 28.1 46.1
Corn-cabbage-tomato 71.5 45.2
aThe first, second and the third crops represent January, May
and September plantings, respectively.
plots. Several large stones were dug out and trees were uprooted from this plot. Simulated annual soil loss
for fallow-fallow-fallow sequence was 22.9 Mg ha−1
as opposed to the 23.7 Mg ha−1measured value.
With a single vegetable crop per year, average soil loss was least (Table 7) when a fallow was established well before the most erosive time of the year (Septem-ber/October). However, if not constrained by capital or labour, farmers would be unlikely to leave land in fallow during the rainy season, the season when crop-ping is favoured under the essentially rain-fed system. If a single crop was taken in the summer period, fal-low was not sufficiently well established to control soil erosion by September/October. If, however, the fallow followed corn (as seen in the double cropping), the soil loss was considerably less than if following cabbage. Not all multiple cropping sequences were equally effective in reducing soil erosion (Table 7). The crop-ping sequence of tomato-cabbage-tomato (98.3 Mg
ha−1) resulted in nearly three times more soil loss
than that of cabbage-tomato-cabbage (28.1 Mg ha−1).
Hence, consideration must be given to the crops and planting seasons when designing a multiple crop-ping pattern to minimize erosion on steeplands. The reason for low annual soil loss from the cabbage-tomato-cabbage cropping sequence is attributed to
greater canopy cover by cabbage than tomato (Poudel et al., 1999b) during erosive rainfall events that generally occur in the months of March–May and August–October. The importance of canopy cover to minimize soil loss, especially during the months of September–October, was also manifested by the sim-ulated soil loss for the cabbage-tomato-corn sequence (Table 7). Poudel et al., (1999b) reported a greater canopy cover for corn than tomato and cabbage.
Fig. 2 shows the effect of selected multiple-cropping sequences on annual soil loss for different slope cat-egories. It shows that the difference in the amount of soil loss between cropping sequences increases as slope increases. These data support the suggestion by Phillips et al. (1993) that on gentle slopes, cropping sequences have less influence on soil erosion than on steeper slopes.
As for measured soil loss in researcher-managed site (Poudel et al., 1999b), on average, two-thirds of the total soil loss across all cropping sequence x slope simulations occurred during September–December, whereas one-third occurred during May–August (data not shown). Emphasis should be given to those crop-ping sequences that include less erosive crops in their September plantings. Simulation results indicated that
Table 8
Simulated seasonal soil loss with 42% natural slope for May and September plantings of corn, tomato, and cabbage in the Manupali watershed, Mindanao, the Philippines
Cropping seasons Soil loss
(Mg ha−1)
I. May–August
Corn after cabbage 23.4
Cabbage after tomato 20.2
Corn after tomato 17.8
Tomato after cabbage 9.2
II. September–December
Tomato after cabbage 55.4
Tomato after corn 24.1
Cabbage after corn 24.4
Corn after tomato 27.8
in-crop erosion can be affected by the prior crop in the sequence. Table 8 presents simulated soil loss for different crops in their May and September plant-ings at 42% natural slope. The September planting of tomatoes after cabbage showed 130% greater soil loss
(55.4 Mg ha−1) compared to that of planting tomatoes
after corn (24.1 Mg ha−1). This relatively lower soil
loss after corn compared to that after cabbage is at-tributed to the differences on the root systems of the preceding crops. Tan and Fulton (1985) reported the total root length of corn to be twice that of tomato, with a much greater spatial density. This greater root system of corn probably holds soil more strongly, re-sulting in a reduced soil erodibility. Soil loss in tomato is also believed to have been aggravated by the prac-tice of earthing up twice, i.e. taking soil from between the tomato rows forming deep furrows between them, and applying that soil to the base of the tomato plants. This was done only once in corn and cabbage. Earth-ing up apparently increases the erodibility of the soils by lowering the surface bulk density and breaking aggregates. Such soil disturbance would be expected to enhance entrainment and re-entrainment in sloping tomato fields. Rose (1988) described entrainment and re-entrainment of the deposited sediments as impor-tant processes in erosion in sloping lands. Hence, minimizing soil disturbance (i.e. minimal tillage) to reduce soil erodibility appears to be a valid erosion reduction practice in these landscapes.
