Soil Science Society of America Journal 67:1213-1219 (2003)
© 2003 Soil Science Society of America
DIVISION S-6—SOIL & WATER MANAGEMENT & CONSERVATION
Compaction and Subsoiling Effects on Corn Growth and Soil Bulk Density
Nidal H. Abu-Hamdeh*
Postal Code 21110, P.O. Box 422, Irbid, Jordan
* Corresponding author (nidal{at}just.edu.jo)
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ABSTRACT
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Many soil physical properties and crop yield are affected by compaction. The effects of compaction of two axle loads resulting from wheel traffic on soil bulk density and crop growth were investigated. Subsoiling as a method to alleviate or reduce effects of soil compaction was evaluated. As a result of compacting loads in 2000, corn (Zea mays L.) yield was reduced by 26.8 and 14.5% in 2000 and 2001. Even two years after compaction, soil bulk density was 1.6 to 6.1% greater than the zero-load from 10- to 50-cm depths by 8- and 19-Mg axle loads. Plants in compacted plots had a greater concentration of roots near the base of the plant compared with the plants in the zero-load plots. Plants in the subsoiled plots had fewer roots concentrated near the base of the plant compared with the plants in the nonsubsoiled plots for each load. Plant height was also significantly reduced by compaction. Compaction affected plant and soil properties, but subsoiling removed the compaction effect and improved soil properties, growth, and yield.
Abbreviations: LCML, lower critical mechanical limit • UCML, upper critical mechanical limit
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INTRODUCTION
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IN RECENT YEARS, heavier and more powerful tractors and machinery have been used on farms throughout the world (Swan et al., 1987). Reasons for this trend include reduced human labor and a corresponding increase in farm size with a need to increase individual operator productivity. However, problems have been noted due to increased loads on the soil surface. Some of the most serious problems resulting from increased machinery size are soil deformation, compaction, and destruction of established soil structure (Hakansson et al., 1988; Hakansson and Petelkau, 1991; Lowery and Schuler, 1991).
Soil compaction is a phenomenon that involves significant interrelationships between most recognized physical and biological properties of soils. Soil compaction is often measured in terms of soil density, water infiltration, or soil air porosity. One of the most frequently used measures of compaction is soil bulk density. Bulk density is found by determining the mass of dry soil that occupies a known volume. Ngunjiri and Siemens (1993) observed an increase in bulk density of the topsoil from 0.96 to 1.31 g cm-3 as a result of the first pass of a tire in a study of effects of multiple wheel traffic passes on soil compaction. Gameda et al. (1987) reported that cumulative high axle loading contributed to an annual increase in soil bulk density.
The degree of soil compaction is dependent on the axle load (Hakansson et al., 1988). Vehicles with high axle loads have reduced crop yield and caused detrimental environmental effects as reported by Hakansson et al. (1988). Champers et al. (1990) reported on a compaction amelioration study that used a tandem axle liquid manure tank with a 6.4-Mg-per-axle load to compact field plots that were subsequently seeded to barley (Hordeum vulgare L.) and alfalfa (Medicago sativa L.) on a clay soil. The compaction treatment significantly decreased crop yield, increased cone penetration resistance, and increased dry bulk density.
Subsoiling is often prescribed to alleviate subsoil compaction. Reeder et al. (1993) evaluated five subsoiler types operated at the 0.3-m depth on a compacted 25-ha field in fall 1990, and measured soil physical properties and yields during the subsequent two growing seasons. Air porosity and cone penetration measurements showed continuing benefits, from all types of subsoilers, 2 yr after tillage in areas not trafficked. However, two passes of a tractor recompacted the soil to a density greater than before subsoiling. Precision traffic is, therefore, considered essential to obtain long-term benefits from subsoiling. Erbach et al. (1992) evaluated the effect of four tillage treatments—no tillage, chisel plow, moldboard plow, and paraplow systems—on three soils (poorly drained, medium, and fine textured) in Iowa. Results showed that all tillage tools reduced bulk density and penetration resistance to the depth of tillage. However, after planting, only the soil tilled with the paraplow remained less dense than before tillage.
The effect of compaction on crop yields is the primary concern behind much compaction research. Lowery and Schuler (1991) conducted a study on two silt loam soils in Wisconsin to investigate the effect of compaction on corn yield. The axle loads applied were 8 and 12.5 Mg. Either chisel plowing or moldboard plowing was used after the application of loading treatments. Corn yield was significantly reduced in the first year for both soils and in the second year following loading for one soil but not for the other soil. Carter and Tavernetti (1968) observed cotton yield decreased from 1.78 to 0.6 bales ha-1 when soil bulk density increased from 1.48 to 1.63 g cm-3.
