effect of tillage-induced soil macroporosity on water infiltration

Soil& Tillage Research, 17 (1990) 153-165 153 Elsevier Science Publishers B.V., Amsterdam

Effect of tillage-induced soil macroporosity on water infiltration*

J .L. P i k u l Jr . , J .F . Z u z e l a n d R.E. R a m i g U.S. Department of Agriculture, Agricultural Research Service, Pac(lTc West .4 rea, P.O. Box 370,

Pendleton, OR 97801 (U.S.A.)

(Accepted for publication 22 March 1989 )

ABSTRACT

Pikul, Jr., J.L., Zuzel, J.F. and Ramig, R.E., 1990. Effect of tillage-induced soil macroporosity on water infiltration. Soil Tillage Res., 17:153-165.

In some areas of the inland Pacific Northwest, surface runoff is a major source of water loss. Chis- eling or plowing in the fall of the year, following wheat harvest, are farming practices to increase water infiltration especially in areas where frozen soil is anticipated. Water infiltration, overwinter water storage, and soil macroporosity were measured in fall tillage treatments of: ( 1 ) no tillage (NT): ( 2 ) chiseled stubble (C); (3) Paraplowed stubble (P). In spring, final infiltration rates for the respective treatments were 9.2, 22.8 and 23.5 mm h - j . Over winter, all treatments stored the same amount of water in the 3.35-m profile, however there were differences in the distribution of water in the profile. The fraction of macroporosity in a horizontal cross-section of soil, was different for each treatment and depth of cross-section. For the C treatment, macroporosity decreased from about 20% at the 7.6- cm depth to < 1.0% at 25.4 cm. In contrast, macroporosity decreased with depth to a minimum of 6.9% at the 12.7-cm depth and increased to a maximum of 17.2% at 25.4 cm for the P treatment. NT had < 1.0% macroporosity. Over winter soil settling is uncertain, however, water infiltration and macroporosity measurements conducted in March attest to the overwintering stability of the macropore structure on both C and P treatments.

INTRODUCTION

M o r e t h a n 1.6 M h a o f c r o p l a n d in t h e i n l a n d P a c i f i c N o r t h w e s t , e a s t o f t h e

C a s c a d e M o u n t a i n s , s u f f e r s e v e r e w a t e r r u n o f f a n d e r o s i o n ( H y d e et al . ,

1 9 8 4 ) . T h i s w h e a t - p r o d u c i n g r e g i o n h a s l o w - i n t e n s i t y w i n t e r r a i n f a l l , l o e s s i a l

so i l s a n d s t e e p i r r e g u l a r t o p o g r a p h y . F r o z e n so i l , s n o w m e l t a n d r a i n o n s n o w

h a v e b e e n i d e n t i f i e d as k e y f a c t o r s c o n t r i b u t i n g to m a j o r so i l e r o s i o n e v e n t s ( Z u z e l e t a l . , 1982 ) . A v e r a g e a n n u a l so i l l o s s r a n g e s f r o m 5 t o 50 t h a - j .

C r o p r e s i d u e o n t h e s u r f a c e o v e r w i n t e r r e d u c e s w a t e r e r o s i o n o n a g r i c u l -

*Joint contribution of the U.S.D.A.-A.R.S. and Oregon State University Agricultural Experi- ment Station, Technical Paper No. 8552.

154 J.L. PIKUL JR. ETAL.

tural lands in eastern Oregon (Allmaras et al., 1980). Ramig and Ekin (1984) found that crop-residue mulches increase soil water storage during the first winter, compared with bare soil in a fallow-wheat rotation. Surface residue also reduces frost penetration by retarding soil heat loss. Even in the absence of snow cover, surface residue reduced frost penetration by an average of 35% compared with treatments where surface residue was removed (Pikul et al., 1986).

