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a Dep. of Plant Prod., Faculty of Agron., Univ. of Buenos Aires, 1417, Av. San Martín 4457, Buenos Aires, Argentina, and Nitragin Argentina S.A., Calle 10 y 11, Parque Industrial Pilar, 1629, Pilar, Buenos Aires, Argentina
b Dep. of Agron., Univ. of Kentucky, Lexington, KY 40546-0091
c Dep. of Earth and Planetary Sci., Univ. of Tennessee, Knoxville, TN 37996-1410
* Corresponding author (mdzorita{at}speedy.com.ar)
Received for publication December 14, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: GMD, geometric mean diameter • LogGSD, log of the geometric standard deviation • NT, no-till • SWC, soil water content • Till, chisel plus disk tillage • TN, total nitrogen • TOC, total organic carbon
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The development of structure in soils is also important for controlling wind and/or water erosion processes (Lal and Elliott, 1994; Skidmore et al., 1994). For example, the presence of large stable soil fragments in seedbeds promotes the occurrence of surface roughness, reducing soil erodibility by wind (Chepil, 1953) as well as water erosion (Deizman et al., 1987). Well-structured soils have higher infiltration rates and can maintain their pore system in a stable state longer than soils formed by weak structural units (Kay, 1990).
In agricultural soils, tillage and traffic operations are major factors involved in soil structure degradation through fragmentation and compaction processes (Kay, 1990). Fragmentation involves the break down of large structural units into smaller fragments. Compaction involves a reduction in the void space. In both processes, the final soil structural status will depend on the amount of mechanical stress applied and soil properties [e.g., organic matter, texture, soil water content (SWC)].
The surficial accumulation of soil organic materials favored by minimum tillage disruption in conservation tillage systems is recognized to be a major factor contributing to the development of structural units similar to those observed under sod or native conditions (Lal et al., 1994). No-tillage practices have been reported to maintain and sometimes enhance soil aggregation when compared with other tillage practices (Mahboubi and Lal, 1998; Díaz-Zorita, 1999). Another advantage of NT systems is conservation and storage of soil water due to reduced water evaporation and increased infiltration in the presence of surface mulches (Mahboubi et al., 1993; Unger and Jones, 1994).
The maintenance of continuous NT systems may produce soil conditions (higher soil compaction, lower soil temperatures) that reduce crop productivity (Lal et al., 1994). For example, greater soil penetration resistance and bulk density levels are commonly observed in NT soils compared with frequently tilled soils (Van Ouwerkerk and Boone, 1970; Kruger, 1996). Several researchers have observed winter wheat grain yield reductions when planted after maize crops in NT systems and attributed this negative effect to either prolonging low temperatures during spring or to poor stand establishment in the presence of large amounts of residues (e.g., Mead and Chan, 1988; Kiger and Grove, 1999). Winter wheat plant, tiller, and spike densities increased when the distance between maize residue and the wheat row was increased (Kiger and Grove, 1999). Thus, tilled winter wheat seedbeds are the common practice currently used by producers to avoid winter wheat grain yield reductions.
Pierce et al. (1994) studied the effects of periodic moldboard plowing on soil properties of soils in NT farming systems in eastern Minnesota. They observed that plowing, compared with NT, decreased bulk density and increased total and macroporosity levels. Within 4 to 5 yr after plowing, the effects of plowing were dissipated, and the soil was physically similar to NT. Tilling a soil previously subjected to NT redistributed soil nutrients within the plow layer, but the surface stratification of NT was not entirely lost. Residual effects of periodic plowing were minimal for soil physical properties but significant for chemical properties including organic C levels (Pierce and Fortin, 1997).
The information available supporting the regular implementation of tillage practices in otherwise NT systems is not abundant (Pidgeon and Soane, 1977; Campbell et al., 1984). Varsa et al. (1997) and Díaz-Zorita (2000) showed that periodic loosening of NT soils with deep-tillage operations does not always result in increased crop yields. West et al. (1996) concluded that deep loosening improved maize yields on dark, poorly drained soils but not on well-drained soils where NT is traditionally well adapted.
