Crop Science 43:288-294 (2003)
© 2003 Crop Science Society of America
FORAGE & GRAZING LANDS
Soil Type and Moisture Level Influence on Alamo Switchgrass Emergence and Seedling Growth
Gerald W. Evers* and
Margaret J. Parsons
Texas A&M University Agricultural Research and Extension Center, P.O. Box 200, Overton, TX 75684
* Corresponding author (g-evers{at}tamu.edu)
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ABSTRACT
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Attempts to establish lowland types of switchgrass (Panicum virgatum L.) in the Lower South have frequently been unsuccessful. A greenhouse study was conducted to determine the influence of soil texture and moisture on ‘Alamo’ switchgrass emergence and seedling growth. Soils used were a very fine sandy loam, loamy fine sand, silt loam, and clay that were watered every 3 to 4, 7, 10 to 11, or 14 d. Emergence was recorded daily for the first 28 d and seedlings were harvested at 6 wk to determine survival and seedling traits. The study was initiated on 30 March 2001 and repeated on 29 May and 24 July. Run date caused significant (P 
0.001) interactions for all measurements with soil series and watering interval because of increasing greenhouse temperatures with each succeeding run. Watering interval affected emergence only at the 24 July run with emergence decreasing as watering interval increased. However, watering interval of at least 7 d was necessary for greater than 90% seedling survival in all soils. Only the clay soil consistently had good seedling survival (
94%) at all watering intervals. Shoot stage and weight and root stage and weight decreased as watering interval increased. These seedling traits were usually higher in the loamy fine sand and very fine sandy loam if watered at least every 10 d. The trend for root/shoot ratios was to increase as moisture became limiting because shoot weight decreased faster than root weight. Rainfall occurring at greater than 10 d intervals is one of the factors for unreliable switchgrass establishment on sandy soils in the Lower South.
Abbreviations: DAP, days after planting
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INTRODUCTION
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AS WITH MOST native warm-season perennial grasses, obtaining good stands of switchgrass is difficult because of small seed size, slow and erratic germination, and poor seedling vigor. Several studies have been conducted on switchgrass seed size, seeding rates, planting depth, and planting date to identify optimum establishment practices (Vassey et al., 1985; Newman and Moser, 1988; Aiken and Springer, 1995; Smart and Moser, 1999). The influence of temperature on switchgrass seedling development has been studied in growth chambers (Hsu et al., 1985). Essentially, all these studies have been conducted with upland types that are adapted to the temperate climates of the Great Plains (Smart and Moser, 1997), northern USA (Vassey et al., 1985), and Mid-Atlantic States (McKenna and Wolf, 1990). Lowland types that originated at lower latitudes are later maturing and more productive in the southeastern USA than the upland types originating at northern latitudes (Hopkins et al., 1995).
Obtaining good stands of lowland types in the Lower South is more of a problem than with upland types (Miller and Owsley, 1994). Soils in the southeastern USA are more weathered and acidic than those in the Great Plains and the northern USA. Soil texture has influenced establishment of other native warm-season perennial grasses (Cox and Martin, 1984; De Alba-Avila and Cox, 1988). Miller and Owsley (1994) investigated planting depths from 0 to 2.5 cm for Alamo switchgrass in a loamy sand and clay soils under greenhouse conditions. Planting depth had only small effects on seedling emergence in the clay soil but seedling emergence of the surface sown seed on the sandy soil was only about 25% of the other planting depths. Except for the surface sown seed, seedling emergence from the sandy soil was more rapid than from the clay soil. Using both lowland and upland switchgrass types, Aiken and Springer (1995) reported greater emergence in sand than in two silt loam soils. Soil moisture was not limiting in either of the two studies.
