Cropping and Management Factor CP

The original USLE separated the cropping factor (C) and management factor (P) into two different terms. In this discussion, they have been combined. The cropping factor is used to estimate the relative effectiveness of the practice to retard soil loss when compared to a continuously tilled field. The management factor (P) is an estimation of the effect of conservation practices in reducing soil loss. These factors combined are a function of the crop raised, the management practice, the planting date, the amount of residue on the surface, and the tillage practice. Values representing these factors can be found in Table 18.2.

Problem: Estimate the annual erosion for a field having a rainfall factor of 220, consisting mainly of loamy sand with 2% organic matter, and averaging a 4% slope with a slope length of 400 ft, which has been in continuous small grain, normally planted in June, with moderate residue worked under and farmed up and down the slope.

Solution: Using the USLE and the appropriate tables and figure:

where R = 220; K = 0.10, Table 18.3. LS = 0.73, Figure 18.2. CP = 0.29, Table 18.4.

A = R x K x LS x CP = 220 x 0.10 x 0.73 x 0.29 = 4.6574 or 4.6 T/ac/yr

Table 18.3. K factor for sample problem.

Organic matter content %

Organic matter content %

Table 18.3. K factor for sample problem.

Textural class

0.5

2.0

4.0

Fine sand

0.16

0.14

0.10

Very fine sand

0.42

0.36

0.28

Loamy sand

0.12

0.10

0.08

Loamy very fine sand

0.44

0.38

0.30

Sandy loam

0.27

0.24

0.19

Very fine sandy loam

0.47

0.41

0.33

Silt loam

0.48

0.42

0.33

Clay loam

0.28

0.25

0.21

Silty clay loam

0.37

0.32

0.26

Silty clay

0.25

0.23

0.19

The correct K factor is 0.10.

The correct K factor is 0.10.

Slope length (ft) FIGURE 18.2. LS for sample problem.

Table 18.4. CP value for sample problem.

Cropping practices

Table 18.4. CP value for sample problem.

Cropping practices

Management

Up and down

Terraces and

On the

factors

the slope

field boundary

contour

Continuous small grain MRU (6/20)

G.29

0.21

0.15

Continuous small grain HRU (6/20)

G.22

0.16

0.11

Continuous small grain MRU (8/1)

G.22

0.16

0.11

Continuous small grain HRU (8/1)

G.18

0.13

0.09

Continuous small grain ROS

G.12

0.09

0.06

Continuous cotton MF no WC

G.59

0.42

0.30

RC Continuous grain sorghum (25-30 bu)

G.48

0.34

0.24

RC Continuous grain sorghum (35-45 bu)

G.42

0.30

0.21

Continuous peanuts with WC

G.43

0.30

0.22

Continuous peanuts no WC

G.54

0.38

0.27

Alfalfa 5 yr/small grain 2 yr

G.G5

0.05

0.05

MRU = Moderate Residue Under; HRU = Heavy Residue Under; RC = Row Crop; ROS = Residue on Surface at Seeding Time; MF = Moderate Fertilizer; WC = Winter Cover.

MRU = Moderate Residue Under; HRU = Heavy Residue Under; RC = Row Crop; ROS = Residue on Surface at Seeding Time; MF = Moderate Fertilizer; WC = Winter Cover.

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