Surface runoff and evaporation are major sources of water loss, and water limits crop yields throughout most of the region. Redinger et al. (1984) estimated that water loss from runoff may be up to one-third the annual precipitation. Accordingly, fall plowing or chiseling following wheat harvest, are practices used to increase water infiltration. Shallow tillage has been shown to improve water intake in frozen and unfrozen soil (Wischmeier, 1973; Mu- khtar et al., 1985; Zuzel and Pikul, 1987 ) suggesting that there are preferential water flow paths through voids created by tillage.

The structure of tilled soil has been studied by impregnating it with paraffin wax (Ojeniyi and Dexter, 1983), polyester resin (Bullock et al., 1985), and polyester resins containing u.v. fluorescent dye (Shipitalo and Protz, 1987). Macroporosity has been quantified using line-intercept methods (Dexter, 1976 ) and image analysis (Bullock and Murphy, 1980).

Our hypothesis is that large voids created by fall tillage are stable over the winter period and that these voids provide high overwintering infiltration rates. Thus, our objectives were to: ( 1 ) develop a simple field method to ex- amine the pore space created by tillage; (2) measure water infiltration rates for 3 tillage treatments in the spring after normal winter precipitation and soil freezing and thawing.

MATERIALS AND METHODS

Cultural practices

Field experiments were conducted near Pendleton, OR on a Walla Walla silt loam (Typic Haploxeroll, coarse-silty, mixed, mesic). Average slope was 15%. The experimental area was in a summer fallow-winter wheat ( Triticum aestivum L. ) rotation for at least 5 years. However, in the fall of 1985, winter barley (Hordeum vuigare) was planted instead of winter wheat. The normal tillage sequence for this cropping system after wheat harvest was spring disk- ing, followed by spring tooth harrow, injection of anhydrous ammonia fertil- izer, 4 rod weedings during summer and seeding in late October.

Barley was harvested during August 1986 leaving 3.4 Mg ha- l crop residue on the soil surface. In early October 1986, three tillage treatments were established: ( 1 ) no tillage of barley stubble (NT); (2) chiseled barley stubble (C); (3) Paraplowed barley stubble (P). Tillage treatments were 18 × 36 m, with

TILLAGE-INDUCED SOIL MACROPOROSITY AND WATER INFILTRATION 1 5 5

the long axis parallel to the slope, and 15-m separation between treatments. The Paraplow I fractures soil without inversion. Details of the Paraplow de- sign have been given by Pidgeon (1983). Chiseling was done to a depth of 20-25 cm with 7.6-cm-wide twisted shank chisels spaced at 33 cm. Paraplow- ing was done to a depth of 20-25 cm. Shank spacing was 50 cm. Average water content of the top 30 cm of soil was 34 mm at the time of tillage. Tillage speed was 5.5 and 7 km h - ~ for the paraplow and chisel, respectively.

Infiltration and soil water content

Soil water content of the 3.35-m soil profile was measured at ~ 2-week intervals during October 1986 through March 1987. In each tillage treatment 3 neutron access tubes were installed and water content was measured in 30-cm depth increments.

A Palouse rainfall simulator (Bubenzer et al., 1985 ) was used to apply water at a constant rate to l-m 2 infiltration plots in each of three treatments. Water- drop size created by the simulator emulates the raindrop size of natural storms occurring in the inland Pacific Northwest, but the application rate exceeds natural storms. Two bordered, infiltration plots were established on each treatment in March 1987. Borders were constructed of heavy-gage sheet metal and driven into the soil to a depth of 25 cm. Prior to water application the inside edges of the infiltration plots were sealed with bentonite clay to prevent leakage along the metal/soil interface. Profiles of soil water content in the top 1.5 m of soil were obtained using a neutron probe, before and after water application. Application rates were determined by collecting the runoff from a l-m 2 calibration pan placed over the plot frame at the beginning and end of each test. Infiltration rates with respect to time were calculated as the difference between application rate and runoff rate.

Runoff

Runoff occurring from natural winter precipitation and snowmelt events, was measured in 18.3-m-long 3.05-m-wide bordered runoff plots on each of the 3 treatments. Precipitation, snow water equivalent, and frost depth were also measured throughout the winter. Additional details on these runoff plots are given by Zuzel et al. ( 1982 ).

