Effect of Disturbed Soil Thickness on Soil Water Use and Movementunder Perennial Grass1

S. D. MERRILL, S. J. SMITH, AND J. F. POWER 2

ABSTRACTPrevious research has shown that plant growth on minespoil of

lower plant productivity is dependent upon thickness of soil re-placed. Sodium-affected strip-mine spoils are a significant recla-mation problem in the northern Great Plains. The purpose of thiswork was to develop understanding of plant growth response to soilthickness over sodic minespoils by measurement of water movement,water use, and root growth. Soil water storage and use were mea-sured in each of three growing seasons under crested wheatgrass(Agropyron desertorum) grown on 0.25-, 0.5-, 0.75-, and 1.0-m thick-nesses of disturbed soil. Soil profiles were constructed from Hap-loboroll topsoil (0.2 m for all treatments), placed over varying thick-nesses of subsoil (B and C horizon materials), which in turn wasplaced over sodic (Sodium adsorption ratio = 30) drag-line spoil ata semiarid, steppe-land site in western North Dakota. Forage yieldwas 2- to 3.5-fold greater on 1.0 m soil thickness than on 0.25 m.Both total soil water potentials and root weight densities were sim-ilar in minespoil and in subsoil at the same profile depths. Rootwater uptake was much less from the minespoil (mean saturatedhydraulic conductivity (HC) = 1 X 10 3 cm/d) than from subsoil(mean HC = 0.2 cm/d). Low HC per se appeared to be the dominantfactor limiting sodic minespoil as a plant growth medium becauselow HC resulted in less use of stored soil water from minespoilcompared to subsoil. Depletion from a 120-cm profile was 0.2, 3.3,7.9 and 9.8 cm for 0.25-, 0.50-, 0.75- and 1.0-m soil thickness, re-spectively. Relative differences in evapotranspiration (ET) betweenthe 0.25-m and 1.0-m soil thickness treatments were much less thanyield differences, reflecting progressively reduced water use effi-ciency with less soil thickness.

Additional Index Words: Agropyron desertorum, bromide tracer,crested wheatgrass, hydraulic conductivity, mined-land reclamation,root growth, sodic soil, total water potential

Merrill, S. D., S. J. Smith, arid J. F. Power. 1985. Effect of disturbedsoil thickness on soil water use and movement under perennialgrass. Soil Sci. Soc. Am. J. 49:196-202.

THE NEED to restore plant growth potential to drast-ically disturbed lands has given rise to investi-

gations in which topsoil and/or subsoil or overburdenmaterials are laid over minespoils in various thick-nesses and configurations to observe depth and qual-ity effects on plant growth. Schuman and Power (1981)

and Doll et al. (1984) have reviewed a number of theseexperiments conducted in the Northern Great Plains.These studies, which may be called soil constructionexperiments, serve as useful general models for studyof the relation between soil thickness and soil pro-ductivity because many elements of soil structure arecontrolled. Results of these experiments may be in-terpreted by principles generally similar to those em-ployed in soil loss-soil productivity studies: (i) Be-cause limited water is the chief constraint to cropgrowth, soil reconstruction will increase yield to theextent that water availability to plants is increased,(ii) Available water-holding capacity and nutrientavailability of covering-soil and minespoil determineyield responses to soil thickness. Additional relativeyield response to topsoil additions has been observedas sodicity and dispersibility of minespoil increased(Merrill et al., 1981) or as textural coarseness of un-derlying material increased (Halvorson et al., 1980;Doll et al., 1984). These principles have been illus-trated also by soil construction experiments con-ducted by Power et al. (1981), in which interpositionof marginal quality, saline subsoil was shown to in-crease crop yields above levels obtained with 0.2 or0.6 m of surface-soil material alone over sodic mi-nespoils.

Minespoils high in clay and sodium content are asignificant problem in disturbed land reclamation inthe Northern Great Plains. The purpose of the workreported here was to increase understanding of plantresponses to varying soil thicknesses over such mi-nespoils by measurement of water movement, wateruse, and root growth in soil-minespoil profiles. A par-ticular goal was to understand why sodic minespoilsare poor media for plant growth. A companion paper(Power et al., 1985) describes observations of nitrogenmovement and use in response to soil thickness oversodic minespoils.

