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DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION

Tillage Influence on Soluble Salt Movement in Silt Loam Soils Cropped to Paddy Rice

^a P.O. Box 3508, Southeast Research and Extension Center, Univ. of Arkansas, Monticello, AR 71656 USA
^b P.O. Box 48, Northeast Research and Extension Center, Univ. of Arkansas, Keiser, AR 72351 USA
^c 115 Plant Sciences Bldg., Dep. of Agronomy, Univ. of Arkansas, Fayetteville, AR 72701 USA
^d 403 Agricultural Engineering Bldg. 2, Dep. of Agric. Economics, Univ. of Kentucky, Lexington, KY 40546 USA
^e Dep. of Agricultural Economics and Agribusiness, Univ. of Arkansas, 221 Agriculture Bldg., Fayetteville, AR 72701 USA
^f P.O. Box 351, Rice Research and Extension Center, Stuttgart, AR 72160 USA

wilson{at}uamont.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
Rice (Oryza sativa L.) produced in a dry-seeded, delayed-flood system common to the southern USA is extremely sensitive to excessive accumulation of soluble salts during the seedling growth stage. Recent observations have been made by producers, county agents, and researchers that the use of conservation tillage systems, particularly no-till systems, may increase the level of salinity in the rice root zone in dry-seeded, delayed-flood systems during the 4- to 6-wk period prior to permanent flood establishment. With the use of conservation tillage practices for rice production steadily increasing in the southern USA, it is important to determine if these practices increase potential for accumulation of soluble salts, to determine the mechanisms involved, and to develop management strategies to overcome the problem. A 2-yr study was initiated in the fall of 1994 to monitor salt distribution within the soil profile under different tillage regimes. A conventional system, a para-till operation, a chisel plow operation, and a no-till system were implemented in the fall and Br^- was applied to monitor salt movement. `Kaybonnet' rice was seeded during 1995 and 1996 and soil samples were collected from each plot at the two- to three-leaf growth stage. Salt accumulation at the rice seedling growth stage near the soil surface was higher in the no-till treatment than in any of the other tillage treatments based on higher electrical conductivity, Cl^- concentration, and NO^-₃ concentration in the top 2.5-cm depth. At this depth, the electrical conductivity was 30 to 40% greater, the Cl^- concentration was 30 to 160% greater, and the NO^-₃ concentration was 10 to 160% greater in the no-till treatment than in the other tillage treatments. Salt distribution within the profile was similar for all treatments beyond the 2.5-cm depth. The data suggest that tillage does tend to reduce the potential for salt accumulation in the root zone.

Abbreviations: EC, electrical conductivity

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
SALINITY has normally been associated with heavily irrigated areas of the world that also have arid or semiarid climates (Letey, 1984; Rhoades and Loveday, 1990). In the southern USA, substantial dry-seeded rice acreage is affected each year by the accumulation of soluble salts in the root zone when the rice is at the seedling or early vegetative growth stage (Gilmour et al., 1985). Rice is generally tolerant to salinity after the mid-tillering growth stage and is often used for saline soil reclamation (Kaddah et al., 1975). Flushing is the only current management practice routinely used to provide protection from salinity injury at the seedling growth stage (Slaton et al., 1994). Current research is underway to develop management strategies that minimize losses due to salinity in this region.

The silt loam soils used for rice production in the southern USA typically have a clay layer or fragipan whose upper surface is usually <120 cm below the soil surface. These layers have significantly reduced hydraulic conductivity compared with the surface horizon and are essentially impermeable (Keisling et al., 1984; Scott et al., 1986). While this impermeable layer is beneficial by allowing efficient flood irrigation for rice production, the reduced hydraulic conductivity results in slow leaching of soluble salts that are added in irrigation water. Since loss of soluble salts in runoff water is relatively small (Gilmour and Marx, 1981), accumulation of soluble salts in these soils to levels that are detrimental to rice seedlings is significant (Gilmour et al., 1985). Soluble salts move in the horizons above the restrictive horizon as a function of the soil moisture potential and hydraulic conductivity (Gilmour et al., 1985, 1986). As evaporation occurs, salts move toward the soil surface and accumulate in the rice root zone. If sufficient salt accumulates near the soil surface when rice is at the seedling or early vegetative growth stage, significant injury and stand loss can result (Gilmour, 1981; Gilmour et al., 1981).

