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Soil macroporosity, hydraulic conductivity and air permeability of silty soils under long-term conservation tillage in Indiana
Soil & Tillage Research, 11 (1988) 1-18 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
1
Soil Macroporosity, Hydraulic Conductivity and Air Permeability of Silty Soils under Long-Term Conservation Tillage in Indiana JOHN R. HEARD, EILEEN J. KLADIVKO, and JERRY V. MANNERING
Agronomy Department, Purdue University, West Lafayette, I N 47907 (U.S.A.) (Accepted for publication 29 June 1987 )
ABSTRACT Heard, J.R., Kladivko, E.J. and Mannering, J.V., 1988. Soil macroporosity, hydraulic conductivity and air permeability of silty soils under long-term conservation tillage in Indiana. Soil Tillage Res., 11: 1-18. With the increasing use of conservation tillage, many questions about the long-term effects of tillage system on soil physical properties have been raised. Studies were conducted to evaluate saturated hydraulic conductivity (KsAT), macropore characteristics and air permeability of two silty soils as affected by long-term conservation tillage systems in the state of Indiana. Measurements were taken during the tenth year of a tillage study on a Chalmers silty clay loam (Typic Haplaquoll) and the fifth year of a study on a Clermont silt loam (Typic Ochraqualf). Tillage systems were moldboard plow, chisel, ridge till-plant, and no-till in a rotation of corn (Zea mays L. ) and soya beans (Glycine max L. ). Saturated hydraulic conductivity was measured on large soil columns (25 X 25 × 40 cm) before spring tillage, and macropore size and continuity were assessed with staining techniques. Intact soil cores (8 cm diam × 10 cm) were collected in early July in the row and non-tra•cked interrow at three depths (10-20, 20-30, and 30-40 cm) and were analyzed for air permeability (Kair ), air-filled porosity and bulk density. Saturated hydraulic conductivity values were in the order plow > chisel > ridge till > no-till for the Chalmers soil and were significantly greater in the plow treatment than in the other 3 tillage systems on the Clermont soil. Differences in K S A w between the 2 soils were generally greater than differences among tillage systems, and coefficients of variation were lower for treatments that did not include any fall tillage operations. At the 10-cm depth on the Chalmers soil, the chisel treatment had the greatest number of stained cylindrical channels, whereas for the Clermont soil the ridge till had the greatest number at this depth. Although the no-till treatment had similar or fewer total channels, it had the most continuous channels from the 10-cm depth to the 20- and 30-cm depths on both soils. Tillage system, row position and depth all affected K~r. On the Chalmers soil, plow, chisel and ridge systems had lower g a i r between rows than in the row at the 10-20-cm depth, whereas no-till had constant Kair in the row and between the row. On the Clermont soil, ridge till had the highest Kair of all treatments at the 10-20-cm depth, and no-till had the highest Ka~rof all treatments at the 20-30-cm depth. Journal Paper No. 11 090 of the Purdue University Agricultural Experiment Station. Contribution from the Department of Agronomy.
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© 1988 Elsevier Science Publishers B.V.
INTRODUCTION Conservation tillage systems have been shown to reduce soil erosion on many soils (Moldenhauer, 1985). However, the long-term effects of conservation tillage on soil physical properties have not been adequately documented. Flow processes ( i.e. water, air, heat) in particular have not been studied adequately enough to enable reliable tillage response models to be developed. In addition, the spatial variation of physical properties with row position and depth is important for quantitative tillage models (Cassel and Nelson, 1985) but has often been ignored. More measurements of soil physical properties, as affected by long-term tillage treatments, are needed to model and predict adequately crop response to a given tillage system. Most studies on fine-textured soils show that soil bulk density is higher under no-till than under conventional-tillage systems ( Gantzer and Blake, 1978; Kladivko et al., 1986), but some studies have found no differences (Blevins et al., 1983 ). Usually infiltration rates for no-till have been higher than for tilled treatments (Triplett et al., 1968; Edwards, 1982 ) but in some cases they were lower (Lindstrom et al., 1981 ). The hydraulic conductivity of non-tilled soils can be significantly affected by macropores formed by earthworms, soil insects or roots, but these macropores are often destroyed in tilled soils (Ehlers, 1976). Increased earthworm activity has been observed under no-till systems compared to tilled treatments (Boone et al., 1976; Lal, 1976; Barnes and Ellis, 1979; Mackay and Kladivko, 1985 ). Studies of water flow in earthworm channels indicate that infiltration rate is governed by diameter and depth of the channels, as well as by the number of channels per unit area (Ehlers, 1975; Edwards et al., 1979; Bouma et al., 1982). Douglas et al. (1980) measured greater hydraulic conductivity at the interface of topsoil and subsoil under direct-drilling than plowing, owing to continuous earthworm channels at this depth. Gantzer and Blake (1978), despite finding two to four times more macropore channels in no-till, measured greater hydraulic conductivity on tilled soil. Air permeability is another measurement that has been used to assess the transmission aspects of pore geometry, in order to evaluate the effects of soil compaction (Phillips and Kirkham, 1962), cropping rotations (Evans and Kirkham, 1949; Groenevelt et al., 1984), and tillage systems (Janse and Bolt, 1960; Ball, 1981; Hamblin and Tennant, 1981; Mielke et al., 1986; Douglas, 1986). Air permeability (Kair ) was usually found to be greater under tilled than under non-tilled conditions, owing to greater porosity and larger pores. Douglas (1986) found a good relationship between continuous macropores and air permeability, especially under no-till conditions. Ball (1981) identified the g r e a t e r K a i r of conventional plowing compared to direct drilling as being due to more pores > 150/lm diameter. In compaction studies on corn growth, Phillips and Kirkham (1962) found that Kair values accurately reflected the corn-
pacted treatments. Both Evans and Kirkham (1949) and Groenevelt et al. (1984) measured higher Kair when corn followed other crops in a rotation than in continuous corn culture. The objectives of this study were to measure hydraulic conductivity, air permeability, and pore characteristics of 2 silty soils as affected by 4 long-term conservation and conventional-tillage systems in Indiana, and to assess variation in air permeability with row position and depth. MATERIALSAND METHODS Field sites and tillage systems Soil samples were collected in 1984 from long-term tillage experiments located in north-central and southeastern Indiana. Soils at both sites freeze during the winter, with the annual number of freeze-thaw cycles varying with the weather regime each year. The north-central location was established in 1975 on a Chalmers silty clay loam ( fine-silty, mixed mesic, Typic Haplaquoll). The soil is dark-colored, well-structured, naturally poorly drained and contains 59% silt, 32% clay and 4.0% organic matter in the Ap horizon. Drain tiles (20-m spacing) are located at approximately 1-m depth and run perpendicular to the rows. Crop rotations studied were continuous corn (Zea mays L. ), corn/soya bean ( Glycine max L.) rotation and continuous soya beans. Treatments were replicated 4 times, and tillage plots were 12 rows wide and 46 m long with 76cm row spacing. The southeastern location was established in 1980 on a Clermont silt loam (fine-silty, mixed mesic, Typic Ochraqualf). A fragipan-like horizon below a depth of about 90 cm can restrict drainage in the spring. This soil is lightcolored, poorly structured, naturally poorly drained, and contains 73% silt, 10% clay and 1.3% organic matter in the Ap horizon. Crop rotations studied were continuous corn and corn/soya bean rotation. Experimental design was similar to that of the Chalmers, except that plots were either 46- or 61-m long. Tillage systems included conventional moldboard plow, chisel plow, ridge till-plant, and no-till. Conservation tillage has been defined as any tillage system that reduces loss of soil or water relative to conventional tillage (Mannering and Fenster, 1983), by a combination of surface residue cover and/or soil roughness. Moldboard plow has been the "conventional" form of tillage for corn and soya beans in this region, and all other systems studied were to some extent "conservation" tillage. Moldboard plowing to a depth of 20-25 cm was performed in the autumn on the Chalmers and in the spring on the Clermont. Chiseling (10-cm twisted shanks) to a depth of 20-25 cm was performed in the autumn on both soils. Secondary tillage for both the plow and chisel systems consisted of discing and field cultivation to depths of about 10 cm in the spring prior to planting. The ridge till-plant system consisted of planting
in pre-formed ridges by scraping the top 2.5 cm of soil from the ridge and reforming the ridge with a disk-hiller or winged-sweep, during a cultivation operation when the corn was about 45-60 cm tall. Ridges for soya beans were formed after harvest of the beans rather than at cultivation. For no-till planting, the planter was equipped with 2.5-cm-wide non-powered fluted coulters to open a slot for the seed. Anhydrous NH3 was injected in each interrow area of all plots prior to planting, and was the only "tillage" the no-till plots received. Crop yields and selected soil physical properties from those plots have been reported previously ( Kladivko et al., 1986 ). During 1984, on the Chalmers soil, interrow cultivation was done on 25 June for ridge-till soya beans and on 3 July for plow and chisel soya beans. On the Clermont soil, plow, chisel, and ridgetill soya beans were cultivated on 28 June.
