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Published online 7 November 2007
Published in Crop Sci 47:2495-2503 (2007)
© 2007 Crop Science Society of America
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SEED PHYSIOLOGY, PRODUCTION & TECHNOLOGY

Soil Compaction and Soybean Seedling Emergence

Jessica Hyatt, Ole Wendroth, Dennis B. Egli and Dennis M. TeKrony*

Dep. of Plant and Soil Sciences, Univ. of Kentucky, Lexington, KY 40546-0312. Published and approved by the Director of the Kentucky Agricultural Experiment Station as Paper no. 07-06-057

* Corresponding author (dtekrony{at}uky.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Soil compaction can cause nonuniform emergence and poor field stands. The effect of soil compaction on soybean [Glycine max (L.) Merr.] seedling emergence in controlled (greenhouse or growth chamber) environments was evaluated using seed lots from six cultivars with high standard germination but variable seed vigor (accelerated aging test). Soil compaction treatments providing a range of compactive efforts (CE) were imposed after planting in near-cylindrical containers. As compaction increased from low (4.6 kJ m–3 CE) to high (22.9 kJ m–3 CE), emergence declined; however, high-vigor seed consistently emerged better than low-vigor seed. Emergence of high-vigor seed lots remained >80% until compaction increased to 13.7 kJ m–3 CE, while low-vigor seed lots had low emergence (<50%) even at the lowest compaction (4.6 kJ m–3 CE) level. Three seeds per container (spaced 3.81 cm apart) had consistently higher seedling emergence in compacted soil than one seed; however, seed size had no effect on emergence at any level of compaction. Seeds that did not emerge either germinated but remained under the soil crust or were dead (occurred more frequently in low-vigor seed lots). Seedling emergence at 15 or 20°C in the growth chamber was less than at 25°C, and the effect of compaction was enhanced at lower temperatures. Seed lots with high vigor provided adequate seedling emergence following moderate levels of soil compaction (4.6 and 9.2 kJ m–3 CE). Thus, planting high-vigor seed would be advantageous on soils that are susceptible to compaction.

Abbreviations: AA, accelerated aging • CE, compactive efforts

Soil Compaction and Soybean Seedling Emergence

Jessica Hyatt, Ole Wendroth, Dennis B. Egli and Dennis M. TeKrony*

Dep. of Plant and Soil Sciences, Univ. of Kentucky, Lexington, KY 40546-0312. Published and approved by the Director of the Kentucky Agricultural Experiment Station as Paper no. 07-06-057

* Corresponding author (dtekrony{at}uky.edu).

Soil compaction can cause nonuniform emergence and poor field stands. The effect of soil compaction on soybean [Glycine max (L.) Merr.] seedling emergence in controlled (greenhouse or growth chamber) environments was evaluated using seed lots from six cultivars with high standard germination but variable seed vigor (accelerated aging test). Soil compaction treatments providing a range of compactive efforts (CE) were imposed after planting in near-cylindrical containers. As compaction increased from low (4.6 kJ m–3 CE) to high (22.9 kJ m–3 CE), emergence declined; however, high-vigor seed consistently emerged better than low-vigor seed. Emergence of high-vigor seed lots remained >80% until compaction increased to 13.7 kJ m–3 CE, while low-vigor seed lots had low emergence (<50%) even at the lowest compaction (4.6 kJ m–3 CE) level. Three seeds per container (spaced 3.81 cm apart) had consistently higher seedling emergence in compacted soil than one seed; however, seed size had no effect on emergence at any level of compaction. Seeds that did not emerge either germinated but remained under the soil crust or were dead (occurred more frequently in low-vigor seed lots). Seedling emergence at 15 or 20°C in the growth chamber was less than at 25°C, and the effect of compaction was enhanced at lower temperatures. Seed lots with high vigor provided adequate seedling emergence following moderate levels of soil compaction (4.6 and 9.2 kJ m–3 CE). Thus, planting high-vigor seed would be advantageous on soils that are susceptible to compaction.

