In this paper soil compaction is described in relation to soil physical factors, root growth, and nutrient uptake by crop plants; rice growth and yield. In compaction, soil solids are rearranged with compression of liquid and gaseous phases accompanied by volume change. Soil compaction affects water retention characteristics, water intake rates, and gaseous exchange. In compacted soil, bulk density, microvoids, thermal conductivity, and nutrient mobility increase and macrovoids, hydraulic conductivity, and water intake rates decreases. Medium textured soils are most susceptible to compaction. Plants response in relation to root growth and nutrient uptake varies depending on particular stage of development under a particular environment.
Soil compaction, which cuts down percolation losses and reduces the water requirements of rice, appears to be a more practical and economical tillage practice than puddling for increasing rice growth and yield and water use efficiency. Compaction as a tillage practice is simpler, consumes less energy, is easily designed, and shows possibility of mechanization of wetland rice cultivation by replacing messy and difficult operation of puddling, especially on medium textured soils.
The compaction of soil can be defined as an increase in its dry density,and closer packing of solid particles or reduction in porosity (McKyes,1985).In other words, soil compaction is a dynamic soil behavior by which the state of compaction is increased (Gill and Vanden Berg, 1967). Soehne (1958 ) determined that for arable soils compaction could be described by the following relationships: n = – A ln P + C Where, n = porosity, C = porosity obtained by compacting loose soil at a pressure of 10 psi, A = slope of the respective plotted curves, and P = applied pressure
Compaction can result from natural causes including rainfall impact, soaking, internal water tension and the like. Compaction of agricultural soils below the cultivated layer commonly results from the passage of vehicular traffic (Soane et al., 1982).
In agricultural fields compaction is produced more often in the surface layer and in the layer below the plow layer. Medium textured soils are most susceptible to compaction. Compaction causes a rearrangement of the soil particles and many properties of the soil are influenced as a result. Pore size distribution is altered, total porosity is decreased, and there are changes in the movement and content of heat, air, water and nutrients in the soil (Grable and Siemer, 1968; Kemper et al., 1971; Tackett and Pearson, 1964a and 1964b; Warkentin, 1971; Wills and Raney, 1971). The restricted growth of roots commonly observed in compacted soil has been variously attributed to all these properties, and to the high mechanical resistance which compacted soil presents to plant roots (Taylor and Gardner, 1963; Taylor and Ratliff, 1969).
Compaction of soil decreases the number of large pores i.e. those similar in size to the diameter of plant roots, and since roots can not enter pores which have a smaller diameter than themselves (Goss, 1977 and Wiersum, 1962) they must exert force to deform or displace the soil particles (Whiteley and Dexter, 1981). The ability of roots to overcome the mechanical resistance of the soil varies with species (Taylor and Gardner, 1968) but how the differences in ability arise is not yet clear. The differences may be related to the diameter of the roots ( Barley, 1953; Phillips and Brown, 1965 ; Whiteley and Dexter, 1981) although data on this and the maximum forces which the roots of different species can exert are scarce (Goss,1977).
The effects of compaction on nutrient uptake by roots have recieved much less attention than effects on growth itself. While compaction might be expected to increase the movement of ions to roots by diffusion (Kemper et al., 1971; Phillips and Brown, 1965), restricted growth of roots generally results in smaller amounts of nutrients being absorbed from compacted than uncompacted soil (Boone and Veen, 1982; Weirsum, 1957). However, where only part of the root system or particular root members ( e.g. axis ) are subjected to unfavourable conditions, the growth of the other parts of the root system or other root members ( e.g.lateral roots) may be enhanced ( Crosett et al., 1975; Russell and Goss, 1974). Hence, compaction of the soil need not necessarily result in a decrease in the total amount of nutrients absorbed, although the location from which the nutrient derived may vary.
The use of harrow and rollers for seedbed compaction and for pressing seedlings into the soil are historical examples of the understanding that looseness in a soil can be adverse for plant growth. In Bangladesh, puddling is an essential practice for transplanted rice culture, and is widely practiced even though it is high energy consuming and detrimental to soil health. The difficulty of moving heavy machinery on a soft puddle seriously limits the mechanization of rice cultivation and continues to be a challenging problem.
Studies of the physical requirements of the soil (Ghildyal, 1969) for rice growth show that major needs of rice plants can be met with a simple tillage practice of soil compaction. An engineer can easily design the compaction tillage system for a given soil type and thus eliminate puddling. The simpler practice of compaction indicates a possibility for the mechanization of rice cultivation. The state of compaction of a soil largely determines the physical and related chemical soil conditions that control the responses of crop plants required for effective farming and that control the conservation of soil and water needed for a permanent agriculture. The state of compaction of soil largely influences seed germination, seedling emergence, root growth and in fact all phases of crop production.