Predicted annual runoff values for three cropping
sequences: tomato-cabbage-tomato,
cabbage-corn-Fig. 3. Simulated annual runoff for tomato-cabbage-tomato, cabbage-corn-tomato, and cabbage-tomato-cabbage cropping se-quence in the Manupali watershed, Mindanao, the Philippines.
tomato and cabbage-tomato-cabbage are presented in Fig. 3. Although soil loss differed between the three cropping sequences, annual runoff for each was very similar. This suggests that different cropping sequences with a relatively low runoff volume may not reduce soil erosion on sloping lands. As there was no relationship between runoff and soil erosion across cropping sequences, factors other than runoff (e.g. gravity loss, soil disturbance, root distribution) were responsible for the soil loss differences between cropping sequences.
On average, runoff quadrupled as the slope in-creased from 15 to 65% (Fig. 3). This trend suggests that farmers on steeper slopes should adopt practices, such as mulching, to conserve water in their vegetable fields.
3.4. Future prospects of FPR on soil conservation
integrated pineapple with trees in his farm land. As-paragus had poor growth in most farmer’s plots, while pigeon pea suffered from insect pest damage. Lemon-grass was a good hedgerow in all erosion-runoff plots.
Based on the survey results, this participatory research helped farmer cooperators in learning the benefits of contour hedgerows (9 out of 12) and of planting in contour (6 out of 12) for soil conservation. Ten out of 12 farmer cooperators mentioned that they had gained a quantitative impression of the soil loss from their vegetable fields. Most farmer cooperators considered the participatory research approach as an effective tool for technology transfer (11 out of 12). But, farmer cooperators also pointed out several problems in relation to participatory erosion control research. The majority of cooperators (9 out of 12) agreed that input cost and time were the main con-straints to this research. At least two farmer coopera-tors complained about the obstacles to the movement of their draft animals during field operations imposed by iron sheets used in the erosion-runoff plots. Other problems related to this research included: technical-ity in soil collection and air-dry, frustration on the part of farmer cooperators due to poor attendance of their fellow cooperators in regular meetings, labour availability, and capital requirement for the research. Farmer cooperators suggested several measures for the future improvement of FPR on soil conservation. These included: careful selection of farmer coopera-tors who can spend more time for the research, finan-cial assistance to conduct field experiments, regular visits of a farmer’s field experiment by other fellow cooperators, farmer-to-farmer technology transfer, a clear agreement between the researchers and farmers about the use of research materials after the comple-tion of the research, regular meetings, detailed plan of activities, and a strong monitoring of the program.
4. Conclusions
High-value contour hedgerows (e.g. asparagus, pineapple, pigeon pea, lemongrass, and tea) are effec-tive measures to reduce soil erosion on steepland veg-etable systems, and are more acceptable to vegveg-etable growers than are perennial leguminous tree-based hedgerows. Strong fertility gradients exist along the
slope positions of steepland vegetable fields. There-fore, variable fertility management strategies are necessary to improve productivity of these produc-tion systems. A cropping sequence approach to soil erosion management appears to have great potential, since, farmers can plan their least erosive cropping sequences. Choice of appropriate cropping sequence to minimize soil erosion becomes more critical on steeper slopes. Based on simulations in this study, farmers cultivating steep lands can minimize soil ero-sion by adopting a multiple cropping sequence that includes corn or cabbage during the most erosive period of the year.
Acknowledgements
We acknowledge SANREM-CRSP funding through USAID Grant No. LAG-4198-A-00–2017–00 that supported this study. We appreciate farmer coop-erators as well as our field staff Ferdinand Banda, Agamer Mendez, and Jurnito Daguinlay in Lanta-pan for their great enthusiasm and hard work on this participatory research.
