Spatial and Temporal Variability of Soil Physical Properties Following Tillageof Norfolk Loamy Sand1

D. K. CASSEL2

ABSTRACTPrimary and secondary tillage practices are used to control weeds,

incorporate soil amendments, and establish conditions favorable for rootgrowth. Field data showing the degree to which various soil physicalproperties are altered by and following tillage are rare. This study wasdesigned to evaluate spatial and temporal changes for nine soil physicalproperties following tillage and planting of Norfolk loamy sand (TypicPaleudult). The soil was moldboard plowed 25 cm deep 1 month priorto seeding. It was disked three times prior to seeding soybeans [Glycinemax (L.) Merr.] on 16 May 1977. The following physical properties wereevaluated on 16 May (after seeding), 3 June, and 8 July 1977: bulkdensity (Dt), weight percent soil water (PJ, cone index (CI), total po-rosity (P), macroporosity (M), saturated hydraulic conductivity (A~M,),and soil water characteristic at soil water pressures of —0.4, —1.0, and—10.0 kPa. All properties were measured within the 0- to 14-, 14- to28-, and 28- to 41-cm depth increments for three positions: row (R),trafficked interrow (TI), and nontrafficked interrow (I). Temporal andspatial variations in soil physical properties after seeding were limitedprimarily to the 0- to 14-cm depth. For the 0- to 14-cm depth, of thenine soil properties examined, six underwent significant temporal vari-ation; all nine had significant temporal (date) X position interactions.Within this depth, Db and CI were significantly greater and P and Mwere significantly smaller for position TI compared with positions R andI. Small significant differences in KM were found even though difficultywas encountered in measuring it for this nearly structureless loamy sandat low Db.

Additional Index Words: bulk density, cone index, total porosity,macroporosity, saturated hydraulic conductivity, soil water character-istic.

Cassel, D. K. 1983. Spatial and temporal variability of soil physicalproperties following tillage of Norfolk loamy sand. Soil Sci. Soc. Am.J. 47:196-201.

PRIMARY AND SECONDARY tillage practices are usedto control weeds, incorporate soil amendments, and

modify soil physical properties, and thus establish morefavorable soil conditions for crop growth. Soil physicalproperties are altered every time a tillage implement isused. For example, bulk density (Db) and mechanicalimpedance (MI) of soils undergo immediate, sometimesradical, changes in response to imposed tillage operationssuch as disking or moldboard plowing.

Statistically significant differences in Db > 0.07 Mg/m3 and in cone index (CI) ranging from 200 to 7,000kPa due to tillage have been reported by several inves-tigators (Gooderham and Fisher, 1975; Cassel et al.,1978). Some soil physical properties, e.g., soil texture,are not usually altered by tillage over short time periods.However, Lal et al. (1980) have reported that even soilsurface textures of credible soils can be altered over aone-year time span following tillage due to selective ero-sion of clay particles.

1 Contribution from the Dep. of Soil Science, North Carolina StateUniv. Paper no. 8724 of the Journal Ser. of the North Carolina Agric.Res. Serv., Raleigh, NC 27650. Received 15 Apr. 1982. Approved 6Oct. 1982.

2 Professor of Soil Science, North Carolina State Univ., Raleigh, NC27650.

In general, it is difficult to quantify soil physical prop-erty changes imposed by tillage. These tillage-inducedchanges are not uniform spatially, but vary both with soildepth and distance normal to the direction of travel ofthe tillage implement. Cone index was found to increasewith depth for three tillage treatments for a Norfolk sandyloam (Typic Paleudult) (Cassel et al., 1978). When CIwas measured at in-situ field capacity on a transect nor-mal to a row of soybeans, little variation was observedfor the chiselplow and moldboard plow treatmentswhereas large differences were apparent for the subsoil-bedded treatment. When CI was measured at drier soilwater contents, all three treatments showed significantdepth and position effects.

In addition to the spatial variability imposed by tillage,a soil physical property may also undergo temporal var-iation. The physical condition of the soil immediatelyafter passage of a tillage implement is not static but var-ies with time. Temporal changes in soil properties suchas Db or CI may stem from soil compaction resultingfrom vehicular traffic or induced by natural processessuch as rainfall or dessication. Regardless of the specificreason, it is recognized by field researchers that manysoil physical properties, e.g., Db, MI, hydraulic conduc-tivity (ATsat), thermal conductivity, and infiltration rate,undergo temporal variation during the year.