Abbreviations: AA, accelerated aging • CE, compactive efforts


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
SOIL COMPACTION and crusting has been a prevalent problem in crop establishment and production in many regions of the world dating back to the early nineteenth century (Awadhwal and Thierstein, 1985; Soane and van Ouwerkerk, 1994). Soil compaction is the compression of soil by external forces that decrease the volume of pore space while increasing the soil density (Harris, 1971). Soil compaction and crusting have reduced seedling emergence of various crops (Hanks and Thorp, 1956, 1957; Taylor, 1962; Bennett et al., 1964; Kolp et al., 1967; Radford et al., 2000, 2001; Nabi et al., 2001; Bayhan et al., 2002).

Soil compacted at five pressures ranging from 0 to 0.110 MPa reduced coleoptile length, and the rate and total emergence of wheat (Triticum aestivum L.) seedlings (Kolp et al., 1967). Seedling emergence was reduced as the thickness of a molded wax surface crust increased for wheat, grain sorghum [Sorghum bicolor (L.) Moench], and guar [Cyamopsis tetragonoloba (L.) Taub.] seeds (Taylor, 1962). Hanks and Thorp (1956) found that bulk density was indirectly related to reduced wheat seedling emergence on compacted soils because any change in bulk density affected the O2 diffusion rate and soil crust strength; however, it was difficult to determine which of the two factors was responsible for reduced emergence. Followup quantitative studies determined that crust strength (0–140 kPa) and moisture content (field capacity to 0.25 m–3 m–3) resulted in significant reductions in seedling emergence of wheat, grain sorghum, and soybean (Hanks and Thorp, 1957). At a given crust strength, seedling emergence was the lowest where the soil water content was lowest, while at a constant moisture content, seedling emergence decreased with increasing crust strength.

Species with hypogeal seedling emergence (grain sorghum, wheat, and pea [Pisum sativum L.]), had greater seedling emergence following compaction and crusting than those with epigeal emergence (guar, lupine [Lupinus angustifolius L.], and medic [Medicago polymorha L.] (White and Robson, 1989; Parker and Taylor, 1965). Dicotyledonous species with epigeal emergence may encounter considerable resistance as the large cotyledons are pushed through the soil crust, which prevented emergence or severely damaged the hypocotyls or cotyledons of soybean (Rathore et al.,1981).

Planting seed quality, seedbed conditions, seedling emergence, and planting rate combine to determine plant population, which is often related to final yield. Standard germination is the traditional measure of seed quality; however, in recent years seed vigor has also been used as an indicator of field performance. Egli and TeKrony (1995) evaluated the relationship between measures of seed quality of a large number of soybean seed lots and emergence in 26 field experiments. All tests accurately predicted field performance under ideal soil conditions, but the ability to predict decreased as soil stress increased. Seed vigor tests were generally better predictors than standard germination under less-than-ideal seedbed conditions, but when seedbed conditions deteriorated enough, even seed vigor would not predict performance. Although the specific soil conditions causing stress were not determined in these experiments, soil compaction was thought to have played a role.

The effects of soil compaction and crusting on soybean emergence have been investigated in controlled environments (Hanks and Thorp,1957; Rathore et al., 1981); however, seed quality (germination and vigor) was not included in those evaluations. Thus, the objective of this study was to quantitatively relate the effects of soil compaction and seed vigor on soybean seedling emergence in controlled environments (greenhouse or growth chamber).


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Greenhouse and growth chamber experiments were conducted from May 2004 through September 2005 to evaluate the effect of soil compaction on soybean seedling emergence. The soil used was taken from the surface A horizon of a Maury silt loam (fine, mixed, mesic Typic Paleudalf) at the Spindletop Research Farm near Lexington, KY (38°02' N), air dried, and sieved to a particle size of <2 mm. The sand, silt, and clay contents were 78, 707, and 215 g kg–1, respectively, and the dominating clay mineral was kaolinite.

Compaction was imposed on the surface soil after the seed was planted in near-cylindrical plastic containers (11 [diameter] by 13 cm [height]), with two drainage holes at the bottom of the container (Fig. 1 ). Approximately 500 mL of soil (230 g water kg–1 soil moisture content) was placed in each container and approximately 225 mL of water was added to the soil and allowed to drain for 24 h. One or three seeds were lightly pressed into the soil surface before 300 mL of soil with a water content of 230 g kg–1 was placed on the seeds and the compaction treatment was applied. The planting depth was 5.0 cm at zero compaction and was reduced to 3.8 cm following compaction. Although some spatial variation in compaction may have existed, compaction was established above and to some depth below the seed.


