Solute Movement in Uncropped Ridge-Tilled Soil under Natural RainfallDan B. Jaynes* and J. B. Swan

ABSTRACTPoint injection of N fertilizers within the ridge of ridge-tilled soils

has shown promise in reducing nitrate leaching in short-term rainfallsimulation studies, but it has not been studied under natural rainfallthroughout the growing season. At two uncropped locations, we mea-sured the leaching of tracers injected into different ridge positions inestablished ridge-tilled plots that were leached by natural rainfall. Apoint injector was used to inject conservative anionic tracers 100 mmbelow the soil surface every 38.1 mm along ridge-top, ridge-shoulder,and furrow positions. Movement of tracers was measured four timesafter tracer application by triplicate soil sampling with a 38.1-mm-diam. soil probe to a maximum depth of 1.22 m. Sample collectionspanned a period of 102 d with 282 mm of cumulative precipitationafter tracer application at the Boone site and 97 d with 314 mm ofprecipitation at the Treynor site. Mass recoveries of furrow-appliedtracer was significantly lower (P = 0.05) than ridge-applied tracer onthe last two sampling dates at both sites. There was no consistentdifference in mass recoveries between tracers applied at ridge topcompared with shoulder positions. Furrow-applied tracer had lowerconcentrations in the soil profile than tracers applied to ridge-top andshoulder positions after 40 d and 76 to 129 mm of precipitation. Tracermovement was essentially vertical below the ridges, with £96% ofthe recovered ridge-applied tracer found below the ridges. Lateralspreading was more pronounced under the furrows, with £49% ofthe recovered furrow-placed tracer found below the furrows. Fertilizerinjection within the ridge shows promise for reducing leaching andpotentially increasing nutrient availability to plants.

RDGE TILLAGE HAS PROVEN TO BE an effective manage-ment system for reducing runoff and sediment loss

from fields (Laflen et al., 1978). By moving residue fromthe ridges to the furrows, ridge tillage helps warm anddry the soil in the ridges during the spring (Radke, 1982;Radke et al., 1993), improving germination and seedlinggrowth while protecting the soil surface of the furrowsfrom direct rainfall.

Blaylock and Cruse (1992) proposed that the ridgeconfiguration could also be exploited to increase N fer-tilizer availability. They hypothesized that placing Nfertilizer within the ridge would result in less denitrifica-tion and leaching than N placed in the furrow or broad-cast across the surface. Their results confirmed thatpoint injection of urea-NH4NO3 fertilizer in ridge tillageincreased N-uptake efficiency by corn (Zea mays L.) vs.broadcast applications, but they found no consistentdifference in N leaching between furrow vs. ridge-injected treatments.

Although Blaylock and Cruse (1992) found no consis-tent difference in N leaching between ridge and furrowplacement, several studies have shown differential

Dan B. Jaynes, USDA-ARS, National Soil Tilth Lab., 2150 PammelDr., Ames, IA 50011-3120. J.B. Swan, Dep. of Agronomy, Iowa StateUniv., Ames, IA 50011. Received 1 Dec. 1997. *Corresponding au-thor (jaynes@nstl.gov).

Published in Soil Sci. Soc. Am. J. 63:264-269 (1999).

leaching of solutes as a function of solute placement.Hamlett et al. (1990) found less leaching of Br~ andNO3~ during single artificial rainfall events when thesolutes were applied within ridges than when the soluteswere applied below level ground. Benjamin et al. (1990),using a two-dimensional water and heat transportmodel, demonstrated less water movement below ridgesthan below furrows. Bargar et al. (1999) confirmed thisfinding using field measurements of changes in soil wa-ter content within a ridge-tilled soil. The difference inwater movement is apparently due to a fraction of therain running off the ridges and infiltrating through thefurrows and to less evaporation occurring through theresidue layer covering the furrows. In contrast, Clayet al. (1994a) found greater nitrate leaching loss fromanhydrous ammonia knifed into ridge positions vs.knifed into furrow positions. The authors attributed thedifference in leaching to the slot formed by knifing inanhydrous ammonia fertilizer remaining open in theridges while the slot in the furrows closed during rainfall.The slot served to funnel runoff through the ridge ratherthan off the ridge, which indicates the importance ofmicrotopography on infiltration and leaching.

