5.3.5 Erosion Control Practice Factor

     The erosion control practice factor (P-factor) is defined as the ratio of soil loss with a given surface condition to soil loss with up-and-down-hill plowing. P-factor values involve treatments that retain liberated particles near the source and prevent further transport.^(a) The P-factor accounts for the erosion control effectiveness of such land treatments as contouring, compacting, establishing sediment basins, and other control structures. At sites where there is or has been much human and vehicular traffic, the P-factor reflects the roughening of the soil surface by tractor treads or by rough grading, raking, or disking. Practices that reduce the velocity of runoff and the tendency of runoff to flow directly downslope reduce the P-factor (Goldman et al. 1986; Novotny and Chesters 1981).

Table 5.11. C-Factor Values for the Universal Soil Loss Equation (after Kay 1983; USDA 1975;
Wischmeier and Smith 1978; as reported by Goldman et al. 1986)

Type of Cover	Cfact	Soil Loss Reduction, %
None	1.0	0
Native vegetation (undisturbed)	0.01	99
Temporary seedings:
90% cover, annual grasses, no mulch	0.1	90
Wood fiber mulch, 3/4 ton/acre (1.7 t/ha), with seed(a)	0.5	50
Excelsior mat, jute(a)	0.3	70
Straw mulch (a)
1.5 tons/acre (3.4 t/ha), tacked down	0.2	80
4 tons/acre (9.0 t/ha), tacked down	0.05	95
(a) For slopes up to 2:1.

      Table 5.12 presents typical values for sites with varying practice conditions and structures. Note that most of the P-factor values are close to unity; changing the surface conditions does not provide much direct reduction in P-factor, although other USLE factors may change and may have a significant effect on the amount of soil loss.

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(a)	Note that protection of the soil surface against the impact of rain droplets and subsequent loss of soil particles is reflected in the C-factor (Novotny and Chesters 1981).