Hydraulic conductivity, water-conducting channels, and bulk density Saturated hydraulic conductivity (KsAT) was measured by the method of Bouma and Dekker (1981) in the corn/soya bean rotation plots during the spring following soya beans. Two soil columns per plot were collected in late April and early May, prior to any spring tillage and while the soil was close to saturation. The column edge was positioned about 5 cm from the row and extended into the untrafficked interrow almost to the midline between rows. It was felt that this column size and position would adequately represent the average field conditions. The final column size was 25 × 25 × 40-cm deep and included the soil surface and the interface between the plow layer and subsoil. Four replicate blocks were sampled at the Clermont site (32 individual columns), but only 3 blocks were sampled at the Chalmers site (24 columns). Saturated hydraulic conductivity was measured by maintaining a constant head of about 2 cm water on the soil surface and collecting the percolate in large plastic basins. For the ridge-till system the entire surface was submerged, and the average water head was assumed to be the head at the midplane between the top and bottom of the ridge. Rates were measured at 10-min intervals until steady state was reached. Saturated hydraulic conductivity values represent the 0-40-cm depth of the soil. For those tilled treatments with different bulk densities in the tilled zone and the undisturbed soil below, the measured KSAw would actually be the effective KSAw of the layered system. A 0.1% solution of methylene blue dye was ponded on the soil surface ( Bouma et al., 1977) prior to attaining steady-state KSAw o n the Clermont soil, and after attaining steady-state on the Chalmers soil. Pores that were continuous to the soil surface, and were actively conducting water, would be stained with this solution. The columns were sectioned at depths of 10, 20, and 30 cm. Ridgetill depths were measured from the average height of the surface (midplane between ridge top and ridge valley). Both stained and unstained visible channels (cylindrical) were counted and sorted into four size classes by their di-
ameter: small (1-2 m m ) ; medium (2-4 m m ) ; large (4-6 m m ) ; very large ( > 6 m m ) . Stained cracks and non-circular voids were also observed, but no attempt was made to characterize them. Bulk density of the Clermont was determined with a can sampler (cans 3.5 cm X 5.5 cm diam), during sectioning of the soil columns, at depths of 2.5-6, 10-13.5, 20-23.5, and 30-33.5 cm. Bulk density was measured on the Chalmers by an excavation technique developed by the Soil Conservation Service (R.B. Grossman, personal communication, 1983), at depths of 0-3, 7.5-12.5, and 17.5-22.5 cm in April 1985 prior to any tillage operations. The technique is basically a modification of rubber balloon excavation techniques, and involves excavating a shallow hole, lining the hole with thin plastic film, and filling the hole with water to determine the excavated soil volume.
Air permeability, air-filled porosity, and bulk density Samples for air permeability, air-filled porosity, and bulk density were collected from continuous soya bean and corn/soya bean rotation plots on the Chalmers soil, and from corn/soya bean rotation plots on the Clermont soil, during early July ( soya-bean crop year). Intact 8-cm diameter cores were taken with a hydraulic probe truck in the crop row, and in the non-trafficked interrow, to a depth of 45 cm at 2 sites in each plot. Cores were then cut into 3 depth sections (10-20, 20-30 and 30-40 cm ), and dipped in saran ( liquid plastic ) to maintain the undisturbed soil structure. The total number of 10-cm core sections sampled was 384 and 192 from the Chalmers and Clermont soils, respectively. In the laboratory, the cores were initially wet up on a sand tension table at - 0.5 kPa water potential. A 5.0-cm long core was cut from the least disturbed portion of the Chalmers soil cores, and from the middle of the Clermont soil cores. E n d surfaces were carefully picked to expose natural ped faces, and to prevent smearing the soil surface. Core volumes were calculated from height and diameter measurements. Cores were encased with paraffin wax into 9.8cm diameter aluminum rings. After saturation the samples were equilibrated at - 5 kPa water potential on a sand tension table, and air permeability was measured. The Chalmers soil was further equilibrated at - 10 kPa water potential and air permeability remeasured. After the final air permeability reading, the core was oven dried and weighed for bulk density calculations, and airfilled porosity was calculated by the difference between saturated core weight and equilibrated weight at - 5 or - 10 kPa water potential. Air-filled porosity at - 5 kPa water potential (60 ~m equivalent pore diameter) has been used as an index of "aeration porosity" ( Kohnke, 1968), and a water potential of - 10 kPa is often used as an approximation of"field capacity" (Cassel and Nielsen, 1986). A constant-pressure air permeameter, similar to that of Green and Fordham
(1975), was designed. The system was operated at low pressure (1.4 kPa ), and viscosity of the air was maintained relatively constant by bubbling air through a water bath at 24-27 ° C. Air permeability coefficients (Groenevelt et al., 1984) were calculated from: F--gai r Ah/L where F is the measured flux of air (cm 3 cm -2 sec-1), zih is the air pressure head difference across the soil core (cm H20), and L is the length of the soil sample (cm).
Statistics The 2 columns per plot that were collected for KSAT w e r e treated as subsamples, and data were averaged from them before performance of analysis of variance (ANOVA). Because hydraulic conductivity is often found to be lognormally distributed in the field (Nielsen et al., 1973), the geometric means for conductivity were also calculated and analyzed by ANOVA. Air permeability data were transformed to approximate a normal distribution. The experimental design for this part of the study was a split-split-split plot for the Chalmers and a split-split plot for the Clermont. For the Chalmers experiment, rotation was randomly assigned to the main plots and tillage was randomly assigned within those plots as the subplots. Row position was a subsubplot and depth was the sub-sub-subplot and both were systematically arranged. For the Clermont experiment, only one rotation was sampled, but the rest of the design was the same as the Chalmers. Analysis of variance was performed for the 3 soil properties on the 2 soils. The error term used to test the significance of any effect that included position or depth (systematically arranged factors) was the block × effect interaction for that effect. As discussed by Cassel and Nelson (1985), this procedure gives an extremely conservative F-test for significance, but is more appropriate because position and depth cannot be randomized. Mean separations of main effects and interactions were then performed using Student-Newman-Keuls and LSD procedures, respectively. In order to evaluate whether treatment differences in Kair were due mainly to differences in air-filled porosity (fa), values of Kair were divided by fa, and ANOVA procedures were rerun on the scaled values ( Groenevelt et al., 1984 ). If the scaled values still show significant treatment effects, it is likely that differences in K ~ are due to differences in pore size and continuity. RESULTS AND DISCUSSION
Hydraulic conductivity, water-conducting channels, and bulk density Water-conducting channels All of the larger cylindrical channels observed in the columns appeared to be formed by earthworms, owing to the presence of cast material and the lack
7 TABLE I Numbers of stained channels at 10-cm depth as affected by tillage system on the Chalmers and Clermont soils. {Pore numbers shown as number m -z) Soil
Tillage
Pore size 1
S
M
Chalmers silty clay loam
Plow Chisel Ridge No-till
119 105 97 139
132d 2 260c 158cd 177cd
Clermont silt loam
Plow Chisel Ridge No-till
603 527 511 275
454 470 408 235
L
VL
T
24d e 91c 20d lld
3 8 0 0
278 464 275 327
64cd 2 41d 116c 38d
3 0 5 10
1124 1038 1040 558
~S=small (1-2 ram), M=medium (2-4 mm), L=large (4-6 mm), VL=very large ( > 6 mm) diameter; T = total 2Means in this column for the same soil that are followed by the same letter are not significantly different (Student-Newman-Keuls at 95% level). Columns with no letters had no significant differences among treatments.
of any decomposing root tissue. Some small channels were formed by decomposing roots, but most appeared to be earthworm holes. Numbers of stained channels at the 10-cm depth, as affected by tillage system, were quite variable, and few significant t r e a t m e n t differences were found ( Table I ). On the Chalmers soil, the chisel t r e a t m e n t had significantly greater numbers of medium and large stained channels t h a n other treatments. On the Clermont soil, ridge till had significantly more large channels than either chisel or no-till at this 10-cm depth. Higher numbers of channels in the chisel t r e a t m e n t were not expected. However, since chiseling was done in the early fall, and earthworms are most active in the fall and early spring (Barnes and Ellis, 1979), the earthworm population may have recovered sufficiently from the tillage operation to form new burrows. Chiseling may actually have encouraged earthworm activity at this 10cm depth, because of the incorporation of crop residue as a food source in this layer. Gantzer and Blake (1978) suggested that a high activity of channel formation in the spring may compensate for destruction of continuous pores by tillage. There are no apparent reasons for the greater number of large channels under ridge till than chisel and no-till, or the comparatively low channel numbers under no-till on the Clermont soil. Table II indicates the numbers of stained channels at 20- and 30-cm depths, as a percentage of those stained at 10 cm, for each tillage t r e a t m e n t and poresize class. This percentage expresses the continuity of pores from the 10-cm
8
TABLE II Stained channels at 20- and 30-cm depths as a percentage of stained channels at 10 cm, as affected by tillage system on the Chalmers and Clermont soils Tillage
Depth
Pore size 1
(cm) S Chalmers silty clay loam Plow Chisel Ridge No-till Clermont silt loam Plow Chisel Ridge No-till
M
L
VL
T
20 30 20 30 20 30 20 30
28.6 13.4 99.0 81.9 71.1 88.7 79.1 65.5
43.9 28.0 38.1 20.0 29.7 33.5 54.8 51.4
100.0 41.7 6.6 5.5 15.0 15.0 145.5 0
366.7 0 0 0 0 0 0 0
45.7 22.7 45.0 30.8 44.4 41.8 68.2 55.7
20 30 20 30 20 30 20 30
55.2 116.4 119.0 112.7 78.7 117.4 176.0 145.1
48.9 40.7 50.4 42.3 52.0 50.7 80.9 75.3
35.9 43.8 12.2 17.1 22.4 8.6 52.6 86.8
0 0 0 0 100.0 0 0 0
51.4 81.4 83.7 77.1 62.0 78.6 124.4 109.1
~S=small (1-2 ram), M=medium (2-4 mm), L=large (4-6 mm), VL=very large ( > 6 mm) diameter, T = total. d e p t h to the 20- a n d 30-cm depths. U n d e r t h e C h a l m e r s soil at 20 a n d 30 cm, a greater relative n u m b e r o f c h a n n e l s was p r e s e n t in no-till t h a n u n d e r o t h e r tillage t r e a t m e n t s , owing chiefly to a g r e a t e r relative n u m b e r of m e d i u m c h a n nels. In t h e C l e r m o n t soil, a l t h o u g h t h e no-till h a d t h e least n u m b e r of s t a i n e d c h a n n e l s at t h e 10-cm d e p t h c o m p a r e d w i t h t h e o t h e r t r e a t m e n t s ( T a b l e I ) , it t e n d e d to m a i n t a i n m o r e c o n t i n u o u s c h a n n e l s w i t h d e p t h ( T a b l e I I ) . T h e s e o b s e r v a t i o n s suggest t h a t , despite similar or fewer n u m b e r s of c h a n nels u n d e r no-till c o m p a r e d w i t h o t h e r tillage t r e a t m e n t s , t h e s e c h a n n e l s were m o r e c o n t i n u o u s with depth. C h a n n e l s n e a r the surface u n d e r t h e tilled syst e m s were n o t as c o n t i n u o u s , p r o b a b l y owing to the d e s t r u c t i v e n a t u r e of tillage. T h e n u m b e r o f t o t a l s t a i n e d c h a n n e l s ( T a b l e I ) r a n g e d f r o m 1.7-4.0 t i m e s g r e a t e r u n d e r t h e C l e r m o n t t h a n t h e C h a l m e r s . T h i s p r o b a b l y results f r o m t h e difference in t h e s t a i n i n g p r o c e d u r e , since t h e t o t a l n u m b e r s of s t a i n e d plus u n s t a i n e d c h a n n e l s u n d e r t h e C l e r m o n t r a n g e d f r o m 0.46 to 1.96 t i m e s t h o s e of t h e C h a l m e r s . Owing to high variability, few significant t i l l a g e - t r e a t m e n t differences in t o t a l c h a n n e l s ( s t a i n e d plus u n s t a i n e d ) were detected, c o n t r a r y
T A B L E III Soil bulk density of the Chalmers and Clermont soils as affected by tillage system (Mg m -:~) Tillage
Chalmers silty clay loam 1
Clermont silt loam 2
0-3
7.5-12.5
17.5-22.5
2.5-6
10-13.5
20-23.5
30-33.5
1.03 0.95 0.93 1.02
1.15d a 1.18d 1.29d 1.43c
1.27d 4 1.36d 1.45d 1.53c
1.44 1.45 1.40 1.48
1.47 1.46 1.45 1.52
1.49 1.50 1.54 1.55
1.45 1.55 1.49 1.44
Depth (cm)
Plow Chisel Ridge No-till
~Bulk density measured by excavation technique, April 1985, in field plots. 2Bulk density measured with can sampler, May 1984, on individual KSAT columns. :~4Means in this column that are followed by the same letter are not significantly different at the 95% and 99% levels, respectively. Columns with no letters had no significant differences among treatments.
