Soil Conservation An Assessment of the National Resources Inventory, Volume 2 (1986) / Chapter Skim
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7. Erosion Control Practices: The Impact of Actual Versus Most Effective Use
7. Erosion Control Practices: The Impact of Actual Versus Most Effective Use
Pages 204-236
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From page 204... ...
The agricultural sector, on the other hand, suffered from low prices relative to production costs and the resulting downward pressure on net farm income. Most farm programs have sought to control acreages in crop production, thus reducing the cropland base.
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From page 205... ...
This paper explores the extent and severity of sheet and rill erosion, the extent to which conservation practices have reduced potential erosion, and the ability of conservation practices to resolve sheet and rill erosion problems. The national survey of lands used primarily for agricultural purposes by the Soil Conservation Service (SCS)
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From page 206... ...
Table 1 summarizes the sheet and rill erosion on cropland documented in the 1982 NRI for the United States. These rates are annual averages of soil movement within a field or sample point.
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From page 207... ...
. ~ 1 Row Crops Close Grown Hayland Other FIGURE 1 Cropland use and erosion in the United States in 1982.
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From page 208... ...
FACTORS IN CROPLAND EROSION As Table 1 suggests, averages do not tell the whole story. Valuable insights can be gained, however, by reviewing the factors responsible for sheet and rill erosion on land used to grow row crops and small grains in 1982.
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From page 209... ...
. This suggests that if the 417.5 million acres used for cropland had been in fallow and had been tilled up and down the slopes, an average 21.8 tons/acre/year would be lost to sheet and rill erosion.
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From page 210... ...
. A comparison of C-factor values indicates that plant matter and residue are most effective in reducing erosion rates on hayland and least effective on land in row crops
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From page 211... ...
. Thus, the overall impact of supporting practices was to reduce sheet and rill erosion by 9 percent (1.0 - .91)
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From page 213... ...
The black bars are much closer to zero, indicating greater ability to decrease erosion. Thus, from a national policy viewpoint, it would seem that land use changes reflecting plant and residue changes are more important than supporting practices.
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From page 214... ...
The main point is still clear: Namely, that the value of the C factor for minimum tillage systems can vary from extremely effective to only as effective as fall-plowed row crops. This variation is probably caused by the wide range of residue quantity that can be left on the soil surface and still qualify as minimum tillage.
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From page 215... ...
215 oo a~ tn U]
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From page 216... ...
Minimum tillage has a small distribution around the typical P-factor values of .45 to less than .60 (see Figure 5)
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From page 217... ...
The record of P-factor values of .90 and above indicates that contours and striparopping occur on lands that have slopes or slope lengths that all but eliminate the impacts of the supporting practices. To some extent, these phenomena cannot be prevented due to the use of a variety of slopes and slope lengths in close proximity.
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From page 218... ...
. For land in row crops, small grains, and hay, average RKLS values are higher for terraced cropland.
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From page 219... ...
If all slope lengths were set at 90 feet, the total soil movement would be reduced by 2.5 billion tons, and 7 fewer tons/acre would be moved (see Table 15)
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From page 220... ...
Within slope groups, a dichotomy exists when slope lengths are increased. Slope groups A and B have ranges from low to high that are within 3 tons/acre of their respective weighted averages (see Table 16)
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From page 221... ...
If the soil loss tolerance limit is 4 or 5 tons/acre, then RKLSTr1 will be less than 1 and yet the soil may be experiencing twice the tolerable amount of soil movement. Testing the magnitude of the potential error is beyond this paper.
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From page 222... ...
Overall Comparison The interaction of supporting practices, land use, C and P factors, and RKLS groups is established by sorting the NRI cropland data into those points with one or all supporting practices (minimum tillage, contouring, stripcropping, or terracing) and those points without any such practices.
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From page 223... ...
Land in other crops is similar to close-grown crops for low RKLS values, but the C factor values are about half as high when the RKLS is high. Nationally, the impact of supporting practices on row crops causes the weighted average C-factor value to fall from 0.35 to 0.28 (see Figure 7)
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From page 224... ...
