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USDA-ARS Grassland, Soil and Water Research Lab., 808 East Blackland Rd., Temple, TX 76502
* Corresponding author (torbert{at}ars.usda.gov)
Received for publication August 1, 2000.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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Abbreviations: Chisel-bed, chisel plow tillage system with bedding • chisel–no bed, chisel plow tillage system without bedding • no-till, no-tillage system with bedding
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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Vertisols, in the Blackland Prairie, are difficult to manage because of their physical characteristics, including high shrink–swell potential, high water holding capacity, high plasticity, increased strength when dry, and a limited range of soil water content in which soil tillage can be performed (Potter and Chichester, 1993). When wet, these soils have slow infiltration rates, which can lead to high runoff rates; because such soils slake into fine aggregates, they are easily eroded by water. The most common tillage system used in this region for corn production has been a chisel tillage system. A management system using raised wide beds has been proposed as a conservation tillage system for these soils (Morrison et al., 1990). Furrows between the wide beds were used as surface drain ways and controlled-traffic lanes. This system has been found to support corn grain yields comparable to the conventional chisel tillage systems (Potter et al., 1996), but the influence of these soil tillage systems on fertilizer N management has not been determined.
Fertilizer N management can be greatly affected by changes in tillage. For example, conservation tillage systems may increase both N immobilization (Gilliam and Hoyt, 1987) and N losses from leaching (Tyler and Thomas, 1977) and denitrification (Gilliam and Hoyt, 1987). Soil moisture and temperature, which are greatly affected by tillage, will also impact soil N dynamics (Torbert and Wood, 1992; Nadelhoffer et al., 1991). The use of conservation tillage has been reported to increase short-term N immobilization due to the slower plant decomposition process when tillage is limited (Gilliam and Hoyt, 1987; Wood and Edwards, 1992). Generally, in conservation tillage fertilizer N rates have been increased by as much as 25% to prevent yield limitations from short-term N immobilization (Randall and Bandel, 1991). It has been hypothesized that application efficiency of fertilizer N can be enhanced by synchronizing fertilizer application with plant uptake needs (Keeney, 1982). Fertilizer applied during peak plant N demand can limit fertilizer N immobilization and/or losses from the soil–plant system due to leaching and denitrification and to increase N use efficiency (Olson and Kurtz, 1982; Keeney, 1982). Synchronization of residue N mineralization, fertilizer-N application time, and subsequent crop demand for N can improve N use efficiency of crops planted in conservation tillage systems (Reeves et al., 1993). Studies conducted on C and N cycling in Vertisols have shown that changes in potential N mineralization levels were impacted more due to changes in tillage intensity than fertility management (Torbert et al., 1997), but the influence of these soil tillage systems on fertilizer N rates or timing on corn production in Vertisols of the Blackland Prairie has not been determined.
In addition, in the Blackland Prairie, the soil and climate conditions that make managing the soil physical properties difficult also cause difficulties with fertilizer N management. For example, substantial losses of fertilizer nutrients could occur with surface application of granular fertilizer under wet soil conditions (Torbert et al., 1999). Using simulated rainfall on a chisel plowed field, Torbert et al. (1999) reported an increase of 3.0 to 18.9 kg NH4–N ha-1 losses in a runoff event when fertilizer was applied to wet soil (0.30 kg kg-1 soil) when compared to dry soil (0.10 kg kg-1 soil). Covering the soil with residue reduced NH4–N losses from 18.9 to 3.9 kg ha-1. Physical restraints due to the soil and climate conditions also impact fertilizer application management due not only to the limited time for field operations, but also to the interaction of application equipment with the physical condition (i.e., stickiness) of soil (Morrison and Chichester, 1988; Morrison and Potter, 1994). Because soil conditions are usually wet during planting operations, fertilizer N commonly is applied during the fall or early winter to expedite field operations. However, the potential fertilizer N loss during this fallow period and the impact on crop yields due to these losses is not well understood.