Under many situations organic matter in the soil which helps resist compaction tends to become depleted. Further, many of the new farming practices introduced to implement our intensified and so-called more scientific and progressive agriculture tend to promote continuous farming with a single crop. In many instances these crops are subjected to intensive cultural practices which contribute to the problem of soil compaction. The increasingly rapid spread of irrigation and recent rapid increase in the mechanization of field operations are likely to create compaction problems. Hence, an attempt is made in this paper to explore soil compaction with the following objectives — 1. To understand the mechanism of soil compaction and physical changes due to soil compaction. 2. To explore the effects of soil compaction on crop root growth. 3. To evaluate the effects of soil compaction on plant nutrition. 4. To explore the scope of replacing puddling with compaction for transplanted rice culture.
Review of findings
1.Mechanism of soil compaction
Compaction is the increase in soil density caused by dynamic loading. During compaction soil particles move to a closer state of contact and bulk density, porosity, etc. change. The degree of compaction depends on soil moisture, compactive energy, nature of the soil and amount of manipulation. On dynamic loading, the soil solids are rearranged and liquid and gas within the soil pores are compressed. As solids and water are incompressible, the state of compaction depends on their movement and reorientation of soil particles as given by the following relationship (Harris, 1971) e = eo – C log P/Po where, e = void ratio, eo = void ratio at initial pressure Po, C = slope of the curve on semi logarithmic plot, and P = applied pressure. The relationship between the change in void volume and applied load is exponential, not linear.
The particle size distribution will affect the change in void volume because of the rearrangement of soil particles. During compaction soil behaves like a plastic material because of the orientation of soil particles, but the volume change due to deformation of individual particles is elastic in nature. A definite relationship exists between soil moisture and the bulk density to which a soil can be compacted. When soil density is plotted, a smooth hyperbolic curve is obtained (Ghildyal, 1978).
As a soil becomes finer in texture the maximum density tends to decrease and occur at higher moisture content (Kemper et al., 1971). The medium textured soils are therefore more susceptible to compaction. 2. Physical changes due to soil compaction
2a. Hydraulic properties — Water retention and transmission
Both water retention and transmission are affected as compaction changes void size distribution and fabric geometry. Compaction alters the water content and movement in soils by modifying the void size distribution. Large macropores in the soil fabric are the first to be reduced in volume. The largest voids being affected most, compaction changes the amount of water held at any suction (Warkentin, 1971).
In compacted soils the water retained at low suction and saturation decreases but more water retained at high suction. Thus, compaction increases plant available water in a soil. But as non -capillary pore-space conducts most of the water, compaction, by decreasing the large voids, decreases the hydraulic conductivity of the soil. The reduction in hydraulic conductivity of soils is generally logarithmic with changes in soil dry density, or with void ratio. Some of the ranges of observed alterations in clay and loam soils are given in Table 1.
Table 1: Observed changes in saturated hydraulic conductivity at different soil dry densities Soil typeRange of dry density
(tm-3)Range of saturated hydraulic conductivity (10-6cm s-1)Source Beach sand1.40-1.6655,000-20,0001
Dam filter sand1.66-1.8313,000-7,0002
Yolo silt loam1.20-1.503,000-304
North Carolina silt1.43-1.6660-0.6
When a lateritic sandy clay loam soil was compacted from a bulk density of 1.53 g cm-3, to 1.83 g cm-3, the capillary porespace decreased from 23.9 % to 5.9 %, and the hydraulic conductivity decreased from 99 x 10-4 cm/second to 7.0 x 10-4 cm/second ( Ghildyal and Satyanarayana, 1965 ). 2b. Gas
Gas exchange within the soil and between the soil and atmosphere is primarily a function of air – filled pores, or aeration porosity. By destroying air – filled pore spaces, soil compaction limits gas transfer in a soil. With an increase in bulk density from 1.4 to 1.8 g cm-3, the aeration decrease from 10.2 to 3.8 %. The oxygen diffusion rate was reduced from 16.2 x 10-8 cm-2 per mm to 5.23 x 10-8 g cm-2 per mm as the bulk density increased from 1.4 to 1.8 g cm-3, as measured by the platinum microelectrode (Varade and Ghildyal, 1967). Compaction may thus alter the gas compaction of soil if porosities are reduced sufficiently. 2c. Soil Temperature
Soil temperature is important in determining the energetics of soil-plant interactions. Compaction affects the thermal characteristics of soil by altering the fabric geometry, soil water relations, etc. At a given soil moisture content and as bulk density increases, thermal conductivity increases ( Ghildyal and Tripathi, 1971 ). 2d. Soil Strength
In the dynamic sense soil compaction is a physical deformation ( volumetric strain ) ; in the static sense it is the soil characteristics related to resistance to increase in bulk density ( Chancellor, 1971 ). At a given moisture content the strength of the soil increases with compaction (Kumar et al., 1971 )
For a given density, soil strength decreases with increasing moisture content and increases with decreasing moisture content. 3. Effect of Soil Compaction on Root Growth
Mechanical impedance is one of the most common limitations to root growth. Roots will grow through continuous pores with a diameter bigger than themselves (about 300 – 500 μm), but as these are usually not enough big pores roots must also displace soil particles. They can only do this if the pressure exerted by the elongating cells at the root tip can overcome the resistance of the soil matrix to compression. This resistance depends on the composition of soil and its bulk density, and becomes greater as the soil
Bulk density values of 1.55 – 1.85 g cm-3, depending on particle size distribution, may reduce root growth. These values are only a rough guide to the restriction of root growth, first because of the effect of soil water content on compressibility, and secondly because roots may grow through cracks and pores even in soils of high bulk density (Wild, 1993).