Optimum crop yields are dependent on optimum root growth, and when soil is in good condition, root systems are large, deep, and expansive (Trouse, 1977). Roots are capable of reaching depths of 180 cm in less than a month, and spreading laterally more than 100 cm. Dominant roots of most plants are able to elongate rapidly for many days and to develop branch roots that supply the plants with the nutrients and moisture they demand. Bulk densities associated with decreased root growth or crop yield have been measured for several crops. Decreased root penetration by cotton was associated with an increase in soil bulk density to 1.65 g cm-3 (Taylor and Gardner, 1963). Reeves et al. (1984) found that spring wheat in Australia grown in soil with a bulk density of 1.52 g cm-3 in the 0- to 20-cm depth had less root growth than that grown in soil with a bulk density of 1.32 g cm-3. Laboski et al. (1998) conducted field experiments to determine if soil strength and/or available water could be the factors limiting corn rooting depth on an irrigated fine sandy soil. They found that a compacted soil layer confined roots almost entirely to the top 0.60 m of soil because it had high soil strength and bulk density and the compacted layer, in turn, retained more water for crop use. Boone et al. (1986) defined the lower critical mechanical limit (LCML) as the soil strength where root growth was reduced to 50% of unimpeded growth. The upper critical mechanical limit (UCML) was the soil strength where root growth ceased or was completely a function of mechanical resistance. They determined that the UCML for corn was 3.0 MPa and 1.5 MPa for the LCML. These numerical limits were developed for homogeneous soils, where soil texture or organic matter show no significant variation with depth.
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Objectives
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This research was started in the fall of 1999 on a clay loam soil in northern Jordan, with the following objectives: (i) Investigate the effect of soil compaction resulting from wheel traffic with 8- and 19-Mg axle loads and subsequent subsoiling on soil bulk density, corn yield, root density, and plant height, and (ii) Evaluate subsoiling as a method to alleviate or reduce effects of soil compaction.
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MATERIALS AND METHODS
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The experiment was initiated in 1999 at a farm located in the northern part of Jordan. The soil is a clay loam soil classified as fine, mixed, mesic Typic Haplustox under the USDA Soil Taxonomy classification system. The clay loam contains 200 g sand kg-1, 390 g silt kg-1, 410 g clay kg-1. The experiment was arranged in 15 plots representing five replications for three compaction treatments (0-, 8-, and 19-Mg axle loads). The plots were randomly selected for the treatments and each plot was 9 m wide and 15 m long. All plots received primary tillage by chisel plowing in the spring of 1999 (more than a year before applying the traffic treatments) to a depth of 
15 cm. This was the only tillage done before the traffic treatments.
In this experiment, application of anhydrous ammonia using a six-row applicator was the first operation in the spring. Compaction treatments were then applied on 1 Mar. 2000. Compaction was applied with a two-axle, four-wheel drive off-road truck, which had single tires on both axles, to simulate combines and other heavy machines. Tires (0.66 by 0.64 m) were bias-ply and inflated to 300 kPa. The 19-Mg-per-axle treatment was obtained by loading the truck with gravel until a load of 19 Mg is exerted by each axle. The first level of compaction was imposed to prespecified plots by the loaded truck. The second compaction level (8 Mg per axle) was then imposed by the empty truck to the specified plots. The two levels of compaction were imposed before planting in 2000. The zero-load plots were not compacted by any level of axle load. All compactive loads were applied such that the entire area of each plot (8 and 19 Mg) was covered completely with wheel tracks twice. Following compaction, one half of each plot was subsoiled parallel to the rows before planting in 2000 using a paraplow to a depth of 45 cm. All nonsubsoiled plots received primary tillage by chisel plowing to a depth of 15 cm. The six treatments investigated consisted of (i) zero-load, nonsubsoiled; (ii) zero-load, subsoiled; (iii) 8-Mg load, compacted and nonsubsoiled; (iv) 8-Mg load, compacted and subsoiled; (v) 19-Mg load, compacted and nonsubsoiled, and (vi) a 19-Mg load, compacted and subsoiled. After that, herbicides were applied and incorporated with a 4.7-m wide field cultivator before planting. Incorporated herbicides were atrazine (1-Chloro-3-ethylamino-5-isopropylamino-2,4,6-triazine) at 2.8 kg a.i. ha-1 and alachlor (Methoxymethyl-2',6'-diethylanilide chloroacetate) at 3.36 kg a.i. ha-1. Next, corn was planted on 4 March and 5 March in 2000 and 2001, respectively, in rows spaced 0.76 m apart and a seeding rate of 105 kg ha-1. Conventional farming practices were then performed. All tillage and planting operations were performed by an 80-kW two-wheel drive KUBOTA M8030 tractor weighing 5 Mg (front tires were 14.9R30 set to recommended level of 190 kPa and rear tires were 18.4R46 bias-ply set to recommended pressure of 110 kPa). These operations, chisel plowing, field cultivating, and planting were performed such that the tractor wheels operated off the area from which soil and crop measurements were taken.