The objectives of this research were (i) to characterize the effects of periodic tillage disruption in otherwise NT systems on soil properties and crop productivity in a winter wheat–double-crop soybean–maize rotation in western Kentucky and (ii) to determine if soil structural changes occurring in tilled soils are independent of changes in other soil properties.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Two winter wheat seedbed planting systems, NT winter wheat and tilled winter wheat (Till), were compared using a complete randomized block design in plots 9 m long by 3 m wide with three replicates. No-till winter wheat was planted directly into mechanically shredded maize stalks with a NT drill. Tilled winter wheat plots were chisel-plowed and disked twice before planting. Maize and double-crop soybean crops were always planted with NT practices in both winter wheat planting systems (NT and Till) so that the only tillage difference in the entire rotation was tillage for winter wheat. Each year, the three crops were planted at an optimum time: winter wheat, early to mid-October; double-crop soybean, mid-June; and maize, early April. Crop cultivars changed from year to year. Lime and N, P, and K fertilizer applications were made according to recommendations (Anonymous, 2003) and were similar for both treatments (NT and Till).
Crop Measurements and Weather Information
Maize, soybean, and winter wheat grain yields were determined yearly since 1992 until 2000 by harvesting the center rows of each plot with a plot combine. All grain yields were adjusted to a moisture content of 130 g kg–1. Maize grain yields for the 1996 season were not reported because of failures during the harvest operation.
Daily precipitation records for the studied period, from the weather station located at the University of Kentucky Research and Education Center in Princeton (KY), were downloaded from the University of Kentucky Weather Center (http://wwwagwx.ca.uky.edu; verified 22 July 2004).
Soil Measurements
Composite soil samples from the midinterrow space of each plot, where low machinery traffic occurs, were taken in the spring of 2000, 20 and 8 mo after the fourth tillage operation (spring 1998 and 1999). At soil sampling, maize and soybean crops were within the first month after planting.
Total Organic Carbon, Nitrogen, pH, and Soil Test Phosphorus and Potassium
Twelve composite soil samples from the 0- to 10- and 10- to 20-cm layers were taken from each plot using a 1.6-cm-diam. cylindrical core sampler. Soil total nitrogen (TN) and total organic carbon (TOC) concentrations were determined by dry combustion using a LECO CN-2000 (Leco Corp., St. Joseph, MI) analyzer (Tabatabai and Bremner, 1970). Soil water pH in a 1:1 slurry was measured with deionized water electrometrically (Thomas, 1996). Soil test extractable P and K were determined according to the Mehlich-III procedure (Mehlich, 1984).
Bulk Density and Saturated Hydraulic Conductivity
Duplicate soil core samples were taken in the 0- to 10- and 10- to 20-cm soil layers with a 5.2-cm-diam. and 137.4-cm3 cylinder for measuring soil oven dry bulk density (Blake and Hartge, 1986). The mean coefficient of variation between the duplicates was 16.3%. Only in the plots cultivated with maize, 20 mo after the last tillage operation, a separate triplicate set of samples was taken from the 0- to 10-cm soil layer for determining saturated hydraulic conductivity using a constant head permeameter (Klute and Dirksen, 1986). The average coefficient of variation between triplicates was 27.8%.
Dry Fragment Size Distribution
Duplicate soil samples in the 0- to 10- and the 10- to 20-cm layers were taken with a 5.2-cm-diam. cylinder when the gravimetric SWC averaged 219 g kg–1. No significant differences in SWC between tillage treatments, 212 g kg–1 for NT and 226 g kg–1 for Till, were observed. Each field-moist sample was dropped from a height of 1.6 m over an aluminum tray to induce fragmentation and air-dried for at least 14 d before sieving. The mean SWC after air drying was 14.1 g kg–1. Dry sieving was performed with a Fritsch vertical vibratory sieve shaker (model Analysett-3, Fritsch, Idar-Oberstein, Germany) for 30 s using an oscillation amplitude of 2 mm and a frequency of approximately 50 Hz (Díaz-Zorita et al., 2000). The dry fragment size distribution was determined using a nest of sieves that consisted of 13 aperture sizes between 16.00 and 0.85 mm using the mass of fragments remaining on each sieve after sieving to calculate the distribution of fragments normalized with respect to the total mass. The fragment size distributions were characterized using the geometric mean diameter (GMD) and the log of the geometric standard deviation (LogGSD) parameters from the cumulative log-normal distribution equation fitted using the NLIN procedure of PC-SAS (SAS Inst., 1997). The upper size of fragments was estimated as the maximum length of the sampled soil (i.e., 100 mm).