None of these studies followed solute movement un-der natural rainfall over the growing season. While ridgetopography is important in determining leaching pat-terns during the large rainfalls simulated in these earlierstudies, it is unclear whether significant differenceswould result under typical weather patterns in whichlight and heavy rainfalls are interspersed with periodsof redistribution and evaporation. It is also unclearwhether chemical injection must be into the top of theridge or if injection into the side of the ridge wouldalso affect leaching. The objective of this study wasto quantify the leaching patterns in ridge-tilled soil oftracers applied to furrow, ridge-top, and ridge-shoulderpositions and leached by natural rainfall over the grow-ing season.

MATERIAL AND METHODSField sites were located in western Iowa at the USDA

Deep Loess Research Station near Treynor and at a farm nearBoone in central Iowa. The Treynor site was on a Mononasilt loam (fine-silty, mixed, mesic, Typic Hapludoll) developedfrom deep loess deposits, and the Boone site was on a Clarionloam (fine-loamy, mixed, mesic, Typic Hapludoll) developedfrom glacial till. Soils at both sites were well drained, with thewatertable below 20 m at the Treynor site and from 1 to 2 mat the Boone site. Both sites had been in ridge tillage for>10 yr with rows oriented in the north-south direction. Rowspacing at the Boone site was 0.76 m with a ridge height after

Abbreviations: DFBA, 2,6-difluorobenzoate; DOY, day of year;PFBA, pentafluorobenzoate; TDR, time-domain reflectometry;TFMBA, o-(trifluoromethyl)benzoate.

264

JAYNES & SWAN: SOLUTE MOVEMENT IN UNCROPPED RIDGE-TILLED SOIL 265

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tions of soil cores, neutron access tubes, and wells. No wells wereinstalled at Treynor.

planting of 0.08 m. Row spacing at the Treynor site was 0.97 mwith a ridge height after planting of 0.11 m.

Corn was planted in 1992 with a 12-row planter at Booneand a 4-row planter at Treynor. After planting, test areas 16 mlong by 12 rows wide at Boone and 16 m long by 16 rowswide at Treynor were established. After they emerged, cornplants were removed from the areas by hand. Seven contiguous1-m-long plots were established within each area (Fig. 1). Oneplot was used for installation of time-domain reflectometry(TDR) probes at specific depths below the ridge and furrows,and one plot was used for the installation of thermocouples.The plot between these two was used as an access pit to installthe equipment [see Bargar et al. (1999) for details on TDRmeasurements]. Because of instrumentation in the plot, culti-vation and ridge reforming, which are typically practiced inridge tillage, were not performed. Thus, the ridges tended toerode slowly during the period of the experiments.

The tracers Br~, 2,6-difluorobenzoate (DFBA), o-(triflu-oromethyl)benzoate (TFMBA), and pentafluorobenzoate(PFBA) were applied to the remaining plots. These tracerswere chosen since they are unique to soil systems, and theyare anionic, nonabsorbing, and conservative. They have alsobeen shown to have nearly identical transport properties inthese soils (Jaynes, 1994), making direct comparison of theirtransport behavior possible. Applying multiple tracers simul-taneously allowed us to follow solute leaching from differentsource locations within the same furrows and ridges. Tracerswere applied at rates of 8 g m~2, with bromide applied tofurrow positions, TFMBA and DFBA applied to ridge-shoul-der positions, and PFBA applied to ridge tops (Fig. 2). Bro-mide was applied at Boone using a 174 g LT1 solution of KBr.Benzoate tracers were applied using a solution made by mixing118 g Lr1 of the benzoic acid analog with equal molar amountsof NaOH. At Treynor, a 221 g L"1 solution of KBr was used,and 149 g Lr1 of the benzoic acid analogs were mixed withequal molar amounts of NaOH. Tracer application and sam-pling dates are shown in Fig. 3.

Rather than the knifing in of anhydrous ammonia, applica-tion of tracers was designed to mimic point injection of liquidfertilizer (Baker et al., 1989) to prevent the slot formationobserved by Clay et al. (1994a, 1994b). To reduce solute leach-ing to a two-dimensional pattern, a quasi line-source was cre-ated for each tracer by inserting a 6.35-mm-diam. steel rod,100 mm into the soil every 38.1 mm along the 1-m length ofeach plot. A pistol grip veterinary syringe (Benjamin et al.,

O BrA TFMBA

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Fig. 2. Schematic of plot profile showing tracer placement and soilsample locations with respect to ridge configuration. Ridge spacingequaled 0.76 m at Boone and 0.97 m at Treynor.