Crop yields over the subsequent two growing seasons and soil bulk density were measured to evaluate subsoiling as a method to alleviate or reduce effects of soil compaction. Corn harvest was begun at the end of July using a combine equipped with a four-row corn header to determine crop yield. Soil bulk density was measured after harvest in 2001. Soil bulk density was measured using samples obtained by a manually operated sampling tool. These cores were 5 cm in diameter and 5 cm in length and 100 cm3 in volume (Blake and Hartge, 1986). The soil was sampled at five locations in each plot and at 10 depths (5, 10, 15, 20, 25, 30, 35, 40, 45, and 50 cm) in each location. The soil samples were taken randomly in each plot. For each individual treatment of the three compaction treatments, there were 25 sites for soil samples. The data for each treatment were compiled and individual values were averaged for each 10-cm depth increment to a depth of 50 cm. The top two soil cores (top 10 cm) were not included in the analysis because all plots had been spring chisel plowed (except subsoiled plots), which tends to eliminate the effects of compaction near the surface. Some of the soil samples were taken to measure soil water content at specified depths. Soil water content (from the surface to the 50-cm depth) at the time of compaction ranged from 8.4 to 18.3 kg kg-1. Wet bulk density of the soil sample was obtained by weighing the known volume of the core filled with soil and then subtracting the weight of the core itself. The effect of vehicle axle load was evaluated by measuring changes in soil bulk density in comparison with nontrafficked plots. After the moisture content evaluation, dry bulk density was calculated using the equations by Hillel (1982).
In both growing seasons, root density and distribution and plant height were monitored and evaluated. Growth rate was assessed by measuring the height of corn plants at various times during the growing season. Height was first measured 30 d after emergence; thereafter, every 20 d until it was evident that all vegetative growth had stopped. Root density was assessed using the procedure presented by Ngunjiri and Siemens (1993). A hole was dug to expose the plant roots to a depth of 50 cm and a width of 50 cm centered across a plant row. A 2- by 2-cm grid was placed on the exposed surface and a root index recorded for each square of the grid. Root indices ranged from a low of 0 for squares with no roots to a high of 8 for squares with numerous roots (Fig. 1). The indices shown in Fig. 1 were used to estimate root density using the relationship presented by Ngunjiri and Siemens (1993). The density of roots in the 0- to 50-cm soil layer reported here is the sum of root densities with depth for a given distance from the row.
Statistical analysis was performed on the experimental data using the statistical analysis software, MINITAB (1994). Statistically significant differences are reported for the 0.05 probability level in case of plant height and for the 0.1 probability level in case of root density, crop yield, and soil density.
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RESULTS AND DISCUSSION
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Plant Height and Root Density
Compaction and subsoiling were found to significantly influence plant height (Table 1). The 8- and 19-Mg axle loads significantly reduced corn height in 2000 and 2001. Thirty days after emergence in 2000, average plant height was decreased with increasing levels of compaction. At 50, 70, and 95 d after emergence in 2000, the height continued to be shorter in 19-Mg load plots, between 6 to 19 cm, compared with plant height in the 8-Mg axle load and zero-load treatments, respectively. In 2000, the 8-Mg axle load reduced the final height of corn plants by 6% while the 19-Mg axle load reduced the final height by 10% compared with plant height in zero-load plots. Most of the growth reduction occurred in the earlier vegetative period. The data in Table 1 show that the greatest reductions in plant growth, as measured by height, from compaction occur early in the growing season. While there was less of a difference in height at the end of the season between compacted and uncompacted plots, the difference was still significant. Compaction may have restricted water infiltration and limited gas diffusion and plant development.
In 2001, compaction appeared to have less of an effect compared with 2000. The percentage decreases in the final height of corn plants compared with the zero-load plot were 5 and 6% for the 8- and 19-Mg loads in 2001, respectively. This agrees with Erbach et al. (1988) who found that compaction due to tractor traffic reduced corn height by 6%. Lowery and Schuler (1991) found that plant heights were reduced 13 and 26% on 8 and 12.5-Mg-per-axle compaction treatments, respectively, compared with the control.
