Water erosion is the detachment and transport of soil by the energy contained in moving water, particularly raindrop impact and surface runoff. Water erosion results in the loss of the fertile topsoil required for healthy plant growth. Water erosion is particularly severe in sloping upland areas of Vietnam due to farming methods, steep slopes, shallow poorly structured soil and heavy rainfall. Green manure (green plant material) contains more nutrients then dry plant material, potentially increasing plant growth while providing erosion control.
 
Local sources of green manure are readily available in the upland areas of Vietnam. These sources can be weeds, remains of a previous crop or a poorly utilised waste product from other processes e.g. leaves from forest trees and elephant grass stalks. In locations where sources of green manure are poorly available, strategic planting of a source with increase the availability, such as hedgerows or cover crops, resulting in minor reductions in productive area. Local field trials need to show it is possible to offset the reduction in productive area by the increase in productivity in the crop.
 
Not all green manures are the same. Using a simple field trial different green manures can be assessed to determine there respective effectiveness at reducing erosion and increasing plant yield.
 
The structure of this report focuses on relatively simple and universal methods and considerations to conduct a field trial. The methods assess soil erosion by raindrop impact, mulch decomposition rate, plant biomass productivity, plant yield, weed growth, soil moisture. It is not essential to use all of the methods below together. In addition, most methods will need customisation to fit the situation. Make the trial as simple and efficient by only concentrating on the important issues. For example to assess the affect of time of application of the same green manure additions there is little need to assess decomposition rate.