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USDA-ARS, National Sedimentation Laboratory, P.O. Box 1157, Oxford, MS 38655 USA
rhoton{at}sedlab.olemiss.edu
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMethods and materialsResults and discussionConclusionsREFERENCES |
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0.05) differences compared to conventional tillage (CT). The surface 2.5 cm of the NT treatments had higher levels of SOM, exchangeable Ca, and extractable P, Mn, and Zn, but lower extractable K, Fe, and Cu. Tillage had no effect on exchangeable Mg and pH. No-till also resulted in higher AS, and lower MR, WDC, and TC in the top 2.5 cm, relative to CT. The differences in soil properties between tillage treatments were essentially independent of crop. Instead, the results are controlled by relative amounts of SOM and clay, and the extent to which these properties change with time. Undoubtedly, NT practices can improve several fertility and erodibility-related properties of this soil within 4 yr, and enhance its sustainability.
Abbreviations: AS, aggregate stability • CT, conventional tillage • MR, modulus of rupture • NT, no-till • OC, organic carbon • SOM, soil organic matter • TC, total clay • WDC, water dispersible clay
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMethods and materialsResults and discussionConclusionsREFERENCES |
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The drastic loss of SOM was perhaps the most critical of these changes, because it has resulted in soil aggregates so unstable that only relatively low rainfall energies are required for their disruption. This promotes surface seal formation, and enhanced runoff and erosion. When combined with a sloping topography (0–12%) and high rainfall energies [8121 MJ·mm·(ha·h)-1] (McGregor et al., 1995) common to the area, these soil characteristics create one of the most erosion prone soil regions in the USA. Estimates of average erosion losses in this region range from 34 to 56 Mg ha-1 yr-1 (Langdale et al., 1985).
Given the excessive erosion losses, and their adverse effect on soil properties, the implementation of conservation tillage systems is essential for the sustainability of row crop agriculture in the region. Several studies conducted on similar soil types have indicated that the introduction of NT management practices lead to improved soil conditions, especially with respect to increased SOM contents (Tyler and Overton, 1982; Tyler et al., 1983; Hill, 1990; Havlin et al., 1990). Others (Ismail et al., 1994; Karathanasis and Wells, 1989) have also indicated that extractable P, exchangeable cations, cation-exchange capacity, and pH increase in NT environments.
The fact that NT improves soil conditions with time is well established. However, the rate at which improvement proceeds has not been thoroughly addressed, particularly from the standpoint of time required for statistically significant changes to occur. Dick et al. (1991) indicate that changes in soil properties, created by the imposition of NT practices, are most rapid in the first 10 yr, and that it is difficult to detect changes after only 2 or 3 yr. Other research in which changes in SOM contents were monitored have indicated increases after time periods ranging from 4 to 28 yr (Blevins et al., 1983; Lal et al., 1994; Ismail et al., 1994; Karathanasis and Wells, 1989; Edwards et al., 1992; Havlin et al., 1990; Shuman and Hargrove, 1985). These measurements, however, were apparently not made on a periodic basis. Therefore, the rate of SOM increase is not available. Other soil property changes under NT conditions reported by some of these researchers are: increased extractable Ca, Mg, Mn, and Zn after 5 yr (Hargrove et al., 1982); greater P contents after 8 yr (Shuman and Hargrove, 1985); lower, or no difference in bulk densities after 10 yr (Edwards et al., 1992; Blevins et al., 1983); and increased aggregate stability after 5 yr (West et al., 1991). Again, no indications were given for changes occurring in these soil properties during interim time periods.
Time required for soil properties to improve substantially after NT management practices are introduced is important from three perspectives: (i) for determining an approximate length of time required to continue a given practice before beneficial changes occur in a soil property; (ii) farmers would have a better idea of when to expect NT crop yields to reach or exceed those previously obtained from conventionally tilled conditions; and (iii) information of this nature should improve our abilities to predict changes in soil loss rates once management practices are changed from CT to NT.
The current study was conducted to determine the time required for fertility and erodibility-related soil properties to change in response to NT practices.
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Methods and materials |
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TOPNOTESABSTRACTINTRODUCTIONMethods and materialsResults and discussionConclusionsREFERENCES |
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6 m thick (Lindbo et al., 1994). The predominant soil type was Grenada silt loam. Slopes within the experimental plot areas ranged from 2 to 4%. Land-use immediately prior to initiation of the study was pasture.