The purpose of this study was to quantitatively eval-uate the magnitude of the spatial and temporal variationfor nine soil physical properties on a conventionally tilledsoil on the Atlantic Coastal Plain. This information shouldbe useful in helping soil scientists develop sampling re-gimes to assess spatial and temporal changes in soil prop-erties in future studies.

MATERIALS AND METHODSThe study was conducted in 1977 on Norfolk loamy sand at

the Central Crops Research Station, Clayton, N.C. Total thick-ness of the A horizon was 40 ± 10 cm and was comprised ofa 25-cm thick Ap overlying a highly leached A2 horizon. Pre-vious tillage studies on this field indicated that a tillage-inducedpan which severely restricted plant root proliferation was pres-ent at the 25-cm depth. The effect of the tillage pan at this siteupon soybean [Glycine max (L.) Merr.] growth was reportedby Kamprath et al. (1979).

The 0.4-ha field was moldboard plowed 25 ± 3 cm deepapproximately 1 month prior to seeding. The land was diskedthree times prior to planting. The first disking was broadcastdisking without regard to wheel track locations. Two of thedisking operations occurred on the morning of 16 May, the dateof plant, and wheel traffic was confined to position TI whichwill be defined later. Soybeans cv. Ransom were planted at 95-cm row spacings with a two-row planter having 20-cm widepress wheels. The planter was mounted on a tractor with 30-cm wide tires. Total mass of the tractor-planter combinationwas approximately 1,600 kg. After planting, no machinery trafficwas permitted in or adjacent to those soybean rows where soilphysical properties were measured.

Four 12.3- by 7.6-m sites were selected at random in thefield. All soil physical property measurements taken at the foursites were either measured in situ or measured on undisturbedsoil cores.

196

CASSEL: SPATIAL AND TEMPORAL VARIABILITY OF SOIL PHYSICAL PROPERTIES FOLLOWING TILLAGE 197

Cone index, soil water content on a weight percent basis (Pw),and Dj, were measured immediately after planting on 16 May.Cone index was measured with a recording, hydraulically dri-ven cone penetrometer which was mounted on a frameworkattached to a tractor. The design of the frame-mounted pene-trometer allowed CI to be measured on a given row withoutrequiring the tractor to traffic that row. The penetrometer conehad a 60-degree included angle and a shoulder diameter of 20.0mm. The 11.1-mm diameter of the 41-cm long recessed pene-trometer shaft eliminated friction between the shaft and thesoil. At each site CI was measured at three positions on a tran-sect normal to the third row of soybeans. The three positionswere the soybean row (R), nontrafficked interrow (I), and traf-ficked interrow (TI). Cone index measurements were replicatedtwice at each site as illustrated in Fig. 1. The second set ofmeasurements was located approximately 50 cm down the rowfrom the first. The mean maximum CI value (CImax) for eachpair of replicated measurements was calculated for each depthincrement (0 to 14, 14 to 28, and 28 to 41 cm) for each positionand was used in the analysis.

Immediately after taking penetrometer measurements, soilsamples for PH, determination were collected using a hand-pushedprobe. These samples were taken within 5 cm of the locationwhere the penetrometer measurements were taken. Soil coreswith 2.1-cm diameters were collected from the 0- to 14-, 14-to 28-, and 28- to 41-cm depth increments (hereafter calleddepths 1, 2, and 3, respectively) and placed in moisture tins (18samples per site). The Pw value reported for each depth-positioncombination is the mean of the two replicates. The samplingarrangement is shown in Fig. 1.

Undisturbed 7.6- by 7.6-cm diameter soil cores were takenon the same day from the three depth increments at each po-sition at the midpoint between the locations of the penetrometermeasurements using a Uhland sampler (Blake, 1965). Each soilcore was collected from the midpoint of its respective depth.After each core was trimmed to size, snug-fitting end caps wereattached to prevent spillage, and they were transported to thelaboratory and stored at 4°C, for a time not exceeding 1 month,before being processed.