Soil Water Retention Curves
Composite-disturbed samples from the 0- to 10-cm layer of the soil were taken with a 1.6-cm-diam. cylinder and used for constructing soil water retention curves. Dew point water activity meters (WP4 Dewpoint PotentiaMeter and AquaLab, Decagon Devices, Inc., Pullman, WA) were used to measure soil water tension (
) over a range of SWCs from approximately 100 kPa to oven dryness (Gee et al., 1992). The corresponding gravimetric water contents were measured, and the data was fitted to the Ross et al. (1991) soil water retention model using Eq. [1],
 | [1] |
d is the tension at oven dryness, c is an empirical constant related to the pore-size distribution, and A (Eq. [2]) is a compound constant related to the SWC at saturation (SWCsat), tension at the air entry value (a), d, and c,
 | [2] |
The A, d, and c in Eq. [1] were estimated using the NLIN procedure of PC-SAS (SAS Inst., 1997). All of the fits converged and had adjusted r2 values greater than 0.98.
Soil water retention curves were also measured using disturbed and undisturbed samples. A knife was used to cut a 0.5-cm-thick rectangular soil block from a depth between 2 and 3 cm below the surface. This was performed 1 d after a rainfall, permitting excavating and trimming to a cross-sectional area of 1 cm2 with minimal disturbance. For transportation, the undisturbed samples were placed into the same plastic containers provided for making the water tension measurements with the dew point water activity meter. Additional samples, taken from directly adjacent soil, were placed in a plastic bag and broken by hand into small fragments under field moisture conditions. Equation [1] was fitted to the two data sets using the NLIN procedure of PC-SAS (SAS Inst., 1997), yielding estimates of A, d, and c. All of the fits converged and had adjusted r2 values greater than 0.97.
Soil Water Content
In 1999 and 2000 maize growing season, beginning one month after planting, 15 composite samples were taken every 2 wk from the 0- to 15-, 15- to 30-, and 30- to 45-cm layers of the soil with a 1.6-cm-diam. cylinder from all plots planted with maize and used to determine the gravimetric SWC.
Penetration Resistance
In each plot in its second year after tillage, 4 d after a rainfall, six sampling points were located along the midinterrow for measuring the soil strength in the 0- to 30-cm layer at 1-cm intervals using a recording cone penetrometer (Eijelkamp Agrisearch Equipment, Giesbeek, the Netherlands). The gravimetric SWC, for the 0- to 10- and 10- to 20-cm soil layers, was determined by compositing samples taken close to each point of penetration.
Data Analysis
Analysis of variance was performed for interpreting soil management effects on seasonal crop grain yield for a randomized block design using the GLM procedure and protected LSD test of PC-SAS (SAS Inst., 1997) at a 95% level of confidence. Seasonal mean yield effects on treatment (NT and Till) yields were evaluated comparing regression lines (Snedecor and Cochran, 1989) between both variables for each crop (Analytical Software, 2000).
For each crop sequence (winter wheat–soybean or maize plots), soil management (NT and Till) effects on TN, TOC, pH, soil test P and K, bulk density GMD and LogGSD parameters, and penetration resistance levels were analyzed using analysis of variance (SAS Inst., 1997) for a randomized block design split by depth of sampling. Saturated hydraulic conductivity and soil water retention parameters were evaluated using one-way analysis of the variance (SAS Inst., 1997).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Mean maize grain yields ranged from 9277 to 11903 kg ha–1, and in most of the seasons, four out of six, the yields under continuous NT management were significantly greater than those where tillage was done before winter wheat planting (Table 1). Averaged over all six seasons, maize under NT management yielded 7.0% more than in Till treatments. The slopes of the linear relationship between the average seasonal yield and treatment yields were not statistically different (P = 0.60). However, a statistically greater intercept (P = 0.02) was determined for continuous NT maize compared with Till for winter wheat treatments (Fig. 2c).
Maize grain yields were increased by continuous NT management, independent of seasonal productivity. Nevertheless, greater enhancement of maize yields under continuous NT management occurred in 1994 and 1999 seasons with greater water deficiencies than when adequate rainfall occurred (Table 2). The percentage of yield change under continuous NT with respect to Till management was negatively related to the total amount of rainfall in May [yield change (%) = 22.3 – 0.13 x rainfall (mm), r2 = 0.76, P = 0.023].
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The yields obtained in our study translate to an economic benefit for the continuous NT system. This conclusion was based on the average grain yields, total cost, and grain prices for farmers in western Kentucky for the studied period (1991 to 2000). For the complete maize–winter wheat–double-crop soybean sequence, the production costs were only different for winter wheat. We assume that the cost of chisel tillage, with one secondary disking, was about $50 ha–1, including the fuel and the maintenance of a larger tractor for primary and secondary tillage. The cost of herbicide use, including spraying, was estimated to be $40 ha–1. Thus, net returns for the complete maize–winter wheat–double-crop soybean sequence were estimated to be $73 ha–1 higher under NT than under Till treatments. This comparison does not include government payments and promotions.