Soil bulk density and surface cover

Soil bulk density, Pb, in 2-cm depth increments on the NT treatment, was measured to a depth of 50 cm with a tube sampler having an inside diameter

Wrademarks and company names are included for the benefi t of the reader and do not imply endorsement or preferential t r ea tmen t of the product by the U.S.D.A.

156 J.L. PIKUL JR. ETAL.

of 2.04 cm. This sampling method has been described by Allmaras et al. ( 1988 ). At each 2-cm depth, 12 cores were taken to describe Pb as a function of depth. Samples were taken in March 1987. For the P and C treatments, 15- cm depth increments were used because of the difficulty in obtaining a 2-cm core from the loose, unconsolidated soil found in the tillage zone.

Surface residue was measured by collecting all of the residue on the surface within an area of 1 m 2. Residue attached to the soil was clipped at the soil surface.

Soil structure

Void space created by tillage was measured on the P, C and NT treatments. Three undisturbed soil cores 20 X 20-cm × 25-cm-deep were taken from each treatment by pressing a heavy-gage sampling tube into the soil. Samples obtained in March 1987 measured the void structure following winter precipitation and freeze-thaw cycles. For the treatments with tillage, samples were taken directly over the path of the tillage tool shank, samples were taken directly over the barley row of the NT treatment. This sampling layout was used because surface geometry diverts water toward the shank opening in the case of the P and C treatments, and barley row in the case of the NT treatment.

Soil blocks were heated to 58°C, the melting point of Paraseal ~ canning wax, for 12 h. The Walla Walla silt loam has low shrink/swell potential and did not crack upon drying. Paraseal was heated to about 58 °C and poured on the surface of the block to infiltrate into the soil. Molten wax was continually added until ponding occurred. Blocks were then cooled at room temperature. Paraplast I tissue embedding medium, a compound of paraffin and plastic polymers, was also used to impregnate soil blocks. For this application Para- seal or Paraplast worked equally well.

Wax-impregnated soil blocks were cut horizontally into serial sections that were 2.5-cm thick, and 20 X 20-cm square. Cutting was done on a radial arm saw using a 35.56-cm diameter, 54 tooth, carbide-tipped combination wood- cutting blade. Each soil face was scraped with a razor blade after cutting to remove smeared wax, and to reveal soil and void features on horizontal planes.

Each face of each section was photographed using a 35-ram camera with a 50-mm lens. A 32 400 m m 2 frame was used to outline a known sample area on each face. Good results were obtained using Kodachrome 64 t color slide film. Diffuse natural lighting in a fiberglass-covered greenhouse provided ad- equate lighting.

Areal macroporosity of each cross-section was quantified using ATLAS ~ geographic information systems image-processing software. Areal macroporosity is the fraction of pores in a representative cross-section and is consid- ered equal to porosity, determined on a volume basis (Hillel, 1971 ). Quan- tification of macropore area, at one depth, using a line-intercept method

TILLAGE-INDUCED SOIL MACROPOROSITY AND WATER INFILTRATION 15 7

provided a comparison of results obtained using image analysis. Digitized images were obtained by illuminating the color slides with a Gordon' high- intensity chromatic light source and digitizing with an Eikonix 1 image-scan- ning system. Each image contained about 360 × 360 pixels. Any void feature having a square area >__ 0.25 mm 2, the area of I pixel, is operationally defined as a macropore.

RESULTS AND DISCUSSION

Weather and soil conditions

During the test period of October 1986 through March 1987 precipitation at the study site was 242 mm, which is close to the average of 216 mm. Aver- age air temperature for the period was 4.4°C which is average for the site. There were two major freeze-thaw cycles in December and January where the soil remained frozen for 20 and 21 days, respectively. Maximum frost depth on the P, C and NT treatments during December was 5, 8 and 5 cm, respectively; maximum frost depth during January on the P, C and NT treatments was 14, 12 and 10 cm, respectively. Residue cover, measured in March, for P, C and NT treatments was 1.7, 1.6 and 2.9 Mg ha- 1, respectively.