CropsCropsHayCropsTotal 0 -- <52.61.80.10.75.3 5 -- <1016.124.20.58.849.6 10 -- q518.133.60.77.860.2 15 -- <2026.835.51.55.369.1 20 -- <2526.534.11.84.266.5 25 -- <3026.127.72.14.360.3 30 -- <3522.420.41.63.047.3 35 -- <4018.113.11.23.335.7 40 -- <5035.617.62.53.959.5 50 -- <7556.320.35.57.189.4 75 -- <10034.47.23.44.349.3 100 & up77.715.28.013.6114.5 Total360.7250.728.966.3706.7 TABLE 14 Sheet and Rill Erosion on Unterraced Cropland by Land Use and RKLS Factor, United States, 1982 (Million Tons) RKLSRowClose-Grown Other (Tons/Acre/Year)
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From page 225... ...
The weighted average P-factor values for each land use are as follows: for land in row crops, .86 with supporting practices and 1.0 without; for close-grown crops, .87 with and .98 without; for hayland, .81 with and 1.0 without; and for land in other crops, .88 with supporting practices and .99 without them.
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From page 226... ...
Using 1982 NRI data, this paper explores the extent and severity of sheet and rill erosion problems, the extent to which conservation practices have reduced potential erosion, and the ability of conservation Arc: tm rc~ land c! h^~ =^A -; 1 1 ~;~ ~_~1 ~ ~ ~ _~_ v ~ ~= ~ ~ ~ ~ ~ ~ ~ ~ V- ~ Ill CAL ~JLJ1~:~;:~ e The NRI data indicate that 417 e 5 million acres are readily available for crop production in the United States.
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From page 227... ...
Because these factors were not present, erosion was much less severe and averaged only 4.3 tons/acre/year The difference is due to the impact of current managerial decisions regarding cover and management practices, which are reflected in the C factor of the soil loss equation; to supporting practices, which are reflected in the P factor; and to shortened slope lengths in terraces.
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From page 228... ...
Thus, if these 25 million acres could be taken out of cultivation, excess erosion could be decreased by almost one-third. Another way to lower soil erosion is through supporting practices that lower the value of the P
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From page 229... ...
One reason for this small impact is that only 40 percent of cropland has a supporting practice that lowers the P-factor value. From a national policy viewpoint, it appears that land use changes reflecting plant and residue changes are more important than supporting practices.
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From page 230... ...
SOURCE: 1982 NRI. systems, striparopping, and terrace systems in the United States 1 including 81 percent of row crops and 40 percent of all cropland.
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From page 231... ...
In a market economy, every force that operates to limit the private and social value of farm outputs also operates to limit the value of soil and the volume of soil conservation that is privately and socially justifiable. Forces affecting the value of soil in farm production bear on the erosion process through their impact on the farmer.
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From page 232... ...
The literature suggests that where crop yields decline in response to erosion, the range of decline is from 1 to 9 percent per acre inch of soil loss and the average decline is about 5 percent. Assuming corn yields of 120 bushels/acre and soil weight of 150 tons/acre inch, 1 ton of soil loss coverts to a yield reduction of 0.04 bushel annually.
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From page 233... ...
A major impact of applying artificially low interest rates to soil conservation practices and programs, or any program, is to penalize future resource endowments in favor of poor current performance. I submit that insofar as the purpose of soil conservation is to ensure maximum availability of resources for future generations, conservation programs and practices should be planned and administered using interest rates at least as high private market rates.
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From page 234... ...
demonstrated that the value of exhaustible resources must rise at an annual rate equal to the interest rate if the privately and socially optimum rates of exhaustion are to coincide. Applying Hotelling's "law" to agriculture, if the price of farm outputs rises at annual rates equal to the interest rate, market forces will give farmers every incentive to conserve erosive soil to the extent socially justifiable.
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From page 235... ...
Great progress was made during the second phase in terms of technical capability to quantify erosion problems and the impact of alternative erosion control practices on them both at the farm level and in terms of national policy. Too often, however, the drive to reduce erosion rates to absolute T values established without regard to nonsoil resources caused farmers to question whether the federal government really knew much about "scarce resources." It is time for a third phase in the erosion control effort.
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From page 236... ...
1. Methodology in Soil Conservation and Agricultural Adjustment Research.
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Key Terms
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