Experimental Design
In the fall of 1987 experimental field plots (12.2 x 5.5 m) were installed at the site. The experiment was a split-split plot with the main unit having a randomized complete block design with 10 replicates and a 3 by 2 factorial treatment structure (crop x tillage). The split plot factor was three depth increments, and the split-split plot factor was 3 yr. An LSD at
was used to separate the means. All statistical analyses used the PROC MIXED procedure of SAS Version 7 (SAS Inst., 1997). Each block contained 14 different plots (treatments) as part of a larger, separate study, but this study used only six plots per block (CT and NT grain sorghum–corn, cotton, and soybean–wheat).
Plot Management and Characterization
Fertilizer (672 kg ha-1, 0–20–20) and lime (5600 kg ha-1) were applied to all plots based on soil test results. These amendments were incorporated with a chisel plow and disk, then a wheat cover crop was seeded on all plots. In the spring of 1988, whole plots were planted to either cotton, grain sorghum, or soybean using chisel plow or disk followed by a finishing implement (Do-All) that pulverized, mixed and firmed the seedbed (CT), and no-tillage (NT) treatments on separate plots. Additionally, the NT soybean treatment was double-cropped with winter wheat (hereafter referred to only as soybean), the NT cotton was planted into a killed wheat cover crop, and the NT grain sorghum was planted into a killed hairy vetch (Vicia villosa Roth) cover crop. Beginning with the 1992 growing season, all grain sorghum treatments were changed to corn (hereafter referred to only as corn). All crops were planted in 91.4-cm rows except wheat which was drilled in 17.8-cm rows. Wheat straw from the double-crop soybean was left on the surface.
An initial set of soil samples was collected for baseline characterization data immediately prior to the 1988 growing season. Core samples were collected to a depth of 15.2 cm at six randomly selected points within each of the plots, and sectioned into 0- to 2.5-, 2.5- to 7.6-, and 7.6- to 15.2-cm increments. These individual sections were composited to form one sample per depth increment, or three samples per plot replicated 10 times across the study site. These sampling procedures were repeated again in 1992 and 1996.
The plots were fertilized each year, based on soil test data, once the soil characterization samples were collected each spring. Cotton and corn treatments received a broadcast application of 13–13–13 to adjust N, P2O5, and K2O levels to 50 kg ha-1. Cotton and corn were later sidedressed with NH4NO3 at rates of 50 and 134 kg N ha-1, respectively. Fertility levels of the soybean treatments were adjusted to 56 kg ha-1 P2O5 and K2O by broadcast application of 0–20–20. Additionally, double-crop wheat received 90 kg N ha-1 in two applications. Fertilizer applications were incorporated by the tillage operations previously mentioned for CT, but left on the surface of the NT plots.
Laboratory Analyses
Soil samples were air-dried, crushed, and sieved to <2 mm in the laboratory. The <2 mm fraction of all samples was extracted separately with double acid (0.05 N HCl + 0.025 N H2SO4) and 1 M NH4OAc to determine extractable P, K, Fe, Mn, Zn, and Cu (Olsen and Sommers, 1982), and exchangeable Ca and Mg (Thomas, 1982), respectively. Elemental concentrations in these extracts were measured by atomic absorption spectroscopy. Phosphorus in the extracts was determined colorimetrically. Soil pH was measured in a 1:1 soil/distilled water suspension (McLean, 1982). Organic carbon (OC) was determined by combusting 2-g samples in a Leco CR-12 carbon analyzer (Leco Corp., St. Joseph, MI). The OC data were converted to SOM by multiplying OC by a factor of 1.72. Soil samples used for particle-size distribution were pre-treated with H2O2, dispersed overnight in Na-hexametaphosphate, and then evaluated by the pipette method of Day (1965). Water dispersible clay contents were estimated by the same pipette method except that the H2O2 pretreatment was eliminated, and distilled water was used instead of a chemical dispersant. Aggregate stability measurements were made according to the procedure of Kemper (1965), with the exception that no sample pre-wetting was involved (i.e., only air-dry aggregates were used). Modulus of rupture was determined according to the method of Richards (1953).
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Results and discussion |
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TOPNOTESABSTRACTINTRODUCTIONMethods and materialsResults and discussionConclusionsREFERENCES |
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0.05) higher and lower SOM contents than the CT treatments, respectively.
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0.05) increases in SOM contents were identified in the first 4 yr of the current study for all three crops, the discrepancy is likely due to differences in sampling depths. Specifically, Hargrove et al. (1982) collected surface samples to a depth of 7.5 cm compared to 2.5 cm in this study. Thus, tillage effects were probably diluted by the greater sample volume.
