2012-06-20_PERMIT FILE - C2010089 (60)

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POWER ET AL.: EFFECTS OF TOPSOIL AND SUBSOIL THICKNESS ON SOIL WATER CONTENT 100- 80- 60- 40- • 90 70 50 1 No Topsoil -1 d 60 cm Topsoil 30 0 `5' 60 ova 120 180 240 0 180 240 SOIL THICKNESS, cm Fig. 1— Percent of maximum yield of alfalfa, crested wheatgrass, native grasses, and spring wheat as affected by topsoil and subsoil thickness. (Fig. 1). To develop this figure and to reduce vari- ability in the resulting curves, data in Tables 3 -6 were used to calculate a three - sample moving average with increasing subsoil thickness. The moving average was determined from the average yield of the thickness under consideration plus yields from the immediately adjacent samples both upslope and downslope. For example, the 90 -cm thickness moving average would be the mean value for the 70 -, 90 -, and 110 -cm thick- ness samples. After the moving average was calculated, the largest yield obtained was given a value of 100%, and all other yields were expressed as a percent of this largest value. Best yields with no topsoil were 62, 75, 88, and 73% of the maximum for alfalfa, crested wheatgrass, native grasses, and spring wheat, respectively (Fig. I). Thus, relative growth on exposed subsoil was greatest for native grasses and least for alfalfa. In all instances, relative yields increased with subsoil thickness up to at least 60 to 90 cm. When less than 30 cm of soil material (topsoil plus subsoil) was present, only crest- ed wheatgrass yielded appreciably over 50% of maxi- mum. Highest relative yields of most crops occurred where 20 or 60 cm of topsoil was used. Generally, the re- sponse for 20- cmtopsoil was very similar to that for 60 -cm topsoil i dicating little, if any, benefit from more _than 20 cm of topsoil. Relative yields > 90% of maximum were obtained at some soil thickness for all crops —at 60- to 90 -cm thickness for all crops but crested wheatgrass growing on 20 cm of topsoil and at 90- to 130 -cm thickness for all crops when 60 cm of topsoil was returned. Relative yields in almost all instances declined somewhat when total soil thickness exceeded about 150 cm, probably because of traffic patterns and snow distribution. For the 20 -cm top- ! •C 60 20 cm Topsoil X—X wheat o - -o Alf elf o Greeted w � 120 6 - —a Native G Mixed 127 soil treatment, relative yields for crested wheatgrass were usually somewhat lower than for other crops, whereas for the 60 -cm topsoil treatment, the native grasses generally produced lowest relative yields. When subsoil and topsoil were mixed in a 3:1 ratio, crop responses were variable. Relative yield of al- falfa increased as soil thickness increased to 130 cm and then decreased. Crested wheatgrass yields in- creased to about the 70 -cm soil thickness and then remained relatively constant. Response by native grasses was similar to that of alfalfa except that yields did not decrease beyond the 130 -cm thickness. Spring wheat responded similarly to crested wheatgrass, with relative yields leveling off at about the 70 -cm soil thickness. However, highest relative yields for wheat were about 80% of maximum compared with over 95% for the other crops for this treatment. Generalized conclusions to be drawn from the data in Tables 3 -6 and Fig. 1 are that: 1) All crops studied respond to increased soil thick- ness up to a total of about 75 to 120 cm, with minor or inconsistent differences between crops. 2) Maximum yields obtained by all crops equaled or exceeded average yields for undisturbed, well - managed Temvik silt loam. 3) Greatest yields of all crops occurred when about 20 cm of topsoil was placed over 55 to 110 cm of subsoil, although at least 90% of maximum yield of all crops except wheat were obtained without separating topsoil and subsoil into discrete layers. 4) With no topsoil, only native grass produced yields over 75% of maximum. 5) In most instances, yields of most crops decreased when total soil thickness exceeded about 150 cm. Frequently, for dryland crop production, there is a good correlation between plant growth and water use