The sustainability of any crop production system depends on maintaining adequate soil plant nutrients such as N. To fully develop conservation tillage systems for the soils of the Texas Blackland Prairie an understanding of the dynamics of applied N fertilizer in this new conservation tillage system is needed. The objectives of this study were: (i) to investigate the impact of tillage systems on fertilizer N application rates needed; and (ii) to examine the impact of fertilizer application timing within these tillage systems on corn production.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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Tillage plots (244 m long and 18.3 m wide) consisted of either a chisel plow tillage system with bedding (chisel-bed), chisel plow without bedding (chisel–no bed), or a no-tillage system with bedding (no-till). The chisel–no bed system consisted of flail-shredding residue, tandem disking, chisel tilling, tandem disking, and field cultivating. This system is the most common management system in the area. The no-till system consisted of no preplant tillage and planting with a slot planter. Percent residue cover for the wheat residue (residue into which corn was planted) in this study was reported at 99.3% with a standard deviation of 1.5 for the no-till treatment, while the chisel-till treatment resulted in a 30.0% residue cover with an 8.6 standard deviation (Torbert et al., 1996; Morrison et al., 1996). In the chisel–no bed and the no-till systems, the beds were raised soil areas 0.15 m high, 1.5 m wide, and each was separated by 0.5-m furrows (Morrison et al., 1990). In the chisel-bed system, the beds were chisel plowed annually and then reconstructed each year. Once every 3 yr in the no-till system, the furrows between the wide beds were maintained with a sweep that cleaned the furrow and reformed the bed shoulder.
Crop production in these plots consisted of an annual rotation of wheat (Triticum aestivum L.), followed by corn (Zea mays L.), which was followed by grain sorghum [Sorghum bicolor (L.) Moench]. Corn row spacing was 76 cm. A starter fertilizer (10–34–0 solution) placed adjacent to the seed at planting provided 5.6 kg N ha-1 and 19.1 kg P ha-1 to all treatments. Yearly fertilizer N applications previous to initiation of this study were 168 kg ha-1 applied to corn, 140 kg ha-1 applied to grain sorghum, and 112 kg ha-1 applied to wheat. All three crops were present in the study each year, but only corn will be discussed. Corn was planted on 3 Mar. 1994, 23 Feb. 1995, 21 Feb. 1996, and 31 Mar. 1997 at a rate of 66700 seed ha-1.
The main plots were split into seven fertilizer N management treatments, each of 30.5 m in length and 4 m wide, which included four fertilizer N rate and three fertilizer N application timing treatments. Fertilizer N rate treatments consisted of 0, 56, 112, and 168 kg N ha-1 applied at planting. Fertilizer N application timing treatments included applying 168 kg N ha-1 in the fall, at planting, or a split fertilizer N application consisting of 112 kg N ha-1 applied at planting and 56 kg N ha-1 applied 30 d after planting. Liquid UAN (urea ammonia nitrate, 32–0–0) was applied in a low-disturbance band with a coulter-nozzle applicator, which placed the fertilizer in the surface 5 cm of soil with complete soil coverage of the fertilizer band (Morrison and Potter, 1994). Similar fertilizer N application treatments were applied in the wheat and grain sorghum crop rotations at rates appropriate for each crop to maintain a continuity of fertilizer N treatment plots (data not shown).
At harvest, plant samples were collected for grain and fodder yield by collecting above ground biomass from a total of 5.9 m of row. Plants were collected from across the entire width of the wide bed to account for any difference due to proximity to the furrow. Plant samples were separate for grain and fodder, dried at 65°C (until weight loss was complete), ground to pass a 0.15-mm sieve, and analyzed for total N on a FISON NA1500 nitrogen and carbon determinator (Fison Instruments, Dearborn, MI1). Total biomass and total N uptake were calculated as the sum of the grain and fodder components.
Statistical analysis of data were performed using the Mixed procedure of the Statistical Analysis System at an established a priori level of P 
0.10 (SAS Inst., 1996).
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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In the dry 1996 year, as was observed in 1994, the lowest biomass yields were observed with the chisel–no bed treatment and the highest yields were observed with the no-till treatment. This increase in biomass production also resulted in a corresponding increase in grain yields. While grain yields in 1996 were reduced when compared with other years (especially the wetter years), the no-till treatment provided a yield advantage when compared with chisel–no bed and the chisel-bed treatments (Table 3). Similar effects were observed for N uptake between the tillage treatments in 1996 for both the grain and total biomass measurements (Tables 2 and 3). The yield advantage observed in both biomass and grain production with the no-till treatment when compared to the chisel tillage treatments in the dry years of 1994 and 1996 was most likely due to the 99.3% residue ground cover, which would have reduced soil water evaporation and increased soil water conservation.
