Soil & Tillage Research, 9 (1987) 255-263 255 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Til lage Effects on Shear Strength and Bulk Dens i ty of Soil Aggregates I

J.G. BENJAMIN and R.M. CRUSE

Department of Agronomy, Iowa State University, Ames, IA 50011 (U.S.A.)

(Accepted for publication 31 October 1986)

ABSTRACT

Benjamin, J.G. and Cruse, R.M., 1987. Tillage effects on shear strength and bulk density of soil aggregates. Soil Tillage Res., 9: 255-263.

An experiment was conducted to determine the effect of four tillage systems (moldboard plow, chisel plow, Paraplow and no-till) on soil aggregate shear strength and bulk density. Two soils, a Canisteo clay loam (fine-loamy, mixed (calcareous), mesic, Typic Haplaquoll) and a Haig silt loam (fine, montmorillonitic, mesic, Typic Argiaquoll) were used in this study. Soil samples were collected from the 0.075-0.15-m-depth increment in 1983 and the 0.075-0.15- and 0.225-0.30-m- depth increments in 1985. Shear strength of soil aggregates 0.02-0.03 m in diameter was measured by a fall-cone penetrometer and bulk density of the same aggregates was measured by gamma-ray attenuation. Aggregates were tested at soil water matric potentials (~m) of - 0.2, - 1.1 and - 4.0 kPa in 1983 and at ~m of -0.2, - 1.1, - 4 .0 and -7 .9 kPa in 1985. Tillage for the 1983 growing season was conducted under very wet conditions, whereas tillage for the 1985 growing season was conducted under much drier conditions. Samples collected in 1983 showed little tillage effect on shear strength or bulk density. In 1985, tillage had an effect on shear strength and bulk density for the Haig soil, but not for the Canisteo soil. Much of the tillage effect on soil aggregate shear strength could be explained by tillage-induced changes in the aggregate bulk density. As bulk density decreased, soil aggregate shear strength decreased.

Sampling depth had no effect on soil aggregate shear strength or bulk density. Matric potential had an effect on soil aggregate shear strength and bulk density. As matric potential decreased, both shear strength and bulk density increased.

INTRODUCTION

Soils are tilled to provide a suitable seedbed, reduce competition from other plants, improve surface drainage and change soil structure (Gebhardt et al., 1985 ). One physical characteristic often altered by tillage is soil strength. High

l Journal Paper J-12241 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA. Project No. 2659. Supported in part by Imperial Chemical Industries, Fernhurst, Haslemere, Surrey, U.K.

0167-1987/87/$03.50 © 1987 Elsevier Science Publishers B.V.

256

soil strength can limit root growth (Barley et al., 1965) and seedling emerg- ence (Taylor, 1971). The strength of soil aggregates can affect soil erosion through an influence on particle detachment due to waterdrop impact (Francis and Cruse, 1983), surface seal formation (Johnson et al., 1979) and infiltra- tion rate (Greenland, 1977). Soil aggregate bulk density can affect water and nutrient absorption by plants ( Voorhees et al., 1971 ).

Cooper (1971) described the mechanical reactions of the soil to the tillage tool as follows:

(1) parting of the soil particles due to insertion of the tillage tool; ( 2 ) compressing the soil due to the force applied to the soil and its resistance

to motion; (3) shearing the soil due to indirect tension forces resulting from compres-

sive forces; (4) bending, twisting, or t ransport ing the soil dependent upon the shape of

the tool and the material; (5) accelerating due to, or transporting dependent upon, the shape of the

tool and the material. We hypothesized that (1) different tillage tools will affect both qualitatively

and quantitatively the mechanical reactions occurring in the soil during the tillage tool operation; (2) alterations in the mechanical reactions will affect the forces acting on the soil during the tillage operation; (3) the forces acting in the soil during tillage will, or could, affect aggregate physical properties.

The objective of this study was to determine the effect of four tillage systems on soil aggregate shear strength and bulk density.

MATERIALS AND METHODS

A tillage experiment was established in the autumn of 1982 on two Iowa soils. The first soil, a Haig silt loam (fine, montmorillonitic, mesic, Typic Argia- quoll ), was sampled from the McNay Research Center near Chariton, IA. The second soil, a Canisteo clay loam ( fine-loamy, mixed (calcareous), mesic, Typic Haplaquoll), was sampled from the Agronomy and Agricultural Engineering Research Center near Ames, IA. Selected physical properties of each soil are presented in Table I.

