Soil & Fillage Research, 19 ( ! 991 ) 11 ! - 1 ! 9 Elsevier Science Publishers B.V., Amsterdam

i l l

Effect of soil compaction on root development

H.M. Taylor and G.S. Brar Agronomy. Ilorticulture and Entomology Department, Texas Tech University, Lubbock, TX 79409

(U.S.a.)

(Accepted for publication 9 November 1989)

ABSTRACT

Taylor, H.M. and Brar, G.S., 1991. Effect of soil comv.action on root development. Soil Tillage Res., 19: I l l -119 .

Changes in soil compactness have no direct effect on root development, but indirectly affect rooting through changes in structural arrangement and cracking pattern, soil strength, total porosity, number of large pores, volumetric water content, soil hydraulic conductivity, air filled porosity, and gaseous diffusion rate. Each of the~e indirect effects of soil compaoness changes can directly affect root de- velopment. Increases er reductions in soil compactness usua!!y do not occur uniformly with depth or with location within the field. Wheel traffic will create zones of high strength in strips along the field. These compacted strips may create a root shadow effect in the subsoil, in addition to denying the pla~t water and nutrients located in the compacted strip. Even if root development is altered by changes in soil comi~actness, above-ground growth may be normal, if the plant is able to obtain sufficient water and nutrients.

INTRODUCTION

Soil compact ion affects root deve lopment through :everal indirect mecha- nisms. The increase in compactness arises as a result o f f o ~ e s that act on the soil volume. Some o f these forces are the result o f natural phenomena, such as drying and gravity, but we will l imit our discussion to those forces ti,at are applied externally for short periods and which can be measured readily.

SOIL HETEROGENEITY

When cl imax vegetation has been on a site for many years, the soil usually is very heterogeneous and is considered to have a good structure. In a soil that contains significant amounts o f clay, this structure has a hierarchical nature. Clay platelets are combined into domains or floccules, some o f the domains are combined into microaggregates and some o f the microaggregates are com- bined into macroaggregates (Dexter, 1988 ). This hierarchical order led Dex- ter ( 1988 ) to define soil structure as "the spatial heterogeneity o f the differ-

0167-1987/91/$03.50 © 1991 - - Elses ter Science Publishers B.V.

i 12 H.M. TAYLOR AND G.S. BRAR

ent components or properties of soil." This definition encompasses the many different aspects of soil structure that exist at many different size scales in the soil. Therefore, this definition is equally valid for the arrangement of colloi- dal clay particles in a floccule, for the arrangement of soil clods in a tilled soil, for the array of root and earthworm channels in an untilled stratum of the soil, and for the variability of soil strength in a compacted layer.

When a soil with climax vegetation is brought under cultivation, its heter- ogeneity usually is reduced. Oxidation of organic matter is increased, reduc- ing the soil's ability to resist externally applied forces. During tillage, the soil is stirred, which also reduces heterogeneity. Pressures from the hooves of an- imals or from tires of tractors that pull the tillage implements will increase compactness of the soil volume where the force is applied. Unless care is ex- ercised, random hoof or tire traffic, will soon apply force to all of the soil sur- face, causing increases in compactness throughout the paddock or field.

However, traffic-applied forces or tillages do not always reduce heteroge- neity, because tractor tire traffic often occurs in a pattern that leaves linear strips of compacted surface soil interspersed with previously loosened soil. These strips occur when tram-lines are established during cereal production, when multi-row equipment is used in row crop production or when tires are

Fig. 1. Cotton (Gossypium hirsutum L.) plants may become established on high-strength soils only where the planted rows intersect volumes of soil loosened to 30-cm depth by chisel t,nes. Soil was Amarillo fine sandy loam (Aridic Paleustolls, finedoamy, mixed, thermic). Photo- graph from Taylor and Burneu (1963).

SOIL COMPACTION AND ROOT DEVEI,OPMENT 113

run only in a few pathways in controlled-traffic operations. In addition, tines are often used in a pattern that leaves alternating strips of compacted and loosened soil. When crops are planted across these loosened and compacted strips, plant establishment may be confined to the loosened soil, especially in semi-arid environments (Fig. 1 ).

STABILITY OF SOIL STRUCTURE

A soil that is excellent for plant growth must possess a heterogeneous soil structure that persists for a reasonable period of time. The persistence of a specific at, mgement of particles against the forces that act on the soil is a measure of its stability. Generally, it is net useful to discuss whether or not a specific soil volume has had its structure destroyed. Not all hierarchies of structure will be destroyed by a particular set of forces.