to several published reports (Ehlers, 1975; Gantzer and Blake, 1978). Counts of macropore channels in this study generally exceeded those of Ehlers (1975), and were intermediate between the tilled and untilled numbers reported by Gantzer and Blake (1978) and Barnes and Ellis (1979).
Bulk density On both soils the no-till treatment had a higher bulk density than the other tillage systems in the 7.5-23.5-cm depth zone (Table III). No significant differences in bulk density among tillage systems were found below the tillage layer. Other researchers have also found higher bulk densities under no-till in the topsoil, but few tillage differences at depths greater than 30 cm ( Gantzer and Blake, 1978 ). The higher bulk density for chisel than the other treatments at 30 cm on the Clermont may have resulted from compaction by autumn chiseling to depths of about 25 cm. Bulk densities of the Clermont soil were higher than the Chalmers soil, owing to poor structure and low cohesiveness.
Hydraulic conductivity Saturated hydraulic conductivity (KsAT) data for tillage treatments on both soils are shown in Table IV. On the Chalmers the trend for KSAT was plow > chisel > ridge till > no-till. The coefficient of variation ( C.V. ) values for plow and chisel systems were much larger than for ridge and no-till, suggesting that soil physical conditions were not uniform in space following these primary tillage operations. The lower C.V. values of no-till and ridge till suggest that some spatial uniformity is achieved if the soil is not disturbed by primary tillage. The geometric mean conductivity values for plow, chisel and ridge till
10 TABLE IV Saturated hydraulic conductivity (KsAT) of Chalmers and Clermont soils as affected by tillage system (0-40-cm depth) Tillage Chalmers silty clay loam Plow Chisel Ridge No-till Clermont silt loam Plow Chisel Ridge No-tiU
KSAT (cm hr -1)
C.V. 1 (%)
KSAT2 (cm hr -1)
38.1 30.5 26.8 15.5
68 45 12 16
27.7 27.4 26.4 15.3
12.8" 3.7 4.1 2.3
51 140 65 53
11.6" 2.2 2.9 2.1
1Coefficient of variation, calculated after averaging samples within each block; Chalmers (n = 3 ), Clermont (n--4). ~Geometric mean values. *Significant at the 95% level.
were all similar, and were higher than no-till, but no significant difference was detectable even with this log-normal transformation. Plow and no-till KSAw values for the Chalmers soil were very similar to those of Gantzer and Blake (1978), who collected cores from a clay loam soil at the 7.5-15-cm depth after secondary tillage and corn planting (plow--38.2 cm hr -1, no-till=14.6 cm hr-1). The KSAw of the Clermont silt loam was significantly higher under the plowed treatment (sampled prior to spring plowing) than under any other tillage treatment, and no significant differences in KSAw w e r e observed among no-till, chisel and ridge-till plots, using either arithmetic or geometric mean values. Despite previous indications of a plowpan in the Clermont soil (King and Franzmeier, 1981 ), and observation of a perched water table during excavation of preliminary soil columns under saturated conditions, the conductivity of the spring plow exceeded the other treatments. Bulk density values also gave no indication of a dense plowpan under this treatment. The high K s A w of the plow treatment compared with the chisel and ridge can be attributed partly to the covering of soya bean residue on the soil surface, which prevented surface sealing over the winter and spring. Those treatments involving fall tillage (chisel plowing and fall ridge forming for ridge till) exhibited surface sealing (up to 0.8-cm thick ) in places where residue did not protect the soil surface. The low conductivity of no-till compared with plow may be due to the higher bulk density of the no-till (Table III). The highest C.V. values on the Clermont were associated with fall chiseling and ridge till, whereas the no-till and spring plow,
11
which had been unworked since the previous summer, tended to have lower C.V. values. The KSAw of the Clermont was much lower than that of the well-structured Chalmers, regardless of tillage treatment. This would indicate that other relevant soil properties (texture, organic matter content, etc.) have more effect o n KSAT than does tillage treatment.
Relationship of pore continuity and KSA T Under no-till both soils showed a greater continuity of conducting pores from the 10-cm to the 20- and 30-cm depths than under the other tillage treatments. However, no-till had lower KSAw values than some other treatments (Table IV). On the Chalmers, plow and chisel operations performed in the autumn left the soil very loose and porous. Voids and cracks formed by tillage and residue incorporation were still present and contributed to a large amount of 'non-channel' macroporosity, which was effective in transmitting water through the tilled layer as observed by blue staining. In addition, the Chalmers has a moderate subangular blocky structure which is effective in transmitting water between peds. No attempt was made to quantify these planar cracks and voids. The trend in conductivity was paralleled by the total porosity values, as calculated from bulk density data (Table III). Because the porosities of the tilled systems were different in the tilled zone than in the non-tilled subsoil, KSAT values calculated from the columns should be considered as an effective KSAT for the layered system. Although the conducting pores present at 10 cm in no-till on the Clermont were more continuous to 20 and 30 cm than in other tillage treatments, the number of conducting pores at 10 cm was only about half that of the other systems. In addition, bulk density of the no-till tended to be slightly higher than the plow, which may contribute to decreased conductivity. It appears that a critical bulk density (BD) for low KSAw o n the Clermont soil may exist. Columns that had a bulk density > 1.50 Mg m -3 somewhere in the 0-40 cm depth ( Table III) had a relatively low KSAw ( Table IV). These higher densities in the reduced-tillage plots may still be remaining from intensive conventional tillage performed for many years prior to establishment of the experiment. Probably the most significant contribution to higher KSAT for the plow over chisel and ridge-till treatments is due to surface sealing of those fall-tilled treatments.
Air permeability, air-fiUedporosity, and bulk density Air permeability data were transformed to approximate a normal distribution before further analysis. A logarithmic transformation was best for the Chalmers soil, and a square-root transformation was best for the Clermont soil. Treatment means shown in Figs. 1 and 2 have been transformed back to
12 DEPTH 10-20 cm
~,~:.
,~ oc,,~-
,~
20-:50 cm
J J IT
ILl :30-40 cm 13..
P
C
R
N
1 2 rn
06
13
OD
p
C
R
N
P
C
R
N
12
TILLAGE Fig. 1. Air permeability coefficient (K,,~, at - 5 kPa water potential), air-filled porosity (fa at - 5 kPa water potential) and bulk density (BD) of Chalmers soil as affected by tillage system, row position and depth. Data are the average of samples from the corn/soya bean and continuous soya bean plots. P = plow, C = chisel, R = ridge, N = no-till.
DEPTH 0 - 2 0 cm
~
c.I
P
~ 0 - 3 0 cm
LI.I OC
~'~
p
u.]
r'r" Z~3-40 cm
,sFq )
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TILLAGE Fig. 2. Air permeability coefficient (Kai, at - 5 kPa water potential), air-filled porosity ([a at - 5 kPa water potential) and bulk density (BD) of Clermont soil as affected by tillage system, row position and depth. P = plow, C = chisel, R = ridge, N = no-till.
13 the original scale. Data for the - 5 and - 10 kPa water potentials for the Chalmers showed similar trends, and only the - 5 kPa results will be presented here. For the Chalmers soil, air permeability (Kair) and air-filled porosity (fa) were significantly affected by tillage, row position, depth, and many of the interactions. Bulk density (BD) was significantly affected by tillage, depth, and interactions. Rotation was in general not a significant effect, and all subsequent discussion involves data averaged from both rotations unless otherwise noted. For the Clermont soil, significant effects were tillage for K~i~,depth for fa, and tillage and depth for BD, as well as some interaction effects on all 3 properties.