Soil pH and Exchangeable Calcium and Magnesium
Soil pH was relatively uniform among treatments in 1988 following the initial lime application the previous fall, ranging from 5.8 to 6.5 (Table 2). After four growing seasons the pH of NT corn and soybean were significantly lower than CT plots at 0 to 2.5 cm. There were no tillage differences for cotton at this depth, however, the NT treatment had a significantly greater pH at the 2.5 to 15.2 cm depths. Otherwise, few significant differences occurred regardless of crop, tillage, or depth. The reapplication of lime in the fall of 1992 raised the pH of corn and cotton to similar levels in the CT and NT plots, but the NT soybean plots were even more acidic than CT plots between 0 and 7.6 cm. This greater acidity for NT soybean is the result of 73 kg N ha-1 applied to the double-crop wheat only 2 mo before soil samples were collected. Generally, the greatest differences in soil pH between CT and NT plots were restricted to the surface 2.5 cm where N fertilizer was applied without incorporation. Beyond this aspect, tillage had little effect on the pH of these soils.
In 1988 exchangeable Ca and Mg ranged from 4.2 to 6.7, and 0.9 to 1.6 cmol kg-1, respectively (Table 2). The depth distribution of these cations should reflect the movement of surface-applied liming materials. For Ca, the NT plots generally contained the greatest concentrations after 4 yr for all depths and crops, but most of the differences between tillage treatments were not significant. Tillage means, however, averaged over all crops indicate that NT had significantly greater concentrations in the surface 2.5 cm. The relatively even distributions of Ca between sampling depths is attributed to the incorporation of lime on all plots during the original application in the fall of 1987.
After 8 yr, only the NT treatments for the 0- to 2.5-cm depth had greater exchangeable Ca relative to CT. Again, tillage means indicated that NT treatments contained significantly greater amounts of exchangeable Ca. At the other two sampling depths, lower Ca concentrations in the NT plots are due to a lack of incorporation of lime into the soil, and possibly to the greater SOM contents at the surface that accumulated subsequent to the first liming. The fulvic acid component of SOM can form stable complexes with Ca and retard its activity (Schnitzer, 1978). This may be substantiated by the fact that the NT plots had the lowest pH at the 0- to 2.5-cm depth, but higher exchangeable Ca than CT plots. A substantial decrease in exchangeable Ca from the 0- to 2.5-cm to the 2.5- to 7.6-cm depth was significant in the NT cotton and soybean plots. Tillage means also showed significantly less Ca at that depth. At 7.6 to 15.2 cm, there were no tillage effects.
Exchangeable Mg was uniformly distributed among treatments in the surface 2.5 cm after four growing seasons. No tillage effects were found, but in every case the 1992 concentrations were significantly greater than 1988, presumably due to the progressive dissolution of the liming materials. This trend was constant throughout all sampling depths. The sampling depths below 2.5 cm had less Mg in the NT plots compared to the surface, but only the cotton was significantly different. The tillage means also indicated significantly less Mg in the NT treatments. The 8 yr data indicated no tillage effects in the surface, however, the NT treatments had significantly less Mg at both lower sampling depths for all three crops. The data also indicate that Mg concentrations decreased significantly between 1992 and 1996 at these depths. Apparently, exchangeable Mg in NT treatments is similar to Ca in that the distributions appear to be explained by a lack of mixing and a progressive increase in SOM contents at the surface which can form complexes with these cations.
The distribution of exchangeable Ca in this study is similar to previous reports (Ismail et al., 1994; Lal et al., 1994) of greater concentrations in the surface of NT practices. These same studies also reported more exchangeable Mg in NT surfaces relative to CT, but this study did not find any tillage effects in the surface 2.5 cm.
Extractable Phosphorus and Potassium
In 1988, extractable P concentrations ranged from 5.9 to 29.1 mg kg-1 (Table 3)
. The 7.6- to 15.2-cm sampling depth had the lowest amounts, averaging only 30% of the P concentrations in the upper depths. This suggests that the fertilizer applied in 1987 was not thoroughly mixed to a depth of 15.2 cm. Within 4 yr, the surface application of P without incorporation on NT plots had resulted in concentrations that were, in some instances, three times greater than the CT treatments for cotton and soybean. Although the NT corn plots had 39% more P than the CT treatments they were not significantly different. The low P contents of the corn plots relative to cotton and soybean suggests a greater uptake by the grain sorghum–corn. In contrast, the relatively large P concentration of NT soybean at 0 to 2.5 cm are the result of poor wheat stands that resulted in relatively low fertilizer uptake. Since the soybean plots were fertilized when the double-crop wheat was planted, a larger proportion of P than normal remained when plots were sampled the following spring. Concentrations decreased sharply below 2.5 cm for both treatments, especially NT cotton and soybean, but there were no tillage effects.