Each soil core was transferred to a separate pressure cham-

POSITION

R. 195cm

050cm

.U0 [

Soybeon Rows JL• _ Penetrometer Measurementso_ Water Content Measurement*Q_ Bulk Density Measurements

Fig. I—Sampling scheme for penetrometer, water content, and bulkdensity measurements for each date. Measurement at three depthswere taken at each sampling locus.

ber, slowly water-saturated, and the soil water characteristicdetermined for soil water pressures of 0, —0.4, —1.0, —3.0,-6.0, -10, -16, and -36 kPa. Some of the soil cores settledupon wetting, a point discussed in detail later. None of the coresexhibited swelling. Upon removing each soil core from its pres-sure chamber, it was resaturated with water over a 12-h period

jind saturated hydraulic conductivity (K^t) determined (Klute,•*1965^The core was then oven-dried at 105 °C for 24 h, weighed,and Db calculated. Total porosity (P) was measured as the vol-ume fraction of water held by the soil core at saturation. Macro-porosity (M) is defined as P minus the water-filled porosity atthe soil water pressure of —6.0 kPa.

All sampling and analysis procedures were repeated again on3 June about 1 week after soybean emergence when the soilprofile was near in-situ field capacity (Cassel et al., 1978). Thethird set of measurements was taken on 8 July before the can-opy had completely closed.

The oven-dried soil cores for the 3 June sampling date werecrushed, passed through a 2-mm sieve, and a 60-g subsamplewas used to determine soil texture using the hydrometer method(Day, 1965). Duplicate subsamples of crushed soil were alsoused to determine soil water content at —33 and —1,500 kPa(Richards, 1965).

Tensiometers were installed at each of the four replicatedsites to measure soil water pressure (SWP) throughout thegrowing season. Tensiometers were constructed with 2.5- by 1-cm diameter cups (Arya et al., 1975) and were installed priorto 21 June. Banks of tensiometers were placed at positions I,R, and TI. In each bank, one tensiometer was placed at eachof the following depths: 10, 20, 30, 45, 60, 90, 120, and 150cm. Tensiometers that ceased to function when SWP < — 85kPa were reserviced after each precipitation event.

All soil property data were analyzed using depth and positionclasses as discussed by Cassel et al. (1978). Measurements weretaken in this manner in order to ascertain soil property differ-ences which may occur either temporally or spatially at onlyone particular depth rather than throughout the profile as awhole. In this manner, significant changes in the value of a soilproperty at a given depth may be detected, whereas the changemight not be statistically significant if all depths were pooledin the analysis.

RESULTS AND DISCUSSIONBefore the results are presented it is necessary to un-

derstand why sample intervals of 0 to 14, 14 to 28, and28 to 41 cm were chosen. The plow pan occurred at adepth of 25 ± 3 cm when measured prior to moldboardplowing. After plowing and disking, i.e., just beforeplanting, the soil is uneven and the plow pan occurs atdepths ranging from 28 to 32 cm below the soil surface.The problem that arises is that of a changing point ofreference during the study, i.e., the soil settles resultingin the pan being closer to the surface. I chose to use adepth of 28 cm as the base of the second soil layer be-cause it represents a mean distance for the two extremes.This changing reference point problem must eventuallybe addressed and algorithms included in tillage and bulkdensity models.

Soil texture and water contents at —33 and —1,500kPa vs. depth for the Norfolk soil are presented in Table1. The low standard deviations indicate that the field wasrelatively uniform. For depth 3, percent sand was sig-nificantly less than and percent clay significantly greaterthan their respective values in the upper two depths. Thestandard deviations for sand, silt, and clay in the plowlayer, i.e., depths 1 and 2, were < 1.8% on a total soilbasis indicating a relatively uniform Ap horizon with re-

198 SOIL SCI. SOC. AM. J., VOL. 47, 1983

Table 1—Mean and standard deviation of selected soil physicalproperties of Norfolk loamy sand on 3 June 1977

at Clayton, N.C.

Table 2—Significance levels for analyses of variance by depthfor field-measured soil physical properties

for Norfolk loamy sand.