The use of conservation tillage practices in rice production has increased substantially during the past few years (N.A. Slaton, 1998, personal communication). Although these practices tend to reduce labor, improve surface water quality, and potentially reduce production costs, casual observations by scientists and producers indicate that salinity stress may be enhanced in the predominant dry-seeded, delayed-flood production systems. Since the micropores in the soil under no-till systems are not disrupted as they are under conventional tillage, we hypothesize that water and salt movement toward the soil surface is more efficient. As a result, salt may tend to accumulate more at the soil surface under conservation tillage practices. Research evaluating the effects of conservation tillage on salt dynamics in the soil profile has been limited. Therefore, we implemented this study to evaluate the effects of various tillage systems on salt movement and distribution within the profile of silt loam soils cropped to rice.

	Materials and methods

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The study was conducted at the Pine Tree Branch Experiment Station near Colt, AR, during 1995 and 1996 on a soil association consisting of a Calloway silt loam (fine-silty, mixed, thermic Glossaquic Fragiudalf), Calhoun silt loam (fine-silty, mixed, thermic Typic Glossaqualf), and Henry silt loam (coarse-silty, mixed, thermic Typic Fragiaqualf) in areas that had a history of salinity injury to rice. The experiment was arranged in a randomized complete block design with three replications. Four tillage treatments (conventional, para-till, chisel plow, and no-tillage) were implemented during the fall of 1994 and 1995 in areas that had been previously cropped to soybean [Glycine max (L.) Merr.]. The plots were 18.2 m wide and 36.8 m long during the 1995 growing season and 12.2 m wide and 36.8 m long during the 1996 growing season. The conventional plots were disked in the fall and then disked and smoothed with a land-plane (float) in the spring prior to planting. The para-till deep-tillage plots were tilled with a para-till subsoil implement in the fall prior to the cropping season. The chisel plow deep-tillage plots were tilled with a chisel plow in the spring. The para-till and chisel plow treatments were then disked and planed prior to planting. No tillage operations were used for the no-till treatment. Glyphosate [N-(phosphonomethyl)glycine] was applied to the no-till treatments at a rate of 1.12 kg a.i. ha^-1 10 d prior to seeding.

Cultural practices, including seeding, fertilization, and water management, were conducted for dry-seeded, delay-flood rice production according to standard recommendations (Slaton et al., 1994). Kaybonnet rice was seeded into each plot at a rate of 100 kg ha^-1 on a 15-cm row spacing with a no-till drill at a depth of 0.6 to1.3 cm. Nitrogen was applied at a rate of 84 kg N ha^-1 at the four- to five-leaf growth stage and was followed by establishment of a permanent flood. The flood was maintained at an approximate depth of 10 cm until physiological maturity. Grain yields were determined at maturity by harvesting a 6.1 by 30.4 m section of each plot with a commercial combine and then weighing on a grain cart with a computerized weight system attached. Yields are reported after correcting to 12% moisture.

Bromide was surface applied in the fall of 1994 and 1995 after the fall tillage operations at a rate of 187.7 kg ha^-1 as KBr to monitor salt movement following deposition of salts. Soil samples were collected from 20 locations within each plot when the rice reached the two- to three-leaf growth stage. The samples were then composited into one sample for the depth increments of 0 to 2.5, 2.5 to 5, 5 to 7.5, 7.5 to 15, 15 to 30, and 30 to 45 cm. During 1996, soil was collected within 24 h of a rainfall event of 2.5 cm. After 7 d of drying conditions, additional soil cores were then collected (as described above) based on the prediction that more salts would have accumulated in the surface horizon following the drying period.