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0.01) was obtained for SOM vs. extractable P using 1996 data. Thus, extractable P in these soils is strongly related to SOM contents. At depths between 2.5 and 15.2 cm, the P distributions between NT and CT plots varied by crop, but in most instances the concentrations were relatively similar and 5 to 10 times lower than the surface 2.5 cm. When averaged over crop, NT treatments contained significantly greater amounts of P in the surface 2.5 cm after 4 and 8 yr, and after 8 yr at 2.5 to 7.6 cm. The NT treatments at 7.6 to 15.2 cm contained significantly less P than CT at the end of the eight growing seasons.
Extractable K concentrations ranged from 45.3 to 93.0 mg kg-1 in 1988 (Table 3). The distribution of this nutrient differed from P in terms of tillage effects. Specifically, after 4 yr the CT plots contained higher levels of extractable K in the surface 2.5 cm compared to NT, but only corn and soybean plots were significantly different. The same relationship existed at 2.5- to 7.6-cm depths, and at 7.6 to 15.2 cm for corn. The trend of greater extractable K in CT plots continued through eight growing seasons for corn and soybean at 0 to 2.5 cm, but NT cotton plots had greater concentrations than CT. At the lower sampling depths, few tillage effects by crop were found in 1996. The tillage means, however, showed that after 4 yr, NT treatments had significantly less K at all depths, but after 8 yr these differences were not significant.
Higher concentrations of extractable K in CT practices have also been reported by Edwards et al. (1992) and Hargrove et al. (1982) for soils in similar climatic zones. Other reports from more northern climates (Ismail et al., 1994; Follett and Peterson, 1988; Triplett and van Doren, 1969) found more extractable K in NT relative to CT practices.
Extractable Iron, Manganese, Copper, and Zinc
Extractable Fe ranged from 35.2 to 45.8 mg kg-1 (Table 4)
in 1988. In the surface 2.5 cm, Fe was most concentrated in CT plots for all years, but significant differences in tillage treatments occurred only in corn and cotton data for 1996. The tillage means averaged over crops indicate significantly greater Fe in the CT plots. The general trend for Fe in these soils was increased concentrations with time and depth. Relative to tillage, NT cotton and soybean plots had significantly greater Fe contents at 2.5 to 7.6 cm in 1996. This may be explained by the lower pH of NT compared to CT plots at this depth, since the solubility of Fe increases as acidity levels increase. The pH of the CT and NT plots was more similar in the soil surface, but the greater SOM contents may have reduced the extractability of the Fe by formation of fulvic acid–Fe complexes. Similar distributions of Fe have been reported by Shuman and Hargrove (1985) between CT and NT systems.
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Extractable Cu, which ranged from 0.42 to 0.56 mg kg-1 (Table 4) was most concentrated in the CT plots, and increased progressively with time for all treatments and depths. Although most CT treatments had significantly greater Cu concentrations in the surface 2.5 cm within 4 yr, no tillage effects were found at the lower sampling depths. Other researchers (Hargrove et al., 1982; Shuman and Hargrove, 1985; Edwards et al., 1992) have reported no tillage effects for extractable Cu. Shuman (1979), however, indicated that higher Cu contents can be expected in finer textured, higher organic matter soils due to greater cation-exchange capacities relative to coarse-textured soils. This relationship seems to explain Cu distributions with time and between tillage treatments for these soils. Clay contents (discussed later) progressively increased with time in the CT plots, due to continued erosion losses from 1988, as did SOM contents in the NT. According to Shuman (1979), both of these changes should enhance extractable Cu contents. Apparently, for these soils, the increase in clay contents at the surface contributed more to higher extractable Cu than did the increases in SOM. Thus, the erosion enhanced clay contents in CT plots resulted in significantly greater amounts of extractable Cu in the surface 2.5 cm within 4 yr.
The distribution of extractable Zn coincides with Cu, relative to the effects of clay and SOM contents (Shuman, 1979). For these soils, extractable Zn ranged from 2.0 to 7.6 mg kg-1 (Table 4), and was concentrated in the 0- to 2.5-cm depth of the NT treatments. The only tillage effects identified were restricted to this zone. The data suggest that extractable Zn is controlled more by SOM than clay contents or soil pH, the opposite of Cu. For example, both SOM and Zn contents increased in the surface 2.5 cm of the NT plots within 4 yr. During this same period, Zn levels dropped as SOM contents of the CT plots decreased and clay contents increased in the soil surface. A response to soil pH is also indicated by the minimum Zn contents of 1992 which coincide with the lowest soil pH recorded in the study (Table 2). These findings are supported by a regression equation derived by Edwards et al. (1992) which showed that the prediction of extractable Zn was improved when the soil pH factor was added to SOM contents in the equation.