Soil bulk density, measured in March, is shown in Fig. 1 for the P, C and NT treatments. By sampling the .NT treatment in 2-cm depth increments, two distinct density layers were identified. A layer in the top 20 cm had a maximum Pb value of 1.36 g c m -3. This layer is thought to be a consequence of shallow rodweedings during the summer of 1985. Rodweeding during the fallow period creates a special combination of soil layers designed to conserve

'•-20 0 c- Ck ¢'~ -40

* 0

00000 P 0 * ~ ***** C . . . . . NT

BULK DENSITY (g/era 3) -6C '

1.00

Fig. 1. Depth distribution of soil bulk density for the Paraplow (P), chisel (C) and no-tillage (NT) treatments.

158 J .L . P I K U L J R . E T AL.

soil water during the summer months (Leggett et al., 1974). The layer, at about 30 cm, has a max imum Pb of 1.28 g cm -3 and coincides with the ap- proximate depth of previous moldboard plowing.

Soil bulk density on the P and C treatments for the 0-15-cm layer was less than the NT treatment (Fig. 1 ). There was no difference in Pb between the P and C treatments. At depths greater than 15 cm there was no difference in Pb between the P, C and NT treatments. Standard deviation of mean Pb, pooled with depth for the P, C and NT treatments were 0.025, 0.03 and 0.03 g c m -3, respectively.

During the winter of 1986-1987 there were no differences of stored water in the 3.35-m profile between the P, C and NT treatments. Soil water storage, between 3 October and 16 March, on the P, C and NT treatments was 77, 73 and 72%, respectively, of the 242 m m of precipitation falling at the site.

Total water stored in each treatment during October through March was not significantly different, but there were differences in cumulative water ac- cretion and depletion during some time periods (Fig. 2 ). From 12 to 23 De- cember 1986 the soil was frozen in all 3 treatments and precipitation was I 1 mm. The NT treatment had no additional storage, while the P and C treatments stored 6 and 1 mm, respectively. Similarly, during 9-28 January 1987 the soil was also frozen. Precipitation was 25 m m and 4 m m of runoff occurred on the NT treatment. Soil water storage increased by 21, 19 and 6 m m for the P, C and NT treatments, respectively.

Water storage measurements (Fig. 2) also suggest that evaporation was

200

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I-- Z 0 120

ry

I - - <~

~: so

40

¢~wO P

F=. : L~,J I tl.I j

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I I I I i i i i i i J i t i i i i i i i l l l l l l l i l l , , l l l l l l , l , 1 i

1 NOV 1 JAN 1 MAR

Fig. 2. Cumulat ive differences in water content measured in the 3.35 m soil profile of the Para- plow (P) , chisel (C) and no-tillage ( N T ) treatments. T ime intervals when the soil was frozen are indicated. Initial profile water content was measured after tillage on 3 October.

TILLAGE-INDUCED SOIL MACROPOROSITY AND WATER INFILTRATION 159

120

E E

k - Z W F-- Z 0 80 0

n*,

I.--

^ ^ ^ ¢vvv~) P

z ~ z N-- N--J O0 O 0

1 NOV 1 JAN 1 MAR

Fig. 3. Soil water content of the 0-30-cm soil layer for the Paraplow (P), chisel (C) and no- tillage (NT) treatments. Time intervals when the soil was frozen are indicated.

greater on the P and C treatments during warm, dry and windy conditions. For example, there was high evaporat ion potential during 10-25 February 1987, characterized by above normal air temperatures and wind travel. The P and C treatments, with less surface residue cover, lost 1 and 3 mm of water, respectively, while the NT treatment gained 10 mm of water. Precipitation during this t ime was 17 mm.