The soils were air-dried, ground, and sieved through a 2-mm mesh screen. A 10-g subsample of each soil was oven-dried to determine moisture content and results were reported on an oven-dry basis. A 50-g subsample of each soil was extracted with 100 mL of deionized water for 24 h and electrical conductivity (EC_1:2) was determined on the suspension with a YSI model 32 conductivity bridge (YSI Inc., Yellow Springs, OH). Although standard EC determinations are usually made on saturated paste extracts (EC_sat) or soil solution EC determinations, the 1:2 soil suspension determination used routinely in soil testing laboratories (because of the reduced time and labor involved) relates well to EC_sat (Gilmour et al., 1985; Sriyotai and Gilmour, 1976). The suspension was then centrifuged for 30 min at 5858 g at 20°C, and the centrifugate was passed through a 0.45-µm filter before analysis for Cl^-, Br^-), NO^-₃, and SO^-2₄ with a Dionex (R) ion chromatograph (Dionex Corp., Sunnyvale, CA).

Analysis of variance procedures were conducted with PROC GLM in SAS (SAS Institute, 1985). The grain yields were analyzed as a randomized complete block design, and the soil analyses were analyzed as a split-plot design with the tillage treatment as the main plot and the soil depth as the subplot.

	Results

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Salt Distribution
Tillage significantly decreased the salt accumulation near the soil surface at the time of sampling (Table 1) . The soils were collected when the rice was in the seedling growth stage, which is the time when the rice was most sensitive to salinity (Kaddah et al., 1975). Electrical conductivity was highest near the soil surface and exponentially declined with depth for all treatments. However, all of the tillage operations accumulated less salt in the top 2.5-cm soil depth than the no-till both in 1995 and 1996. The salinity was much greater in 1996 than in 1995 for all treatments. However, an EC_1:2 of 0.4 to 0.5 dS m^-1 in the root zone during the seedling growth stage has been shown to be sufficient to cause salinity injury (Gilmour et al., 1985). Note that no-till treatments had the highest EC_1:2 in the root zone and had corresponding lowest yield (Table 2) . The distribution of total salts within the soil profile is similar for all treatments below the 2.5-cm depth. During 1996, the para-till treatment had significantly less salt in the surface layer (0–2.5 cm) than the conventional tillage treatment.

Comparison of Wet and Dry Soil Conditions
When the distribution of soil EC_1:2 measured after a significant rainfall event was compared with that after a drying period, a substantial difference was observed in salt concentrations with depth (Fig. 1) . Soil samples were collected on 30 May 1996 24 h after a rainfall event measuring 2.5 cm had occurred. Soil samples were also collected on 6 June 1997 after 7 d of drying conditions. The EC_1:2 was significantly higher near the soil surface (0–2.5 cm depth) following the drying period than following the rainfall. Data from both weather scenarios were found to significantly decrease exponentially with depth (Fig. 1). The predictions of EC_1:2 near the soil surface immediately after the rainfall event is on the order of one-half that observed after drying. These data demonstrate the salt concentration effects near the soil surface because of drying and evaporation. The EC_1:2 observed in top 2.5 cm are well within the range where salinity injury to seeding rice would be expected, particularly after the drying event. Since the samples were collected during the most sensitive growth stage for rice, seedling injury to the rice would be expected.

View larger version (18K): [in this window] [in a new window]   Fig. 1 Distribution of electrical conductivity (EC1:2) with respect to soil depth as a function of sampling date during 1996. Rainfall occurred on 29 May. (LSD = 0.17 dS m-1)

	Discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
Many of the silt loam rice soils in much of the southern USA are underlain at some depth, usually <120 cm, with an essentially impermeable layer typically characterized as a fragipan or argillic horizon (Keisling et al., 1984; Scott et al., 1986). Rice producers in this region often create a very dense plow pan beginning at a depth of 7 to10 cm and extending downward to a depth of 15 to 20 cm. This pan is desirable for rice production because it is somewhat restrictive to downward water flow, allowing for a more rapid flood establishment. Also, the layer assists in supporting harvesting equipment when fields are very wet.