Taylor et al. (1966) measured the number of taproots of the cotton plant which penetrated compacted layers of different soils, and characterized the degrees of compaction by means of measurements with a cone penetrometer. The number of roots penetrating the soil was reduced drastically as the penetration resistance approached 2 MPa pressure. In fact, at soils compacted to more than 2 MPa resistance, virtually no roots at all were able to grow (Taylor et al., 1966).
Taylor et al. (1967) found a curvilinear decrease in cotton taproot elongation rates as soil strength increased . Sugarcane ( Monteith and Banath, 1965 ), garden peas and wheat (Barley et al., 1965), tobacco (DeRoo, 1960), oats ( Schuurman, 1965 ), alfalfa, sugarbeets, tomatoes ( Scott and Erickson, 1964 ) follow the same general pattern of decrease in root elongation rate associated with increase in soil strength. 4. Effect of Compaction on Plant Nutrition
As the mineral phase is incompressible, compaction merely implies a decrease in that part of the soil occupied by air and water. As the process of soil compaction tends primarily to the collapse of the large pores, that is, those pores responsible for effective drainage and aeration, the net effect of compaction is to increase soil moisture at the expense of soil air. 4a. Beneficial effects of Compaction on Plant Nutrition
Soil compaction increases the mobility of nutrient ions to roots by both diffusion and mass flow. Compaction increases the number of platelets per tactoid, thereby increasing the amount of exchangeable ions (Blackmore and Miller, 1961).
Better particle to particle contact in a compacted soil tends to increase the average inter-diffusion of ions. An increase in bulk density from 1.1 to 1.6 g cm-3 may increase the inter-diffusion rate by 50 %. As compaction increased the volumetric water content by 10 %, the diffusion coefficient of phosphorus doubled ( Olsen et al., 1965 ).
Compaction increases the amount of soil per unit volume, thereby increasing the amount of phosphorus available at a given chemical potential of phosphorus. Moreover, due to increase in microvoids, the unsturated hydraulic conductivity at suctions greater than field capacity can be doubled by increasing density from 1.1 to 1.5 g cm-3. With conductivity doubled and suction gradients constant, the nutrients in the solution move twice as fast in the compacted soil. With a very fertile soil provided adequate water is available soil volume is not critical for maximum growth.
Therefore, even though compaction reduces root growth it does not interfere with the nutrition of the plant. Phosphorus uptake by maize showed no significant ( p < 0.05 ) effect of bulk density ( Fig. 9 ) but in ryegrass maximum uptake of phosphorus per unit length of root occured at intermediate bulk densities ( 1.35 – 1.45 g cm-3 ) ( Shierlaw and Alston, 1984 ). 4b. Adverse effects of Soil Compaction on Plant Nutrition
Diffusion coefficients at high soil solution contents increase initially with compaction, due to the production of a more continuous aqueous system in the soil pores, and hence a less tortuous path for diffusing ions, increasing compaction however, eventually reduces diffusion because of the increasing obstructional effects of solid soil particles (Graham – Bryce, 1965; Phillips and Brown, 1965). As calcium and agronomically much more important nitrate are taken up by the plant by the mass flow route, any check on this movement will reduce their uptake.
Thus, as the water held in small pores is not available to the plant any nitrate and calcium it contains will also be unavailable. Compaction of a low fertility soil but of good structure would, however, by mechanical impedence of root development reduce the amount of soil exploitable by the roots and thus reduce growth and development of plants grown on the soil.
Unfortunately, most researchers have not in fact studied the effect of compaction on crop nutrition. In many cases the plants were stunted because factors other than nutrients limited the growth. Flocker et al. ( 1959 ) showed, however that soil compaction as measured by changes in bulk density (BD) reduced phosphate uptake of tomato plants grown on a Yolo sandy loam and a Hispera sandy loam ( Table 2 ).