The stability of different hierarchical orders aepends on different bonding agents (Oades, 1984, 1987). Microaggregates are bound by poiysacchaddes exuded by plant roots and by microorganisms. Stability of microaggregates is also increased by polyvalent cations, such as C a 2+ (Edwards and Bremnei ~, 1967).

Stability of microaggregates usually is associated with organic matter that is relatively protected from decomposition; therefore, their stability is rela- tively unaffected by cropping history. On the other hand, macroaggregates derive part of their stability by being enmeshed in plant roots, root hairs and fungal mycelia. These agents depend on recent additions of organic residue. Macroaggregates are very susceptible to cropping history and soil manage- ment. Interactions between soil management and aggregate stability have been reviewed by Quirk ( 1979 ).

SOI L COMPACTION AND PLANT GROWTH

Soil structure per se provides nothing that is essential for plant growth. To grow rapidly in volume, mass or complexity, higher plants require adequate quantities of water, oxygen for aerobic respiration, carbon dioxide and ra- diant energy for photosynthesis, some 15-17 essential elements, temperature within an acceptable range and the absence of toxic agents and severe patho- gens. Changes in soil compactness may influence fluxes and concentrations of each of the requirements furnished by the plant roots but these changes will not affect growth unless the particular requirement becomes the limiting agent to growth.

SOIL COMPACTION AND ROOT GROWTH

Roots increase in length because new cells are formed ill meristematic tis- sue near the root tip and these newly formed cells increase in volume, pushing

i 1 4 H.M. TAYLOR AND G.S. BRAR

the root tip forward if growth conditions are satisfactory. Four processes oc- cur simultaneously for this expansion to occur and continue.

Hsiao and Bradford (1983) describe these four processes. Before the cells start the process of expansion, water potential inside the cell, C/inside , is equal to the water potential outside, ~7 °''tside, and ¥insid~= ~,v+ ¢/o where ~p is turgot pressure and ¢/o is osmotic potential. The first process is that of cell-wall loos- ening or relaxation. This process causes ¢/outsid~ to be greater than ~,~nsid~. In the second process, water flows into the cell, diluting its solutes. This water influx causes the cell walls to expand, the third process. Finally, solutes then accumulate within the cell until v/insio~ and ¢/out~d~ are again nearly equal. Of course, all four processes are occurring simultaneously and continuously.

Turgor (hydrostatic pressure) inside the cells mu~t exceed a threshold of 0.15-0.60 MPa before cells wPd expand irreversibly in nutrient solution (Hsiao and Bradford, 1983 ). If the cells are located in the soil, any additional con- straint imposed by soil surroundings also must be overcome by the expanding roots. Obviously, if the cells of a root are expanding into the void of a crack or worm hole, the ad~:.fional constraint is almost zero but if the root tip must push asidc soil particles to create a pathway, the additional constraint may be severe. If the soil constraint is severe, additional solutes may collect in the elongation zone. These solutes will further rcc~uce the, osmotic potential and may increase turgor within the cells (Greacen, 1987 )

The additional constraints imp ased by the soil on the expanding root cells are the result of increased resistaace of the soil to deformation. Soil s, trength, a measure of deformation resistance, increases with increased soil bulk den- sity or with decreased soil matric potential (Fig. 2 ). When the proportion of cotton taproots that penetrated cores of four medium- to coarse-textured soils

4.0"

3.0"

2.0"

1.0

0- 1.0

Matric potential

(kPa)

-1250 /

Chesterfield loamy sand / /

-42 j -19

- - ~ 1 i i i i i.1 1.2 1.3 1.4 1.5 1.6

Bulk density (Mg-m-~)

Fig. 2: Penetrometer resistance of Chesterfield loamy sand as influenced by soil bulk density and matric potential (data of Taylor and Ratliff, 1969).

SOIL COMPACTION AND ROOT DEVELOPMENT 115

was plotted against soil strength, the data could be expressed as one curvilin- ear relationship, even though soil bulk density varied from 1.25 to 1.85 Mg m-3 and soil matric potential varied from-0.02 t o - 0 . l MPa (Fig. 3 ). The elongation ;ate of peanut (Arachis hypogaea L. ) roots decreased with an in- crease in soil strength, as measured by penetrometer resistance (Fig. 4).

100

A

60

== ~. 40

g r £

20

o •

• o :,r,

O

• QUINLAN x COLUMBIA

NARON o MILES

0 V i

0 1.0 4.0 5.0

O•<

~ l ~ ~ o v v w x ! J

2.0 3.0

Soil strength (MPa)

Fig. 3. Relationship between soil strength (as measured by penetrometer resistance) and the percentage of cotton (Gossypium hirsutum L. ) taproots that penetrated 2.5-cm thick soil cores (data of Taylor et al., 1966 ).