350

^ 300 -

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P 200 -

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E 50 -

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Boone sitetracers applied on day 139

I samples taken

140 160 180 200 220 240

Day of year

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200 -

150 -

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Treynor sitetracers applied on day 155

I samples taken

150 170 190 210 230 250

Day of yearFig. 3. Cumulative precipitation after tracer application and dates of

soil sampling at Boone and Treynor.

266 SOIL SCI. SOC. AM. J., VOL. 63, MARCH-APRIL 1999

1988) was used to apply 2 mL of the appropriate tracer to thebottom of each hole. The holes were manually filled with loosesoil to prevent channeling of runoff down the holes as wasobserved by Clay et al. (1994a) for knife applicator slits. Wefound formation of the holes before tracer application neces-sary to prevent plugging of the syringe. Before applying tracerin the furrows, the surface residue was removed by hand andthen returned after tracer application.

Four wells were installed between plots at the Boone site(Fig. 1) to monitor groundwater level and collect water sam-ples. No wells were installed at Treynor because of the greatdepth (>15 m) to groundwater. Depth to ground water ineach well at Boone was measured weekly. The wells werethen purged and water samples collected the following dayfor tracer analysis.

Soil samples at Boone and Treynor were collected fromthree replicate transects across each plot on four dates aftertracer application. Soil cores were taken at each furrow, shoul-der, and ridge position that tracer was applied, for a total ofnine cores per transect (Fig. 1). Soil cores were taken to adepth of 0.91 m on the first sampling date at both locationsand to depths of 1.22 m on all other dates. Soil cores werecollected by pushing a 38.1-mm diam. steel soil probe fittedwith a removable acetate liner into the soil with a hydraulicram. On occasion, soil cores showed signs of compaction andwere discarded; a replacement core was then taken from thesame ridge or furrow position. The soil core and liner wereremoved from the steel probe, capped on each end, and frozenuntil tracer extraction. The frozen soil cores were cut into101-mm-long sections, removed from the liners, thawed, andmixed by hand. A 20-g subsample was removed, weighed,dried at 105°C for 24 h, and reweighed to calculate the soilwater content. The remainder of the sample was weighed andmixed with =225 mL of a 0.001 M CaSO4 solution for 5 minon a wrist shaker. The solution was then filtered and refriger-ated at 4°C until analysis for tracer concentrations.

The analysis of the tracers was performed on a DionexSeries 4500i ion chromatograph (West Mont, IL)1 using themethod described by Bowman and Gibbens (1992). For thefluorobenzoates, a SAX column (Regis Chemical Co., MortonGrove, IL) was used with 30 mM KH2PO4, adjusted to a pH of2.65 with H3PO4, and 20% (v/v) acetonitrile as eluting solution.

Flow rate was 1 mL min"1 and the detection wavelengthwas set to 205 nm. Bromide could not be quantified with theabove procedure due to interferences caused by high nitratelevels. Instead, Br~ was determined using a Dionex AG9 guardcolumn followed by an AS9 separator column. The elutingsolution was 1 mM Na2CO3 and 0.75 mM NaHCO3 at a pHof 10.4, with 12.5 mM H2SO4 used for suppression. Flow ratewas 1 mL min"1 and electrical conductance was measuredwith a conductivity detector. The detection limit for both Br~and benzoates was 0.1 mg L"1 in the extract solutions.

Tracer concentrations were calculated by multiplying mea-sured concentrations by the sum of the mass of soil water plusadded CaSO4 solution then dividing by the mass of soil water.Tracer distributions across the plot profile were graphed usingSurfer (Golden Software, Golden, CO) software. Since tracermovement was two-dimensional from pseudo-line sources,mass recoveries were calculated by integrating the recoveredtracer mass (g m~2) over the plot profile for each replicateusing the same software. Statistical comparisons were con-ducted using SigmaStat software (Jandel Scientific, San Ra-fael, CA).

Table 1. Relative mass recovered each sampling date for tracersapplied on DOY 139 at the Boone site and DOY 155 at theTreynor site. ___ ___

Relative mass recovery!