Figure 1
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  		Figure 1. Compaction equipment used in greenhouse and growth chamber experiments: (a) weight used to apply compaction treatments to soil in plastic container; (b) plastic container with metal disk on top of soil.

 
Each experiment was planted on one greenhouse bench in a randomized complete block design with 10 replications (each container was an individual plot [replication]). Following planting, the containers were not watered again and emergence counts were made daily for 10 d. All greenhouse experiments were conducted under a 60% shade cloth to decrease the daily fluctuation in air and soil temperatures and reduce evaporation. Air and soil (depth of planting) temperatures were recorded at hourly intervals for 10 d following planting. Seed lots from six soybean cultivars with acceptable standard germination (79–98%) but a range in seed vigor (AA germination 0–92%) were used in these experiments (Table 1 ). All seed lots were stored at 10°C before use.


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  		Table 1. The characteristics of the experiments relating seed quality and compaction to soybean seedling emergence.

 
The soil was compacted by dropping a 2.5-kg weight on the soil as described by the American Society for Testing and Materials (2000) (Fig. 1). A metal disk (99-mm diameter) that covered the entire soil surface in the container was placed on top of the soil before dropping the weight. The weight was dropped from heights of 15 and 30 cm above the soil and in combinations of drops providing a total of 45 (30 + 15), 60 (30 + 30), or 75 (30 + 30 + 15) cm to provide 0.0, 4.6, 9.2, 13.7, 18.3, and 22.9 kJ m–3 compactive effort (CE), respectively (Diaz-Zorita et al., 2001):

Formula

Formula

This laboratory procedure has previously been shown to be positively correlated to bulk density as measured with the standard Proctor test for nine soils (Diaz-Zorita et al., 2001). By compacting the soil sample from the top, a compactive force acts on the soil surface. Unlike a field experiment where a heavy machine causes compaction primarily from the top, in this laboratory procedure the side walls of the containers imposed a resistance to soil deformation and expansion. Therefore, compaction not only occurred from the surface, but also from the side walls and was probably not uniform with depth.

Soil Measurements
Shear stress of the soil was measured on containers without seeds using a field inspection vane tester (Eijkelkamp Agrisearch Equipment, Giesbeek, the Netherlands) with a 16- by 32-mm vane, with the top of the vane level with the soil surface (American Society for Testing and Materials, 2001). The tester measures the resistance to vane rotation. Six shear stress measurements were made for each CE treatment (0.0, 4.6, 9.2, 13.7, 18.3, 22.9 kJ m–3) at 4, 7, and 10 d after compaction.

The Hydrus-2D simulation model (Simunek et al., 1998, 1999) was used to determine if water or air deficiency were limiting factors during the greenhouse experiments. The model solves the Richards equation for water flow using a finite-element approximation. Water evaporation was measured from all containers of the three-seed-per-container treatment during the seed spacing experiment by weighing the cups at 0800 and 1700 h for 10 d. Soil hydraulic properties (water retention curve and hydraulic conductivity function) were estimated with an inverse algorithm supported by measurements of soil water pressure head with tensiometers. Tensiometers were inserted through the side of the plastic container into the soil just below the seed planting depth in containers compacted at 9.2, 18.3, and 27.5 kJ m–3 CE. Measurements were recorded twice daily until pressure head became less than –700 cm.

Seed Quality Measurements
The standard germination (SG) test (Association of Official Seed Analysts, 2000) (4 x 50 seeds at 20/30°C for 7 d) was conducted 1 to 3 mo before or after planting all experiments. The accelerated aging (AA) vigor test was conducted by aging 42 g of seed over 40 mL of water at 41°C for 72 h (Hampton and TeKrony, 1995), after which SG was determined. A vigor index was used to combine the results of SG and AA germination for each seed lot (TeKrony and Egli, 1977), with an index ≥8.0 classified as high vigor, 6.0 to 7.9 as medium vigor, and <6.0 as low vigor (Table 1).

Seedling Measurements
Emerged seedlings were counted daily for 10 d (15 d for temperature experiments) following planting in all experiments and included normal seedlings at the VE stage (one or two cotyledons above the soil surface, Fehr and Caviness, 1977) and abnormal seedlings (cotyledons either missing or not emerged). The soil above the planting depth was removed after the final count and the number of seedlings that germinated under the soil, but did not emerge, and the number of dead seeds (no indication of radicle emergence from the seed [Association of Official Seed Analysts, 1992]) were determined.