Field experiments have usually shown no effect on nutrient uptake; change in oxygen diffusion rates and in hydraulic conductivity have usually been considered as the principal factors affecting yields ( Rosenburg and Willets, 1962 ). Possibly the most informative field experiment, carried out by Phillips and Kirkham (1962) showed that some reduction in phosphorus and potassium uptake occured with increasing compaction (Table 3).
Table 3 : Effect of level of compaction on leaf composition and yield of maize after the second year of growth on non fertilized plots ( Phillips and Kirkham, 1962 ) Soil type and bulk densityFresh weights of plants at specified rate of P2O5 application per acre
Yolo fine sandy loam:(1)(lb)(lb)(lb)(lb)
Hesperia sandy loam:(2)
(1)LSD (2)LSD Density (0.01) 3.8 Density (0.01) 5.6 Rate (0.01) 7.4 Rate (0.01) 6.7 Interaction** Interaction ns
In soils of low fertility with the available nutrients homogeneously distributed, even a slight reduction in root exploration of the profile will by reducing the uptake of immobile nutrients adversely affect plant growth. As the value of soil needed for maximum plant growth is not critical, provided adequate moisture and aeration and nutrients are available, then on highly fertile soils any effect of compaction on yields will be due primarily to moisture and aeration effects.
As we move from sands to clays we move from soils in which water movement is extremely rapid and root development is physically impeded, to soils which present problems of internal drainage and in which root development may be seriously restricted. 5. Soil Compaction for Rice Culture
With wetland cultivation of flooded rice, repeated plowing for intensive tillage of the saturated and flooded soil creates a soft puddle. Puddling is an essential practice for transplanted rice culture, and is widely practiced even though it is high energy consuming. Energy requirement experiments at Pantnagar (India) showed that tractor puddling requires 300 hp ha-1, and bullock power may require 100 hp ha-1. The difficulty in moving heavy machinery on a soft puddle seriously limits the mechanization of rice cultivation and continuous to be a challenging problem.
Studies of the physical requirements of the soil (Ghildyal, 1969) for rice growth showed that major needs of rice plants can be met with a simple tillage practice of soil compaction. In this system the soil is first plowed and harrowed, fertilized, and brought to an optimum moisture content. Heavy rollers or tractors with loaded wheels are passed repeatedly over the soil until desired density is achieved. The soil is then flooded and the crop is transplanted. Or, rice seeds can also be drilled directly into the compacted soil with a suitable drill. An engineer can easily design the compaction tillage system for a given soil type and thus eliminate puddling. The simpler practice of compaction indicates a possibility for the mechanization of rice cultivation.
Summary and Conclusions
A change in the state of compaction of a soil results from a change in the volume of the soil. A change in volume is caused by forces that may originate either from mechanical sources such as machines, or from natural sources such as drying and wetting.
When a soil is subjected to an applied load that is sufficient to cause a volume change, there are four possible factors to which the change could be attributed : 1. A compression of the solid particles.
2. A compression of the liquid and gas within the pore spaces. 3. A change in the liquid and gas contents in the pore spaces. 4. A rearrangement of the soil particles. Since the solid and liquid phases are relatively incompressible and do not undergo appreciable volume change under loads usually applied to the soil mass, the change in state of compaction depends on movement of either the liquid or the solid phase, or both. The change in state of compaction resulting from rearrangement of the particles is due primarily to a change in the volume of the voids. The following empirical relationship has been determined from uniaxial compression tests in the general field of soil mechanics: e = eo – C log P/Po where, e = void ratio, eo = void ratio at the initial pressure Po, C = slope of the curve on semilogarithmic plot, and P = applied pressure. The relationship between the change in void volume and applied load is exponential, not linear.
Compaction changes water content and transmission by altering the void size distribution of soils; causes an increase in density with a resultant increase in thermal diffusivity. When a soil is compressed, root proliferation is usually inadequate to meet the requirements of a plant, although some roots are capable of developing within the soil.
Soil compaction can affect the nutrient status of soil and nutrient uptake by plant in both detrimental and beneficial ways.
Compaction increases the rates at which most nutrients move to the roots by diffusion and mass flow. On the other hand, soil compaction results in a decrease in the amounts of nutrients mineralized from organic matter.
The effects of compaction on the facility with which a crop can obtain nutrients from the soil will not necessarily be adverse or beneficial, but will vary depending on fertility level, with root distribution and with the moisture and aeration regimes.
Therefore, any particular state of soil compaction will not produce a particular plant response except for a particular plant at a particular stage of development under a particular environment.
Soil compaction, which cuts percolation losses and reduces the water requirements of rice, appears to be a more practical and economical tillage practice than puddling for increasing rice yield and water use efficiency. Compaction as a tillage practice is simpler, consumes less energy, is easily designed, and shows the possibility of mechanization of wetland rice cultivation by replacing high energy consuming operation of puddling, especially on medium textured soils.