Depth Sandt Silt Clay

-33kPa -l,500kPawater water

content content

123

86.9±1.8a85.1±1.3a80.9±5.0b

9.8±1.0a11.4±1.2a11.8±1.2a

3.4±1.0a3.6±1.0a7.4±4.1b

7.7±1.2a 2.2±0.6a7.9±1.5a 2.2±0.4a9.1±1.9a 3.2±1.3a

t Data in a given column followed by the same letter are not significantlydifferent at the 0.10 level. Each datum is the mean of 12 measurements(4 sites x 3 positions). Depths 1, 2, and 3 refer to depths of 0 to 14,14 to28, and 28 to 41 cm, respectively.

spect to these soil properties. The trend was for the —33and —1,500 kPa water content values to increase at depth3 in response to the higher clay content although signif-icant differences were not found.

Table 2 summarizes the results of the analyses of var-• iance by depth for nine soil physical properties that wereexpected to change following tillage. Examination of Ta-ble 2 reveals that significant differences of soil propertiesdue to position (Pos), date (D), and Pos X D interactionwere common for depth 1. All nine properties exhibiteddifferences with respect to Pos and Pos X D effects. Fordepths 2 and 3, fewer significant differences were found.The Pos X D means for each depth for six of the nineproperties are listed in Table 3. The soil properties arediscussed individually below.

Bulk Density (Db)As shown in Table 2, Pos, D, and Pos X D effects for

Db at depth 1 were highly significant. From Table 3, wesee that Db for depth 1 was greater at position TI thanfor positions I and R for all dates. Furthermore, Db av-eraged across the three positions (3cPos) increased from1.44 Mg/m3 on 16 May to 1.63 Mg/m3 on 3 June andremained at this value. The Db increase from 16 May to3 June ranged from 0.24 Mg/m3 for position I where nocompaction during planting occurred to 0.14 Mg/m3 forposition TI which experienced tractor wheel compactionduring planting. The increase in Db between 16 May and3 June is attributed primarily to compaction associatedwith wetting of the soil caused by rainfalls of 2.6 and3.9 cm occurring on 25 and 28 May, respectively. Nofurther change in Db occurred between 3 June and 8 July.This observation is not surprising when one considers thatthe Ap horizon was nearly structureless.

Bulk density increased with soil depth (Table 3) al-though it was invariant with Pos, D, and Pos X D fordepths 2 and 3. The greater Db on 16 May at depth 2compared with depth 1 is attributed to soil settling duringthe month after moldboard plowing but before diskingand compaction of depth 2 during the disking operation.The tillage-induced pan at depth 3 causes the 1.81 Mg/m3 Db value.

Water Content (Pw)No attempt was made to measure the soil properties

on dates when Pw values were identical. In fact, this couldonly have been partially achieved and would have re-quired irrigation or rainfall 1 or 2 d before sampling(Bishop and Grimes, 1978). On 16 May and 8 July, Pw

Depth 2(14-28 cm)

Depth 3(28-41 cm)

Soil property

Depth 1(0-14 cm) ______ _________

Pos Pos PosPos§ D x D Pos D x D Pos D x D

Bulk density (Db)Water content} (PJCone index (CI)Total porosity (P)PVI-0.4)PV(-l.O)PV(-IO.O)Macroporosity {M )Ksat

*********

»******NSNSt*

* NS

*****t**

*****#*

NS

NSNSNSNSNSNS

T

NS

**NSNSNS*NSNS

NS

TNSNSNSt

NSNS

NS

tNSNSNSNSNSNS

t

**NSNSNSNSNSNS

NS

NSNSNSNSNSNSNS

**,*, andf represent significance levels of 0.01,0.05, and 0.10, respectively.NS is not significant at 0.10 level.

t Water content units are percent by weight. PV(-0.4), PV(-l.O), andPV(-IO.O) are water content in percent by volume at SWPs of -0.4,-1.0, and -10.0 kPa, respectively.

§ Pos and D refer to position and date of sampling, respectively.

values for depth 1 were similar. Otherwise, the Pos, D,and Pos X D effects for Pw were significantly differentat all depths (Table 2). Water content differences as af-fected by Pos and D decreased with depth. Although Pwfor any date did not vary with position by > one to twopercentage points, the statistical design made it possibleto detect significant differences within each depth as in-dicated by the LSDs for the Pos X D interactions.

