2002-07-08_GENERAL DOCUMENTS - M2002004 (4)

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d#t! Of rooting zone below mI my its a reservoir for nutrients }1!__ productive undisturbed Salt Willi be suitable for extensive root end _water movement and retention. aubsoll characteristics that develop ormation do not have as marked an ef 11011 productivity as do the characteristics s011. Subsoil replacement serves two pur- ta Was discussed by Omodt et al. (1975) and - non and Schroer (1980). First, if the underlying I Is = coarse- textured with a low water - holding Ity, finer-textured second lift materials will in- aae the available water - holding capacity within root - Zone. Second, If the underlying spoil has =undesirable properties for plant growth, replaced aubsoll serves as a buffer by placing the undesirable spoil materials below the root zone. If the properties - Of the underlying spoil are such that it is a favorable plant growth material, it can serve as the lower por- tion Of the root zone. The depth of subsoil, or second ilft, that must be replaced to restore optimum pro- ductive levels therefore depends upon the character- istics of the underlying spoil. Water - holding capacity: The water - holding capacity of a soil is a fixed pro- perty which is dependent primarily upon texture and bulk density. While the water - holding capacity can be slightly altered by changes in organic matter and degree of aggregation, coarse - textured soils will always be more droughty than finer - textured soils. The water - holding capacity of reclaimed soils can be Improved by selective placement of medium and flne - textured materials in the root zone or by increas- ing the effective depth of the root zone when available second lift materials are coarse - textured. When sandy loam topsoil was placed over moderately sodlic clay loam spoil in the Knife River experiment (Tables 4 and 5), yields of both wheat and corn after six years tended to be higher when 12 inches of topsoil was replaced than when 24 inches was replaced. This is attributed to the higher water - holding capacity when the top 2 feet of the root zone consisted of 12 Inches of sandy loam plus 12 inches Of clay loam Instead of 24 inches of sandy loam. Higher yields were Initially obtained with 24 inches of topsoil, probably because moisture levels in the spoil were low Immediately after reclamation. In the Stanton wedge experiment (Table 12), yields at total soil depths of 28 and 36 inches were consistently higher with 8 inches of sandy loam topsoil than with 24 inches; the underlying subsoil was clay loam in texture. The yield data for this experiment (Tables 7, 0, 9, and 10) suggest that about another foot of sub - _aOli Was needed for optimum yields with 24 inches =than with 8 Inches of sandy loam topsoil. Subsoil materials of different texture were com- pared In the double wedge experiment at Zap (Figure 17 5 and Table 14), interpretation of differences due to subsoil characteristics are complicated by the in- teracting effects of slope, aspect, and soil and crop management. Wheat yields were higher on the fine - textured subsoil, irrespective of soluble salt and sodicity levels (Table 14); crested wheatgrass yields followed the same trend in 1978 when the highest yields were obtained, but when moisture was more limiting (1979 and 1981), yields were highest on the coarse- textured subsoil which was also lowest in soluble salts and sodium. In general, these results are consistent with the effects of topsoil texture discussed previously for the Knife River and Stanton experiments. When the overburden was nonsaline and non - sodic, as in the Falkirk trench plots, replacement of 27 inches of loam topsoil over gravelly loamy sand was not sufficient for optimum yields (Table 16). The replacement of clay loam subsoil over gravelly loamy sand resulting in a total soil depth of 5 feet was pro- bably more than adequate for optimum yields. Yields with 27 inches of topsoil without subsoil were about 85 percent of maximum yields in this experiment and by interpolation suggest that at least 32 to 35 inches of medium or fine - textured materials would be need- ed for optimum yields over gravelly loamy sand. When the texture of the overburden was silty clay loam or clay loam, optimum yields were obtained with 18 or 27 inches of topsoil. Throughout this ex- periment, yields tended to increase as topsoil depths increased from 9 to 27 inches, although yield differences between the 18 and 27 inch depths were minimal. Relative yield differences between topsoil depths tended to decrease as the experiment pro- gressed, and differences due to topsoil depths would be expected to decrease as structure and root channels were formed in the overburden material. These results emphasize the importance of the water - holding capacity of the upper portion of the root zone and show that spoil materials which have favorable chemical and physical characteristics are acceptable for the lower portion of the root zone. However, these spoil materials have not been sub- jected to the weathering processes which take place in the lower horizons during soil development. Yields immediately after soil replacement in the Knife River (Table 4) and Falkirk (Table 16) ex- periments suggest that the development of favorable conditions for plant root development may require several years. Conversely, yields on the reclaimed soils at Beulah and Center (Table 19) tend- ed to approach yields on similar undisturbed soils after two or three years. Until more definitive data are available concerning the development of favorable characteristics for root growth in spoil materials, replacement of at least 1 foot of subsoil, in addition to 1 foot of topsoil, over good quality spoil materials seems to be desirable unless proper- ties of the subsoil indicate that it is less desirable for plant growth than the underlying spoil.