Br~ TFMBA PFBA DFBALocation Sampling date (furrow) (shoulder) (ridge) (shoulder)

Boone

Treynor

DOY170190218241176197217252

„ !.„-!

469b$200b138b1381)838a250b75b16c

6 "650a562a2381)362a675a663ab300ab200b§

•g395b350ab500a400a838a1025a412a625a

494ab312b175b275ab388a450ab475a-fl

' Trade and company names are used for the benefit of readersand do not imply endorsement by the USDA over similar products.

t Recovery values calculated as mass of tracer recovered divided by massof tracer applied. TFMBA = o-(trifluoromethyl)benzoate; PFBA =pentafluorobenzoate; DFBA = 2,6-difluorobenzoate.

t Values followed by the same letter within a row are not significantlydifferent at P = 0.05.

§ Based on soil samples collected from under only one of the two ridge po-sitions.

II Tracer not measured.

RESULTS AND DISCUSSIONCumulative precipitation after tracer application and

dates when soil samples were taken are shown in Fig.3. Rainfall at Boone occurred on 25 of the 101 daysbetween tracer application and the last soil samplingand had a cumulative total of 282 mm. A total of 314mm of rain fell at the Treynor site distributed in 26events within the 197 d between tracer application andthe last soil sampling. The smallest one-day rainfall re-corded was 0.254 mm at both sites, while the largestone-day rainfall was 30.5 mm on day of year (DOY)195 at Boone and 53.8 mm on DOY 249 at Treynor.This compares to a range of 24 to 72 mm that was appliedas single events in the rainfall simulation experiments byHamlett et al. (1990). The maximum rainfall intensitiessustained over any half-hour period were 40.6 mm h"1

at Boone and 65.6 mm h"1 at Treynor, which are of thesame order as the 48.6 mm h"1 rate used in the rainfallsimulation study by Clay et al. (1994a). Thus, the maxi-mum natural rainfall events during this study are compa-rable to the artificial rainfall used by Hamlett et al.(1990) and Clay et al. (1994a).

Mass recoveries of tracers ranged from 138 to 650 gkg"1 at the Boone site and from 16 to 1065 g kg"1 atthe Treynor site (Table 1). Data for samples collectedon DOY 252 at Treynor are incomplete because somesamples collected on this date were inadvertently dis-carded before complete benzoate tracer analysis. Afteronly 30.1 mm of rain at the Boone site, tracer massrecovered by the first soil sampling did not exceed 650g kg"1 tracer applied. Tracer mass recovered by the firstsoil sampling at Treynor was slightly higher than atBoone after receiving slightly less rain (25.7 mm). Nomore than half of any tracer was recovered within thetop 1.22 m by the third soil sampling at either site. Lowrecoveries of conservative tracers are not unusual infield experiments (Hamlett et al., 1990; Starr and Glot-felty, 1990; Jaynes et al., 1992; Timlin et al., 1992; Timlinet al., 1998) and have been attributed to leaching belowthe depth of sampling. Water samples from the shallow

JAYNES & SWAN: SOLUTE MOVEMENT IN UNCROPPED RIDGE-TILLED SOIL 267

Table 2. Analysis of variance of the effects of sampling date andtracer placement on tracer mass recovery.

Mean SignificanceSite Source of variation df squares F value levelBoone

Treynor

Sample datePlacementDate X placementResidualCorrected totalSample datePlacementDate X placementResidualCorrected total

339

3247236

2435

46785105 345199444217

1640097567

113 034484091416633276

11.09624.9844.730

6.8887.9793.417

<0.001<0.001<0.001

0.004<0.001

0.014

wells at the Boone site contained measurable concentra-tions of the furrow-applied Br~ after DOY 219, indicat-ing deep leaching of this tracer. Nevertheless, no detect-able levels of the tracers applied to the ridge-top orshoulder positions were found in the well samples. Lowrecoveries may also be due to the possible radial trans-port geometry that results from applying the tracer asa line source, although little lateral spreading was foundfor any but the furrow applied tracer (see below).