Plots were arranged in randomized complete-block design with four repli- cations. The tillage systems included in the study were moldboard plow in the autumn (MP) , chisel plow in the autumn (CP) , Paraplow" in the autumn (PP) and no-till (NT) . The MP and CP treatments were autumn-plowed 0.2 m deep, disked 0.1 m deep the next spring, and harrowed with a spike-tooth

"Paraplow is the registered trademark of the Howard Rotovator Company. Mention of trade prod- ucts in this paper does not imply endorsement over similar products from other companies and is provided for convenience of reference only.

TABLE I

Characteristics of soils used for aggregate strength determination

257

Soil D e p t h % S a n d a % S i l t a

(m) 2-0.5 mm 50-20 zm 20-2 gm

% Clay " % Organic ~' pH' < 2 pm carbon

Canisteo 0.075-0.15 33.1 18.6 19.5 28.8 2.8 7.5 0.225-0.30 33.9 13.0 21.6 31.5 2.3 7.9

Haig 0.075-0.15 3.5 28.6 42.3 25.6 1.8 6.2 0.225-0.30 3.5 27.5 41.7 27.3 1.6 6.7

"Particle size analysis by pipette method (Day, 1965). h0rganic carbon by Wakley-Black method (Nelson and Sommers, 1982). CpH was measured in a 1:1 soil to water mixture ( McLean, 1982 ).

harrow before planting. The P P t rea tment was autumn-t i l led 0.3 m deep, with no additional tillage before planting corn (Zea rnays L.). No primary or sec- ondary tillage was used on the N T treatments .

Autumn tillage for the 1983 growing season was done under very wet con- ditions at both sites. No moisture samples were taken at the time of tillage, but the soils were tilled at moisture contents above the plastic limit of each soil (plastic limit of 28 and 26% gravimetric water content for the Canisteo and Haig soils, respectively). Autumn tillage for the 1985 growing season was con- ducted under much drier conditions. The gravimetric water content of the upper 0.15 m at the time of tillage was 21% for both the Canisteo and Haig soils.

Soil aggregate samples were collected in early July of 1983 and 1985. No samples were collected in 1984. A 0.35 X 0.15-m trench was dug centered on, and perpendicular to, a row with non-tracked inter-rows. Samples were col- lected from the 0.075-0.15-m-depth increment in both 1983 and 1985. Addi- tional samples were collected from the 0.225-0.30-m-depth increment in 1985. A block of soil at the designated depth was removed from the trench and bro- ken by hand into aggregates smaller than 0.05 m. Soil aggregate shear strength (~) and bulk density (Pa) were measured on aggregates of 0.02-0.03 m in diam- eter. Soil aggregate r was measured with a fall-cone penetrometer, and soil aggregate Pa was measured by gamma-ray a t tenuat ion (Benjamin and Cruse, 1985 ). Shear strength andpa measurements were made at three soil water matric potentials ( - 0 . 2 , - 1 . 1 and - 4 . 0 kPa) in 1983 and four soil water matric potentials ( - 0 . 2 , - 1 .1 , - 4 . 0 and - 7 . 9 kPa) in 1985. Three aggregates were tested as subsamples for each yearly combinat ion of tillage, replication, depth and matric potential. The average ~ and pa of the subsamples were used for statistical analysis. For comparison of tillage systems between years, data for aggregates sampled at the 0.075-0.15-m depth and tested at - 0 .2 , - 1 . 1 and

- 4.0 kPa matric potentials were combined for analysis. Analysis of variance was used to determine year, tillage, matric potential

258

(F) and depth effects on soil aggregate shear strength and bulk density. Aggre- gate bulk density can also have an effect on soil aggregate shear strength {Chancellor, 1971; Benjamin and Cruse, 1985). Analysis of covariance was used to adjust soil aggregate z for effects Ofpa. A non-linear analysis of covar- lance was performed by applying a linear analysis of covariance to logl0-trans- formed z data. The null hypothesis that tillage, ~,, or depth had no effect on z or Pa was rejected if a significant t rea tment effect could be detected at the 0.05 confidence level. A least significant difference (LSD) was used to show differ- ences among t rea tment means only if there was a significant t rea tment effect.