10-20-cm soil depth Figure la,b,c illustrates tillage and row position effects on Kair, fa, and BD at the 10-20-cm depth of the Chalmers. Although tillage effects on K, ir were not significant at this depth, the row position had Kair values 3.8, 5.9 and 2.1 times greater than the between-row position for plow, chisel, and ridge till, respectively (Fig. la). The values for no-till were constant at both row positions. Air-filled porosity of ridge till was significantly higher than other treatments at both row positions (Fig. lb). Chisel, ridge till and plow tended to have higher fa and lower BD (Fig. lc) in the row than between the rows, whereas the no-till remained relatively constant across positions. Air permeability values for the Clermont soil ranged from 0.43 to 0.72 of those for the Chalmers soil when averaged across tillage treatments. This difference is probably due to the inherently weaker soil structure of the Clermont, resulting from lower organic matter and a high silt content. For the Clermont soil, ridge till had significantly higher Kair than plow or no-till at the 10-20-cm depth, owing to high permeability in the row position (Fig. 2a). Air-filled porosity and BD of the ridge-till treatment were significantly higher and lower, respectively, for the row position than between rows (Fig. 2b,c). At the 10-20-cm depth, differences in gair on the Clermont could be explained by the magnitude of fa alone, but differences in Kai~ on the Chalmers were only partially due to differences in fa. The large differences in hair, fa and BD with row position for plow, chisel and ridge on the Chalmers suggest deterioration of soil pore structure between rows. This between-row area did not receive wheel traffic, so the structural deterioration may be a result of interrow cultivation. Interrow cultivation did not appear to affect these soil physical properties adversely in the chisel and plow treatments on the Clermont silt loam. The much greater K~ir and fa for ridge till in the row suggests some improvement in soil properties in the ridge on the Clermont, and is consistent with the greater number of stained large channels at 10 cm in the ridge ( Table I).
14
20-30-cm soil depth At the 20-30-cm depth there were significant differences in soil properties among tillage treatments on both soils. On the Chalmers (Fig. l d ) , the Kair of the no-till was significantly higher than the chisel, when averaged across row position. Few differences in fa (Fig. le) or BD (Fig. lf) among tillage treatments were found. Plow, chisel and ridge till had significantly higher K,~ in the row than between rows, while no-till again was constant across row positions. This significant position effect for plow, chisel, and ridge was not removed by scaling individual K~r values by the magnitude of/a, suggesting that the greater K,i~ in the row resulted from more continuous or more large pores (Groenevelt et al., 1984). Air permeability of no-till on the Clermont (20-30 cm) was significantly higher than all other tillage treatments, and ridge and chisel were greater than plow (Fig. 2d). The high K,~r of no-till versus the other treatments was greater than could be explained by the magnitude of fa alone. The higher K,~ of ridge till and chisel than plow could be explained by the greater air-filled pore space. Values of K~i~,fa and BD (Fig. 2d, e,f) all suggest the existence of a dense, slowly permeable layer at this 20-30-cm depth for the plow treatment, which is consistent with the observations of other researchers ( King and Franzmeier, 1981 ) but not consistent with the higher measured KSATvalues in the column study. A tendency for a higher Ka~r in the row than between rows was also observed on the Clermont for plow, ridge and no-till treatments.
30-40-cm soil depth Because samples from 30-40 cm were below the depth of primary tillage, no treatment differences were expected (Fig. lg,h,i, Fig. 2g,h,i). On the Chalmers, no significant treatment differences were found for/{air (Fig. lg). No significant treatment differences were observed for K,i~ on the Clermont, but ridge till tended to have higher hair in the row than between rows (Fig. 2g). A significant position X tillage interaction for fa showed ridge till to be greater than other tillage treatments in the row position, and for ridge till to have greater fa in the row than between the row (Fig. 2h).
Row position and depth effects Reasons for the significant row position effects on soil physical properties in the plow and chisel treatments, especially on the Chalmers, are not totally clear. These position effects were significant for much deeper zones (10-20 cm and 20-30 cm) than the typical depth of interrow cultivation (5-10 cm). Significant interactions of row position X depth o n g a i r for the Chalmers reflect the large decreases in Kair with depth in the row of the chisel and ridge treatments (Fig. la,d,g). The tendency for larger decreases in Kair with depth (Fig. 1 ) in the 3 tilled treatments, compared with no-till, is consistent with the ob-
15 servation of greater continuity of conducting channels in no-till in the large column study (Table II). On the Clermont soil, K~ir for no-till increased in both row positions from the 10-20-cm depth to the 20-30-cm depth, whereas Kai r decreased at this middle depth for the other 3 treatments (Fig. 2a,d,g), causing a significant depth X tillage interaction. This increased K~i~with depth for no-till is consistent with the increased relative number of stained channels with depth in the large columns (Table II). With the exception of no-till, the minimum Ka~r values were measured in the 20-30-cm depth zone. Douglas et al. (1986) measured air permeability on a weakly-structured silty soil, and they also found the lowest values in samples from just above the topsoil/subsoil interface.
Relationship of Kai r and KSAT The greater KSAT measured for plow than for no-till in the large columns (Table IV) was not consistent with the greater Kair for no-till than for plow and the evidence for a slowly-permeable pan in the plow treatment at the 20-30cm depth of the Clermont (Fig. 2). Measurements of Kair and BD (Fig. 2 ), as well as observations made in this study and by King and Franzmeier (1981), suggest that a tillage pan does exist in the Clermont under conventional tillage, and that it can restrict water movement. The two types of samples were taken at different times during the season and at different field water contents, and may illustrate the importance of temporal variations in soil physical properties. The g a i r measurements were made within two months after several tillage operations (tillage, planting and interrow cultivation ), whereas the KSAw m e a s u r e m e n t s were made about 10 months after the last tillage operation on the plow treatment. The different relationships between plow and no-till at different times during the season suggest that long-term changes resulting from different tillage systems cannot be viewed in isolation from seasonal changes. The two types of samples were also taken from slightly different positions with respect to the row. Cores were taken directly in the row and midway between rows, whereas the 25-cm-wide columns extended from about 5 cm to about 30 cm from the row, thus sampling neither directly in the row nor in the middle. SUMMARYAND CONCLUSIONS The data suggest that tillage system may have a greater relative effect on soil properties of the Clermont than on those of the Chalmers. Because of the initially low organic matter and poor structure of the Clermont soil, management systems that maintain surface cover or reduce tillage disruption would probably improve soil structure more quickly than on a soil that is initially well-structured. Improvements in soil-aggregate stability and crop yield of notill relative to plow in the Clermont soil have been noted previously ( Kladivko et al., 1986). Some improvement in soil structure was found in all the conser-
16 vation tillage systems in this study, as measured by increased air permeability. Row position tended to have a greater effect t h a n tillage t r e a t m e n t on measured soil properties on the Chalmers, suggesting a decline in soil structure between rows of tilled systems. In general the no-till system had similar or smaller numbers of stained channels at the 10-cm depth t h a n plow, chisel and ridge systems, but those channels were more continuous with depth. Air- permeability data also indicated greater pore continuity with depth for no-till, suggesting t h a t a lack of tillage leads to more uniform soil properties with depth. Some evidence for more uniform soil properties across space in the absence of tillage was also found in the lower variability of KSAw data for no-till. Row position effects on soil properties were most pronounced in the 3 tilled treatments, again suggesting some spatial uniformity with no-till. Soil type had a greater effect on measured soil properties t h a n did tillage treatment. As discussed by Cassel and Nelson (1985), much more data are needed on the spatial and temporal variations in soil properties as affected by tillage system. Adequate models of crop response to tillage system must incorporate soil and climatic differences as well. In some soils (i.e. Clermont) tillage system may have a major effect, whereas in other soils (i.e. Chalmers) row position may be of greater importance. ACKNOWLEDGEMENTS The authors gratefully acknowledge the technical assistance of Doug Taylor. This research was supported in part by funds from Pioneer Hybrid International, Inc.
REFERENCES Ball, B.C., 1981. Pore characteristics of soils from two cultivation experiments as shown by gas diffusivitiesand permeabilities and air-filledporosities. J. Soil Sci., 32: 483-498. Barnes, B.T. and Ellis, F.B., 1979. Effects of different methods of cultivation and direct drilling, and disposalof straw residues, on populations of earthworms. J. Soil Sci., 30: 669-679. Blevins, R.L., Thomas, G.W., Smith, M.S., Frye, W.W.and Cornelius,P.L., 1983. Changesin soil properties after 10 years continuousnon-tilledand conventionallytilled corn. SoilTillageRes., 3: 135-146. Boone, F.R., Slager, S., Miedema,R. and Eleveld, R., 1976. Some influencesof zero-tillageon the structure and stability of a fine-textured river levee soil. Neth. J. Agric. Sci., 24: 105-119. Bouma, J. and Dekker, L.W., 1981. A method for measuring the vertical and horizontal KSAw of clay soils with macropores. Soil Sci. Soc. Am. J., 45: 662-663. Bouma, J., Jongerius, A., Boersma, 0., Jager, A. and Schoonderbeek,D., 1977. The function of different types of macropores during saturated flowthrough four swellingsoil horizons. Soil Sci. Soc. Am. J., 41: 945-950.