Subsoiling in all cases increased plant height in all treatments (0-, 8-, and 19-Mg loads). The greatest height benefit of subsoiling occurred in 2001 (Table 1). In general, the corn heights in the compacted and subsoiled plots were all less than in the zero-load and nonsubsoiled. The plant final heights of the subsoiled 19-Mg treatments were significantly less than the zero-load and nonsubsoiled plot: 2.7 and 3.8% in 2000 and 2001, respectively. Final height in the subsoiled plots of the 8-Mg load was not significantly less than plant final height in the zero-load, nonsubsoiled in 2000, while it was significantly less than plant final height in the zero-load, nonsubsoiled plot in 2001. Generally speaking, subsoiling improved plant height in all plots. Heights in the compacted and subsoiled plots remained significantly less than the plant heights in the zero-load and nonsubsoiled, but they were significantly greater than the plant heights in the compacted and nonsubsoiled plots. It appears that subsoiling nearly obliterated the effect of the axle load on soil strength in the top 50-cm layer and resulted in improved plant development and growth.
Mapping the root density showed that the 19-Mg load treatment significantly restricted root penetration between the rows the most as resulted from the ANOVA procedure (P value = 0.09, F = 2.53). Plants in the 19- and 8-Mg load treatments had a greater concentration of the roots near the base of the plant compared with the plant in the zero-load treatment (Fig. 2). From the same figures it can be seen that the roots were more widely spread between rows in the zero-load treatment than in the 8- and 19-Mg load treatments. This agrees with Ngunjiri and Siemens (1993), who found that soil compaction caused by 8.5 Mg per axle restricted plant root distribution. Plant root growth is sensitive to soil compaction. It materially inhibits or diverts root growth from normal patterns (Gaultney et al., 1980). Trouse (1971) observed that even under reduced compaction conditions roots elongate more slowly with resulting slower plant development. The effect of subsoiling on root density in all treatments (0-, 8-, and 19-Mg loads) was evaluated using the ANOVA procedure. Subsoiling in all cases significantly increased root density at a given distance from the row in all treatments (0-, 8-, and 19-Mg loads). The roots between the rows in the compacted and subsoiled plots had less root density at a given distance from the row than in the zero-load, nonsubsoiled plot but had greater root density at a given distance from the row than in the equivalent compacted and nonsubsoiled plots. In general, subsoiling improved root density between the rows in all plots. It seems that the subsoiling eased root penetration in the top 50-cm layer due to its ability to loosen the soil in the top 50-cm layer.
Crop Yields
As shown in Fig. 3, the yield reductions caused by compaction decreased with time, perhaps because of drying/wetting processes or what is called natural recovery (Al-Adawi, 1995). The 8- and 19-Mg loads significantly reduced corn yields in 2000 and 2001 (Fig. 3). Lowery and Schuler (1991) found that corn yield was reduced significantly with increasing levels of compaction from 8 to 12.5 Mg per axle. In their field experiments conducted to evaluate the effects of vehicle axle load resulting from wheeled traffic on soil tilled in the fall, Abu-Hamdeh and Al-Widyan (2000) found that barley yields decreased with increasing vehicle axle load. This suggests that permanent traffic lanes (controlled traffic) is a possible solution to restrict the effect of high axle loads.
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Fig. 3. Effects of soil compaction and subsoiling on corn yield. Letters a–e indicate the statistical difference within a year at the 0.1 probability level.
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Compaction caused the greatest yield reduction in 2000, the first growing season after compaction was applied. The 8-Mg load level reduced corn yield by 16.6%. The 19-Mg load level reduced corn yields by 26.8%. Compaction may have restricted water movement and reduced infiltration rate resulting in water standing on the soil surface which may have limited gas diffusion and plant development (Carpenter et al., 1985). In 2001, two growing seasons after compaction was applied, compaction appeared to have less effect (percentage reductions were 9.9 and 14.5% for the 8- and 19-Mg loads, respectively) compared with its effect in 2000. The 8-Mg load reduced average corn yield by 13% for 2000 to 2001 and the 19-Mg load reduced average corn yield by 20.4% for 2000 to 2001. Total cumulative corn yield losses (2000 to 2001) caused by the 8-Mg load were 2529.6 kg ha-1, while the 19-Mg load yield losses were 3939.3 kg ha-1. Both the 8- and 19-Mg loads reduced yields significantly in 2001, two growing seasons after the last compaction was applied.