3.0

2.5

E g 2.o

• - 1.5

~ 1.0

~ 0.5

0

Y = 2.69 0.84X + 0.07X 2

- Matric poi~atla. I ,~u (kPa)

o ~ o -19 a A -42

x ~ o -1250

o

1.0 2.0 3.0 4.0 5.0 6.0 7.0

Penet rometer res is tance lMPa)

Fig. 4. Effect of soil strength (as measured by penetrometer resistance) and soil water content on peanut (Arachis hypogaea L.) root e23ngation for th,: period 40-80 h after transplant;ng (data of Taylor and EBtliff, 1969).

! 16 H . M . T A Y L O R A N D G . S . B R A R

Water must move from outside into the root at a rate sufficient to maintain the expanding root cells at a nearly constant turgor. Sufficient water must also be supplied to satisfy the water flows toward_ *..issue away from the root tip. Soil hydraulic conductivity and soil matric potential will affect the rate of water supply to the root cells. Soil compaction affects the soil hydraulic con- ductivity - water content relationship (Fig. 5 ). At matric potentials near zero, soil compaction reduces soil hydraulic conductivity because it reduces the number of large pores. At matric potentials n e a r - 1.0 t o - 1.5 MPa, increases in soil compactness usually increase hydraulic conductivity because it in- creases particle to particle contact. Soil compaction also affects the quantity of water retained at high matric potentials (Box and Taylor, 1962 ).

Metabolic energy must be supplied to the expanding cells in order for them to increase their solute concentration. Molecular oxygen is required for the efficient conversion of metabolites into energy. These oxygen supplies will. be reduced below critical levels when eoil air-filled pol'osity is reduced below min imum values that depend on soil texture, water content and soil structure. Soil compaction causes a decrease in air-filled porosity.

Supplies of metabolic energy, indolacetic acid and possibly ethylene are re- quired in the cell-wall loosening process. Root growth ceases in cotton and soybeans within 2-3 min after nitrogen gas replaces air that surrounds the roots (Huck, i 970). ~rom this tact, we can conclude that lack of adequate oxygen probably affects the cell-wall loosening process. Increases in soil com- pactness or soil water content may cause a similar response to that of nitrogen gas, although at slower rate.

~ ' 1 0 " "ID E E >,

"~ ;= o

"~0~ 10"2

0 .2

z,.

~ " lO-a o

x'.. . . Bulk density (Mg m -3)

i I , I , I I I I

-0 .02 -0 .04 -0 .06 -0 .08 -0 .10 -0 .12 -0 .14

Matric potential (MPa)

Fig. 5. Effect of soil bulk density and soil matric potential on the hydraulic conductivity of Fort Collins clay loam (data of Kemper et al., 1971 ).

SOIL COMPACTION AND ROOT DEVELOPMENT I 17

SOIL COMPACTION AND ROOT GROWTH PRESSURE

Pfeffer (1893) directly measured the force that roots exerted against a bar- rier and the cross-sectional area of the root at the time it was exerting that force. His calculated root growth pressures ranged from 1.2 to 2.5 MPa when oxygen partial pressures, temperatures and water supplies to the elongation zones probably were adequate. Reduction in oxygen concentration around the elongation zone reduces root growth pressure (Ea.f.s et al., 1969; Souty and Stepniewski, 1988 ). Reductions in temperature below that required for maximum elongation rate in an optimal environment also reduces root growth pressure (Greacen, 1987). Soil compaction alters the fluxes and storage of oxygen and heat and may affect the pressures that roots can exert against the soil matrix. Some type of anchorage is also neces~acy far the roots to exert their maximum force against the soil matrix. If the anchorage is not present, the root tip may remain stationary and the growth in the elongation zone will cause spiralling or some other root distortion.

SC, IL COMPACTION AND LATERAL ROOT FORMATION

An accumulation of ethylene around the meristematic tissue can cause a dramatic increase in formation of root hairs and lateral roots (Crossett and Camg'bell, 1975). The ethylene gas accumulates under soil conditions where diffusion of oxygen is low and biological respiration is high. This condition occurs most frequently in wet compacted clay soils.

PLOW LAYER HETEROGENEITY AND ROOT DISTRIBUTION

Many different arrangements of compacted and loosened soil can occur in a cultivated field. These different arrangements can affect the spatial distri- bution of roots, not only in the plow layer but also in the subsoil.