Competition from weeds also hinders switchgrass establishment. Higher rainfall and milder winters in the Lower South than in temperate regions favor warm-season annual weeds such as crabgrass (Digitaria sp.), foxtail (Setaria sp.), panicums (Panicum sp.), and crotons (Croton sp.). Screening selective herbicides for upland types in the Great Plains has been successful in some cases (Masters et al., 1996). However, efforts to identify selective herbicides for lowland types in the Lower South have been disappointing (Nerada et al., 2001). Data are available on the influence of soil moisture on the establishment of some native warm-season perennial grasses, but not on switchgrass (Winkel et al., 1991). Consequently, a greenhouse study was conducted to provide information on the interaction of soil type and moisture level to help understand some of the difficulty in obtaining switchgrass stands of lowland types in the Lower South.
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MATERIALS AND METHODS
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The greenhouse study was conducted using the top 15 cm of four diverse soil series which were obtained from cultivated fields in the eastern half of Texas. Soils included in the study were: Bowie very fine sandy loam (fine-loamy, siliceous, thermic Plinthic Paleudults) and Darco loamy fine sand (loamy, siliceous, thermic Grossarenic Paleudults), which are upland Coastal Plain soils from Rusk County; Weswood silt loam (fine-silty, mixed, superactive, thermic Udifluventic Haplustepts) is a Brazos River bottom soil from Burleson County and Houston clay (fine, smectitic, thermic Udic Haplusterts) is an upland soil from Bell County.
Soils were sterilized in a Model SS-15 soil sterilizer (Pro-Grow Supply Corp., Brookfield, WI) for 72 h at 49°C. They were air dried and ground to pass through a 20-mesh screen. Samples of each soil were analyzed at the Texas A&M University Soil, Water, and Forage Testing Laboratory by standard methods of analysis. Twenty-four plastic pots (12-cm diam x 12-cm width) for each soil were filled up to 2.5 cm from the top. Potted soils were watered twice to allow two wet–dry cycles to settle the soil.
Seed of Alamo switchgrass, a lowland type, was harvested in Oklahoma in 1997. Original analysis in 1997 was 73.1% pure seed with 42% germination and 31% dormancy. A germination test (30°C d for 12 h, 20°C nights) conducted before the study in March 2002 had 67% germination. Twelve seed (with 67% germ. 
8 live seed) were evenly spaced on the soil surface in 16 of the 24 pots of each soil. Approximately 1.25 cm of the soil was placed over the seed. Four planted and two unplanted pots of each soil were assigned to one of four watering treatments. All pots received 200 mL of water at the beginning of the study and again every 3 to 4, 7, 10 to 11, or 14 d. Watering intervals of 3 to 4 and 10 to 11 d will be referred to as the 3- and 10-d treatments. On the first day and every 2 wk thereafter, 200 µg mL-1 of 15N-5P-15K-5Ca-2 Mg plus micronutrients (Miracle-Gro Excel, Scotts Co.) solution were applied in the 200 mL of water.
Pots were arranged in a randomized split-plot design with water treatments as main plots and soils as subplots with four replications. The study was repeated three times beginning on 30 March, 29 May, and 24 July 2001. The greenhouse temperature controller was set at 16°C for the minimum and 26°C for the maximum temperatures. A 7-d chart temperature recorder was placed among the pots for a week during the middle of each run. The greenhouse cooling system was unable to maintain the set maximum temperature of 26°C.
Switchgrass seedling emergence and death were monitored daily for the first 28 d. Cumulative emergence is reported as a percentage of eight viable seed planted per pot. Any emerging seedlings other than switchgrass were removed. Percent survival was calculated as the ratio of the number of seedlings present at 6 wk when the runs were terminated to the total number of seedlings emerged x100. Soil samples were taken from the top 2.5 cm of the unplanted pots every 7 d before any scheduled watering treatments to monitor soil moisture. Soil moisture samples were weighed and then placed in a force draft oven at 65°C until dry and weighed again to determine percent soil moisture.