In the 2 yr with higher rainfall (1995 and 1997), biomass production was higher when compared with the dry years and differences between tillage systems were smaller (Table 2). In 1995, the chisel-bed system had the highest biomass production and chisel–no bed had the lowest yields. No significant difference was observed for biomass production between tillage treatments in 1997 (Table 2). However, differences were observed in total N uptake between the different tillage treatments in the 2 wet years (Table 2). In both 1995 and 1997, the chisel–no bed treatment had lower N uptake when compared with the bedded treatments, and in 1995 the no-till system had higher total N uptake when compared with the other tillage systems (Table 2).
As with biomass production, grain yields were higher in the higher rainfall years (1995 and 1997) than the low rainfall years (1994 and 1996) (Table 3). In 1995 and 1997, the chisel–no bed treatment had lower yields than the no-till treatments (Table 3). Similarly, N uptake in grain mirrored these differences (Table 3).
In almost all cases, the chisel–no bed treatment resulted in numerically lowest yield and N uptake in both grain and biomass (Tables 2 and 3). Likewise, the highest numeric yields and N uptake were observed with the no-tillage system. This indicated that a yield advantage may be realized by using a tillage system that utilized soil bedding in the heavy clay soils of the Blackland Prairie. Likewise, this also indicates that the no-tillage system may be the most reliable tillage system for corn production in this region.
Corn response to fertilizer N application rate was highly dependent on the rainfall conditions. In the two low rainfall years (1994 and 1996), limitations to plant growth resulted in a limited effect for fertilizer N application in corn biomass production (Table 4). In both 1994 and 1996, only the no-till treatment had a positive response to increasing N fertilizer for biomass production (Table 4). In 1994, the increased N application rates increased N uptake with the no-till system. A significant increase in N uptake was also observed for the chisel–no bed treatment, although no increase in biomass production was observed. With the chisel-bed treatment, there was no significant effect for either biomass production or N uptake with fertilizer rate treatments (Table 4). Again, in 1996 only the no-till treatment had a significant response to N fertilizer application for biomass production, and a positive response for fertilizer N application in total N uptake for the no-till treatment (Table 4). A significant increase in N uptake was observed for the chisel-bed treatment, but this increase did not result in a significant increase in biomass production for this treatment. No significant effect was observed for either biomass or total N uptake for the chisel–no bed treatment (Table 4).
In the 2 yr with higher rainfall conditions, a significant increase was observed for biomass production and total N uptake with increased fertilizer N application in all tillage systems (Table 4). In general, fertilizer N application up to 168 kg ha-1 increased both the biomass yield and N uptake in both years (Table 4).
Grain yield and grain N uptake response was severely limited in the 2 dry years, with no significant effect to increasing N application with any of the tillage treatments for either 1994 or 1996 (Table 5). The small fertilizer N response that was observed for total biomass production and total N uptake in these 2 yr (Table 4) was not observed as a N response for grain production (Table 5).
In both 1995 and 1997, increasing fertilizer N rate increased corn yield within all of the tillage systems up to 168 kg ha-1 (Table 5). Likewise, N uptake in grain increased with increasing N application in 1995 and 1997 (Table 5). Generally, the larger the yield response from tillage systems, the more positively corn yields increased with increasing fertilizer N. For example in 1995, grain yield in chisel–no bed increased from 3601 to 4809 kg ha-1 while no-till increased from 4161 to 7612 kg ha-1 for fertilizer application of 0 to 168 kg N ha-1, respectively. Similar results were observed in 1997, with an increase of 3929 to 8303 kg ha-1 for chisel-bed and 6094 to 8435 kg ha-1 for fertilizer N application of 0 to 168 kg ha-1, respectively.
