Innovative farming techniques minimize soil erosion in Prek Thnot watershed

Sokh Heng1, So Thea2 and Edward V. Maningo3

October 2018

Farming has long been identified as a major contributor to siltation in Prek Thnot watershed. Land cultivation disturbs and exposes the soil to wind and rain, hastening topsoil erosion. While the condition of streams and rivers deteriorates due to siltation, farmers suffer soil fertility losses. Surface runoff strips the soil of organic matter and nutrients, which are carried downstream during heavy rains. But these negative impacts of farming on the watershed could soon be a thing of the past if farmers adopt the sustainable farming technologies introduced through a project led by the Institute of Forest and Wildlife Research and Development (IRD). The project is one of the research activities under the Asia-Pacific Network for Sustainable Forest Management and Rehabilitation (APFNet) - funded project “Landscape Approach to Sustainable Management of Forests in Prek Thnot”.

Prek Thnot watershed was selected for the demonstration of agroforestry and soil and water conservation technologies because it is one of the most severely degraded watersheds in Cambodia. Most of the land is devoid of forests as the watershed is dominated by cultivation. The area is drained by the Prek Thnot River and its tributaries, which are used as sources of irrigation water in lowland areas. The APFNet project introduced agroforestry and soil and water control and conservation measures to farmers. The aim was to minimize the harmful impacts of cultivation by preventing topsoil erosion while increasing farm productivity. Agroforestry technology was implemented in farmers’ fields through participatory action research. The farmers were involved in piloting agroforestry and soil and water conservation techniques. The design 1 Director, Institute of Forest and Wildlife Research and Development (IRD)2 Deputy Director, IRD3 Project Technical Advisor

combines vegetative cover, soil control structures and cultural practices.

Four farmer collaborators from Trapeang Chour and Krang Dei Vay communes in Prek Thnot watershed eagerly agreed to test the soil and water conservation technologies on their farms. Support included the provision of elephant grasses that are very useful for their cattle, and technical knowledge and skills. The soil and water control measures include contour canals planted with hedgerows, and also fascines.4 The agroforestry farms were planted with fruit trees inter planted with vegetables. In order to understand the importance of the soil and water control measures, the farmers were involved by the project in the monitoring the soil erosion and the rainfall in their respective farms. Along the contour canals, soil traps were installed to collect the soils eroded and rain collecting jars were used to record rainfall.

As an incentive, the project provided farmer-cooperators with planting materials and training. Seeds and seedlings were selected based on the crops the farmers were interested in planting. Training covered agroforestry and soil and water conservation techniques. Farmers were shown how mixing grasses and trees naturally produces humus within the topsoil; how establishing contour canals on sloping land helps retain topsoil and slow surface runoff; and how to use an A-frame, a simple tool, to locate contour lines on their land.

The project guided the farmers on the planting arrangement and the establishment of soil and water conservation measures. The crops planted on the pilot farms include Napier or elephant grass (Pennisetum purpureum), which is ideal for controlling runoff on fine sandy soils. Some farmers planted lemongrass to stabilize the contour bunds. Moringa, mango, jackfruit and papaya were also planted in some parts. The fruit trees were mostly planted along farm boundaries and under planted with cash crops such as ginger, pineapple and galangal. Intercropping was implemented to diversify agricultural production. Some farmers planted premium tree species such as Kranhuong (Dalbergia cochinchinensis), Thnong (Pterocarpus mcrocarpus), Chheuteal (Dipterocarpus alatus) and Korki (Hopea odorata). The farmers believe that these native trees can help restore valuable timber species that have been lost through clear cutting. 4 Fascines are bundle of twigs, branches that are laid along the contour of the

farm to control erosion

Small bunds established around the fruit trees to contain topsoil and litter (top); contour canal stabilized with lemongrass (bottom)

Farmers learning how to use an A-frame to locate the contours on their farmland

Crops planted by the farmers

All the farmer-cooperators reported that the agroforestry techniques and soil and water conservation measures are beneficial to them. After only a few rainfall occurrences, farmers could see how contour canals and fascines trap a considerable amount of nutrient-rich topsoil (Table 1), which otherwise would have been lost). Throughout project implementation, the farmers worked with the project team to analyze soil erosion from their fields. The results indicate that soil erosion ranges from 14.82 ton/ha/year in Krang Dei Vay commune and 32.97 ton/ha/year in Tropeang Chour. The results also indicate a significant increase in economic benefits compared to baseline (Table 2).

Table 1. Result of soil analysis Soil Proper-

tiesLocation Difference Remarks

Outside contour

canal

In the contour

canal

Soil acidity (pH)

7.65 6.57 -1.08 lower

Organic mat-ter %

2.92 313.00 310.08 significantly higher

Carbon % 1.70 1.82 0.12 increased

Nitrogen % 0.16 0.17 0.01 increased

Phosphorus % 0.044 0.045 0.001 improved

Potassium % 0.96 1.44 0.48 improved

Farmers collecting topsoil deposited in the contour canal

Table 2. Total value of agroforestry benefits5

FarmerLand size (ha)

(agroforestry and housing)

Total benefit (USD/Yr)Baseline

(2016) Endline (2018)

Siv Lim 0.50 81 204

Men Vorn 0.80 80 203

Kim Chap 0.21 296 395

Kim Mao 0.80 119 179

Owing to the small size of their landholdings, most of the plants the farmer-cooperators harvested, such as water melon, morning glory, jackfruit, mango, banana, soursop and longan, were used for household consumption rather than selling. They sold only a small percentage of their surplus, mainly to intermediaries or collectors in the village. They also shared some of their produce with neighbors and relatives. Growing fruits and vegetables on their farm has significantly reduced their food expenses. The initial benefits of the project are expected to catch the interest of other farmers.

Mr Kim Mao on his agroforestry site in Chrak Teak village

5 Source: EDI, Oct 2018

Agroforestry has also contributed to the economic welfare of the project beneficiaries. Mr Kim Chap, for instance, said that his income has improved since adopting the new techniques and he has been able to save enough money to build a new house. Mr Kim Mao, another farmer cooperator, was able to buy a new motorcycle, which he uses for local transport. Mr Siv Lim was able to buy a new phone and can now easily help his daughter who is studying in primary school in Krang Deivay. And Mr Men Vorn said that because of his increased income, he can support the education of his daughter who is at primary school.