Cone Index (CI)Cone index measurements provide information which

allows valid comparisons of mechanical impedance or rel-ative hardness of a given soil. Variables known to affectCI are Db, soil texture, Pw, and/or SWP. Significant dif-ferences in CI occurred for all depths. Table 3 showsthat CI at depth 1 was nearly three times greater on 16May for position TI compared with the other two posi-tions. Cone index, averaged across all positions, de-creased significantly from 16 May to 3 June and thenincreased by 8 July. On 16 May, Db was 1.44 ± 0.14Mg/m3 and Pw was 4.2 ± 0.9%. On 3 June when CIwas actually less than on 16 May, Db had increased from1.44 ± 0.14 to 1.63 ± 0.12 Mg/m3 while Pw had in-creased from 4.2 ± 0.9 to 8.1 ± 1.2%. Cone index variesdirectly with Db and inversely with Pw. Clearly, on 3 JuneCI was more influenced by higher Pw than by higher Db;the net result was a decrease in CI. No difference in Pwoccurred for the 16 May and 8 July dates, hence thehigher CI values on the latter date are attributed to the0.19 Mg/m3 increase in Db.

On 16 May, the mean CI for depth 2 was 55 kg/cm2

greater than for depth 1. This higher CI is attributed tothe higher Db at depth 2 (Table 3). Temporal effects forCI for depths 2 and 3 were significant whereas positioneffects were significant for only depth 3.

Total Porosity (P)Analysis of variance of P showed that Pos, D, and Pos

X D effects were significantly different only for depth 1(Tables 2 and 3). For each date, P within depth 1 wassmallest for position TI; P was invariant for depths 2 and3.

CASSEL: SPATIAL AND TEMPORAL VARIABILITY OF SOIL PHYSICAL PROPERTIES FOLLOWING TILLAGE 199

Table 3—Means, standard deviations, levels of significance, and L.S.D.s for bulk density, water content, cone index, total porosity,macroporosity, and saturated hydraulic conductivity for Norfolk loamy sand, Clayton, N.C., 1977.

Depth 1 (0-14 cm)

Date I R TI *Pos I

Depth 2 (14-28 cm)

R TI

Depth 3 (28-41 cm)

*pos I R TI *PosBulk density (Db)

16 May3 June8 July

5D

1.32±0.021.56±0.071.64±0.051.51 ±0.15

§Pos**D**Pos x D**

1.39±0.051.57±0.101.54 ±0.091.50±0.11

L.S.D. (05)L.S.D. (05)L.S.D. (05)

1.62 ±0.041.76 ±0.041.72±0.021.70±0.07

= 0.05= 0.08= 0.07

1.44±0.141.63 ±0.121.63 ±0.10

1.72 ±0.031.77±0.071.72±0.031.74 ±0.05

PosDPos x D

———— Mg/m' ————1.69±0.06 1.71 ±0.021.72 ±0.02 1.76 ±0.021.70±0.04 1.77 ±0.041.70 ±0.04 1.75 ±0.06

NSNSNS

1.71 ±0.041.75 ±0.051.73 ±0.05

1.82±0.031.81 ±0.051.83 ±0.051.82 ±0.04

PosDPos x D

1.81 ±0.011.84±0.031.84 ±0.061.83±0.04

NSNSNS

1.78 ±0.031.81 ±0.101.78±0.151.79±0.10

1.81 ±0.031.82 ±0.061.81 ±0.09

Water content (PJ

16 May3 June8 July

XD

4.2 ±0.98.0 ±1.04.1 ±0.35.4 ±2.0

Pos**D*Pos x D**

4.1 ±1.07.5 ±0.62.4 ±0.74.7 ±2.3

L.S.D. (05)L.S.D. (05)L.S.D. (05)

4.4±1.18.9 ±1.64.4 ±0.25.9 ±2.4

= 0.3= 1.1= 0.8

4.2 ±0.98.1 ±1.23.6 ±1.0

6.9 ±0.69.6±1.15.6±0.57.4 ±1.9

Pos**D**Pos x D**

- percent by weight —7.1 ±0.5 6.6 ±0.69.6±1.3 10.5±2.35.0 ±0.6 5.9 ±0.87.2±2.1 7.6±2.5