Seed Vigor Experiments
Eight seed lots (Table 1) from four soybean cultivars were planted in the greenhouse to evaluate the effect of soil compaction and seed vigor on soybean seedling emergence. The initial standard germination of the seed lots was ≥89%, but there was a range in seed vigor for the two seed lots for each cultivar. Each container contained three seeds (3.81-cm spacing between seeds) and there were six CE treatments (0.0, 4.6, 9.2, 13.7, 18.3, 22.9 kJ m–3) for each seed lot. The three seeds were planted in a triangular pattern with 3.81 cm between the seeds. The two seed lots of each cultivar were planted on the same day in separate greenhouse experiments from May 2004 through October 2004 (Table 1).

Seed Spacing Experiment
The effect of seed spacing and soil compaction was evaluated in the greenhouse using three high-vigor seed lots (three cultivars) (Table 1). Three seeds (3.81-cm spacing, described above) or one seed were planted before imposing four CE treatments (0.0, 4.6, 9.2,18.3 kJ m–3) in separate experiments for each cultivar from February through April 2005 (Table 1).

Seed Size Experiment
The variation in seed size in three seed lots from three cultivars was determined using round-hole screens ranging from 5.2 to 7.1 mm in diameter at intervals of 0.4 mm and the largest and smallest sizes from each seed lot (with enough seeds for testing) were selected. The small and large seed sizes for each cultivar were: CF 492, 92 and 186 mg seed–1; CF 461, 113 and 183 mg seed–1; and B323, 119 and 214 mg seed–1. The two sizes from CF492 and CF491 were classified as high vigor, while those from B283 were low vigor (Table 1). Each experiment included 20 replications (containers), with one seed being planted per container followed by four CE treatments (0.0, 4.6, 9.2, 18.3 kJ m–3) in a randomized complete block design. The largest and smallest seeds from each cultivar were planted in separate greenhouse experiments during August and September 2005 (Table 1).

Temperature Experiment
High- and low-vigor seed lots of two cultivars were planted on separate dates in growth chambers maintained at 15, 20, and 25°C from November 2004 through January 2005 (Table 1). A split-block design was used, with the three temperatures (growth chambers) as the main blocks and vigor and compaction treatments as sub-blocks arranged in a randomized complete block design with six replications. Three seeds were planted in each container (3.81-cm spacing) and four CE treatments (0.0, 4.6, 9.2, 18.3 kJ m–3) were used.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Soil Measurements
There was a significant quadratic relationship between CE and shear stress measured at 4, 7, and 10 d after compaction (Fig. 2 ). Shear stress at 4 d after compaction was less than at 7 and 10 d. Shear stress continued to increase with increased compaction until it reached a maximum level at 18.9 to 20.3 kJ m –3, which was slightly below the maximum CE.


Figure 2
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  		Figure 2. Relationship between compaction and shear stress measured 4, 7, and 10 d after compacting the soil (n = 6). Bars represent ±1 SEM.

 
Tensiometer readings and the results of the Hydrus 2D simulation models showed that water was not a limiting factor during the 10 d after compaction (data not shown). Simulated pressure heads were still above the permanent wilting point (PWP, –1.5 MPa) 10 d after compaction, i.e., the soil water content was well above the PWP throughout the soil at the 0.0 kJ m–3 CE. It fell below the PWP only in the upper 1.5 cm of the soil depth for the 9.2 and 18.3 kJ m –3 CE treatments, indicating that water was becoming a limiting factor near the soil surface, but was above the PWP at planting depth. On the other hand, under the moist soil conditions at the beginning of the experiment, air-filled porosity was still sufficiently high that O2 deficiency would not limit germination (Ehlers and Goss, 2003).

The soil temperature at planting depth during the greenhouse seed vigor experiments was nearly ideal for seedling emergence and was not affected by compaction. The average maximum soil temperature was 27.2 and 27.7°C and the average minimum soil temperature was 21.3 and 22.9°C for the 0.0 and 22.9 kJ m–3 CE treatments, respectively (average mean temperature was 24.2°C). Similar soil temperatures were recorded following compaction during the seed size and seed spacing experiments.