Enhanced water infiltration in frozen soil caused by surface connected ma- croporosi ty can be seen, indirectly, by the increase in water content of the 0 - 30-cm layer on 28 January (Fig. 3) for the P and C treatments. Prior to 28 January, the site was snow-covered with a snow water equivalent of about 14 mm. Frost depth on the P, C and N T treatments was 14, 12 and 10 cm, respectively. Over the next 5 days, rainfall was 14 mm and average daily air temperature rose from - 6 . 7 °C on 22 January to 5.8 °C on 27 January. Water content o f the 0 -30-cm layer increased to 106 m m on the P and C treatments as a consequence of water infiltrating through the frozen layer by way of open macropore channels. There was no change in water content of the NT treatment. Standard deviat ions of water contents at this depth, for the P, C and NT treatments were 5.0, 2.6 and 4.8 mm, respectively.

Macroporosity and infiltration

Beven and Germann ( 1982 ), classified and interpreted the significance of macropores in soil. The choice of an effective size to delimit macropores is arbitrary and often related more to the details of the experimental technique

160 J.L. PIKUL JR. ET AL.

than to considerations of flow processes; accordingly, experimental technique used to determine macroporosity for water flow should distinguish between two types of large voids: voids that are surface connected and hydrologically active in channeling water flow and those that are not.

Surface crusting of silt loam soil from rainfall and freezing and thawing can seal the surface connectivity of macropores and reduce hydrological effective- ness of the macropore. Weathering of soil structure is a continuous process, but the measurements reported here characterize the soil physical state at only one t ime in March 1987. However, 67% of the winter precipitation is nor- mally received by March and there is low probability that additional freezing will occur (Zuzel et al., 1986). The March measurements can thus be viewed

' I 2 c r n ! i

Fig. 4. Photograph of horizontally oriented soil slab from the 12.7-cm depth of the Paraplow t reatment.

o,, .

• H 2

Fig. 5. Photograph of horizontally oriented soil slab from the 12.7-cm depth of the chisel treatment.


Fig, 6. Photograph of horizontally oriented soil slab from the 12.7-cm depth of the no-tillage t reatment.

TABLE 1

Areal macroporosity at different soil depths for the Paraplow and chisel treatments. Areal macroporosity for the no-till treatment was less than 1% for the depths shown

Soil depth (cm)

Percent soil macroporosity

Paraplow Chisel

7.6 14.5 20.7 10.2 12.4 13.7 12.7 6.9 14.8 15.2 9.7 10.8 17.8 7.9 7.1 20.3 11.0 5.1 25.4 17.2 0.5

Mean 11.4 10.4

as a conservative index of the infiltration and pore structure that was present in mid-winter during the peak period of precipitation and soil-erosion potential.

Tillage tools on each of the P and C treatments created a unique arrangement of voids and clods. Typical soil structure created by tillage at a depth of 12.7 cm is shown for the P and C treatments, (Figs. 4 and 5, respectively) compared with the NT treatment (Fig. 6). Direction of tillage on the P and C treatments is from the bottom to the top of the page. Paraplow tillage created only slight mixing of the soil. Large clods with orientation in the direction of tillage and large fissured macropores were features of this tillage (Fig. 4 ). In contrast, the twisted shank chisel created a mixture of solids and voids

162 J .L. P I K U L JR . E T AL.

of nearly uniform size and shape (Fig. 5 ). The few macropores present on the NT treatment were cylindrical root and earthworm pores (Fig. 6 ).

Macroporosity in a representative cross-section of soil was different for each treatment and depth of cross-section (Table 1 ). For the C treatment macroporosity decreased nearly linearly from about 20% at a depth of 7.6 cm to less than 1% at a depth of 25.4 cm. In contrast, macropore area on the P treatment decreased to a minimum of 6.9% at a depth of 12.7 cm and increased to a

120

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r ~

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Z

2O

. . . . . . . . ; . . . . . . . . . ~ . . . . . . . . . ; . . . . . . . . : , i . . . . . . . . .

TIME (hours)

Fig. 7. Cumulative water infiltration for the Paraplow (P) , chisel (C) and no-tillage (NT) treatments. Curves are averages o f two replications on each treatment.

-5

- 4 5

E o - 8 5

-1- I- . Q.