1 Contribution from Northern Great Plains Research Center,USDA-ARS, P. O. Box 459, Mandan, ND 58554. Received 27 Apr.1984. Approved 20 Sept. 1984.2 Soil Scientist and Soil Scientists (Research Leaders). Second andthird authors are located at USDA-ARS, Durant, OK and USDA-ARS, Univ. of Nebraska, Lincoln, NE, respectively.

MERRILL ET AL: EFFECT OF DISTURBED SOIL THICKNESS ON SOIL WATER UNDER PERENNIAL GRASS 197

METHODSThe experiment was conducted in Motion County, west-

ern North Dakota (48 °N 101°W), within the mixed-grasssteppe zone, which has a semiarid, continental climate. Meanannual precipitation is 38 cm, 80% of which falls from Aprilto September. Mean January and July temperatures are- 14°C and 22°C, respectively.

Six microplots (.093 m2) were established at each of thefour positions on a tract of constructed mine soil (Power etal., 1981). Total thicknesses of replaced soil materials were0.25, 0.50, 0.75 and 1.0 m over minespoil. The profiles ateach site consisted of 0.2 m of topsoil (mostly A horizonmaterial) over variable depths of subsoil (B and C horizonmaterial) underlain by graded, sodic minespoil (see Table1). The unmined soil was classified as Temvik-Williams siltloam: fine-silty, mixed Typic Haploboroll and fine-loamymixed Typic Argiboroll. The non-rocky minespoil resultedfrom a dragline mining operation. An additional two plots(1 m2), used for soil water and allied measurements, werelocated immediately adjacent to each group of microplots.Microplots were isolated around perimeters to a depth of0.75 m by trenching and placement of plywood and plasticsheeting; the 1-m2 plots were isolated with plastic alone. Ply-wood or sheet metal borders extended 5 cm above the soilsurface around perimeters of all plots. Vegetation on plotsand surrounding area consisted of crested wheatgrass (Agro-pyron desertorum), a perennial, established 3 yr previously.

The area received 35 kg P/ha broadcast as concentratedsuperphosphate prior to seeding and 55 kg N/ha broadcastas ammonium nitrate in early spring for 3 yr prior to startof the present study in 1978. Two microplots from eachtreatment received 48.6 kg N/ha as KNO3 of 27.484 atomper cent excess I5N and 200 kg/ha of Br as KBr in April,1978 (exchangeable K in treated soil was usually over 400kg/ha and not considered deficient). The N and Br wereapplied in 0.22 cm of water. At this and subsequent appli-cations of labelled N fertilizer, the other microplots, the1-m2 plots, and areas between plots received non-labelledKNO3 at 50 kg N/ha. Labelled-N microplots were excavatedwhole in July 1978, and sectioned by 0.125 m segments foranalysis. The four remaining microplots received labelled Nand Br, in the same manner and rate described above, inNovember 1978; 1-m2 plots also received Br and unlabelledN at that time. Unlabelled N was applied again to all plotsand remaining microplots in November 1979 at 50 kg N/

Table 1. Average properties of disturbed Temvik-Williams soiland minespoil at various profile positions.

Material zoneand profiledepth position

Percent sandPercent siltPercent clayHC, cm/dj

rangegeometric avg.

Bulk density, Mg/m'Water content, m'/m3:

at -0.033 MPaat - 1.5 MPa

Saturation percentageSaturation extracts:

EC, dS/mNa, mmol/LCa + Mg, mmol/LSAR, (mmol/L)1"

Topsoil

0-0.2 m

533116

61-3.220

1.45

0.330.13

46

1.55.54.22.7

Subsoil

0.2-0.5 m

234434

0.79-0.0080.221.59

0.350.17

58

4.829.917.84.8

Subsoilf

0.5-1.0 m

1.64

58

7.057.315.214.7

Minespoil0 to 0.5 m

below subsoil

125336

0.021-0.00020.00111.45

0.430.26

119

2.626.40.8

29.5

fThis category of subsoil present in 0.75- and 1.0-m soil-thickness treat-ments only.

$ Hydraulic conductivity, n = 12 to 18.

ha. Two microplots from each treatment were excavated inJuly 1979 and the remaining two in July 1980.