Seedling Emergence Experiments
The initial standard germination was ≥86% for all seed lots used in the greenhouse experiments (Table 1). Although the seed lots were stored at 10°C until the experiments were conducted, the range in initial greenhouse seedling emergence for the control treatment (0.0 kJ m–3) across all seed lots was 57 to 100% (Table 1). The high- and medium-vigor seed lots emerged ≥80%; however, emergence of some of the low-vigor seed lots was <80%. Due to the variation in emergence levels at zero compaction, seedling emergence in two experiments (seed vigor and seed spacing) was normalized relative to the level at 0.0 kJ m–3 CE (set to 100% for each seed lot) to facilitate evaluation of the effects of compaction.

Seed Vigor Experiment
As soil compaction increased, there was a significant reduction in normal seedling emergence for all cultivars and seed vigor levels (Fig. 3 ) and high-vigor seed lots always had significantly greater emergence (P = 0.05) than low-vigor seed lots. The lowest level of compaction (4.6 kJ m–3 CE) resulted in <50% emergence for two of the three low-vigor seed lots. Seedling emergence of the high- and medium-vigor seed lots of CF492 was not significantly different, except at the highest compaction levels, where the high-vigor seed exceeded the medium-vigor seed. The emergence of high-vigor seed lots declined to <80% of the uncompacted control as compaction increased to 9.2 kJ m–3, except B283, which did not decline to <80% until compaction reached 18.3 kJ m–3.


Figure 3
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  		Figure 3. Normal seedling emergence at 10 d after planting four cultivars in the greenhouse following six compaction treatments. Emergence means were normalized relative to 100% emergence at 0.0 kJ m–3 compactive effort (n = 30). (Actual emergence levels are shown in Table 1.) LSD bars compare vigor levels within compaction treatments.

 
When averaged across seed lots and cultivars within each vigor level, there were very few abnormal emerged seedlings (0–9%) and there was little effect of compaction or seed vigor (Fig. 4 ). As soil compaction increased, the seedlings that germinated but did not emerge increased for all seed vigor levels. More seedlings germinated but did not emerge in low-vigor seed lots than medium- and high-vigor seed lots, except at the highest CE, where there was little difference. As soil compaction increased, the proportion of dead seeds increased significantly (P = 0.05) for the low- and medium-vigor seed lots, but remained at low levels for the high-vigor seed lots.


Figure 4
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  		Figure 4. Abnormal seedlings, seedlings that germinated but did not emerge, and dead seed following six compaction treatments averaged across vigor levels at 10 d after planting (n = 90, 90, and 60 for high-, low-, and medium-vigor treatments, respectively). Bars indicate SEM.

 
Seed Spacing Experiment
Seedling emergence for three high-vigor seed lots was significantly (P = 0.05) greater (33 and 44%) when three seeds were planted per container rather than one seed at the two lower compaction levels, 4.6 and 9.2 kJ m–3 CE (Fig. 5 ). Emergence was extremely low at high compaction (18.3 or 22.9 kJ m–3 CE) levels, and there was little effect of seed number on emergence. Reduced seedling emergence in the seed spacing experiment was primarily due to the seeds that germinated but did not emerge, compared with small percentages of dead or abnormal emerged seedlings (data not shown).


Figure 5
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  		Figure 5. Effect of seeding density on normal seedling emergence in the greenhouse following compaction for three high-vigor seed lots of three cultivars. Emergence means were normalized relative to 100% emergence at 0.0 kJ m–3 compactive effort (n = 27). (Actual emergence levels are shown in Table 1.) LSD bars compare seedling emergence within compaction treatments.

 
Seed Size Experiment
Seed size had no significant effect on normal seedling emergence of high- or low-vigor seed lots (Fig. 6 ). There was also little difference between larger and smaller seeds in the proportion of seeds that germinated under the crust and did not emerge or were dead (data not shown).


Figure 6
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  		Figure 6. Effect of seed size and vigor on normal seedling emergence in the greenhouse following compaction (n = 20). LSD bars compare seed size within compaction treatments.