Conversion to conservation tillage has been reported to increase short-term N immobilization, and therefore, to prevent yield losses due to N stress, many agronomist recommend that fertilizer rates should be increased (Randall and Bandel, 1991; Wood and Edwards, 1992). In our study, there was no indication that short-term N immobilization reduced yield in the no-till system (Table 5). Even at the lower N rates, no reduction in yields were observed in the no-till compared with the chisel tillage systems in any year (Table 5). In fact, in 1997 the no-till resulted in a significant increase of grain yield compared with the chisel tillage treatments with 6094 kg grain ha-1 for no-till compared with 3929 kg grain ha-1 (P = 0.015) for chisel-bed and 4217 kg grain ha-1 (P = 0.034) for chisel–no bed. Likewise, no effect was observed for N uptake with the use of no-till compared with the chisel tillage systems, including the 2 yr that where highly responsive to fertilizer N application (Table 5). In fact, as was observed with grain yield, end of the year N uptake in grain was significantly increased in no-till compared with the chisel tillage systems, even when no fertilizer N was applied, with 70 kg N ha-1 for no-till compared with 43 kg N ha-1 (P = 0.023) for chisel-bed and 40 kg N ha-1 (P = 0.010) for chisel–no bed. If a short-term N immobilization occurs in these systems it is not sufficient to either reduce N uptake or to decrease grain yields by the end of the growing season. It is note worthy to remember that these were well established no-till systems that had been in place for 8 yr previous to the establishment of this fertilizer N study. It is likely that the short-term N imbalance in no-till systems compared with conventional tillage systems does not prevail long term. The data from this study indicates that for well-established no-till systems in the Blackland Prairie, there is no need to increase fertilizer N application rate to compensate for N immobilization.
In this heavy clay soil, splitting the N applications did not increase yield (Table 6). Even in the 2 wet years (1995 and 1997) when corn yield responded well to increased N fertilizer application rates, no significant impact to split application was observed and in general, a numerical reduction in yield was observed with the split N application in all the tillage treatments for the 4 yr of the study (Table 6). Likewise, no significant increase in N uptake was observed from applying split application of 56 kg ha-1 after planting (except for chisel-bed in 1994) (Table 6). This indicates that N availability was not greatly limiting plant N uptake or grain yields in the no-till system when compared with the chisel tillage systems. This is further evidence that in this soil, short-term N immobilization either did not occur or has no lasting effect over the growing season on corn production. Further, this data would indicated that the extra expense incurred with time and equipment with split fertilizer N applications would not be warranted.
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The N uptake reduction observed in this study with the fall N application treatment was most likely due to soil N loss mechanisms during the winter fallow period before planting (i.e., leaching, erosion, denitrification). In these heavy clay soils, water movement through the soil profile is generally very slow (Potter et al., 1995); therefore, N leaching is normally considered to be nonconsequential. However, differential water flow through soil cracks may greatly contribute to water recharge of the soil profile (Potter et al., 1995), and therefore, could contribute to some N losses from the soil through leaching. Soil erosion may have also contribute to N losses. However, fertilizer N losses from erosion were likely very small since the N uptake reduction observed was as prevalent in the no-till treatment where erosion would be greatly reduced compared with the chisel tillage systems (Torbert et al., 1996). The largest N losses were most likely due to denitrification, since all of the soil conditions required for denitrification were present during the fallow winter period (Ryden and Lund, 1980; Mulvaney and Kurtz, 1984). For example, following fall fertilizer N application in this region soil conditions are commonly saturated and soils temperatures are only rarely below freezing. In addition, these soil are calcareous and denitrification is promoted by elevated soil pH levels. Also, in these tillage systems, the residue of the previous year's crop would provide a readily available soil C source for microbial activity.
Yield losses caused by fall N application were greatest in years with the highest yield potential. For example, the greatest yield levels observed in this study were for the no-till treatments in 1997. In 1997, fall N application to corn grown in no-till resulted in a 48% reduction in yield compared with fertilizer N application at planting (Table 6). The greatest impact to the farmer for expediting fertilizer N application with fall application would be in losing yield potential in those few years when growing conditions were optimal.
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CONCLUSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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1 Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the production, the use of the name by USDA implies no approval of the product to the exclusion of others that may be suitable. 
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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