The pan that develops tends to have holes, such as root channels, and zones of weakness that have very high hydraulic conductivity at water contents close to saturation compared with the surrounding pan matrix (Keisling et al., 1984). When soluble salts are present at the soil surface, for example, through the addition of irrigation water and fertilizer applications, they are subject to being leached downward with subsequent applications of water. An underlying impermeable layer inhibits soluble salts from being leached from the soil profile. At the beginning of infiltration into the layered soil (Ap, 0–7 cm; plow pan 7–20 cm; impermeable layer, >20 cm) described above, the soluble salt will move close to the wetted front. When the wetted front reaches the plow pan, it will move into the plow pan at a very slow rate and the water contents behind the front will increase if surface infiltration rate exceeds the rate of movement into the plow pan. If surface infiltration is less than or equal to the rate of movement into the plow pan, then the water and solutes will pass through the plow pan together, the salt will move similar to piston displacement, and the peak soluble salt concentration will be found deep in the soil profile (van Genuchten and Wierenga, 1976; Wagenet, 1984). However, if the surface infiltration rate exceeds the transmission of water through the plow pan, then preferential flow occurs. Under preferential flow conditions, the soluble salts will be found with an exponential-type distribution with depth (van Genuchten and Wierenga, 1976; Wagenet, 1984). Under preferential flow, the soluble salt distribution with depth is established quite rapidly and remains stable for a relatively long time. Thus, preferential flow would be involved in developing the type of salt distribution with depth of the naturally occurring Cl^- (Table 3) and the added Br^- (Table 4) that was measured in this study and others (Gilmour et al., 1986).

When a surface infiltration and redistribution event described above ceases, evaporation of soil water will occur at the soil surface. The upward flow of water is slow enough that nonpreferential flow occurs. This nonpreferential flow moves the soluble salt from the entire depth cross-section toward the soil surface. When the water evaporates at the soil surface, the soluble salts are deposited near the soil surface. This was observed between the two sampling dates in 1996 of this study (Fig. 1). When these surface infiltration events are coupled with preferential downward flow followed by nonpreferential upward flow, the exponential salt distribution with depth is self-perpetuating (van Genuchten and Wierenga, 1976; Wagenet, 1984).

In a humid climate with in excess of 100 cm of annual rainfall, the conditions necessary for soluble salts to be a problem include a source of soluble salts (such as irrigation) and a soil layer that is essentially impervious to downward leaching. If the salt causes damage primarily as a result of osmotic potential, then there is usually a mechanism of soluble salt concentration. This concentration mechanism has been thought of only in terms of evaporation. A substantial amount of literature is available that describes methods of managing soluble salts in arid and semiarid regions without impermeable soil layers (Rhoades and Loveday, 1990). Research has led to the development of leaching fractions, which determine the amount of irrigation water that should be applied to obtain leaching at large enough rates to avoid soluble salt accumulation. However, in humid regions, the data from our study show that other soil physical properties are involved in salt accumulation near the soil surface in addition to evaporation. The primary phenomenon working in conjunction with evaporation is the preferential flow that takes place during water infiltration. Under some conditions, a coupling between infiltration and evaporation responding similar to a salt pump results in concentrating salt near the soil surface. When the soluble salt concentrations throughout the soil profile are relatively low, almost all of the salt from the entire soil profile must be concentrated near the soil surface to result in high enough concentrations to result in salinity injury to rice.

The increased NO^-₃ observed in the no-till treatments compared with the tillage treatments is interesting. Rice is typically rotated with soybean each year, as was the case in this study. The source of the NO^-₃ measured in the rice root zone at the two- to three-leaf stage of this study is not likely to be fertilizer. Urea is applied to rice at the four- to five-leaf stage and is usually exhausted by the end of the year (Wilson et al., 1989, 1990), so that substantial amounts of carryover are not likely. Nitrogen fertilizer is not applied to soybean, so the only source of fertilizer N is the urea applied to rice. With no-till, previous crop residue and winter weed residue are near the soil surface. In contrast, tillage operations incorporate those residues and distribute them throughout the tilled zone of 10 to 15 cm. It is possible that decomposition of these residues near the soil surface and mineralization could result in greater NO^-₃ concentrations in this surface layer.