Tillage Effect on Soil Properties
Soil test pH, P, K, TOC, TN, and bulk density levels, either 8 or 20 months after the tillage operation in 1999, were similar between NT and Till treatments and independent of the sampling depth (Table 3). Mean values of these properties, averaged by management treatment or by sampling depth, are presented in Table 4. The absence of significant differences in pH, P, or K levels was expected because they were treated equally in both tillage treatments. The TOC and TN levels were probably the same for both treatments because the tillage operations for winter wheat were performed using tillage tools without soil inversion and in the fall when low temperatures as well as high SWCs are the rule. Conservation tillage practices, those that maintain at least 30% soil surface coverage with harvest residues at planting, promote surface accumulation of C because of reduced mineralization and erosion (Thomas, 1985; Golchin et al., 1994; Buschiazzo et al., 1998). Furthermore, soil mineralization is reduced with a reduction in soil temperature and/or aeration (Skopp et al., 1990; Paul and Clark, 1996). Bulk density changes in agricultural soils with similar textures were related to machinery traffic and losses of organic matter after tillage operations (Kay, 1990). In this study, the lack of statistically significant differences in bulk density between tillage treatments can be explained by the fact that soil samples were taken in the interrow space where low machinery traffic occurred and that there were no significant changes in organic matter levels.
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No significant interaction between sampling depth and tillage treatments on fragment size distribution was observed. Averaged over the 0- to 20-cm layer, significantly larger GMDs of dry fragments were found under NT (13.2 mm) than under Till (11.1 mm). No significant effect of tillage practices on the LogGSD of dry fragments was observed. The occurrence of large-sized fragments indicates that organic and inorganic bonds between primary soil particles were stronger under NT systems.
Soil water retention curves based on disturbed samples were different for the two tillage treatments (Fig. 4). In this experiment, because of the lack of difference in bulk density between treatments, differences in gravimetric water content can be interpreted mostly in terms of pore size distribution. At 0.01 MPa of water tension, equivalent to pores 30 µm in diameter, the gravimetric SWC was significantly greater in soils under continuous NT (0.265 g g–1) than in those under tillage for winter wheat (0.215 g g–1). No significant differences (P > 0.10) between the two tillage treatments were observed in the gravimetric SWC at a water tension of 1.5 MPa, equivalent to pores with 0.20-µm diameter. The proportion of mesopores, i.e., pores with equivalent diameter range from 10 to 1000 µm (Luxmoore, 1981), was 23% greater in NT soils (0.189 cm3 cm–3) than in soils tilled once every 2 yr for winter wheat (0.154 cm3 cm–3).
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Under field conditions, the greater SWC in NT soils has been attributed mostly to reduced evaporation due to the presence of surface residue cover, an "organic mulch" (Blevins et al., 1971; Unger, 1994; Tanaka and Anderson, 1997). In 1999, greater SWCs were found under continuous NT management during most of the maize growing season (Fig. 5). No significant differences were found the following season (Fig. 4) when greater rainfall occurred (Table 2). Soil residue cover 20 months after the last tillage for winter wheat was similar between management treatments (data not shown), implying similar protection against evaporative losses between tillage systems. Thus, these results suggest that differences in soil structure resulted in greater water storage in continuous NT soils and improved drought resistance.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The difference in maize yields between continuous NT and Till for winter wheat soils was greater in seasons with few rainfall events, indicating the importance of maintaining good soil structure to facilitate an adequate water supply for the crop. Greater water storage was observed under continuous NT mostly because of the contributions of a greater percentage of mesopores and pore continuity (greater Ksat values).
These results suggest that soil structure could play a role in reducing losses of soil water to the atmosphere, probably due to differences in the proportion of mesopores and the continuity of macropores. In continuous NT soils, greater soil water conservation can be attributed to the cohesion of small soil aggregates with smaller intra-aggregate pore sizes that favor water storage, thereby reducing evaporation. In addition, a lack of tillage disruption allows the stabilization of interaggregate pores with a range of sizes that significantly contribute toward greater soil water infiltration and storage. The period without tillage in the tilled winter wheat treatment, 2 yr, was not long enough to permit soil structure recovery. Similar results have been reported elsewhere, indicating that the benefits of NT soils, in terms of soil structure–related processes, require continuity in NT soil management over much longer periods of time (Voorhees and Lindstrom, 1984; Bergh et al., 2000; Karunatilake et al., 2000; McGarry et al., 2000).
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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