Soil Physical Properties
The response of soil physical properties, that influence erodibility, to NT practices was evaluated for the surface 2.5 cm. The data for AS indicate a significant increase within the first 4 yr for the NT treatment of all three crops compared to CT (Table 5) . Tillage means for 1992 show that AS for all NT crops was 54% greater than CT (54 vs. 35%). The AS of NT corn increased through 8 yr, to a level of 60%. However, NT cotton and soybean decreased between the 4 and 8 yr periods. The decrease from 1992 to 1996 for NT soybean was significant. Aggregate stability also decreased significantly with time on the CT plots of all crops, in the following order: cotton > corn > soybean. In all instances, this relatively large loss of AS with time contributed to significantly lower values compared to NT.
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0.01). The lack of a significant increase in the AS of NT cotton is difficult to account for, considering the increase in SOM during the time. A possible explanation is that a relatively large fraction of SOM derived from cotton residues does not contribute much to AS due to its physical–chemical characteristics. The loss of AS in the NT soybean for 1996 is attributed to the decrease in SOM for this period.
Water dispersible clay contents are an additional indicator of relative erodibility. This property is inversely related to soil stability, and may be a more sensitive measure of a soil's response to erosion by water than AS determined by standard procedures. The WDC data range from 1.9 to 5.1% (Table 5), and indicate a significant increase for CT within 4 yr, with the exception of soybean. This increasing trend continued for CT cotton and soybean for the duration of the study, but WDC contents of CT corn did not increase beyond 4 yr. For NT, there were no time effects on WDC in the first 4 yr. In fact, WDC did not decrease for NT soybean after 8 yr. The NT corn treatment had the only significant decrease in WDC between 4 and 8 yr, however, in all but one case (soybean, 1992), CT had significantly greater amounts than NT. When averaged over crop, the CT treatments had significantly higher WDC contents for both 1992 and 1996. The lack of more significant differences with time in WDC contents of NT treatments is surprising considering the relatively large increases in SOM contents on these plots. The r value for WDC vs. SOM contents was -0.36 (P 0.01). This may indicate that a longer time period is required for SOM to reduce WDC contents.
Total clay contents increased significantly in the CT treatments after 8 yr due to progressive erosion of the soil surface that preferentially removed silt-size materials, and slightly enriched clay contents (Table 5) by gradually incorporating B-horizon materials. Some increases in TC also occurred in the NT treatments during this time period but none were significant. The tillage means averaged over all crops show significantly greater TC contents in the CT treatments for both 1992 and 1996, but such relatively small differences are of little consequence to soil behavior, with the exception of their influence on micronutrient distributions.
Modulus of rupture (Table 5) decreased significantly for all NT treatments and CT cotton within 4 yr. In each case, MR for NT plots was significantly lower that CT. No significant changes occurred for NT corn and cotton between 4 and 8 yr, however, MR for the NT soybean plots increased significantly, again reflecting the loss of SOM for this treatment over this time period. Conversely, all CT treatments increased significantly between 4 and 8 yr. These fluctuations are related to gradual changes in SOM contents, which consists of losses in the CT and gains in the NT treatments. The r value for MR vs. SOM is -0.58 (P 0.01). In all cases, the CT plots have a significantly greater MR relative to NT.
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Conclusions |
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TOPNOTESABSTRACTINTRODUCTIONMethods and materialsResults and discussionConclusionsREFERENCES |
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From the standpoint of extractable nutrients, the higher levels of P in the surface 2.5 cm of the NT plots was most notable. No consistent relationship was identified relative to the distribution of micronutrients between tillage treatments. Extractable Fe and Cu were most concentrated in the CT plots that had relatively higher clay and lower SOM contents compared to NT. Extractable Mn and Zn levels were greatest in the NT plots, due to the higher SOM contents.
The changes in soil properties recorded in the study suggest that the adoption of NT practices will result in substantial improvements in fertility and erodibility parameters within 4 yr. The level of improvement will depend on the extent to which SOM contents are increased by a particular cropping system.SAS Institute 1997
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMethods and materialsResults and discussionConclusionsREFERENCES |
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Received for publication February 4, 1998.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMethods and materialsResults and discussionConclusionsREFERENCES |
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