Subsoiling in all cases increased yields of all treatments (0-, 8-, and 19-Mg loads). The greatest yield benefit of subsoiling occurred in the first growing season after subsoiling, in 2000 (Fig. 3). In general, the corn yields for the compacted and subsoiled plots were all lower than the zero-load, nonsubsoiled plots (Fig. 3). The yields of all subsoiled 19-Mg treatments were significantly less than the zero-load, nonsubsoiled plots: 16 and 6.8% in 2000 and 2001, respectively. Yields of subsoiled plots of the 8-Mg load were significantly less than the zero-load, nonsubsoiled plots in 2000 and were numerically, but not significantly, lower than the zero-load, nonsubsoiled plots in 2001.
Subsoiling of the zero-load plots in 2000 gave a 2-yr (2000 to 2001) total yield increase of 1149 kg ha-1 compared with zero-load plots which were not subsoiled. Comparable total increases for the 8-Mg plots were 1380 kg ha-1. For the 19-Mg plots, corn yields were raised by 1729 kg ha-1. Generally speaking, subsoiling improved yield of all plots. Yields of the compacted and subsoiled plots remained less than the yield of the zero-load, nonsubsoiled plots.
Soil Bulk Density
Measured soil bulk density values were averaged within the depth ranges of 10 to 20, 20 to 30, 30 to 40, and 40 to 50 cm for evaluation of axle load effects. Effects of the compaction loads on bulk density are shown in Fig. 4. The percentage increases in bulk density caused by compaction were calculated for all depths. The 19-Mg load increased dry bulk density significantly to the 50-cm deep (Fig. 4). Deeper sampling may have found compaction deeper in the soil profile. The 8-Mg load significantly increased dry density to 40 cm (Fig. 4). Dry bulk density, on average, was increased by a greater percentage with the 19-Mg load (5%) compared with the 8-Mg load (3.5%) in the 10- to 50-cm depth range. Abu-Hamdeh et al. (2000) found that soil bulk density in the tillage zone and in lower zones was increased with increasing the axle load on the soil surface.
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Fig. 4. Effects of soil compaction and subsoiling on soil bulk density. Letters a–f indicate the statistical difference within a depth range at the 0.1 probability level.
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Generally, the compaction effect on soil bulk density existed two growing seasons after the last compaction. The 19-Mg load had more effect on soil bulk density and to a greater depth than the 8-Mg load. On the basis of the 2001 yields, the effects of the 8- and 19-Mg loads on soil bulk density were significantly restricting plant growth and development (Fig. 3). Increasing axle wheel loads resulted in an increase in shear and vertical soil stresses that led to closer packing of soil particles and then increased the soil bulk density (Abu-Hamdeh and Al-Widyan, 2000). This agrees with Al-Adawi (1995), who found that soil bulk density was increased at the subsurface depths for increasing levels of compaction.
Subsoiling improved soil bulk density of the compacted and zero-load plots (Fig. 4). Generally, subsoiling improved soil bulk density to 40 cm deep. In most cases, bulk density values of the compacted and subsoiled plots were greater, but not always significantly, than those of the zero-load, nonsubsoiled plots. In general, Fig. 4 indicates that soil bulk density of the compacted and subsoiled plots were worse than those of the zero-load, nonsubsoiled plots. The bulk density of the 8- and 19-Mg loads of the subsoiled plots was significantly greater than the zero-load subsoiled plots at all depths. Subsoiling reduced soil density of the zero-load plots by an average of 1.5% (Fig. 4). Subsoiling significantly decreased bulk density by an average of 2.3 and 2.6% for the 8- and 19-Mg plots, respectively. This indicates that subsoiling improved the 19-Mg plots more than the 8-Mg plots. It can be seen that the subsoiling in most cases returned the soil bulk density of the compacted plots close to their original conditions.
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CONCLUSIONS
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On the basis of the results of the experiment which was conducted on clay loam soil, the following can be concluded: (i) Compaction (before planting in 2000) with 8- and 19-Mg axle loads significantly reduced corn yield in 2000 and 2001; (ii) Subsoiling improved yields of the compacted plots. Those yields were greater than yields of the equivalent compacted and nonsubsoiled plots; (iii) Subsoiling significantly increased yields, reduced soil bulk density, and improved root density and plant height of the zero-load plots; (iv) Soil bulk density, root density, and plant height of the zero-load, subsoiled plots were better than those of the compacted, subsoiled plots; (v) Soil bulk densities measured in 2001 were still affected by the compaction treatments. The 19-Mg load affected soil bulk density to a greater depth and a greater percentage than the 8-Mg load. This research is continuing for another two or three years. Information about reconsolidation, subsoiling, and root density and distribution will be forthcoming.
Received for publication April 8, 2002.
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ABSTRACT
INTRODUCTION