~ - 1 2 5

- 1 6 5

/E /

a6,~-~,START ~ FINAL

I i I i

40 60 20

C * * * ' * " * S T A R T : * * * * FINAL

- 2 0 5 ' I , t 20 40 60 20

= I i I

4 0 6 0

WATER CONTENT (mm)

Fig. 8. Soil water content at the start and finish o f the water infiltration measurements on the Paraplow (P) , chisel (C) and no-tillage ( N T ) treatments.

TILLAGE-INDUCED SOIL MACROPOROSITY AND WATER INFILTRATION 16 3

maximum of 17.2% at a depth of 25.4 cm. Average macroporosity of the top 25 cm of soil was the same on both the P and C treatments (Table l ).

Macropore area of the P t reatment at 10.2 cm was also estimated using line intercepts. Percent macroporosity using line intercept was 14.8 compared with 12.4 using image analysis, which represents a difference of about 20%. This agreement supports the use of either method, however, with increasingly complex pore geometry such as that shown in Fig. 5, automated methods such as image analysis are desirable.

Water infiltration was greatest on the P and C treatments and least on the NT treatment. Average cumulative infiltration curves for two replications of the P, C and NT treatments are shown in Fig. 7. Cumulative infiltration for 2 replications of the P, C and NT treatments were 108 and 116, 97 and 104, and 46 and 53, respectively. Average final infiltration rates for the respective treatments were 23.5, 22.8 and 9.3 m m h - i.

The amount of soil water gained (Fig. 8 ) for the P, C and NT treatments supports the water infiltration measurements on the respective treatments (Fig. 7). Profile water gain was 103, 98 and 42 mm, respectively, which is nearly equal to I 12, 100 and 50 m m of infiltrated water, respectively.

SUMMARY AND CONCLUSIONS

Field studies were conducted to measure water infiltration and macroporosity. Tillage was conducted in the fall; measurements were made in March following normal precipitation and freezing and thawing of the soil. Water infiltration, measured in March, was greatest on the Paraplow and chisel treatments and least on the NT treatment. Ancillary measurements of profile water content during the winter showed differences between the treatments in profile position of stored water, however all treatments stored the same amount of water in the 3.35-m profile. Macroporosity created by tillage was important for channeling and storing water in the profile when the soil was frozen.

A paraffin-impregnation technique was used to prepare thick soil sections for macroporosity analysis. Digitized images were analyzed using image processing software. Thick soil sections revealed an arrangement of voids and clods unique to the Paraplow or chisel treatment. Depth distribution of macropore area was different on the Paraplow and chisel treatment, however there was no difference in average macropore area in the top 25 cm of soil. Soil settling over winter is uncertain, however water infiltration and macroporosity measurements conducted in March attest to the overwinter stability of macropore structure on both Paraplow and chisel treatments.

164 J.L. PIKUL JR. ET AL.

ACKNOWLEDGMENTS

T h e au tho r s t h a n k Phi l N u n n e m a c h e r a n d col leagues o f the G e o g r a p h i c T e c h n o l o g y L a b o r a t o r y , D e p a r t m e n t o f G e o g r a p h y , O r e g o n State U n i v e r s i t y for suppor t , in t e r m s o f e q u i p m e n t a n d exper t i se a s soc i a t ed wi th image analysis, also G u n d e r Te r j e son w h o supp l i ed the l a n d u p o n which th is e x p e r i m e n t was c o n d u c t e d . A p p r e c i a t i o n is e x t e n d e d to R i c h a r d G r e e n w a l t a n d Les Ek in for t he i r a ss i s tance wi th f ield work , a n d T h e r e s a Mig l io re t to for he r ass i s tance in p r e p a r i n g w a x - i m p r e g n a t e d soil sect ions .

REFERENCES

Allmaras, R.R., Gupta, S.C., Pikul, Jr., J.L. and Johnson, C.E., 1980. Soil erosion by water as related to management of tillage and surface residues, terracing, and contouring in eastern Oregon. Sci. and Educ. Admin., U.S. Dep. Agric., Washington, DC, ARR-W-10, 53 pp.