17 Bouma, J., Belmans, C.F.M. and Dekker, L.W., 1982. Water infiltration and redistribution in a silt loam subsoil with vertical worm channels. Soil Sci. Soc. Am. J,, 46: 917-921. Cassel, D.K. and Nelson, L.A., 1985. Spatial and temporal variability of soil physical properties of Norfolk loamy sand as affected by tillage. Soil Tillage Res., 5: 5-12. Cassel, D.K. and Nielsen, D.R., 1986. Field capacity and available water capacity. In: A. Klute (Editor), Methods of Soil Analysis, Part 2, 2nd edn., Am. Soc. Agron. Monograph No. 9, pp. 901-926. Douglas, J.T., 1986. Macroporosity and permeability of some soil cores from England and France. Geoderma, 37: 221-231. Douglas, J.T., Goss, M.J. and Hill, D., 1980. Measurements of pore characteristics in a clay soil under ploughing and direct drilling, including use of a radioactive tracer (144Ce) technique. Soil Tillage Res., 1: 11-18. Douglas, J.T., Jarvis, M.G., Howse, K.R. and Goss, M.J., 1986. Structure of a silty soil in relation to management. J. Soil Sci., 37: 137-151. Edwards, W.M., 1982. Predicting tillage effects on infiltration. In: P.W. Unger and D.M. Van Doren (Editors), Predicting Tillage Effects on Soil Physical Properties and Processes. ASA Special Publication No. 44, Madison, WI, pp. 105-115. Edwards, W.M., van der Ploeg, R.R. and Ehlers, W., 1979. A numerical study of the effects of noncapillary-sized pores upon infiltration. Soil Sci. Soc. Am. J., 43:851-856: Ehlers, W., 1975. Observations on earthworm channels and infiltration on tilled and untilled loess soil. Soil Sci., 119: 242-249. Ehlers, W., 1976. Rapid determination of undisturbed hydraulic conductivity in tilled and untilled loess soil. Soil Sci. Soc. Am. J., 40: 837-840. Evans, D.D. and Kirkham, D., 1949. Measurement of the air permeability of soil in situ. Soil Sci. Soc. Am. Proc., 13: 65-73. Gantzer, C.J. and Blake, G.R., 1978. Physical characteristics of Le Sueur clay loam soil following no-till and conventional tillage. Agron. J., 70: 853-857. Green, R.D. and Fordham, S.J., 1975. A field method for determining air permeability in soil. In: Soil Physical Conditions and Crop Production, Min. Agric. Fish Food Tech. Bull. No. 29, H.M.S.O., London, pp. 273-288. Groenevelt, P.H., Kay, B.D. and Grant, C.D., 1984. Physical assessment of a soil with respect to rooting potential. Geoderma, 34: 101-114. Hamblin, A.P. and Tennant, D., 1981. The influence of tillage on soil water behavior. Soil Sci., 132: 233-239. Janse, A.R.P. and Bolt, G.H., 1960. The determination of the air-permeability of soils. Neth. J. Agric. Sci., 8: 124-131. King, J.J. and Franzmeier, D.P., 1981. Morphology, hydrology, and management of Clermont soils. Proc. 1980 Indiana Acad. Sci., 90: 416-422. Kladivko, E.J., Griffith, D.R. and Mannering, J.V., 1986. Conservation tillage effects on soil properties and yield of corn and soya beans in Indiana. Soil Tillage Res., 8: 277-287. Kohnke, H., 1968. Soil Physics. McGraw-Hill, NY, 224 pp. Lal, R., 1976. No-tillage effects on soil properties under different crops in western Nigeria. Soil Sci. Soc. Am. J., 40: 762-768. Lindstrom, M.J., Voorhees, W.B. and Randall, G.W., 1981. Long-term tillage effects on interrow runoff and infiltration. Soil Sci. Soc. Am. J., 45: 945-948. Mackay, A.D. and Kladivko, E.J., 1985. Earthworms and rate of breakdown of soybean and maize residues in soil. Soil Biol. Biochem., 17: 851-857. Mannering, J.V. and Fenster, C.R., 1983. What is conservation tillage? J. Soil Water Cons., 38: 141-143. Mielke, L.N., Doran, J.W. and Richards, K.A., 1986. Physical environment near the surface of plowed and no-tilled soils. Soil Tillage Res., 7: 355-366.
18 Moldenhauer, W.C., 1985. A comparison of conservation tillage systems for reducing soil erosion. In: F.M. D'Itri (Editor), A Systems Approach to Conservation Tillage. Lewis Publishers, Chelsea, MI, pp. 111-120. Nielsen, D.R., Biggar, J.W. and Erh, K.T., 1973. Spatial variability of field measured soil water properties. Hilgardia, 42: 215-259. Phillips, R.E. and Kirkham, D., 1962. Soil compaction in the field and corn growth. Agron. J., 54: 29-34. Triplett, G.B., Jr., Van Doren, D.M., Jr. and Schmidt, B.L., 1968. Effect of corn (Zea mays L.) stover mulch on no-tillage corn yield and water infiltration. Agron. J., 60: 236-239.
1
Soil Macroporosity, Hydraulic Conductivity and Air Permeability of Silty Soils under Long-Term Conservation Tillage in Indiana JOHN R. HEARD, EILEEN J. KLADIVKO, and JERRY V. MANNERING
Agronomy Department, Purdue University, West Lafayette, I N 47907 (U.S.A.) (Accepted for publication 29 June 1987 )
ABSTRACT Heard, J.R., Kladivko, E.J. and Mannering, J.V., 1988. Soil macroporosity, hydraulic conductivity and air permeability of silty soils under long-term conservation tillage in Indiana. Soil Tillage Res., 11: 1-18. With the increasing use of conservation tillage, many questions about the long-term effects of tillage system on soil physical properties have been raised. Studies were conducted to evaluate saturated hydraulic conductivity (KsAT), macropore characteristics and air permeability of two silty soils as affected by long-term conservation tillage systems in the state of Indiana. Measurements were taken during the tenth year of a tillage study on a Chalmers silty clay loam (Typic Haplaquoll) and the fifth year of a study on a Clermont silt loam (Typic Ochraqualf). Tillage systems were moldboard plow, chisel, ridge till-plant, and no-till in a rotation of corn (Zea mays L. ) and soya beans (Glycine max L. ). Saturated hydraulic conductivity was measured on large soil columns (25 X 25 × 40 cm) before spring tillage, and macropore size and continuity were assessed with staining techniques. Intact soil cores (8 cm diam × 10 cm) were collected in early July in the row and non-tra•cked interrow at three depths (10-20, 20-30, and 30-40 cm) and were analyzed for air permeability (Kair ), air-filled porosity and bulk density. Saturated hydraulic conductivity values were in the order plow > chisel > ridge till > no-till for the Chalmers soil and were significantly greater in the plow treatment than in the other 3 tillage systems on the Clermont soil. Differences in K S A w between the 2 soils were generally greater than differences among tillage systems, and coefficients of variation were lower for treatments that did not include any fall tillage operations. At the 10-cm depth on the Chalmers soil, the chisel treatment had the greatest number of stained cylindrical channels, whereas for the Clermont soil the ridge till had the greatest number at this depth. Although the no-till treatment had similar or fewer total channels, it had the most continuous channels from the 10-cm depth to the 20- and 30-cm depths on both soils. Tillage system, row position and depth all affected K~r. On the Chalmers soil, plow, chisel and ridge systems had lower g a i r between rows than in the row at the 10-20-cm depth, whereas no-till had constant Kair in the row and between the row. On the Clermont soil, ridge till had the highest Kair of all treatments at the 10-20-cm depth, and no-till had the highest Ka~rof all treatments at the 20-30-cm depth. Journal Paper No. 11 090 of the Purdue University Agricultural Experiment Station. Contribution from the Department of Agronomy.
0167-1987/88/$03.50
© 1988 Elsevier Science Publishers B.V.
INTRODUCTION Conservation tillage systems have been shown to reduce soil erosion on many soils (Moldenhauer, 1985). However, the long-term effects of conservation tillage on soil physical properties have not been adequately documented. Flow processes ( i.e. water, air, heat) in particular have not been studied adequately enough to enable reliable tillage response models to be developed. In addition, the spatial variation of physical properties with row position and depth is important for quantitative tillage models (Cassel and Nelson, 1985) but has often been ignored. More measurements of soil physical properties, as affected by long-term tillage treatments, are needed to model and predict adequately crop response to a given tillage system. Most studies on fine-textured soils show that soil bulk density is higher under no-till than under conventional-tillage systems ( Gantzer and Blake, 1978; Kladivko et al., 1986), but some studies have found no differences (Blevins et al., 1983 ). Usually infiltration rates for no-till have been higher than for tilled treatments (Triplett et al., 1968; Edwards, 1982 ) but in some cases they were lower (Lindstrom et al., 1981 ). The hydraulic conductivity of non-tilled soils can be significantly affected by macropores formed by earthworms, soil insects or roots, but these macropores are often destroyed in tilled soils (Ehlers, 1976). Increased earthworm activity has been observed under no-till systems compared to tilled treatments (Boone et al., 1976; Lal, 1976; Barnes and Ellis, 1979; Mackay and Kladivko, 1985 ). Studies of water flow in earthworm channels indicate that infiltration rate is governed by diameter and depth of the channels, as well as by the number of channels per unit area (Ehlers, 1975; Edwards et al., 1979; Bouma et al., 1982). Douglas et al. (1980) measured greater hydraulic conductivity at the interface of topsoil and subsoil under direct-drilling than plowing, owing to continuous earthworm channels at this depth. Gantzer and Blake (1978), despite finding two to four times more macropore channels in no-till, measured greater hydraulic conductivity on tilled soil. Air permeability is another measurement that has been used to assess the transmission aspects of pore geometry, in order to evaluate the effects of soil compaction (Phillips and Kirkham, 1962), cropping rotations (Evans and Kirkham, 1949; Groenevelt et al., 1984), and tillage systems (Janse and Bolt, 1960; Ball, 1981; Hamblin and Tennant, 1981; Mielke et al., 1986; Douglas, 1986). Air permeability (Kair ) was usually found to be greater under tilled than under non-tilled conditions, owing to greater porosity and larger pores. Douglas (1986) found a good relationship between continuous macropores and air permeability, especially under no-till conditions. Ball (1981) identified the g r e a t e r K a i r of conventional plowing compared to direct drilling as being due to more pores > 150/lm diameter. In compaction studies on corn growth, Phillips and Kirkham (1962) found that Kair values accurately reflected the corn-
pacted treatments. Both Evans and Kirkham (1949) and Groenevelt et al. (1984) measured higher Kair when corn followed other crops in a rotation than in continuous corn culture. The objectives of this study were to measure hydraulic conductivity, air permeability, and pore characteristics of 2 silty soils as affected by 4 long-term conservation and conventional-tillage systems in Indiana, and to assess variation in air permeability with row position and depth. MATERIALSAND METHODS Field sites and tillage systems Soil samples were collected in 1984 from long-term tillage experiments located in north-central and southeastern Indiana. Soils at both sites freeze during the winter, with the annual number of freeze-thaw cycles varying with the weather regime each year. The north-central location was established in 1975 on a Chalmers silty clay loam ( fine-silty, mixed mesic, Typic Haplaquoll). The soil is dark-colored, well-structured, naturally poorly drained and contains 59% silt, 32% clay and 4.0% organic matter in the Ap horizon. Drain tiles (20-m spacing) are located at approximately 1-m depth and run perpendicular to the rows. Crop rotations studied were continuous corn (Zea mays L. ), corn/soya bean ( Glycine max L.) rotation and continuous soya beans. Treatments were replicated 4 times, and tillage plots were 12 rows wide and 46 m long with 76cm row spacing. The southeastern location was established in 1980 on a Clermont silt loam (fine-silty, mixed mesic, Typic Ochraqualf). A fragipan-like horizon below a depth of about 90 cm can restrict drainage in the spring. This soil is lightcolored, poorly structured, naturally poorly drained, and contains 73% silt, 10% clay and 1.3% organic matter in the Ap horizon. Crop rotations studied were continuous corn and corn/soya bean rotation. Experimental design was similar to that of the Chalmers, except that plots were either 46- or 61-m long. Tillage systems included conventional moldboard plow, chisel plow, ridge till-plant, and no-till. Conservation tillage has been defined as any tillage system that reduces loss of soil or water relative to conventional tillage (Mannering and Fenster, 1983), by a combination of surface residue cover and/or soil roughness. Moldboard plow has been the "conventional" form of tillage for corn and soya beans in this region, and all other systems studied were to some extent "conservation" tillage. Moldboard plowing to a depth of 20-25 cm was performed in the autumn on the Chalmers and in the spring on the Clermont. Chiseling (10-cm twisted shanks) to a depth of 20-25 cm was performed in the autumn on both soils. Secondary tillage for both the plow and chisel systems consisted of discing and field cultivation to depths of about 10 cm in the spring prior to planting. The ridge till-plant system consisted of planting
in pre-formed ridges by scraping the top 2.5 cm of soil from the ridge and reforming the ridge with a disk-hiller or winged-sweep, during a cultivation operation when the corn was about 45-60 cm tall. Ridges for soya beans were formed after harvest of the beans rather than at cultivation. For no-till planting, the planter was equipped with 2.5-cm-wide non-powered fluted coulters to open a slot for the seed. Anhydrous NH3 was injected in each interrow area of all plots prior to planting, and was the only "tillage" the no-till plots received. Crop yields and selected soil physical properties from those plots have been reported previously ( Kladivko et al., 1986 ). During 1984, on the Chalmers soil, interrow cultivation was done on 25 June for ridge-till soya beans and on 3 July for plow and chisel soya beans. On the Clermont soil, plow, chisel, and ridgetill soya beans were cultivated on 28 June.