At 6 wk, switchgrass seedlings were removed from the pots with as little soil as possible, placed in plastic bags, and stored in a refrigerator at 3°C until seedling traits could be measured. Seedlings were removed from the bags and washed to remove any adhering soil. Shoot and root stages were recorded according to Moser et al. (1993). Shoots and roots were separated, placed in an oven at 65°C for 48 h, and weighed to determine dry weights per seedling and to calculate root/shoot ratios. Seedling trait means for each pot were used for statistical analysis.
Cumulative emergence and survival percentages were transformed by an arcsine function before statistical analysis to normalize the data and stabilize the variance (Gomez and Gomez, 1984). Actual percentages are reported. Only cumulative seedling emergence on every fourth day up to 28 d was calculated. Emergence, survival, and seedling traits were analyzed as a 3 x 4 x 4 split-split-plot design with four replications. No seedling survival in some treatments resulted in unbalanced data so Proc GLM ANOVA (SAS Institute, 1991) was used. Run dates were main plots, watering interval subplots, and soil series sub-subplots. When run date x watering interval x soil series interactions occurred, the data were sorted by run date and analyzed as a split-plot with watering intervals as main plots and soil series as subplots. Mean separation was by Fisher's Protected LSD at the 0.05 level of significance.
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RESULTS AND DISCUSSION
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All soils had adequate pH for switchgrass (Staley et al., 1991) (Table 1). Low nitrate-N levels were common for all soils. Bowie very fine sandy loam and Darco loamy fine sand are highly weathered, Coastal Plain soils that are typically low in P, K, Ca, and Mg. However, both soils had moderate P levels probably because of previous fertilization in the field. The high P, K, Ca, and Mg levels in the Weswood silt loam and Houston clay soils are typical for these calcareous soils.
Soil moisture levels by soil series and run date are reported in Fig. 1. The soil moisture level in the 3-d watering treatment in the 29 May run between 14 and 35 d after planting (DAP) was substantially lower than for the other two runs. This was a rain free period with clear days that resulted in rapid drying of soil surfaces. Soil moisture trends were somewhat erratic within each run since the previous watering could have occurred 3, 7, 10, or 14 d before soil moisture samples were taken. Soil moisture levels were highest at the 3-d watering interval and then decreased as watering interval increased with usually similar moisture levels at 10 and 14 d.
The different moisture holding capacities of the soils are reflected in Fig. 1. The very fine sandy loam and the loamy fine sand had similar soil moisture levels. Moisture levels were frequently near 0 g kg-1 at the 10- and 14-d watering intervals with maximum moisture levels of 100 to 150 g kg-1 at the 3 d watering interval. Moisture levels in the silty loam were never below 50 g kg-1, with maximum levels from 200 to 250 g kg-1 at the 3-d watering interval. The clay had the greatest moisture retention with minimum soil moisture levels at approximately 100 g kg-1 and levels up to 300 g kg-1 for the 3-d watering interval. Statistical analysis was not conducted because only one soil sample for each soil series per week from one of two unplanted pots was collected.
Seedling Emergence
Run date x soil series and run date x watering interval interactions (P 0.001) were observed for percent cumulative seedling emergence on every fourth day up to 28 d. Only the 28-d data are presented because of similar trends throughout the 28 d. The run date x soil series interaction averaged across watering intervals at 28 DAP, are presented in Table 2. Highest emergence percentage varied among soils within run dates. Emergence differences among soils began at 20 DAP in the 30 March run, 8 DAP in the 29 May run, and at 4 DAP in the 24 July run (data not shown). Daily maximum temperature ranges for the 30 March run were from 27 to 30°C, the 29 May run from 32 to 34°C, and the 24 July run from 32 to 38°C. As the daily maximum temperature increased with each successive run, moisture holding capacity of the soils became more of a factor.