L.S.D. (05) = 0.4L.S.D. (05) = 0.5L.S.D. (05) = 0.3

6.8 ±0.59.9 ±1.55.5 ±0.7

7.9 ±1.910.4±1.57.5 ±2.68.6 ±2.3

Pos**D**Pos x D*

7.6±2.09.9±1.18.3 ±2.68.6 ±2.1

L.S.D. (05)L.S.D. (05)L.S.D. (05)

10.4±1.611.7±3.210.1 ±3.410.7 ±2.7

= 0.6= 1.6= 0.8

8.6 ±2.110.7±2.18.6 ±2.8

Cone index (CI) x 10-'

16 May3 June8 July

5D

13±914±159±529 ±23

Pos**D**Pos x D*

17±39±3

71±832 ±29

L.S.D. (05)L.S.D. (05)L.S.D. (05)

51±821±594±1155 ±34

= 8= 10= 15

27 ±1915±675 ±20

75 ±2339 ±10

143±3486 ±50

PosD**Pos x Dt

Total porosity (P)

16 May3 June8 July

0.47 ±0.020.39±0.040.40 ±0.01

0.44 ±0.020.42 ±0.030.37 ±0.05

0.34 ±0.030.30±0.010.31 ±0.01

0.42 ±0.060.37 ±0.060.36 ±0.05

0.30 ±0.030.29 ±0.020.30 ±0.03

74±11 96±1234±12 39±12

157 ±22 133 ±2888±55 89±44

NSL.S.D. (05) = 16L.S.D.(05) = 21

cm'/cm' ————0.30 ±0.05 0.29 ±0.020.30 ±0.02 0.26 ±0.030.33 ±0.03 0.27 ±0.01

82 ±1837 ±10

144 ±27

0.30 ±0.030.28 ±0.020.30 ±0.04

95 ±1544 ±15

128 ±5289 ±46

PostD**Pos x D

0.26 ±0.010.24 ±0.060.24 ±0.02

89 ±1943 ±16

165 ±2499 ±55

L.S.D. (05)L.S.D. (05)NS

0.26 ±0.020.25 ±0.020.25 ±0.02

72 ±3940 ±2296 ±6069 ±46

= 26= 21

0.27 ±0.030.26 ±0.040.28 ±0.04

85 ±2643 ±16

130 ±52

0.26 ±0.020.25 ±0.040.25 ±0.03

0.42±0.04 0.41±0.04 0.32±0.03 0.29±0.02 0.31±0.04 0.27±0.02 0.24±0.03 0.25±0.02 0.27±0.03Pos** L.S.D. (05) = 0.03 Pos NS Pos NSD* L.S.D. (05) = 0.03 D NS D NSPos x D L.S.D. (05) = 0.04 Pos x D NS Pos x D NS

Macroporosity (M)

16 May3 June8 July

*D

0.31 ±0.020.20 ±0.040.21 ±0.010.24 ±0.05

Pos**D*Pos x D**

0.26 ±0.020.25 ±0.040.20 ±0.050.24 ±0.05

L.S.D. (05) =L.S.D. (05) =L.S.D. (05) =

0.12±0.020.08 ±0.030.12 ±0.020.11 ±0.030.030.060.05

0.23±0.08 0.13±0.040.18±0.08 0.12±0.020.18±0.05 0.12±0.02

0.12±0.03PosDPos x D

Saturated hydraulic conductivity (K,atft

16 May3 June8 July

5D

4.9 ±3.62.7 ±1.53.0±1.33.5 ±2.4

Pos**DPos x D*

3.9 ±1.22.6 ±0.86.7 ±3.24.4 ±2.5

L.S.D. (05) =NSL.S.D. (05) =

1.4 ±1.20.3 ±0.10.4±0.20.7 ±0.8

= 1.8

= 2.2

3.4 ±2.6 0.4 ±0.21.9±1.4 0.4±0.33.4±3.2 1.0±0.3

0.6 ±0.4PosDPos x D

———— cm /cm ————0.13±0.05 0.12±0.010.13±0.020.15 ±0.040.14 ±0.03

NSNSNS

0.09 ±0.040.10±0.020.10 ±0.03

0.13 ±0.03 0.07 ±0.010.11 ±0.03 0.06 ±0.030.12 ±0.03 0.06 ±0.02

0.06 ±0.02PosDPos x D

0.08 ±0.030.07 ±0.010.07 ±0.010.07 ±0.02

NSNSNS

0.08 ±0.030.07 ±0.010.07 ±0.010.07 ±0.02

0.07 ±0.020.06 ±0.020.06 ±0.01

———— cm/h ———————— ——— —— —— —— —— —— ——0.9 ±0.5 0.4 ±0.30.7 ±0.90.8 ±0.50.8 ±0.6

