Effect of soil compaction on root development
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Soil & Fillage Research, 19 ( ! 991 ) 11 ! - 1 ! 9 Elsevier Science Publishers B.V., Amsterdam
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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
(Accepted for publication 9 November 1989)
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.
Soil compaction affects root development through :everal indirect mecha- nisms. The increase in compactness arises as a result of fo~es that act on the soil volume. Some of these forces are the result of 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.
When climax 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 of clay, this structure has a hierarchical nature. Clay platelets are combined into domains or floccules, some of the domains are combined into microaggregates and some of the microaggregates are com- bined into macroaggregates (Dexter, 1988 ). This hierarchical order led Dex- ter ( 1988 ) to define soil structure as "the spatial heterogeneity of 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 Ca 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 14 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
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 to -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 resistan