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

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loam or coarser) and no more than slightly saline (EC <6) or somewhat sodic (SAR <10), from 24 to 30 in- ches of subsoil (second lift) which is loam or finer in texture should be applied. If the underlying spoil is fine - textured (silt loam or finer), 12 to 18 inches of subsoil should be respread. If the underlying spoil is moderately sodic (SAR 10 -20), the subsoil depth should be increased to 24 to 36 inches. When the spoil is sodic (SAR >20), from 36 to 48 inches of sub- soil should be applied. If topsoil and subsoil materials are sandy loam or coarser, it is proposed that the suggested depths of subsoil replacement be increased by about 12 inches. These suggested guidelines are summarized in Table 21. Table 21. Suggested guidelines for soil replacement based upon spoil properties. Spoil Properties Depth of Soil Replacement Texture EC SAR Topsoil Subsoil Total Coarse' Medium -3 -.3 mmho /cm <6 <6 _3 <12 <12 12 -20 >20 12 12 12 12 'Sandy loam or coarser 2 Loam or finer 3 Not applicable, SAR dominant property inches 24 -30 12 -18 24 -36 36 -48 36 -42 24 -30 36 -48 48 -60 These suggestions are somewhat higher than the optimum levels reported in some of the experiments that were discussed. However, until more research data are available describing the changes that occur in unweathered spoil materials placed within the root zone and until the movement of sodium in soils reclaimed over sodic spoil can be more precisely predicted, care must be exercised to ensure that adequate soil materials are replaced to ensure per- manent restoration to optimum productive levels. It should be emphasized that these guidelines were developed with the assumption that topsoil losses through erosion would be minimized by following good soil management practices. Implications of soil replacement to premine characterization: The importance of adequate premine characteriza- tion of all soil and overburden materials cannot be overemphasized. The properties and volume of soil materials suitable for respreading must be calculated. The amount and location of all desirable and undesirable strata in the overburden must be identified. The average properties of the reshaped spoil and the magnitude of expected variations from this average must be calculated before initial removal of topsoil and subsoil. When the overburden has undesirable properties such as coarse texture or high sodicity, sufficient soil materials must be 19 replaced to ensure optimum production over those sites within the reshaped spoil which have higher levels of these undesirable properties. If, on the other hand, undesirable overburden strata can be selectively placed below the root zone during min- ing, the surface properties of the reshaped spoil may be such that Tess subsoil will need to be replaced. This has two important implications. First, if the amount of available soil materials is not sufficient to result in optimum postmine productive levels over undesirable spoil, selective placement of good quali- ty spoil within the root zone could result in higher postmine productivity. Second, even when sufficient soil materials are available, selective placement of high quality spoil at the surface may justify the replacement of less subsoil. The decision then becomes an economic consideration in which the cost of selective placement is equated to the sav- ings from decreased removal, stockpiling, and respreading of soil materials. Research Needs: The soil replacement guidelines given in the preceding section are based upon the results of ex- periments which have been conducted for relatively short periods. These experiments have shown that productivity can initially be restored to a level equal to or better than that before mining. Studies of chemical and physical changes in these newly reclaimed soils suggest that the initial productivity may be maintained or even increased with time, but this is a tentative observation which must be verified by longer studies. Experiments need to be con- ducted over a long period of time in which the best soil management practices are followed; chemical and physical changes must be closely monitored so that the characteristics of the soils can be predicted when they approach the "steady state" characteris- tic of undisturbed soils. In these studies, movement of soluble salts and sodium need to be precisely described. Changes in physical characteristics such as aggregation, bulk density, and permeability must be quantitatively described. The effects of topog- raphy on all chemical and physical factors need to be delineated. Some new experiments need to be In- itiated on soils reclaimed over spoil materials which differ in both chemical and physical properties. Some of the experiments cited in this review should be seeded to perennial grasses and maintained for periodic cropping and for sampling to follow changes in soil characteristics. The Knife River top- soil experiment (Tables 4 and 5), the Stanton wedge experiment (Tables 6 through 12, the Zap double wedge experiment (Tables 13 and 14), and the Falkirk trench experiment (Tables 15 and 16) should be In= cluded in this long -time study. If the depth of soil replacement is to be related the properties of the reshaped spoil, technlqu need to be refined to estimate the average properti of the reshaped spoil using data from the premi