L.S.D. (05)NSNS

0.3 ±0.30.3 ±0.20.3 ±0.2

= 0.5

0.6 ±0.4 0.4 ±0.40.5 ±0.5 0.2 ±0.10.7±0.4 1.0±1.6

0.6 ±0.9PosDPos x D

0.2 ±0.10.1 ±0.10.3 ±0.20.2±0.1

NSNSNS

0.8 ±1.30.3 ±0.60.5 ±0.40.6 ±0.8

0.4 ±0.80.2 ±0.30.6 ±0.9

**,*, and t Refer to significance levels of 0.01,0.05, and 0.10, respectively. NS is not significant at the 0.10 level.t Ksat values measured on 16 May are affected by experimental artifact.atva

§ Each Pos x D datum is the mean of four observations.

Water contents of the undisturbed soil cores at soilwater pressures of —0.4, —1.0, and —10 kPa were alsodetermined. Table 2 shows that in general the signifi-cance levels for these three properties are similar to thosefor P. Hence, detailed information for these three prop-erties are not included in Table 3.

Macroporosity (M)Macroporosity was previously defined as the air-filled

porosity at a soil water pressure of —6.0 kPa. Table 2shows that differences in M occurred only for depth 1.On 16 May, M at depth 1 ranged from a minimum of0.12 ± 0.02 cm3/cm3 at position TI to a maximum of

200 SOIL SCI. SOC. AM. J., VOL. 47, 1983

0.31 ± 0.02 cm3/cm3 at position I. Rainfall between 16May and 3 June caused the soil in depth 1 to settle, thusreducing M at position I to 0.20 ± 0.04 cm3/cm3. Forpractical considerations, M was invariant with time atdepth 2 because soil at this depth was packed during thedisking operation preceding planting. The value of M atdepth 2 ranged from 0.10 to 0.14 cm3/cm3. For depth 3,M did not attain a value > 0.07 ± 0.02 cm3/cm3. Thislow value is attributed to the fact that few macroporesexist when loamy sand is compacted to a Db of 1.81 Mg/m3.

Saturated Hydraulic Conductivity (Kmt)For depth 1, #sat varied with Pos and Pos X D but

not with D. For depth 2, A"sat varied only with Pos at the0.10 level; no differences in A"sat were observed for depth3.

Wheel compaction at position TI for depth 1 on 16May resulted in Ksai of 1.4 ± 1.2 cm/h compared with4.9 ± 3.6 cm/h at position I (Table 3). However, Ksatwas invariant with time, a result that is somewhat sur-prising when one considers the rather large change in Dbthat occurred in depth 1 from 16 May to 3 June. Thisresult is an experimental artifact and is explained as fol-lows. Bulk density for each soil core collected on 16 Maywas calculated by dividing the mass of oven-dry soil by

-80

Jf

£DCUJ

80

40

0

SO

NORFOLK LOAMY SAND, CLAYTON , 1977

20 CM DEPTH POSITION I

POSITION R

JUN 9 JUL 1 NOV 1

Fig. 2—Temporal traces of soil water pressure at the 20-cm depth ofNorfolk loamy sand planted to soybeans for positions I, R, TI, andthe position mean. The vertical bar above each datum is 1 SD inlength.

the 347-cm3 volume of the core. When the soil was sat-urated prior to determining the soil water characteristic,the soil settled with a concomitant increase in Db. Hence,Asat was determined on a soil sample which had a higherDb than its in-situ Db. For this particular structurelessNorfolk soil, it is not possible to determine K^t at a Dbof 1.32 Mg/m3 (Db for depth 1, position I, 16 May), forexample, because the soil settles to a higher Db uponwetting.