Tardieu and Manichon ( 1986a, b; 1987a, b) mapped maize root distribu- tion within and below the plowed layer, where four types of soil structure had been created. The four types were: ( 1 ) individual small clods and fine earth; (2) mixture of decimeter-sized clods and centimeter-sized voids; (3) plow layer uniformly compacted to the 30-cm depth; and (4) a 50-cm wide wheel compaction to the 30-cm depth in one interrow out oftwo with the remaining soil similar to the condition in type ( l ). The treatments affected the progres- sion of the rooting front with time but rooting depth measured after silking of maize was not affected. Total root weights differed among treatments but the ratios of root weight to total plant weight were similar for all treatments. Total plant weight increased fastest in the "small clods and fine earth treatment." Over the whole profile, the horizontal and vertical spatial distribution of roots was markedly affected by the plow layer treatment. In the treatment that was

118 H.M. TAYLOR AND G.S. BRAR

uniformly compacted to the 30-em depth, appreciable shrinkage cracks ap- peared during the growing season. The irregularities in soil physical condi- tions of the plow layer caused localized changes in rooting density in the non- compacted subsoil, a "shadow effect."

Tardieu (1987) evaluated the effects of soil compact ion on water uptake by maize. Water uptake was reduced to almost 50% of that for the noncom- pacted treatments ( 1 ) and (2) of Tardieu and Manichon ( 1987b ). Soil water content always was greater in the compacted plots. Tardieu (1987) con- chided that differences among treatments in the availability of water reserves are probably caused by differences in the spatial distribution of roots.

Many models of water and ion uptake by root systems contain the stated (or implied) assumption that the roots are uniformly distributed at equal distances in the soil profile. This assumption seldom is satisfied in naturally occurring soil profiles, but comes closer to being true in loose, well-structured soils than in compacted ones.

CONCLUSION

Soil compact ion can change morphology and function of the plant's root system by a number of mechanisms, biologically, chemically or physically. In fact, the morphology of a root system responds so easily to its environment that almost any amount of force applied to a loose well-structured soil will alter root morphology. However, a change in the root system appearance does not necessarily cause an alteration in above-ground growth or yield. A root length density that is reduced substantially from that which developed under well-structured condit ions may still supply all of the water and nutrients de- manded by the plant shoots.

REFERENCES

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Crossett, R.N. and Campbell, D.J., 1975. The effects of ethylene in the root environment upon the development of barley. Plant Soil, 42: 453-464.

Dexter, A.R., 1988. Advances in characterization of soil structure. Soil Tillage Res., 11:199- 238.

Eavis, B.W., Ratliff, L.F. and Taylor, H.M., 1969. Use of a dead load technique to determine axial root growth pressure. Agron. J., 61: 640-643.

Edwards, A.P, and Bremner, J.M., 1967. Microaggregates in soil. J. Soil Sci., 18: 64-73. Greacen, E.L., 1987. Root response to soil mechanical properties, Proc. 13th Conf. Int. Soc.

Soil Sci., ISSS, Hamburg, Vol. V, pp. 20-47. Hsiao, T.C. and Bradford, K.J., 1983. Physiological consequences of cellular water deficits. In:

H.M. Taylor, W.R. Jordan and T.R. Sinclair (Editors), Limitations to Efficient Water Use in Crop Production, American Society of Agronomy, Madison, WI, pp. 227-265.

SOIL COMPACTION AND ROOT DEVELOPMENT 119

Huck, M.G., 1970. Variation in taproot elongation rate as influenced by composhion of the soil air. Agron. J., 62:815-818.

Kemper, W.D., Stewart, B.A. and Porter, L.K~, 1971. Effects of compaction ou soil nutrient status. In: K.K. Barnes, W.M. Carleton, H.M. Taylor, R.I. Throckmorton anct G.E. Vanden Berg (Editors). Compaction of Agricultural Soils. Am. Soc. Agric. Eng., St. Joseph, MI, pp. 178-189.

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Quirk, J.P., 1979. The nature ofaggregate stability and implications for mar agement. In: R. Lal and D.J. Greenland (Editors), Soil Physical Properties and Crop Production in the Tropics. Wiley~ Chichester, pp. 57-74.

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Tardieu, F. and Manichon, H., 1986b. Caract6risation en tant que capteu-" d'eau de l'enracine- ment du ma'is en parcelle cultiv6e. II. Une m6thode d'6tude de la rcpartition verticale et horizontale des racines. Agronomie, 6:415-425.

Tardieu, F. and Manichon, H., 1987a. Etat structural, enracinement et alimentation hydrique du ma'/s. I. Mod61isation d'6tats structuraux types de la couche labour6e. Agronomie, 7:123- 131.

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