Emergence in the silt loam was initially slower than in the other soils but then rapidly increased between 12 and 16 DAP to levels similar to the very fine sandy loam soil. In the 24 July run, seedling emergence was the most rapid in the loamy fine sand, the slowest in the very fine sandy loam and silt loam soils, and the clay soil was intermediate. Miller and Owsley (1994) reported similar Alamo emergence in a loamy sand and clay soils when seeds were planted from 0.5 to 2.5 cm deep. However, Aiken and Springer (1995) reported more rapid and total emergence of both lowland and upland types in pure sand than two silt loam soils.
At 28 DAP, there were no differences in emergence percent among watering intervals at the 30 March and 29 May run dates (Table 3). But there were differences at the 24 July date because of higher greenhouse temperatures. Emergence was high for the 3- and 7-d watering intervals and then decreased as watering interval increased. Attempts to establish lowland types of switchgrass during summer temperatures will probably fail without weekly rainfall or irrigation.
Seedling Survival
The run date x soil series x watering interval interaction for seedling survival was significant (P 0.001). When analyzed by run date, seedling survival in the 30 March run was only influenced by the main effect of watering interval. Seedling survival at the 3- (98%) and 7-d (99%) watering intervals was greater than at the 10 d (92%) and 14 d (87%) watering intervals (data not shown).
Seedling survival was good for all soils in the 29 May run when pots were watered every 3 or 7 d (Fig. 2). None of the seedlings survived in the loamy fine sand with 10- and 14-d watering intervals because of the poor moisture holding capacity. Although recorded soil moisture levels were similar for the very fine sandy loam and loamy fine sand soils (Fig. 1), a sandy loam soil should have better moisture retention than a loamy fine sand. Seedling survival in the silt loam and clay soils was 90% or higher at all watering intervals.
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Fig. 2. Interaction of soil type and watering interval on percent survival of emerged seedlings for the 28 May and 24 July run dates. Vertical bars indicate LSD value (P = 0.05) within watering interval.
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Moisture stress was more pronounced with the higher greenhouse temperatures during the 24 July run (Fig. 2). Seedling survival was 100% in the clay soil for all watering intervals and was the only soil where seedlings survived in the 14-d watering interval. This is a reflection of the higher available soil water retention of the clay compared with the other soils (Fig. 1). Except for the loamy fine sand soil in the 29 May run, seedling survival was 60% or higher at the 10-d watering interval. In all three runs seedling survival was greater than 90% in all soils if watered at 3- or 7-d intervals. Weekly rainfall or irrigation is necessary for good switchgrass seedling survival in sandy and fine sandy loam soils during high temperatures.
Seedling Traits
There was a significant (P 0.001) run date x soil series x watering interval interaction for all seedling traits except root/shoot ratio. When sorted by run date, the soil series x watering interval interaction was significant (P 0.001). The general trend was for seedling development to be more advanced and shoot and root weights to be heavier in the two sandier soils than in the silt loam and clay soils if the seedlings survived (Fig. 3). Shoot stage generally ranged from 4.0 (appearance of collar of second leaf) to 5.0 (appearance of collar of fourth leaf) at 6 wk. In the 30 March and 29 May runs, differences in shoot stage among soil types only occurred at the 10- and 14-d watering intervals. None of the seedlings in the loamy fine sand soil survived in the 29 May run at these two watering intervals. In the 24 July run, shoot development was slow in the clay soil at the 3- and 7-d watering intervals but they were the only seedlings that survived at the 14-d watering interval.
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Fig. 3. Interaction of soil type and watering interval for shoot stage and weight and root stage and weight at 6 wk for three run dates. Vertical bars indicate LSD value (P = 0.05) within watering interval.
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Shoot weight differences among soil types were more pronounced than for shoot stage. There was a general decline in shoot weight as the watering interval increased for all run dates. Shoot weight differences occurred among soils at each watering interval for every date. The highest shoot weight was in the loamy fine sand soil and lowest in the silt loam and clay soils for all three run dates. The run date x soil series x watering interval interaction was significant because shoot weights in the very fine sandy loam soil were lower than the loamy fine sand soil in the 30 March and 29 May runs but similar to the heavier soils in the 24 July run. The higher temperatures during the 24 July run caused a more rapid decline in shoot weight with less frequent watering. The soil series x watering interaction was due to inconsistent differences among soils for each watering interval.
