Measuring Soil and Organic Matter Loss on Farmlands
This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Environmental Science. Please check back later for the full article.
Fields plots are often used to obtain experimental data (soil loss values corresponding to different climate, soil, topographic, crop, and management conditions) for predicting and evaluating soil erosion and sediment yield. Plots are used to study physical phenomena affecting soil detachment and transport, and their size is determined by the experimental objectives and the type of data to be obtained. Studies on interrill erosion due to rainfall impact and overland flow need small plot width (2–3 m) and length (< 10 m), while studies on rill erosion require plot lengths greater than 6–13 m. Sites have to be selected to represent the range of uniform slopes prevailing in the farming area under consideration.
Plots equipped for interrill and rill erosion studies, like the ones used for developing the Universal Soil Loss Equation, measure erosion from the top of a slope where runoff begins, and are sufficiently wide to minimize edge or border effects and long enough for development of downslope rills. Work on an experimental station is based on bounded runoff plots of known area, slope steepness, slope length, and soil type from which both runoff and soil loss are monitored. For a plot having known boundaries defining the measurement area, collecting equipment has to be used to catch plot runoff. A conveyance system (H-flume or pipe) carries total runoff to a sampling unit and a storage system.
Simple methods have been developed for estimating the mean sediment concentration of all runoff stored in a sequence of tanks by using the concentration profile measured along a vertical of each tank. When the number of plots equipped at an experimental area is large, sampling of suspension is highly time-consuming for both the field sampling phase and the oven-drying phase in laboratory. In this last case, a sampler able to extract a column of suspension extending from the free surface to the bottom of the tank can be used.
For large plots, or where runoff volumes are high, a divisor can be used that divides the flow into equal parts and passes one part, as a sample, in a storage tank. Examples of these devices include the Geib multislot divisor and the Coshocton wheel. Because most of the soil organic matter is found close to the soil surface, erosion significantly decreases soil organic matter content. Several studies have demonstrated that the soil removed by erosion is 1.3 to 5 times richer in organic matter than the remaining soil. Soil organic matter facilitates the formation of soil aggregates and increases soil porosity; the improved soil structure, in turn, facilitates water infiltration. Removal of organic matter content can influence soil infiltration, soil structure, and soil erodibility.