Hydraulic conductivity, water-conducting channels, and bulk density Saturated hydraulic conductivity (KsAT) was measured by the method of Bouma and Dekker (1981) in the corn/soya bean rotation plots during the spring following soya beans. Two soil columns per plot were collected in late April and early May, prior to any spring tillage and while the soil was close to saturation. The column edge was positioned about 5 cm from the row and extended into the untrafficked interrow almost to the midline between rows. It was felt that this column size and position would adequately represent the average field conditions. The final column size was 25 × 25 × 40-cm deep and included the soil surface and the interface between the plow layer and subsoil. Four replicate blocks were sampled at the Clermont site (32 individual columns), but only 3 blocks were sampled at the Chalmers site (24 columns). Saturated hydraulic conductivity was measured by maintaining a constant head of about 2 cm water on the soil surface and collecting the percolate in large plastic basins. For the ridge-till system the entire surface was submerged, and the average water head was assumed to be the head at the midplane between the top and bottom of the ridge. Rates were measured at 10-min intervals until steady state was reached. Saturated hydraulic conductivity values represent the 0-40-cm depth of the soil. For those tilled treatments with different bulk densities in the tilled zone and the undisturbed soil below, the measured KSAw would actually be the effective KSAw of the layered system. A 0.1% solution of methylene blue dye was ponded on the soil surface ( Bouma et al., 1977) prior to attaining steady-state KSAw o n the Clermont soil, and after attaining steady-state on the Chalmers soil. Pores that were continuous to the soil surface, and were actively conducting water, would be stained with this solution. The columns were sectioned at depths of 10, 20, and 30 cm. Ridgetill depths were measured from the average height of the surface (midplane between ridge top and ridge valley). Both stained and unstained visible channels (cylindrical) were counted and sorted into four size classes by their di-
ameter: small (1-2 m m ) ; medium (2-4 m m ) ; large (4-6 m m ) ; very large ( > 6 m m ) . Stained cracks and non-circular voids were also observed, but no attempt was made to characterize them. Bulk density of the Clermont was determined with a can sampler (cans 3.5 cm X 5.5 cm diam), during sectioning of the soil columns, at depths of 2.5-6, 10-13.5, 20-23.5, and 30-33.5 cm. Bulk density was measured on the Chalmers by an excavation technique developed by the Soil Conservation Service (R.B. Grossman, personal communication, 1983), at depths of 0-3, 7.5-12.5, and 17.5-22.5 cm in April 1985 prior to any tillage operations. The technique is basically a modification of rubber balloon excavation techniques, and involves excavating a shallow hole, lining the hole with thin plastic film, and filling the hole with water to determine the excavated soil volume.
Air permeability, air-filled porosity, and bulk density Samples for air permeability, air-filled porosity, and bulk density were collected from continuous soya bean and corn/soya bean rotation plots on the Chalmers soil, and from corn/soya bean rotation plots on the Clermont soil, during early July ( soya-bean crop year). Intact 8-cm diameter cores were taken with a hydraulic probe truck in the crop row, and in the non-trafficked interrow, to a depth of 45 cm at 2 sites in each plot. Cores were then cut into 3 depth sections (10-20, 20-30 and 30-40 cm ), and dipped in saran ( liquid plastic ) to maintain the undisturbed soil structure. The total number of 10-cm core sections sampled was 384 and 192 from the Chalmers and Clermont soils, respectively. In the laboratory, the cores were initially wet up on a sand tension table at - 0.5 kPa water potential. A 5.0-cm long core was cut from the least disturbed portion of the Chalmers soil cores, and from the middle of the Clermont soil cores. E n d surfaces were carefully picked to expose natural ped faces, and to prevent smearing the soil surface. Core volumes were calculated from height and diameter measurements. Cores were encased with paraffin wax into 9.8cm diameter aluminum rings. After saturation the samples were equilibrated at - 5 kPa water potential on a sand tension table, and air permeability was measured. The Chalmers soil was further equilibrated at - 10 kPa water potential and air permeability remeasured. After the final air permeability reading, the core was oven dried and weighed for bulk density calculations, and airfilled porosity was calculated by the difference between saturated core weight and equilibrated weight at - 5 or - 10 kPa water potential. Air-filled porosity at - 5 kPa water potential (60 ~m equivalent pore diameter) has been used as an index of "aeration porosity" ( Kohnke, 1968), and a water potential of - 10 kPa is often used as an approximation of"field capacity" (Cassel and Nielsen, 1986). A constant-pressure air permeameter, similar to that of Green and Fordham
(1975), was designed. The system was operated at low pressure (1.4 kPa ), and viscosity of the air was maintained relatively constant by bubbling air through a water bath at 24-27 ° C. Air permeability coefficients (Groenevelt et al., 1984) were calculated from: F--gai r Ah/L where F is the measured flux of air (cm 3 cm -2 sec-1), zih is the air pressure head difference across the soil core (cm H20), and L is the length of the soil sample (cm).
Statistics The 2 columns per plot that were collected for KSAT w e r e treated as subsamples, and data were averaged from them before performance of analysis of variance (ANOVA). Because hydraulic conductivity is often found to be lognormally distributed in the field (Nielsen et al., 1973), the geometric means for conductivity were also calculated and analyzed by ANOVA. Air permeability data were transformed to approximate a normal distribution. The experimental design for this part of the study was a split-split-split plot for the Chalmers and a split-split plot for the Clermont. For the Chalmers experiment, rotation was randomly assigned to the main plots and tillage was randomly assigned within those plots as the subplots. Row position was a subsubplot and depth was the sub-sub-subplot and both were systematically arranged. For the Clermont experiment, only one rotation was sampled, but the rest of the design was the same as the Chalmers. Analysis of variance was performed for the 3 soil properties on the 2 soils. The error term used to test the significance of any effect that included position or depth (systematically arranged factors) was the block × effect interaction for that effect. As discussed by Cassel and Nelson (1985), this procedure gives an extremely conservative F-test for significance, but is more appropriate because position and depth cannot be randomized. Mean separations of main effects and interactions were then performed using Student-Newman-Keuls and LSD procedures, respectively. In order to evaluate whether treatment differences in Kair were due mainly to differences in air-filled porosity (fa), values of Kair were divided by fa, and ANOVA procedures were rerun on the scaled values ( Groenevelt et al., 1984 ). If the scaled values still show significant treatment effects, it is likely that differences in K ~ are due to differences in pore size and continuity. RESULTS AND DISCUSSION
Hydraulic conductivity, water-conducting channels, and bulk density Water-conducting channels All of the larger cylindrical channels observed in the columns appeared to be formed by earthworms, owing to the presence of cast material and the lack
7 TABLE I Numbers of stained channels at 10-cm depth as affected by tillage system on the Chalmers and Clermont soils. {Pore numbers shown as number m -z) Soil
Tillage
Pore size 1
S
M
Chalmers silty clay loam
Plow Chisel Ridge No-till
119 105 97 139
132d 2 260c 158cd 177cd
Clermont silt loam
Plow Chisel Ridge No-till
603 527 511 275
454 470 408 235
L
VL
T
24d e 91c 20d lld
3 8 0 0
278 464 275 327
64cd 2 41d 116c 38d
3 0 5 10
1124 1038 1040 558
~S=small (1-2 ram), M=medium (2-4 mm), L=large (4-6 mm), VL=very large ( > 6 mm) diameter; T = total 2Means in this column for the same soil that are followed by the same letter are not significantly different (Student-Newman-Keuls at 95% level). Columns with no letters had no significant differences among treatments.