Grass in each plot was clipped at 3 cm height after an-thesis in late June or early July, immediately prior to ex-cavation of microplots. Root material was extracted by care-ful handpicking and screening from 5 to 20 kg portions ofair-dried soil from excavated microplots. This method al-lowed rejection of debris and clearly dead, carbonized roots.Following root removal, soil was subsampled for analysis.Soluble soil anions and cations, and electrical conductivitywere determined from saturation extracts by standard tech-niques (Sandoval and Power, 1977). Bromide was deter-mined on 2:1 soil-water extracts with a specific ion electrode(Abdalla and Lear, 1975).

Water content was measured at 0.3-m increments to 1.5-m or greater depth at weekly to monthly intervals with neu-tron moisture probes in access tubes. Four tubes were in-stalled at each soil-thickness treatment position, while twotubes were located in the 1-m2 plots and two were installedin border areas immediately adjacent to microplots. Soil-water matric potential and total water potential was moni-tored with tensiometers and soil psychrometers, respec-tively, installed in the 1-m2 plots. Both screen-covered andceramic-enclosed psychrometers were used for the lattermeasurement. Soil temperature was monitored with cppper-constantan thermocouples installed in the 1-m2 plots in ver-tical arrays.

Because significant downward leaching occurs in this cli-mate only in wetter than normal situations (Power, 1970),and to assure that overall soil-water regimes were at least atmean wetness or greater, 4 to 8 cm of water were appliedannually. Evapotranspiration was calculated from on-site re-cording raingauge data and from soil water content changeswith the assumptions of zero runoff and negligible deep per-colation in underlying minespoil. Hydraulic conductivity wasmeasured on saturated, undisturbed cores 7 cm in diameterand 10 cm long under constant head (Klute, 1965). Bulkdensity was measured with a gamma-backscatter probe inthe access tubes. Water retention at -0.1 MPa or higher ma-tric potential was measured with resin-coated clods on ce-ramic plates, and for potentials lower than -0.1 MPa, withdisturbed samples on pressure plates and membranes.

RESULTS AND DISCUSSIONSoil Water Use and Movement

Properties of materials in the reconstructed minesoil are shown in Table 1. The high saturation per-centage and SAR values of minespoil indicate the dis-persed state of this material, and are consistent withvery low hydraulic conductivity values. Subsoil wasmoderately saline, with higher salinity in the lowerpart of the subsoil zones of the 0.75- and 1.0-m soilthickness treatments. The water-holding capacities ofsubsoil and minespoil were approximately the same.

Soil water content and use over three growing sea-sons is shown in Fig. la. In general, greatest soil waterstorage in the upper 1.2-m depth of profile was in thespring, following snowmelt. Seasonally high evapo-transpiration (ET) rates were evident during the pe-riod of maximum grass growth (Fig. IB), and re-mained evident after grass harvest in July of each year.Limited vegetative regrowth plus high evaporative de-mand maintained ET rates after grass cutting at levelsapproaching rates before cutting. Evapotranspirationrates calculated for 14-d periods ranged from 0.1 to0.5 cm/d, both before and after cutting. The data ofFig 1A show the progressively greater depletion of soilwater as thickness of covering soil increased. For the

198 SOIL SCI. SOC. AM. J., VOL. 49, 1985

first 60 to 75 d after beginning of significant springgrowth (about day 120), average depletion (3 yr) froma 1.2-m profile depth was 1.2-, 4.3-, 6.6-, and 7.8-cmfor the 0.25-, 0.50-, 0.75, and 1.0-m soil-thicknesstreatments, respectively. Snowmelt and early springrecharge were very limited in 1980 (Fig. 1A, 1C), whileearly spring potential ET was higher, consistent withhigher soil temperature at 0.12 m depth on day 100of 1980 (7°C) compared to 0°C on the same date of1979 (Fig. ID). Thus, differences in midseason water

.40

o '35

| .30

'S£ .35Q

.30

I '2Saa.E .30

» .25o

= .20CO•o£ .30o

.25

.20L

EI

10.6»r0.3

50

5 25a'o£ 0a.o°, 20

2 °oa

0.25m

0.75m

1.0m

B

I100 200 300 0

1978100 200 300 0

1979Day of Year

100 200 3001980

Fig. 1.— A. Amount of soil water in upper 1.2 m of profiles for 4 soil-thickness treatments. Dashed lines indicate values in early spring,1978. B. Average crested wheatgrass height during active growingseason. C. Precipitation (solid lines) and irrigation (dashed) in 1-m2 plots by 5-day time intervals. D. Average soil temperature at0.12- and 0.38-m depths.

depletion among treatments was greatest in 1980, withnet depletion since spring 1978 being 0.2, 3.3, 7.9, and9.8 cm for 0.25-, 0.50-, 0.75-, and 1.0-m soil thickness,respectively.