Allmaras, R.R., Pikul, Jr., J.L., Kraft, J.M. and Wilkins, D.E., 1988. A method for measuring incorporated crop residue and associated soil properties. Soil Sci. Soc. Am. J., 52:1128- 1133.

Beven, K. and Germann, P., 1982. Macropores and water flow in soils. Water Resour. Res., 18: 1311-1325.

Bubenzer, G.D., Molnau, M. and McCool, D.K., 1985. Low intensity rainfall with a rotating disk simulator. Trans. ASAE, 28: 35-43.

Bullock, P. and Murphy, C.P., 1980. Towards the quantification of soil structure. J. Microsc. (Oxford), 120: 317-328.

Bullock, P., Newman, A.C.D. and Thomasson, A.J., 1985. Porosity aspects of the regeneration of soil structure after compaction. Soil Tillage Res., 5:325-341.

Dexter, A.R., 1976. Internal structure of tilled soil. J. Soil Sci., 27: 267-278. Hillel, D., 1971. Soil and Water Physical Principles and Processes. Academic Press, New York,

NY, 288 pp. Hyde, G.M., George, J.E., Saxton, K.E. and Simpson, J.B., 1984. Straw-filled slots retard soil

erosion. Agric. Eng. J., 65: 26-29. Leggett, G.E., Ramig, R.E., Johnson, L.C. and Massee, T.W., 1974. Summer fallow in the north-

west. In: Summer Fallow in the Western United States. U.S.D.A.-A.R.S., U.S. Gov. Printing Office, Washington, DC, Conserv. Res. Rep. No. 17, pp. 110-135.

Mukhtar, S., Baker, J.L., Horton, R. and Erbach, D.C., 1985. Soil water infiltration as affected by the use of the paraplow. Trans. Am. Soc. Agric. Eng., 28:1811-1816.

Ojeniyi, S.O. and Dexter, A.R., 1983. Changes in the structure of differently tilled soil in a growing season. Soil Tillage Res., 3: 39-46.

Pidgeon, J.D., 1983. "Paraplow" a new approach to soil loosening. ASAE, St. Joseph, MI, ASAE Paper No. 82-2136.

Pikul, Jr., J.L., Zuzel, J.F. and Greenwalt, R.N., 1986. Formation of soil frost as influenced by tillage and residue management. J. Soil Water Conserv., 41: 196-199.

Ramig, R.E. and Ekin, L.G., 1984. Effect of stubble management in a wheat-fallow rotation on water conservation and storage in eastern Oregon. In: 1984 Columbia Basin Agricultural Research, Oregon, Agric. Exp. Stn, Spec. Rep. 713, pp. 30-33.

Redinger, G.J., Campbell, G.S., Saxton, K.E. and Papendick, R.I., 1984. Infiltration rate of slot mulches: measurement and numerical simulation. Soil Sci. Soc. Am. J., 48" 982-986.


Shipitalo, M.J. and Protz, R., 1987. Comparison of morphology and porosity of a soil under conventional and zero tillage. Can. J. Soil Sci., 67: 445-456.

Wischmeier, W.H., 1973. Conservation tillage to control water erosion. In: Conservation Til- lage, Proceedings of a National Conference, Soil Conserv. Soc. Am. Ankeny, IA, pp. 133- 141.

Zuzel, J.F. and Pikul, Jr., J.L., 1987. Infiltration into a seasonally frozen agricultural soil. J. Soil Water Conserv., 42: 447-450.

Zuzel, J.F., Allmaras, R.R. and Greenwalt, R., 1982. Runoff and soil erosion on frozen soils in northeastern Oregon. J. Soil Water Conserv., 37:351-354.

Zuzel, J.F., Pikul, Jr., J.L. and Greenwalt, R.N., 1986. Point probability distributions of frozen soil. J. Climate Appl. Meteorol., 25: 1681-1686.

effect of tillage-induced soil macroporosity on water infiltration

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