This settling problem posed no problem in measuringthe values of P and M for the 16 May samples discussedabove because the empty space in the 347-cm3 core ringabove the settled soil was considered to be part of themacroporosity. Some natural settling had already oc-curred in the field prior to the 3 June sampling and ATsatof these cores and those obtained on 8 July was not af-fected by this problem.

Soil Water Pressure (SWP)The course of soil water pressure (SWP) at approxi-

mately 10- to 15-d intervals during the major part of thegrowing season at positions I, R, and TI, and the overallmeans for these three positions for the 20-cm depth areshown in Fig. 2. Each datum is the mean of four obser-vations except for the position mean curve for which eachdatum is the mean of 12 observations. One standard de-viation (SD) unit is represented by the length of the ver-tical bar arising from each datum.

Measurement of SWP began on 21 June, 36 d afterplanting, and was <—40 kPa throughout much of thegrowing season except for a 10- to 12-d period beginningaround 17 September and for all dates after 24 October.

Visual examination of the rooting pattern revealed thatroot density was greater at position R than for I andespecially TI. Yet preferential extraction of soil waterfrom position R was not found. Apparently, the root den-sity was sufficient at all three positions to take up waterat approximately the same rate. The absolute value ofthe SD for the mean of all three positions ranged from1.5 to 31.0 kPa. These are appreciable quantities becausethe working range of a tensiometer encompasses a SWPrange of only —80 kPa.

The traces of SWP vs. time at the 60-cm depth (notshown) behaved somewhat differently. Soil water pres-sure changes occurred more slowly and the minimumSWPs were greater. The tillage-induced pan preventedextensive rooting at the 60-cm depth and SWP did notdecrease to —40 kPa until approximately 8 August.

The soil physical property data presented in Tables 2and 3 and in Fig. 2 illustrate some of the spatial differ-ences in soil physical properties that occur. Furthermore,these properties underwent temporal changes dependingupon the spatial location of the soil, i.e., its position anddepth. This variability can be measured in the field andstatistically evaluated if the" correct experimental designis used.

This Norfolk soil was nearly structureless and had atillage-induced pan at the 25-cm depth. The moldboardplowing operation loosened the soil to the 25-cm depth;secondary tillage (3 diskings) compacted the soil belowthe 10- to 15-cm depth. The seeding operation on 16 Mayimposed measurable spatial variation of soil propertiesfor depth 1 due mainly to tractor wheel compaction.Measurable effects of wheel compaction for the other two

SCHWAB & LINDSAY: EFFECT OF REDOX ON THE SOLUBILITY & AVAILABILITY OF IRON 201

depths were absent. Further changes in Db occurred dur-ing or following rainfall events. Soil strength of the nearlystructureless Norfolk soil in depth 1 was so small thatthe soil settled upon wetting due to its own weight. Thisis illustrated by the fact that the undisturbed soil corescollected on 16 May settled when they were slowly watersaturated prior to determining KseLt-

Another energy source that induces temporal variationin the soil properties listed in Table 3 is the kinetic energyassociated with raindrop impact. This energy compactsfield soils that have low aggregate stabilities. The energyassociated with the rainfall events between 16 May and3 June were not recorded; however, it is likely that itcontributed to the overall compaction.

On 16 May Db was highest at depth 1 at position TI.Associated with this highest Db were the highest CI val-ues and the lowest M and Ksat values compared to po-sitions I and R. Compaction occurs at the expense of Pand M thus reducing the mean pore size. Saturated hy-draulic conductivity in turn was reduced due to the re-duction in M. For this soil it is possible that diffusion ofoxygen into the soil, a process that is dependent upon Pand Pw may be reduced below critical levels.

The agronomic effects of spatial and temporal varia-tion in soil properties often can be observed in the field.Soil in the trafficked interrows which have reduced P andATsat often have water standing on the soil surface follow-ing precipitation events whereas water infiltrates morerapidly in the row and nontrafficked interrow positions.During high and medium intensity rainfall events muchof the water falling on the trafficked interrow is lost to

runoff and often causes erosion. In addition, roots do notreadily invade severely compacted soils thus increasingthe plants susceptibility to drought especially if a tillagepan is present which prevents deep root penetration.