 
Temperature Experiment
The high- and low-vigor seed lots of both cultivars responded in a similar manner to compaction and temperature. Thus, the emergence results were averaged across cultivars for the two vigor levels to compare the effects of temperature. Normal seedling emergence following compaction at 25°C was consistently higher for both high- and low-vigor seed lots than emergence at 20 and 15°C (Fig. 7 ). Seedling emergence of the high-vigor seed lots was near 90% at 25°C even at moderate soil compaction (9.2 kJ m–3 CE). Seedling emergence of the low-vigor seed lots was the same at 20° and 15°C, and both were lower than at 25°C for most CE levels. More seedlings germinated but did not emerge at 15° and 20°C than at 25°C (8–41% more than at 25°C, data not shown).


Figure 7
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  		Figure 7. Normal seedling emergence in the growth chamber 15 d after planting averaged across cv. B336 and B283 high- and low-vigor seed lots following four compaction treatments at 15, 20, 25°C (n = 36). Bars indicate ± SEM.

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Soil in plastic containers was compacted by dropping a 2.5-kg weight from several heights and the compaction (CE) significantly increased soil shear stress (Fig. 2) and reduced seedling emergence (Fig. 3, 5, 6, and 7). There was a close relationship between shear stress and bulk density as measurements of soil compaction in other laboratory and field studies (Benjamin and Cruse, 1987; Carter, 1990; Zhang et al., 2001). Thus, we were satisfied that our measurements of shear stress indicated that this technique produced the range in soil compaction needed to test the effects of seed vigor on seedling emergence.

As soil compaction increased, soybean seedling emergence was always reduced; however, the magnitude of the reductions was directly affected by seed vigor for all cultivars and experiments. The high-vigor seed lots had significantly greater emergence than the low-vigor seed lots at all compaction levels and temperatures evaluated. Emergence of all seed lots declined as compaction increased; however, the high-vigor seed lots consistently had 5 to 40% greater emergence than the low-vigor seed lots. Thus, when planting soybean seeds into seedbed conditions that are susceptible to compaction, high-vigor seed lots would have an advantage and may be able to break through the compacted layer while low-vigor seed lots may not. This relationship is consistent with other reports, suggesting that high seed vigor is closely related to improved soybean field emergence under stressful soil conditions (Burris, 1976; TeKrony and Egli, 1977; Johnson and Wax, 1978).

Egli and TeKrony (1995, 1996) indicated that soybean seeds with high vigor (AA germination ≥80%) would provide adequate field performance (≥70% emergence) under a wide range of seedbed conditions. The AA germination of all of the high-vigor seed lots of the four cultivars used in our experiments was ≥80%, except B283, which ranged from 76 to 79% (Table 1). These same seed lots showed high emergence levels in all experiments (except CF492, Fig. 5) until soil compaction exceeded 9.2 kJ m–3 CE. Thus, at low levels of soil compaction, seed lots with high seed vigor (AA germination) exhibited adequate emergence, while seed lots with low seed vigor did not. When compaction became too great, however, all seed lots had poor seedling emergence.

The failure of emergence of normal seedlings as compaction increased was primarily a function of germination failure (dead seeds) or germinated seedlings that were unable to break through the compacted soil. Rathore et al. (1981) monitored soybean seedling development in compacted, crusted soils and found that the hypocotyls became swollen and brittle, and often collapsed at the stress point between the cotyledons, producing an abnormal seedling. We also observed swollen hypocotyls in abnormal emerged seedlings and in germinated seedlings that did not emerge. The percentage of seedlings that germinated but did not emerge was greater for low-vigor seed lots than medium- and high-vigor lots; however, it was ≥20% for all vigor levels at the highest level of compaction (22.9 kJ m–3 CE). Rathore et al. (1981) also observed that when a growing hypocotyl strikes a soil crust, it tries to deviate from vertical growth to circumvent it; however, if it cannot overcome this barrier, it remains buried under the crust. We also found a lower seedling growth rate for low-vigor seed lots than high-vigor seed lots (data not shown), which may have contributed to the greater percentage of germinated seedlings that remained below the soil surface.

As soil compaction increased in all experiments, there was a constant increase in the percentage of dead seeds for the low- and medium-vigor seed lots, but not for high-vigor seed lots (Fig. 4). Seed deterioration follows a progressive decline from normal seedling development during germination to abnormal seedlings to total loss of viability (dead seeds) (Heydecker, 1972; Hampton and TeKrony, 1995). Low-vigor seed lots deteriorate much more rapidly in storage than high-vigor seed lots. Thus, it was not surprising that low-vigor seed lots in this experiment had a higher percentage of abnormal and dead seeds following a soil stress such as compaction compared with high-vigor seed lots.

