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The P factor in Wischmeier's equation
The "erosion control practices factor" (P) is the relation between soil loss on a treated field and that on a neighbouring, untreated plot of similar size, or a control plot. Small erosion plots (100 to 200 m² are generally ill-suited to the study of erosion control practices, and comparisons should be made on small catchments of about one hectare. Therefore here only a comparison is given of results in North and West Africa with the coefficients advocated in the United States after many trials carried out on small catchments (Wischmeier, Smith and Uhland 1958, Roose and Bertrand 1971, Roose 1973, Delwaulle 1973).
Erosion control techniques to divert or totally absorb water are not found in Table 33, since these are encompassed in the topographical factor, in which the length of slope is reduced to the breadth of cropped strips between two banks. In any case, very few studies have scientifically demonstrated a reduction in soil loss from catchment areas after treatment with terracing, for most of them do not differentiate between the effects of terracing and those of the consequent improvement in plant cover on the same catchment (Roose 1974, Goujon and Bailly 1974).
Table 33 clearly shows that biological techniques (maximum soil cover, use of fertilizer, correct tillage, mulching, cover plants, rotations, etc.) are much more effective than mechanical techniques (terracing, ridging, etc.), which are expensive to install and difficult to maintain. Despite this, mechanical techniques receive much fuller treatment in soil conservation manuals, and are advocated more often than not without prior adaptation studies (Roose 1971; 1973; 1974; 1977b). Nevertheless, it should be noted that the drier the area and the more difficult it is to implement biological methods, the more one should use mechanical methods to help the plants to become established and cover the ground as quickly as possible.