RESULTS AND DISCUSSION

Probabil i ty plots of the data indicated a normal distribution for aggregate Pa and a log-normal distr ibution for aggregate z. Analysis of variance and analysis of covariance were conducted on the loglo-transformed z data. The results were retransformed into z for presentat ion in the tables.

Analysis of variance showed a significant location or soil type effect on z and pa of the aggregates. Aggregates from the Haig soil had greater r and p~ than aggregates from the Canisteo soil. Densi ty had a greater effect on r for the Haig aggregates than for the Canisteo aggregates.

There were no significant tillage, depth, or year effects on volumetric water content of soil aggregates within each matric potential. Volumetric water con- tents for Canisteo soil aggregates were 0.46, 0.44, 0.41 and 0.37 m 3 m -3 for ~m of --0.2, --1.1, --4.0 and - 7 . 9 kPa, respectively. Volumetric water contents for the Haig soil aggregates were 0.46, 0.43, 0.41 and 0.40 m a m -3 for ~m of --0.2, --1.1, --4.0 and - 7 . 9 kPa, respectively.

Tillage had a significant effect on z and p~ of aggregates from the Canisteo soil ( Table II) . In 1983, the Paraplow t rea tment had lower z and lower Pa than the other tillage systems, while the moldboard plow t rea tment resulted in the greatest z and p~. In 1985, the moldboard plow t rea tment had the lowest z and the chisel plow t rea tment had the greatest. There was no significant Pa differ- ence between tillage system t rea tments in 1985. The year of sample collection had no overall effect on z or pa, but there was a year × tillage interaction. There was no significant r change between years for the no-till or chisel plow tillage systems. Aggregates from the Paraplow tillage system showed an increased z from 1983 to 1985, and aggregates from the moldboard plow tillage system showed a z decrease. Bulk densi ty did not significantly change between 1983 and 1985 for the no-till, chisel plow, or moldboard plow tillage systems, but p~ of aggregates from the Paraplow system showed an increase between years. Covariate analysis of tillage effects on z with pa as the covariate showed no significant differences among tillage systems.

Year and tillage both affected z and Pa of aggregates sampled from the Haig soil (Table II ). There was a significant decrease in z andp~ from 1983 to 1985.

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In 1983, there was no tillage effect on z orp~. In 1985, there was a tillage effect, with the no-till system having the greatest z and the moldboard plow system having the least z. The no-till and Paraplow t rea tments resulted in similar p~, which were greater than the moldboard plow or chisel plow t rea tment densi- ties. Significant tillage effects were a result of a z decrease for the chisel plow and moldboard plow tillage systems, whereas there was no significant z change for the no-till and Paraplow systems. After z was adjusted for Pa differences, there were still significant t rea tment effects on aggregate shear strength, with the no-till system having the greatest z and the moldboard plow having the least.

Depth of sample collection had no effect on z or pa (Table III) . Tillage by depth had no effect on the Canisteo soil aggregate z, but tillage did affect soil aggregate z differently at different depths for the Haig soil aggregates. At the lower (0.225-0.30 m) depth, tillage had no significant effect on soil aggregate z or p~, but tillage did affect these propert ies in the upper (0.075-0.15 m) depth. The no-till and Paraplow tillage systems had greater aggregate z and p~ than the chisel plow and moldboard plow systems.

Matric potential had a significant effect on z and p~ for aggregates from the Canisteo soil in 1983 and 1985 (Table IV). In general, as ~, decreased, z and p~ increased. There was no difference between years for z or pa averaged over matric potential, but there was an interaction between ~ and year on z. The effect of ~, on z was greater in 1983 than in 1985. In 1983, ~ increased 140% as

decreased from - 0 . 2 to - 4 . 0 kPa. In 1985, z increased 57% over the same decrease. For both years, p~ increased approximately the same as ~, decreased.

Matric potential also had a significant effect on z when p~ was used as a cova- riate, which indicates that matric potential affects v more than can be attrib- uted to changes ofp~ alone.

Matric potential had a significant effect on z for Haig soil aggregates both in 1983 and 1985, but affected p~ only in 1985 (Table IV) . As ~ decreased from - 0 . 2 to - 0 . 4 kPa, z increased 55% in 1983 and 62% in 1985. Shear strength averaged over matric potentials was 38% lower in 1985 than in 1983. Covariate analysis indicated that the z increase due to ~, decrease could be explained by the pa increase in 1985 but not in 1983.