Seedling root stages ranged from 3.4 to 5.2, which indicated the presence of 1 to 6 adventitious roots. The run date x soil series x watering interval interaction resulted from root stage differences among soils at the 30 March and 29 May runs but there were no differences among soils in the 24 July run. There was a general decline in root development as watering interval increased. If watered every 3 d, there were no differences among soils for any run. If the seedlings survived, root stage was more advanced in the two sandier soils than in the loam and clay soils. Development of adventitious roots is essential to the survival of the switchgrass seedling. The absence of adventitious root development may have contributed to seedling mortality.
The trends in root weight were identical to that of shoot weight with differences among soils at every watering interval. The run date x soil series x water interval interaction was due to differences in ranking of soil series for root weight at the same watering interval among the three run dates. Root weight was usually the same in the silt loam and clay soils in the 30 March and 29 May runs but not in the 24 July run. At the 3- and 7-d watering interval in the 24 July run, root weight was greater in the silt loam than the clay. As with the other seedling traits, there was a general decline in root weight as watering interval increased, especially at the higher temperatures in the 24 July run. Root weights were always greater in the loamy fine sand soil and usually in the very fine sandy loam soil if the seedlings survived.
Root/shoot ratio had significant (P 0.001) soil series x watering interval, run date x soil series, and run date x watering interval interactions. The soil series x watering interval interaction was due to the root/shoot ratios in the clay soil that were the lowest at the 3- and 7-d watering interval and one of the highest at the 10- and 14-d watering interval (Fig. 4). The general trend was for the root/shoot ratio to increase as the watering interval increased because shoot weight was restricted more than root weight by limited moisture. This was especially true for the clay and silt loam.
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Fig. 4. Interaction of soil type and watering interval across dates for root/shoot ratio. Vertical bars indicate LSD values (P = 0.05) within watering intervals.
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The run date x soil series interaction on root/shoot ratio was due to the lowest root/shoot ratios occurring in the very fine sandy loam soil in the 29 May run and in the clay soil in the 24 July run (Table 4). Root/shoot ratios in the silt loam soil were the highest, or not significantly different from the highest, at each date. The silt loam would shrink and swell leaving cracks up to 0.5 cm wide in the pots. This obviously affected seedling growth. Seedlings growing in the silt loam soil had some of the lowest shoot weights in all three dates and the lowest root weights in two of the three dates (Fig. 3).
The run date x watering interval interaction occurred because the root/shoot ratios at the 3-d watering interval were lower than the 14-d watering interval at the 30 March run but similar in the other two runs (Table 5). Root/shoot ratios were always high at the 14-d watering interval in all three runs because of the greater moisture stress. Root/shoot ratios were also high at the 10-d watering interval in the 30 March and 29 May runs.
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CONCLUSION
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Greenhouse temperatures increased with each succeeding run from 30 March to 24 July that resulted in interactions with watering interval and soil texture. Minimum soil moisture levels were near 0 g kg-1 for the loamy fine sand and very fine sandy loam soils, 50 g kg-1 for silt loam, and 100 g kg-1 for clay which influenced seedling survival and growth. Watering interval had a greater influence on seedling survival and seedling growth than soil texture. Seedling development and growth decreased as watering interval increased. Weekly rainfall or irrigation is necessary for good switchgrass emergence, survival, and seedling growth. Moisture becomes more critical as daily temperatures increase from spring through summer. Lack of dependable rainfall every 7 to 10 d is a major deterrent to dependable switchgrass establishment on sandy soils in the southeastern USA.
Received for publication January 10, 2002.
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ABSTRACT
INTRODUCTION