of any decomposing root tissue. Some small channels were formed by decomposing roots, but most appeared to be earthworm holes. Numbers of stained channels at the 10-cm depth, as affected by tillage system, were quite variable, and few significant t r e a t m e n t differences were found ( Table I ). On the Chalmers soil, the chisel t r e a t m e n t had significantly greater numbers of medium and large stained channels t h a n other treatments. On the Clermont soil, ridge till had significantly more large channels than either chisel or no-till at this 10-cm depth. Higher numbers of channels in the chisel t r e a t m e n t were not expected. However, since chiseling was done in the early fall, and earthworms are most active in the fall and early spring (Barnes and Ellis, 1979), the earthworm population may have recovered sufficiently from the tillage operation to form new burrows. Chiseling may actually have encouraged earthworm activity at this 10cm depth, because of the incorporation of crop residue as a food source in this layer. Gantzer and Blake (1978) suggested that a high activity of channel formation in the spring may compensate for destruction of continuous pores by tillage. There are no apparent reasons for the greater number of large channels under ridge till than chisel and no-till, or the comparatively low channel numbers under no-till on the Clermont soil. Table II indicates the numbers of stained channels at 20- and 30-cm depths, as a percentage of those stained at 10 cm, for each tillage t r e a t m e n t and poresize class. This percentage expresses the continuity of pores from the 10-cm
8
TABLE II Stained channels at 20- and 30-cm depths as a percentage of stained channels at 10 cm, as affected by tillage system on the Chalmers and Clermont soils Tillage
Depth
Pore size 1
(cm) S Chalmers silty clay loam Plow Chisel Ridge No-till Clermont silt loam Plow Chisel Ridge No-till
M
L
VL
T
20 30 20 30 20 30 20 30
28.6 13.4 99.0 81.9 71.1 88.7 79.1 65.5
43.9 28.0 38.1 20.0 29.7 33.5 54.8 51.4
100.0 41.7 6.6 5.5 15.0 15.0 145.5 0
366.7 0 0 0 0 0 0 0
45.7 22.7 45.0 30.8 44.4 41.8 68.2 55.7
20 30 20 30 20 30 20 30
55.2 116.4 119.0 112.7 78.7 117.4 176.0 145.1
48.9 40.7 50.4 42.3 52.0 50.7 80.9 75.3
35.9 43.8 12.2 17.1 22.4 8.6 52.6 86.8
0 0 0 0 100.0 0 0 0
51.4 81.4 83.7 77.1 62.0 78.6 124.4 109.1
~S=small (1-2 ram), M=medium (2-4 mm), L=large (4-6 mm), VL=very large ( > 6 mm) diameter, T = total. d e p t h to the 20- a n d 30-cm depths. U n d e r t h e C h a l m e r s soil at 20 a n d 30 cm, a greater relative n u m b e r o f c h a n n e l s was p r e s e n t in no-till t h a n u n d e r o t h e r tillage t r e a t m e n t s , owing chiefly to a g r e a t e r relative n u m b e r of m e d i u m c h a n nels. In t h e C l e r m o n t soil, a l t h o u g h t h e no-till h a d t h e least n u m b e r of s t a i n e d c h a n n e l s at t h e 10-cm d e p t h c o m p a r e d w i t h t h e o t h e r t r e a t m e n t s ( T a b l e I ) , it t e n d e d to m a i n t a i n m o r e c o n t i n u o u s c h a n n e l s w i t h d e p t h ( T a b l e I I ) . T h e s e o b s e r v a t i o n s suggest t h a t , despite similar or fewer n u m b e r s of c h a n nels u n d e r no-till c o m p a r e d w i t h o t h e r tillage t r e a t m e n t s , t h e s e c h a n n e l s were m o r e c o n t i n u o u s with depth. C h a n n e l s n e a r the surface u n d e r t h e tilled syst e m s were n o t as c o n t i n u o u s , p r o b a b l y owing to the d e s t r u c t i v e n a t u r e of tillage. T h e n u m b e r o f t o t a l s t a i n e d c h a n n e l s ( T a b l e I ) r a n g e d f r o m 1.7-4.0 t i m e s g r e a t e r u n d e r t h e C l e r m o n t t h a n t h e C h a l m e r s . T h i s p r o b a b l y results f r o m t h e difference in t h e s t a i n i n g p r o c e d u r e , since t h e t o t a l n u m b e r s of s t a i n e d plus u n s t a i n e d c h a n n e l s u n d e r t h e C l e r m o n t r a n g e d f r o m 0.46 to 1.96 t i m e s t h o s e of t h e C h a l m e r s . Owing to high variability, few significant t i l l a g e - t r e a t m e n t differences in t o t a l c h a n n e l s ( s t a i n e d plus u n s t a i n e d ) were detected, c o n t r a r y
T A B L E III Soil bulk density of the Chalmers and Clermont soils as affected by tillage system (Mg m -:~) Tillage
Chalmers silty clay loam 1
Clermont silt loam 2
0-3
7.5-12.5
17.5-22.5
2.5-6
10-13.5
20-23.5
30-33.5
1.03 0.95 0.93 1.02
1.15d a 1.18d 1.29d 1.43c
1.27d 4 1.36d 1.45d 1.53c
1.44 1.45 1.40 1.48
1.47 1.46 1.45 1.52
1.49 1.50 1.54 1.55
1.45 1.55 1.49 1.44
Depth (cm)
Plow Chisel Ridge No-till
~Bulk density measured by excavation technique, April 1985, in field plots. 2Bulk density measured with can sampler, May 1984, on individual KSAT columns. :~4Means in this column that are followed by the same letter are not significantly different at the 95% and 99% levels, respectively. Columns with no letters had no significant differences among treatments.
to several published reports (Ehlers, 1975; Gantzer and Blake, 1978). Counts of macropore channels in this study generally exceeded those of Ehlers (1975), and were intermediate between the tilled and untilled numbers reported by Gantzer and Blake (1978) and Barnes and Ellis (1979).
Bulk density On both soils the no-till treatment had a higher bulk density than the other tillage systems in the 7.5-23.5-cm depth zone (Table III). No significant differences in bulk density among tillage systems were found below the tillage layer. Other researchers have also found higher bulk densities under no-till in the topsoil, but few tillage differences at depths greater than 30 cm ( Gantzer and Blake, 1978 ). The higher bulk density for chisel than the other treatments at 30 cm on the Clermont may have resulted from compaction by autumn chiseling to depths of about 25 cm. Bulk densities of the Clermont soil were higher than the Chalmers soil, owing to poor structure and low cohesiveness.
Hydraulic conductivity Saturated hydraulic conductivity (KsAT) data for tillage treatments on both soils are shown in Table IV. On the Chalmers the trend for KSAT was plow > chisel > ridge till > no-till. The coefficient of variation ( C.V. ) values for plow and chisel systems were much larger than for ridge and no-till, suggesting that soil physical conditions were not uniform in space following these primary tillage operations. The lower C.V. values of no-till and ridge till suggest that some spatial uniformity is achieved if the soil is not disturbed by primary tillage. The geometric mean conductivity values for plow, chisel and ridge till
10 TABLE IV Saturated hydraulic conductivity (KsAT) of Chalmers and Clermont soils as affected by tillage system (0-40-cm depth) Tillage Chalmers silty clay loam Plow Chisel Ridge No-till Clermont silt loam Plow Chisel Ridge No-tiU
KSAT (cm hr -1)
C.V. 1 (%)
KSAT2 (cm hr -1)
38.1 30.5 26.8 15.5
68 45 12 16
27.7 27.4 26.4 15.3
12.8" 3.7 4.1 2.3
51 140 65 53
11.6" 2.2 2.9 2.1
1Coefficient of variation, calculated after averaging samples within each block; Chalmers (n = 3 ), Clermont (n--4). ~Geometric mean values. *Significant at the 95% level.
were all similar, and were higher than no-till, but no significant difference was detectable even with this log-normal transformation. Plow and no-till KSAw values for the Chalmers soil were very similar to those of Gantzer and Blake (1978), who collected cores from a clay loam soil at the 7.5-15-cm depth after secondary tillage and corn planting (plow--38.2 cm hr -1, no-till=14.6 cm hr-1). The KSAw of the Clermont silt loam was significantly higher under the plowed treatment (sampled prior to spring plowing) than under any other tillage treatment, and no significant differences in KSAw w e r e observed among no-till, chisel and ridge-till plots, using either arithmetic or geometric mean values. Despite previous indications of a plowpan in the Clermont soil (King and Franzmeier, 1981 ), and observation of a perched water table during excavation of preliminary soil columns under saturated conditions, the conductivity of the spring plow exceeded the other treatments. Bulk density values also gave no indication of a dense plowpan under this treatment. The high K s A w of the plow treatment compared with the chisel and ridge can be attributed partly to the covering of soya bean residue on the soil surface, which prevented surface sealing over the winter and spring. Those treatments involving fall tillage (chisel plowing and fall ridge forming for ridge till) exhibited surface sealing (up to 0.8-cm thick ) in places where residue did not protect the soil surface. The low conductivity of no-till compared with plow may be due to the higher bulk density of the no-till (Table III). The highest C.V. values on the Clermont were associated with fall chiseling and ridge till, whereas the no-till and spring plow,
11
which had been unworked since the previous summer, tended to have lower C.V. values. The KSAw of the Clermont was much lower than that of the well-structured Chalmers, regardless of tillage treatment. This would indicate that other relevant soil properties (texture, organic matter content, etc.) have more effect o n KSAT than does tillage treatment.
Relationship of pore continuity and KSA T Under no-till both soils showed a greater continuity of conducting pores from the 10-cm to the 20- and 30-cm depths than under the other tillage treatments. However, no-till had lower KSAw values than some other treatments (Table IV). On the Chalmers, plow and chisel operations performed in the autumn left the soil very loose and porous. Voids and cracks formed by tillage and residue incorporation were still present and contributed to a large amount of 'non-channel' macroporosity, which was effective in transmitting water through the tilled layer as observed by blue staining. In addition, the Chalmers has a moderate subangular blocky structure which is effective in transmitting water between peds. No attempt was made to quantify these planar cracks and voids. The trend in conductivity was paralleled by the total porosity values, as calculated from bulk density data (Table III). Because the porosities of the tilled systems were different in the tilled zone than in the non-tilled subsoil, KSAT values calculated from the columns should be considered as an effective KSAT for the layered system. Although the conducting pores present at 10 cm in no-till on the Clermont were more continuous to 20 and 30 cm than in other tillage treatments, the number of conducting pores at 10 cm was only about half that of the other systems. In addition, bulk density of the no-till tended to be slightly higher than the plow, which may contribute to decreased conductivity. It appears that a critical bulk density (BD) for low KSAw o n the Clermont soil may exist. Columns that had a bulk density > 1.50 Mg m -3 somewhere in the 0-40 cm depth ( Table III) had a relatively low KSAw ( Table IV). These higher densities in the reduced-tillage plots may still be remaining from intensive conventional tillage performed for many years prior to establishment of the experiment. Probably the most significant contribution to higher KSAT for the plow over chisel and ridge-till treatments is due to surface sealing of those fall-tilled treatments.