This s tudy showed that tillage may have an effect on soil aggregate z and p~. Much of the z difference resulting from tillage could be at t r ibuted to changes ofp~. Tillage that reduced p~ showed a corresponding decrease of z.

The moisture content of the soil at the time of tillage is very important and can influence the effectiveness of the tillage tool for loosening the soil. In the wet year of this s tudy (1983), tillage had a limited effect on soil aggregate z and p~. Tillage effects on z and pa were greater for tillage conducted during the dry year (1985), particularly for the Haig soil.

Tillage effects on soil aggregate z and p~ occurred only in the upper (0.075-0.15 m) depth. Even though the Paraplow was operated at a deeper

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dep th t h a n the mo ldboa rd plow or chisel plow, there were no s igni f icant tillage effects on soil aggregate z and Pa at the lower (0 .225-0.30 m ) sampl ing depth.

The re was no difference in soil aggregate z or pa be tween years for the no- till t r ea tmen t . Th i s m a y indicate t h a t na tu ra l forces such as we t t ing and dry ing cycles or freezing and thawing cycles have l imited effect on changes of z and p~ for this size of aggregates or t h a t these aggregates are at an equi l ibr ium with the su r round ing phys ica l env i ronmen t .

REFERENCES

Barley, K.P., Farrell, D.A. and Greacen, E.L., 1965. The influence of soil strength on the penetra- tion of a loam by plant roots. Aust. J. Soil Res., 3: 69-79.

Benjamin, J.G. and Cruse, R.M., 1985. Measurement of shear strength and bulk density of soil aggregates. Soil Sci. Soc. Am. J., 49: 1248-1251.

Chancellor, W.S., 1971. Effects of compaction on soil strength. In: K.K. Barnes, W.M. Carleton, H.M. Taylor, R.I. Throckmorton and G.E. Vanden Berg {Editors), Compaction of Agricul- tural Soils. Am. Soc. Agric. Eng., St. Joseph, MI, 190-222.

Cooper, A.W., 1971. Effects of tillage on soil compaction. In: K.K. Barnes, W.M. Carleton, H.M. Taylor, R.I. Throckmorton and G.E. Vanden Berg (Editors), Compaction of Agricultural Soils. Am. Soc. Agric. Eng., St. Joseph, MI, pp. 313-364.

Day, P.R., 1965. Particle fractionation and particle size analysis. In: C.A. Black, D.D. Evans, J.L. White, L.E. Ensminger and F.E. Clark (Editors), Methods of Soil Analysis Part 1. Agronomy, 9: 545-562.

Francis, P.B. and Cruse, R.M., 1983. Soil water matric potential effects on aggregate stability. Soil Sci. Soc. Am. J., 47: 578-581.

Gebhardt, M.R., Daniel, T.C., Schweizer, E.E. and Allmaras, R.R., 1985. Conservation tillage. Science, 230: 625-630.

Greenland, D.J., 1977. Soil damage by intensive arable cultivation: temporary or permanent? Phil. Trans. R. Soc. Lond. B, 281: 193-208.

Johnson, C.B., Mannering, J.W. and Moldenhauer, W.C., 1979. Influence of surface roughness and clod size and stability on soil and water losses. Soil Sci. Soc. Am. J., 43: 772-777.

McLean, E.O., 1982. Soil pH and lime requirement. In: A.L. Page, R.H. Miller and D.R. Keeney {Editors), Methods of Soil Analysis, Part 2.2nd edn. Agronomy, 9: 199-209.

Nelson, D.W. and Sommers, L.E., 1982. Total carbon, organic carbon, and organic matter. In: A.L. Page, R.H. Miller and D.R. Keeney. {Editors), Methods of Soil Analysis, Part 2.2nd edn. Agronomy, 9: 570-571.

Taylor, H.M., 1971. Effects of soil strength on seedling emergence, root growth and crop yield. In: K.K. Barnes, W.M. Carleton, H.M. Taylor, R.I. Throckmorton and G.E. Vanden Berg {Edi- tors), Compaction of Agricultural Soils. Am. Soc. Agric. Eng., St. Joseph, MI, pp. 292-305.

Voorhees, W.B., Amemiya, Min, Allmaras, R.R. and Larson, W.E., 1971. Some effects of aggregate structure heterogeneity on root growth. Soil Sci. Soc. Am. Proc., 35: 638-643.