Soil water use and movement as a function of depthcan be inferred from Fig. 2, where the 0.25- and 1.0-m soil-thickness treatments are compared. Both soilthicknesses show approximately equal recharges anddepletions at 0.15-m depth. The 1.0-m thickness treat-ment shows a large net soil water depletion at 0.45 m(subsoil), relative to the initial water content, and alsoa net depletion at 0.7 5-m (subsoil) beginning in fall1978. In contrast, for the 0.25-m soil thickness treat-ment, soil water content at 0.45 m (in minespoil) var-ied no more than 0.05 m3/ni3 from that originallypresent, and at 0.75 m depth there was a slow increaseover a 2-year period, with a net recharge of 0.07 m3/m3. These contrasting results reflect low plant wateruptake from minespoil and small, slow changes inwater content due to the low hydraulic conductivity(HC) in this material (Table 1) vs. significant wateruptake and higher HC in subsoil.

Thus, despite low HC values measured in mine-spoil, small recharges were evident from the data.Based on a diffusion theory calculation assuming con-stant water diffusivity (Kirkham and Powers, 1972,Chap. 6), a recharge of 0.03 m3/m3 in 15 d over a 0.3-m depth increment of minespoil from topsoil, assum-ing -0.3 MPa initial matric potential in minespoil,would require a HC of 0.0006 cm/d. This is withinthe lower part of the range of measured values (Table1). Water flows of the order of magnitude consistentwith minespoil HC values would be substantially dri-ven by thermal gradients and consist of vapor andmixed liquid-vapor (saltation) fluxes (Nielsen et al.,Chap. 5, 1972). Electrolyte would not be carried bysuch water flow. Thermal gradients measured here (Fig.ID) indicated that flow would be generally upwardbefore day 100, and downwards afterwards.

The relative inability of sodic minespoil zones togain or lose water in comparison to subsoil is reflectedin bromide profile data (Fig. 3). Though Br was usedhere to mimic and trace NO3-N movement (Smithand Davis, 1974), it also serves as an indication ofwater fluxes per se. Comparison of midseason 1979and 1980 profiles (for Br applied Nov. 1978) showsvery little Br movement below the 0.3 m depth forthe 0.25- and 0.5-m soil thickness treatments. Forthicker soil, 1980 Br data indicate considerable waterflow into subsoil below 0.3-m. This Br movement intosubsoil was apparently due to high precipitation (7.8cm) in August 1979 (Fig. 1C) and not to recharge bysnowmelt since snowfall was below normal (Figs. 1A,2).

Plant Growth, Evapotranspiration, and Root WaterUptake

Differences among treatments in utilization of storedwater, and consequent differences in plant water stress,were reflected in differences in yield and plant heights(Table 2). Although forage yields were measured onsmall areas, up to 60 g per microplot represent yieldsup to 6000 kg/ha, comparable to yields up to 5000 kg/ha obtained on 8 m2 harvest areas on the same tract

MERRILL ET AL.: EFFECT OF DISTURBED SOIL THICKNESS ON SOIL WATER UNDER PERENNIAL GRASS 199

1.0 m treatment Depth from

surface

.15m(topsoll)

100 200 300 0 100 200 300 0 100 200 3001978 1979 1980

Day of YearFig. 2.—Time courses of water contents at 3 profile depths for 2 soil-

thickness treatments. Dashed lines indicate levels in early spring,1978.

of mine soil (Power et al., 1981). Smallest relative yielddifferences occurred in 1978, a year with favorabledistribution of spring precipitation, and largest differ-ences occurred in 1980, when early spring potentialET was high and precipitation distribution was lessfavorable for plant growth.