Temperatures <25°C reduced seedling emergence in the growth chamber and increased the effect of compaction treatments. Once again, the effect of compaction on seedling emergence was greater on low-vigor seed lots than on high-vigor seed lots (Fig. 7). The time it takes soybean seedlings to emerge is greater at 15 and 20°C than at 25°C (Hatfield and Egli, 1974). The delayed emergence at low temperatures combined with soil compaction exposed the seeds to additional stress, causing reductions in seedling emergence. Hamman et al. (2002) also found that reduced emergence of low-vigor seeds in nonsterile soil was associated with delayed emergence. Most of the unemerged seedlings at the lower temperatures (15 and 20°C) were seeds that had germinated under the soil but did not emerge, which suggests that seedling growth was so slow that soil compaction prevented the seedling from emerging. Thus, planting high-vigor seed lots would clearly provide an advantage when low soil temperatures and soil compaction are encountered at the time of seedling emergence.

Many have speculated that larger seeds or close spacing between seeds would increase the level of seedling emergence under soil compaction or crusting; however, in our experiments seed size had no effect on emergence following compaction, while close seed spacing (3.81 cm) increased emergence. These results are in contrast to a report by Hanks and Thorp (1957) that seedling emergence of wheat (hypogeal emergence) was not influenced by seed spacing of 2.5 and 5.0 cm, but is in agreement with compaction studies with cotton (Gossypium hirsutum L.) and guar (epigeal emergence), where two seeds planted per hole emerged better than one seed as crust strength increased (Sharma and Agrawal, 1974). Thus, in our experiment, the seeds in the single-seed treatment were germinating but, possibly due to the large cotyledons with epigeal emergence, could not break through the soil compaction layer. In contrast, when three seeds were planted in closer proximity, they were able to assist each other, allowing emergence. Rathore et al. (1981) also observed joint efforts by soybean seedlings pushing up a soil crust and allowing emergence.

The effect of seed size on seed vigor and seedling emergence has been studied extensively, with some reporting that large seeds have the advantage (Burris et al., 1973; Hopper et al., 1979), while others have reported little relationship between seed size and seed vigor (TeKrony et al., 1987). Rathore et al. (1981) reported that large soybean seeds produced seedlings with greater emergence force than small seeds. They also reported that small seeds tended to imbibe and germinate more rapidly, which allowed the seedlings to emerge sooner than those from large seeds and before crust strength was high enough to prevent emergence. In our experiments, the vigor level of large and small seeds was the same (Table 1) and although seedling emergence of larger seeds was slightly higher (5, 6, and 1%) than smaller seeds (Fig. 6), the difference was small and not significant, suggesting that seed size per se had little influence on soybean seedling emergence with or without soil compaction. It should be mentioned that the seed lots used in our experiments were sized following conditioning, which reduced the range in seed sizes compared. Under compacted soils, however, there appears to be little advantage for a seed company to incur the extra cost of sizing seed and selling only the large seed fraction to improve emergence.

In summary, increases in soil compaction and crusting reduced soybean seedling emergence. Seed vigor influenced the effect of soil compaction on seedling emergence, with high-vigor seed lots having consistently higher emergence than low-vigor seed lots at lower levels of soil compaction across all seed lots, cultivars, and temperature treatments. Seed spacing (three seeds planted within close proximity to each other) resulted in consistently higher seedling emergence following compaction than one seed per container; however, seed size had no effect on emergence across all compaction levels. The majority of the seeds that did not emerge following compaction either germinated but remained under the soil or were dead. Dead seeds occurred more frequently in low- and medium-vigor seed lots than in high-vigor seed lots. Soil compaction can be included in the list of the soil variables that can severely reduce soybean seedling emergence. If soils are susceptible to compaction, it would be advantageous to plant high-vigor seed lots at recommended seeding rates to achieve adequate seedling emergence and plant populations.


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 NOTES
ABSTRACT
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 MATERIALS AND METHODS
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Received for publication March 27, 2007.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 





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