The yield reduction associated with the no-till system is important. With increased emphasis on conservation tillage practices, it becomes necessary to understand all of the factors that lead to this yield reduction. Based on the results from this study, the potential for Cl^- or NO^-₃ accumulation in the rice root zone during the seedling growth stage is greater in the no-till systems than in tilled systems. Data from this study were collected during the sensitive growth stage for rice and, subsequently, provide circumstantial evidence of salinity injury to rice. These data suggest that in soils with potential for salinity injury, it may be advantageous to avoid no-till systems. More research is needed in this area to better understand the effects of salt movement on the rice crop.

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
Salt accumulation was greater near the soil surface in the no-till system than in the tilled systems during the seedling rice growth stage. Significant yield reductions were observed with no-till operations, suggesting that some tillage may be advantageous. It is apparent that management practices that encourage the depletion of NO^-₃ from the soil for no-till systems need to be investigated. Also, more information is needed on salt movement in the soil profile under these conditions, as they are affected by rainfall intensity and duration, which will affect water flow rates through the soil profile.

	ACKNOWLEDGMENTS

 
We would like to extend our appreciation to the Arkansas Rice Research and Promotion Board and the Arkansas Soybean Promotion Board for funding this project. Also, we would like to extend our thanks to Mr. Roger Eason and his staff for their assistance with plot maintenance. Valuable comments from the reviewers are appreciated.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
Contribution of the Univ. of Arkansas Agr. Exp. Sta.

Received for publication December 28, 1998.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 

• Baser R.E., Gilmour J.T. Tolerance of rice seedlings to potassium salts. Fayetteville, AR: Univ. of Arkansas Agric. Exp. Stn, 1982 Ark. Exp. Stn. Bull. 860..
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• Gilmour J.T., Baser R.E., Scott H.D., Ferguson J.A. The behavior of soluble salt in Sharkey clay — II. Fayetteville, AR: Ark. Water Resources Res. Center, 1986 Publ. 123..
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• Keisling T.C., Gilmour J.T., Scott H.D., Sadeghi A.M., Baser R.E. Evaluation of drainage tile to alleviate salt build up in heavy soils irrigated with brackish water and cropped to rice and soybeans. Publ. 111. Fayetteville, AR: Ark. Water Resources Res. Center, 1984.
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• Rhoades J.D., Loveday J. Salinity in irrigated agricultre. In: Stewart B.A., Nielsen D.R., eds. Irrigation of agricultural crops. Madison, WI: ASA, CSSA, and SSSA, 1990:1089-1142 Agron. Monogr. 30..
• SAS Institute. SAS user's guide: Basics. Version 5 ed. Cary, NC: SAS Inst, 1985.
• Scott H.D., Ferguson J.A., Sojka R.E., Batchelor J.T. Response of Lee 74 soybean to irrigation in Arkansas. Fayetteville, AR: Univ. of Arkansas Agric. Exp. Stn, 1986 Arkansas Agric. Exp. Stn. Bull. 886..
• Slaton, N.A., R.J. Norman, B.R. Wells, D.M. Miller, R.S. Helms, C.A. Beyrouty, and C.E. Wilson, Jr. 1994. Efficient use of fertilizer. p. 42–54. In Rice production handbook. Misc. Publ. 192. Univ. of Arkansas Coop. Ext. Serv., Little Rock.
• Sriyotai K., Gilmour J.T. Soil salinity measurement by the four-electrode probe technique. Arkansas Acad. Sci. Proc. 1976;30:84-85.
• van Genuchten M.Th., Wierenga P.J. Mass transfer studies in sorbing porous media. I. Analytical solutions. Soil Sci. Soc. Am. J. 1976;40:473-480.[Abstract/Free Full Text]
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