ET calculations for the 1-m2 plots for 60 to 75 dperiods during spring growth are shown in Table 2.In 1978 and 1979, the greatest ET differences werebetween 0.25- and 0.50-m soil thickness treatments.This agrees with the data of Power et al. (1981) forthe same site, which showed no significant differencesin ET between soil thicknesses of 0.5 m and 1.7 m.

Under conditions where water limits growth, a pos-itive and linear relationship generally exists betweenbiomass yield and ET (Hillel, 1980). Factors that in-teract negatively with plant water supply (e.g. soilthickness, and salinity) should increase the yield/ETratio. In our experiment, with ET and yield (Yr) (Table2) set relative to highest values for each season, a rel-atively high yield/ET ratio is obtained: Yr = 2.776 (ETr)- 1.839 (r2 = 0.815, P <.01). Water use efficiencydecreased for lower-yielding treatments because of de-creased soil water availability with less soil thickness.It is likely that an increase in the ratio of soil evap-oration to ET for less thick soil treatments is also in-volved in decrease of water use efficiency. To examinethe nature of the interaction between the soil thickness

0.2

0.4

0.6

0.8

0.4

0.6

0.8

1.0

.25m of soil thickness

.75m

.*,.. 1979

,-V

1.00m

_L0 20 40 60 80 0 20 40 60 80

percentage of bromide In 0.13m increments

Fig. 3.—Bromide profiles in microplots harvested in early July ofyears indicated, resulting from application in November 1978.Percent values are based on total Br recovery from soil. Dashedlines indicate soil-minespoil interfaces. Average values from 2 mi-croplots used.

and plant water supply, root uptake of soil water androot growth pattern with depth must be examined.

During periods of low precipitation and low soilwater contents, water depletion patterns will result al-most entirely from water-uptake by roots. The periodcovered by Fig. 4 data (with 2.1 cm total precipitation)showed depletions of 3.8 cm from a 1.2 m profile forthe 0.25-m treatment, compared to an average of 5.4cm for the other treatments. Comparison of water de-pletion between curves 3 and 5 in Fig. 4 are especiallygood indications of root uptake since water contentswere low enough to greatly reduce soil HC, and the1.3 cm of precipitation was spread over three separatedays. Comparison of the 0.25-m treatment with theother treatments indicates that water uptake wasgreatly restricted from minespoil. However, somewater uptake evidently occurred from minespoil incontact with soil, as indicated by curves for the 0.25and 0.5-m treatments (Fig. 4). As thickness of soil in-creased to 1.0 m, there was an increasing tendency foruptake to predominantly come from subsoil duringthe latter part of the time period (curves 3 to 5, Fig.4). Little water uptake occurred from below 0.7 m dur-ing the period.

Table 2. Grass yield in microplots, maximum plant height,and evapotranspiration in spring.

Yield:dry wt.Maximum

plant heightEvapo-

transpirationSoilthicknesstreatment 1978 1979 1980 1978 1979 1980 1978 1979 1980

m

0.250.500.751.00

-g/plot- cm

32.4 28.9 16.0 0.46 0.46 0.25 14.3 15.7 19.044.9 38.8 18.6 0.53 0.50 0.29 17.1 17.1 20.339.5 64.8 35.7 0.57 0.56 0.35 16.0 17.3 20.458.3 79.3 56.6 0.62 0.62 0.46 17.2 17.7 22.7

LSD (0.05) 10.6 15.6 14.4 0.03 0.05 0.08

200 SOIL SCI. SOC. AM. J., VOL. 49, 1985

Volumetric Water Content - m /m.1 .2 .30 .1 .2 .3

0.9 .

6 4 3 2 1

0.3 .

0.6 .

0.9

1.2

5 4 3 2 1

Fig. 4.— Water content profiles for 4 soil thickness treatments in-dicating successive stages of depletion (1-5) from May 10 to June6, 1978

Root Growth and Factors of Root Water UptakeSoil water potential, soil hydraulic conductivity, and

root length density affect root water uptake. The sig-nificance of each factor is indicated by the followingequations, which represent an approximate consensusof views on root water uptake (Gardner, 1 964; Hillelet al., 1976; Hillel, 1980):

Si = (*, - *,)/(Rp + RSI); S, > 0 ;

where St is water uptake in zone ,, 4>p and $, are totalwater potential of the plant and of bulk soil zone /,respectively, and Rp and Rsi are resistances to watermovement in root tissue and in soil, respectively.Conductance for water uptake, l/Rsi, is also propor-tional to a root soil geometry factor, Bt, unsaturatedHC of the soil, Kj, and root length density, Lt. Thesoil resistance term, Rsi, does not become significant

in comparison to plant resistance, Rp, until soil driesto water potentials of —0.2 to —2 MPa or less (New-man, 1969; Reicosky and Ritchie, 1976; Taylor andKlepper, 1975).