References
Aina, P.O., Lal, R., Taylor, G.S., 1976. Soil and crop management in relation to soil erosion in the rain forest region of Western Nigeria. Symposium Proceedings of the National Soil Erosion Conference 25–26 May, 1976, Lafayette, IN.
Amir, J., 1996. Impact of crop rotation and land management on soil erosion and rehabilitation. In: Agassi, M. (Ed.), Soil Erosion, Conservation, and Rehabilitation. Marcel Dekker, New York, pp. 375–397.
Black, C.K. (Ed.), 1965. Methods Of Soil Analysis Part 2. Chemical and Microbiological Properties. American Society of Agronomy Monograph No. 9. Madison, WI, 1572 pp. Blakemore, L.C., Searle, P.I., Daly, B.K., 1987. Methods for
chemical analysis of soils. Soil Bureau Scientific Report 80, New Zealand.
Cabelguenne, M., Jones, C.A., Marty, J.R., Dyke, P.T., Williams, J.R., 1990. Calibration and validation of EPIC for crop rotations in southern France. Agric. Syst. 33, 153–171.
Cavero, J., Plant, R.E., Shennan, C., Williams, J.R., Kiniry, J.R., Benson, V.W., 1998. Application of Epic model to nitrogen cycling in irrigated processing tomatoes under different management systems. Agric. Syst. 56 (4), 391–414.
Comia, R.A., Paningbatan, E.P., Hakansson, I., 1994. Erosion and crop yield response to soil conditions under alley cropping systems in the Philippines. Soil Tillage Res. 31, 249–261. Cox, P., Cottrell, A., Timms, J., 1996. Strategies for achieving
adoption of new technology or alternative management practices. In: Hunter, H.M., Eyles, A.G., Rayment, G.E. (Eds.), Downstream Effects Of Land Use. Department of Natural Resources, Queensland, Australia. pp. 321–326.
Coxhead, I., 1997. Induced innovation and land degradation in developing country agriculture. Aust. J. Agric. Resour. Econ. 41 (3), 305–332.
Day, P.R., 1965. Particle fractionation and particle-size analysis. In: Black, C.A. et al. (Eds.), Methods Of Soil Analysis, Part I. Agronomy 9, ASA and SSSA, Madison, WI, USA, pp. 545–567. Edwards, C.A., Grove, T.L., Harwood, R.R., Pierce Colfer, C.J., 1993. The role of agroecology and integrated farming systems in agricultural sustainability. Agric. Ecosyst. Environ. 46, 99– 121.
Fujisaka, S., 1989. The need to build upon farmer practice and knowledge: reminders from selected upland conservation projects and policies. Agrofor. Syst. 9, 141–153.
Garrity, D.P., 1993. Sustainable land-use systems for sloping uplands in Southeast Asia. In: Technologies For Sustainable Agriculture In The Tropics. ASA Spec. Publ. 56. ASA, CSSA, and SSSA, Madison, WI, pp. 41–66.
Hashim, G.M., Ciesiolka, C.A.A., Yusoff, W.A., Nafis, A.W., Mispan, M.R., Rose, C.W., Coughlan, K.J., 1995. Soil erosion processes in sloping land in the east coast of Peninsular Malaysia. Soil Technol. 8, 215–233.
Hudson, N.W., 1957. Erosion control research: Progress reports on experiments at Henderson Research Station 1953–1956. Rhod. Agric. J. 54, 297–323.
Jansen, H.G.P., Midmore, D.J., Poudel, D.D., 1995. Sustainable peri-urban vegetable production and natural resources management in Nepal. J. Farming Syst. Res./Ext. 5 (2), 85–107. Jones, C.A., Dyke, P.T., Williams, J.R., Kiniry, J.R., Benson, V.W., Griggs, R.H., 1991. EPIC: An operational model for evaluation of agricultural sustainability. Agric. Syst. 37, 341–350. Kanemasu, E.T., Flitcroft, I.D., Li, B., 1997. Sustainable
agriculture: Research or re-search. J. Agric. Meteorol. 52 (5), 409–418.