Air permeability, air-fiUedporosity, and bulk density Air permeability data were transformed to approximate a normal distribution before further analysis. A logarithmic transformation was best for the Chalmers soil, and a square-root transformation was best for the Clermont soil. Treatment means shown in Figs. 1 and 2 have been transformed back to
12 DEPTH 10-20 cm
~,~:.
,~ oc,,~-
,~
20-:50 cm
J J IT
ILl :30-40 cm 13..
P
C
R
N
1 2 rn
06
13
OD
p
C
R
N
P
C
R
N
12
TILLAGE Fig. 1. Air permeability coefficient (K,,~, at - 5 kPa water potential), air-filled porosity (fa at - 5 kPa water potential) and bulk density (BD) of Chalmers soil as affected by tillage system, row position and depth. Data are the average of samples from the corn/soya bean and continuous soya bean plots. P = plow, C = chisel, R = ridge, N = no-till.
DEPTH 0 - 2 0 cm
~
c.I
P
~ 0 - 3 0 cm
LI.I OC
~'~
p
u.]
r'r" Z~3-40 cm
,sFq )
oo
P
C
R
J --
N
N/
P
C
1 ::D
,~ ,2th )
ca ,7Fi ,
.
,.
I!
TILLAGE Fig. 2. Air permeability coefficient (Kai, at - 5 kPa water potential), air-filled porosity ([a at - 5 kPa water potential) and bulk density (BD) of Clermont soil as affected by tillage system, row position and depth. P = plow, C = chisel, R = ridge, N = no-till.
13 the original scale. Data for the - 5 and - 10 kPa water potentials for the Chalmers showed similar trends, and only the - 5 kPa results will be presented here. For the Chalmers soil, air permeability (Kair) and air-filled porosity (fa) were significantly affected by tillage, row position, depth, and many of the interactions. Bulk density (BD) was significantly affected by tillage, depth, and interactions. Rotation was in general not a significant effect, and all subsequent discussion involves data averaged from both rotations unless otherwise noted. For the Clermont soil, significant effects were tillage for K~i~,depth for fa, and tillage and depth for BD, as well as some interaction effects on all 3 properties.
10-20-cm soil depth Figure la,b,c illustrates tillage and row position effects on Kair, fa, and BD at the 10-20-cm depth of the Chalmers. Although tillage effects on K, ir were not significant at this depth, the row position had Kair values 3.8, 5.9 and 2.1 times greater than the between-row position for plow, chisel, and ridge till, respectively (Fig. la). The values for no-till were constant at both row positions. Air-filled porosity of ridge till was significantly higher than other treatments at both row positions (Fig. lb). Chisel, ridge till and plow tended to have higher fa and lower BD (Fig. lc) in the row than between the rows, whereas the no-till remained relatively constant across positions. Air permeability values for the Clermont soil ranged from 0.43 to 0.72 of those for the Chalmers soil when averaged across tillage treatments. This difference is probably due to the inherently weaker soil structure of the Clermont, resulting from lower organic matter and a high silt content. For the Clermont soil, ridge till had significantly higher Kair than plow or no-till at the 10-20-cm depth, owing to high permeability in the row position (Fig. 2a). Air-filled porosity and BD of the ridge-till treatment were significantly higher and lower, respectively, for the row position than between rows (Fig. 2b,c). At the 10-20-cm depth, differences in gair on the Clermont could be explained by the magnitude of fa alone, but differences in Kai~ on the Chalmers were only partially due to differences in fa. The large differences in hair, fa and BD with row position for plow, chisel and ridge on the Chalmers suggest deterioration of soil pore structure between rows. This between-row area did not receive wheel traffic, so the structural deterioration may be a result of interrow cultivation. Interrow cultivation did not appear to affect these soil physical properties adversely in the chisel and plow treatments on the Clermont silt loam. The much greater K~ir and fa for ridge till in the row suggests some improvement in soil properties in the ridge on the Clermont, and is consistent with the greater number of stained large channels at 10 cm in the ridge ( Table I).
14
20-30-cm soil depth At the 20-30-cm depth there were significant differences in soil properties among tillage treatments on both soils. On the Chalmers (Fig. l d ) , the Kair of the no-till was significantly higher than the chisel, when averaged across row position. Few differences in fa (Fig. le) or BD (Fig. lf) among tillage treatments were found. Plow, chisel and ridge till had significantly higher K,~ in the row than between rows, while no-till again was constant across row positions. This significant position effect for plow, chisel, and ridge was not removed by scaling individual K~r values by the magnitude of/a, suggesting that the greater K,i~ in the row resulted from more continuous or more large pores (Groenevelt et al., 1984). Air permeability of no-till on the Clermont (20-30 cm) was significantly higher than all other tillage treatments, and ridge and chisel were greater than plow (Fig. 2d). The high K,~r of no-till versus the other treatments was greater than could be explained by the magnitude of fa alone. The higher K,~ of ridge till and chisel than plow could be explained by the greater air-filled pore space. Values of K~i~,fa and BD (Fig. 2d, e,f) all suggest the existence of a dense, slowly permeable layer at this 20-30-cm depth for the plow treatment, which is consistent with the observations of other researchers ( King and Franzmeier, 1981 ) but not consistent with the higher measured KSATvalues in the column study. A tendency for a higher Ka~r in the row than between rows was also observed on the Clermont for plow, ridge and no-till treatments.
30-40-cm soil depth Because samples from 30-40 cm were below the depth of primary tillage, no treatment differences were expected (Fig. lg,h,i, Fig. 2g,h,i). On the Chalmers, no significant treatment differences were found for/{air (Fig. lg). No significant treatment differences were observed for K,i~ on the Clermont, but ridge till tended to have higher hair in the row than between rows (Fig. 2g). A significant position X tillage interaction for fa showed ridge till to be greater than other tillage treatments in the row position, and for ridge till to have greater fa in the row than between the row (Fig. 2h).
Row position and depth effects Reasons for the significant row position effects on soil physical properties in the plow and chisel treatments, especially on the Chalmers, are not totally clear. These position effects were significant for much deeper zones (10-20 cm and 20-30 cm) than the typical depth of interrow cultivation (5-10 cm). Significant interactions of row position X depth o n g a i r for the Chalmers reflect the large decreases in Kair with depth in the row of the chisel and ridge treatments (Fig. la,d,g). The tendency for larger decreases in Kair with depth (Fig. 1 ) in the 3 tilled treatments, compared with no-till, is consistent with the ob-
15 servation of greater continuity of conducting channels in no-till in the large column study (Table II). On the Clermont soil, K~ir for no-till increased in both row positions from the 10-20-cm depth to the 20-30-cm depth, whereas Kai r decreased at this middle depth for the other 3 treatments (Fig. 2a,d,g), causing a significant depth X tillage interaction. This increased K~i~with depth for no-till is consistent with the increased relative number of stained channels with depth in the large columns (Table II). With the exception of no-till, the minimum Ka~r values were measured in the 20-30-cm depth zone. Douglas et al. (1986) measured air permeability on a weakly-structured silty soil, and they also found the lowest values in samples from just above the topsoil/subsoil interface.
Relationship of Kai r and KSAT The greater KSAT measured for plow than for no-till in the large columns (Table IV) was not consistent with the greater Kair for no-till than for plow and the evidence for a slowly-permeable pan in the plow treatment at the 20-30cm depth of the Clermont (Fig. 2). Measurements of Kair and BD (Fig. 2 ), as well as observations made in this study and by King and Franzmeier (1981), suggest that a tillage pan does exist in the Clermont under conventional tillage, and that it can restrict water movement. The two types of samples were taken at different times during the season and at different field water contents, and may illustrate the importance of temporal variations in soil physical properties. The g a i r measurements were made within two months after several tillage operations (tillage, planting and interrow cultivation ), whereas the KSAw m e a s u r e m e n t s were made about 10 months after the last tillage operation on the plow treatment. The different relationships between plow and no-till at different times during the season suggest that long-term changes resulting from different tillage systems cannot be viewed in isolation from seasonal changes. The two types of samples were also taken from slightly different positions with respect to the row. Cores were taken directly in the row and midway between rows, whereas the 25-cm-wide columns extended from about 5 cm to about 30 cm from the row, thus sampling neither directly in the row nor in the middle. SUMMARYAND CONCLUSIONS The data suggest that tillage system may have a greater relative effect on soil properties of the Clermont than on those of the Chalmers. Because of the initially low organic matter and poor structure of the Clermont soil, management systems that maintain surface cover or reduce tillage disruption would probably improve soil structure more quickly than on a soil that is initially well-structured. Improvements in soil-aggregate stability and crop yield of notill relative to plow in the Clermont soil have been noted previously ( Kladivko et al., 1986). Some improvement in soil structure was found in all the conser-
16 vation tillage systems in this study, as measured by increased air permeability. Row position tended to have a greater effect t h a n tillage t r e a t m e n t on measured soil properties on the Chalmers, suggesting a decline in soil structure between rows of tilled systems. In general the no-till system had similar or smaller numbers of stained channels at the 10-cm depth t h a n plow, chisel and ridge systems, but those channels were more continuous with depth. Air- permeability data also indicated greater pore continuity with depth for no-till, suggesting t h a t a lack of tillage leads to more uniform soil properties with depth. Some evidence for more uniform soil properties across space in the absence of tillage was also found in the lower variability of KSAw data for no-till. Row position effects on soil properties were most pronounced in the 3 tilled treatments, again suggesting some spatial uniformity with no-till. Soil type had a greater effect on measured soil properties t h a n did tillage treatment. As discussed by Cassel and Nelson (1985), much more data are needed on the spatial and temporal variations in soil properties as affected by tillage system. Adequate models of crop response to tillage system must incorporate soil and climatic differences as well. In some soils (i.e. Clermont) tillage system may have a major effect, whereas in other soils (i.e. Chalmers) row position may be of greater importance. ACKNOWLEDGEMENTS The authors gratefully acknowledge the technical assistance of Doug Taylor. This research was supported in part by funds from Pioneer Hybrid International, Inc.
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