Profiles of root weight per unit soil volume (Fig. 5)indicate that roots were capable of penetrating at least0.25 m into minespoil with a root weight comparableto that in subsoil. Root density did not significantlydiffer in minespoil 0.25- to 0.5-m deep in the profilein the 0.25-m treatment compared to subsoil at thissame depth in the other thickness treatments. Simi-larly, root weights in minespoil at the 0.5- to 0.75-mdepth of the 0.5-m treatment did not differ signifi-cantly from root weights in subsoil at the same depthfor the 0.75-m and 1.0-m treatments. For the 0.25-msoil thickness treatment, only at depths greater than0.5 m were average root weights in minespoil signif-icantly less than those in other treatments. Assumingthat the root weights per volume, as measured herewere correlated with functional root length density,the root factor did not play a significant role in ef-fecting water uptake differences observed between mi-nespoil and subsoil zones at 0.3- to 0.6-m profile depthsof various soil thickness treatments.

Data of total soil water potentials measured withscreen type, in situ soil psychrometers are shown inFig. 6 for the 1979 season. Water potential in mine-spoil at 0.38 m depth under 0.25-m soil thickness av-eraged approximately the same as in subsoil at thisdepth in the other treatments, —1.4 MPa versus —2.0to —1.2 MPa, respectively. For the 0.5-m soil thick-ness treatment, water potential in minespoil at the 0.63-m profile depth was somewhat lower than averagewater potential in subsoil at this depth for the 0.75-mand 1.0-m treatments, —1.3 MPa versus an averageof —0.7 MPa, respectively. If — 2MPa represents theapproximate wilting point in spoil or subsoil, the ma-tric component would be —1.5 to —1.8 MPa (Gardnerand Ehlig, 1963). For a considerable fraction of thetime, values measured in subsoil and minespoil werehigher than —2 MPa. In summary, soil water poten-tial was not an absolute factor restricting water uptake,and water potential level per se appeared to be nomore limiting to root water uptake in minespoil thanin subsoil.

Resistance to water flow in the soil, as compared toresistance in the plant, does not become important in

Root Weight per Unit Soil Volume- g/m3

10 102 103 104 10 102 10s

Q

'5en

.8

1.0

Soil Thickness:

10' 10

.2Sm .50m

Fig. 5.—Profiles of root weight per unit soil volume for 4 soil-thickness treatments representing geometric averages from microplot samplesharvested 1979 and 1980.

MERRILL ET AL.: EFFECT OF DISTURBED SOIL THICKNESS ON SOIL WATER UNDER PERENNIAL GRASS 201

Day ol Year - 1979150 200 250 300 150 200 250 300 150 200 250 300 150 200 250 300

,38m minespoilaoI-

Soil ThicknessTreatment:

-1- 2 - --3--4 -

.13m topsoi

.38m subsoil

.63m minespoil

.13m topsoll

.38m subsoil

.63m subsoil

^W^

.13m topsofl

.38m subsoil

.63m subsoil

0.25m 0.50m 0.75m 1.00mFig. 6.—Time courses of total water potential measured with soil psychrometers for 4 soil-thickness treatments at 3 profile depths. Dashed

lines at -2.0 MPa suggest an approximate wilting point.

plant water uptake until soil water content decreasesto a point at which unsaturated HC becomes very low.Reicosky and Ritchie (1976), supported by a reviewby Newman (1969), indicate that this critical HC levelis approximately 10~6 to 10~7 cm/d. The average sat-urated HC values of topsoil, subsoil, and minespoil(the later two measured in profile depths of 0.25 to0.6 m) were 19.6, 0.279 and 1.3 X 10~3 cm/d, re-spectively, (Table 1), all greater than the thresholdvalue for significant soil resistance to water uptake.The matric potentials at which HC reaches 1 X 10~6

cm/d may be estimated by applying Campbell's (1974)pore class theory to calculate unsaturated HC valuesfrom measured HC values and 5- to 9-point soil waterrelease curves. For topsoil, subsoil and minespoil, thesecalculated values were —3.0, —1.6, and —0.4 MPa,respectively. That significant soil resistance to wateruptake is predicted to occur in the minespoil at a ma-tric potential significantly higher than any conceivablewilting point value is indicative of the constraint thatlow HC places upon root water uptake and wateravailability in minespoil, and contrasts strongly withconditions in the subsoil.