Kang, B.T., Wilson, G.F., Lawson, T.L., 1986. Alley cropping: A stable alternative to shifting cultivation. International Institute of Tropical Agriculture, Nigeria, p. 22.
King, K.W., Richardson, C.W., Williams, J.R., 1996. Simulation of sediment and nitrate loss on a vertisol with conservation tillage practices. Trans. ASAE. 39 (6), 2139–2145.
Lal, R., 1977. Soil-conserving versus soil-degrading crops and soil management for erosion control. In: Greenland, D.J., Lal, R. (Eds.), Soil Conservation and Management in the Humid Tropics. Wiley, London, pp. 81–92.
Lo, K.F.A., 1994. Quantifying soil erosion for the Shihmen reservoir watershed. Taiwan Agric. Syst. 45, 105–116. Midmore, D.J., Poudel, D.D., 1996. Asian vegetable production
systems for the future. Agric. Syst. 50, 51–64.
Midmore, D.J., Jansen, H.G.P., Dumsday, R.G., 1996. Soil erosion and environmental impact of vegetable production in the Cameron Highlands, Malaysia. Agric. Ecosyst. Environ. 60, 29–46.
Midmore, D.J., Poudel, D.D., Nissen, T., 1997. Inducing change among vegetable farmers in environmentally sensitive ecosystems: The process and some practice in the Philippines. In: John D. Smith Memorial Colloquium-Process Of Community Change 31 Oct–1 Nov, Queensland, Australia, pp. 215–222.
Murphy, J., Riley, H.P., 1962. A modified single solution method for the determination of phosphate in natural waters. Anal. Chim. Acta. 27, 31–36.
National Soil Erosion-Soil Productivity Research Planning Committee, 1981. Soil erosion effects on soil productivity: A research perspective. J. Soil Wat. Conser. 36 (2), 82–90. Nelson, D.W., Sommers, L.E., 1982. Total carbon, organic carbon,
and organic matter. In: Page, A.L., Miller, R.H., Keeney, D.R. (Eds.), Methods Of Soil Analysis, Part II, 2nd Edition. American Society of Agronomy, Madison, WI, pp. 581–594. Nissen, T.M., Midmore, D.J., Cabrera, M.L., 1999. Aboveground
NRC (National Research Council), 1991. Toward Sustainability. A Plan For Collaborative Research On Agriculture and Natural Resource Management, National Research Council. National Academic Press Washington DC, 145 pp.
Paningbatan, E.P., 1994. Management of soil erosion for sustainable agriculture in sloping lands. Philipp. Agric. 77 (1), 117–138.
Paningbatan, E.P., Ciesiolka, C.A., Coughlan, K.J., Rose, C.W., 1995. Alley cropping for managing soil erosion of hilly lands in the Philippines. Soil Technol. 8, 193–204.
Phillips, D.L., Hardin, P.D., Benson, V.W., Baglio, J.V., 1993. Nonpoint source pollution impacts of alternative agricultural management practices in Illinois: A simulation study. J. Soil Wat. Conser. 48 (5), 449–457.
Poudel, D.D. (Ed.), 1995. Proceedings of a Farmers Workshop and Follow-up On Commercial Vegetable Production in Lantapan, Bukidnon, the Philippines, April 20–22, May 11. AVRDC, Taiwan. Mimeo.
Poudel, D.D., Midmore, D.J., Hargrove, W.L., 1998. An analysis of commercial vegetable farms in relation to sustainability in the uplands of Southeast Asia. Agric. Syst. 58 (1), 107–128. Poudel, D.D., Nissen, T.M., Midmore, D.J., 1999a. Sustainability
of commercial vegetable production under fallow systems in the uplands of Mindanao, the Philippines. Mt. Res. Dev. 19 (1), 41–50.