Of the factors affecting root water uptake — rootdensity, water potential, and HC — HC appears to bea much more serious constraint than the others. Verylow HC in minespoil also contributes to unsuitabilityas a plant growth medium by restricting acceptance ofwater from overlying soil and its subsequent release.As shown in the companion paper to this study (Poweret al., 1985), the constraint of low minespoil HC onwater uptake and movement affects nitrogen uptakein a qualitatively similar manner — the thicker soiltreatments engendered greater relative use of soil re-sidual-N at deeper depths in the profile while shallowsoil thickness promoted greater proportional use ofsurface-applied fertilizer-N by grass.

The subsoil material, with its low mean HC value(2 to 2-1/2 orders of magnitude lower than topsoil),moderate salinity, and low humus content, is a rela-tively marginal medium for plant growth in compar-ison to topsoil material. This was shown by crop yielddata comparing topsoil and subsoil in a profile surfaceposition (Power et al., 1981). Yet in comparison withthe minespoil, subsoil functions satisfactorily in pro-viding for water uptake by roots and in water storage

and release, thus providing the basis for the soil thick-ness effect observed. There are examples of other dis-turbed soil systems where relative root growth con-straints due to compaction (Fehrenbacher et al., 1982)or adverse spoil chemical conditions (McFee et al.,1981) appear to be the main limiting factors of mine-spoils on productivity. Methodologically, this studyprovided an example of poor correlation between pro-file of root function (water uptake) and root weights,indicating the danger of interpreting function from rootrecovery. This pertains especially to sodic or sodic-saline systems where soil constraint in the lower rootzone is primarily on root function rather than on rootgrowth; the latter is usually the case with systems af-fected by high soil strength.

ACKNOWLEDGEMENTSThe authors thank Basin Electric Power Cooperative and

Consolidation Coal Co. for construction of the experimentalsite and Mr. L.F. Renner and Mr. B.J. Norby for technicalassistance.

202 SOIL SCI. SOC. AM. J., VOL. 49, 1985

of Soil Analysis, Part 1. American Society of Agronomy, Mad-ison, WI.

12. McFee, W.W., W.R. Byrnes, and J.G. Stockton. 1981. Char-acteristics of coal mine overburden important to plant growth.J. Environ. Qual. 10:300-308.

13. Merrill, S.D., P.M. Sandoval, E.J. Doering, and J.F. Power. 1981.The role of gypsum and other amendments in reclamation ofstrip-mined lands in semiarid environments. Vol. 1, p. 218-242. In D.P. Borris and P.W. Boody (ed.). Phosphogypsum:Proc. of the International Symposium on Phosphogypsum.Florida Institute of Phosphate Research, Bart9W, FL.

14. Newman, E.L. 1969. Resistance to water flow in soil and plant.I. Soil resistance in relation to amounts of root: Theoreticalestimates. J. Appl. Ecol. 6:1-12.

15. Nielsen, D.R., R.D. Jackson, J.W. Gary, and D.D. Evans, eds.1972. Soil Water. American Society of Agronomy, Madison,WI.

16. Power, J.F. 1970. Leaching of nitrate-nitrogen under drylandagriculture in the northern Great Plains, p. 111-122. In Rela-tionships of Agriculture to Soil and Water Pollution. CornellUniversity Press, Ithaca, NY.

17. Power, J.F., S.D. Merrill, and S.J. Smith. 1985. Effect of soilthickness on soil and fertilizer nitrogen distribution and use bycrested wheatgrass. Soil Sci. Soc. Am. J. (in press).

18. Power, J.F., P.M. Sandoval, R.E. Ries, and S.D. Merrill. 1981.Effects of topsoil and subsoil thickness on soil water contentand crop production on a disturbed soil. Soil Sci. Soc. Am. J.45:124-129.

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