Poudel, D.D., Midmore, D.J., West, L.T., 1999b. Erosion and productivity of vegetable systems on sloping volcanic-ash derived Philippine soils. Soil Sci. Soc. Am. J. 63, 1366–1376. Poudel, D.D., West, L.T., 1999. Soil development and fertility characteristics of a volcanic slope in Mindanao, the Philippines. Soil Sci. Soc. Am. J. 63, 1258–1273.
Presbitero, A.L., Escalante, M.C., Rose, C.W., Coughlan, K.J., Ciesiolka, C.A., 1995. Erodibility evaluation and the effect of land management practices on soil erosion from steep slopes in Leyte, the Philippines. Soil Technol. 8, 205–213.
Richardson, C.W., King, K.W., 1995. Erosion and nutrient losses from zero tillage on a clay soil. J. Agric. Eng. Res. 61, 81–86. Rhoades, R.E., 1997. Pathways towards a sustainable mountain agriculture for the 21st century: The Hindu Kush-Himalayan Experience. International Centre for Integrated Mountain Development, Kathmandu, Nepal. 161 pp.
Rhoades, R.E., Booth, R., 1982. Farmer-back-to farmer: A model for generating acceptable agricultural technology. Agric. Administration 11, 127–137.
Rose, C.W., 1988. Research progress on soil erosion processes and a basis for soil conservation practice. In: Lal, R. (Ed.), Soil Erosion Research Methods. Soil and Water Conservation Society, Ankeny, IA, USA. pp. 119–140.
Smith, J., Smith, P., Addiscott, T., 1996. Quantitative methods to evaluate and compare soil organic matter (SOM) models. In: Powlson, D.S., Smith, P., Smith, J.U. (Eds.), Evaluation Of Soil Organic Matter Models Using Existing Long-Term Datasets. NATO ASI Series I, Vol. 38. Springer, Hiedelberg, Germany. pp. 181–199.
Sombatpanit, S., Rose, C.W., Ciesiolka, C.A., Coughlan, K.J., 1995. Soil and nutrient losses under rozelle (Hibiscus subdariffa L. var. altissima) at Khon Kaen. Thailand. Soil Technol. 8, 235–241.
Steiner, J.L., Williams, J.R., Jones, O.R., 1987. Evaluation of the EPIC simulation model using a dryland wheat-sorghum-fallow crop rotation. Agron. J. 79, 732–738.
Tacio, H.D., 1993. Sloping agricultural land technology (SALT): a sustainable agroforestry scheme for the uplands. Agrofor. Syst. 22, 145–152.
Tan, C.S., Fulton, J.M., 1985. Water uptake and root distribution by corn and tomato at different depths. HortScience 20 (4), 686–688.
Tautho, C.C., Kumori, F., 1991. Analysis of vegetable farming in Bukidnon, Philippines. Philipp. J. Crop. Sci. 26 (1), 29– 37.
Troeh, F.R., Hobbs, J.A., Donahue, R.L., 1991. Soil and Water Conservation. Prentice Hall, New Jersey, USA, 530p. West, L.T., Lawrence, K.S., Dayot, A.A., Tomas, L.M.,
Yeck, R.D., 1997. Micromorphology and soil development as indicators of ash age on Mindanao, the Philippines, pp. 335–343. In: Shoba, S., Gerasimova, M., Miedema, R. (Eds.), Soil Micromorphology: Studies On Soil Diversity, Diagnostics, Dynamics. 10th International Working Meeting on Soil Micromorphology, July 8–13, 1996, Moscow, Russia. Sub-commission of Soil Micromorphology, International Society of Soil Science, Moscow-Wageningen. 452 pp.
Williams, J.R., 1991. Runoff and water erosion. In: Modeling Plant and Soil Systems. Agronomy Monograph no. 31. ASA, Madison, WI, pp. 429–455.