Results of multifactor analysis of variance indicatedthat sampling date, tracer placement, and the interactionbetween the two (i.e., date X placement) had significanteffects on tracer recovery at both sites (Table 2). Tracermass recoveries decreased over time and with increasingcumulative precipitation. Tracer placement affected themass of each tracer recovered (Table 1). At Boone, Br~placed in the furrows had equal or less mass recoveredthan the tracers applied to the ridge top and shoulders.Similarly, at Treynor furrow applied Br~ had the leastor equal mass recovered of all tracers for every samplingdate but the first. There was no consistent difference inmass recoveries between ridge top- and ridge shoulder-applied tracers. At both sites and for all sampling datesbut DOY 190 there was no significant difference be-tween the mass recoveries of the TFMBA and theDFBA that was placed in the ridge shoulders. This wasas expected because both substances were placed inridge-shoulder positions and have been shown to movethrough soil in a very similar manner (Jaynes, 1994).

Two-dimensional distributions of tracers within theplot profiles illustrate the contrasting leaching patternsof tracers applied at different positions. By DOY 190,following 76 mm of precipitation, measurable tracerconcentrations were present to at least 1.0 m at theBoone site (Fig. 4). Measured Br~ concentrations werelower below the furrows where Br~ was applied thanwere concentrations of the other tracers below the ridge-top and shoulder positions where they were applied.

By DOY 241, after 282 mm of precipitation, concen-trations of all tracers were considerably less than atDOY 190 at the Boone site (Fig. 5). Concentrationsabove 1 mg L~' were found below the 0.80-m depthonly for the furrow applied Br"1 (Fig. 5a) and for theTFMBA applied to one of the ridge-shoulder positions(Fig. 5b). Maximum Br~ concentrations were consider-ably less than those for the other tracers, but Br~ con-centrations above 1 mg Lr1 were measured throughoutalmost the entire plot profile. Tracers placed in the

80 100 120 140 0 20 40 60 80 100 120 140

tracer concentration (mg/L)

20 40 60 80 100 120 140

Lateral distance (cm)Fig. 4. Average concentration for (a) furrow placed Rr . (b) ridge

shoulder-placed TFMBA, (c) ridge top-placed PFBA, and (d) ridgeshoulder-placed DFBA tracers on DOY 190 at Boone.

ridge-top and shoulder positions had concentrationstypically >100 mg L"1 directly below where they wereapplied. The peak concentration for these tracersshowed migration below the 10-cm depth of injectionin only half of the locations (Fig. 5b, 5c, and 5d).

Tracer concentration profiles at Treynor were similarin shape to those at Boone, although measurable con-centrations did not extend as deeply into the profile.By DOY 197 (Fig. 6), after 129 mm of precipitation,

0 20 40 60 80100120140160180

tracer concentration (mg/L)

25 100 200 300

D3"CDX

O

' 0 20 40 60 80 100120140160180

20-0

-20-40-60-80

-100

-12O0 20 40 60 80 100120140160180

,- s Lateral distance (cm)Fig. 5. Average concentration for (a) furrow placed Br , (b) ridge

shoulder-placed TFMBA, (c) ridge top-placed PFBA, and (d) ridgeshoulder-placed DFBA tracers on DOY 241 at Boone.

268 SOIL SCI. SOC. AM. J., VOL. 63, MARCH-APRIL 1999

-20

-40

-60

-80

-100

" 0 20 40 60 80 100 120 140

tracer concentration (mg/L)

100 200 300

2O

' 0 20 40 80 100 120 140

Lateral distance (cm)Fig. 6. Average concentration for (a) furrow placed Br~, (b) ridge

shoulder-placed TFMBA, (c) ridge top-placed PFBA, and (d) ridgeshoulder-placed DFBA tracers on DOY 197 at Treynor.

concentrations of furrow-placed Br~ were lower thanconcentrations of ridge-placed PFBA. Almost no tracerwas present at a concentration >1 mg L~' below a depthof 0.65 m. After 185 mm of precipitation on DOY 217(Fig. 7), concentrations for the shoulder-applied tracersstill exceeded 100 mg L"1, and the ridge-applied tracerconcentrations exceeded 300 nig L"1. In contrast, Br~,which was injected into the furrows, did not exceed 100mg LT1 anywhere in the profile.

tracer concentration (mg/L)IliillliliiiilliJJIJJIiiJJiJtlililJjjjjjjjIIIIIIIJIJIJJr1 25 100 200 300

'0 20 40 60 80 100120140160180 '0 20 40 60 80 100120140160180

Lateral distance (cm)Fig. 7. Average concentration for (a) furrow placed Br~, (b) ridge

shoulder-placed TFMBA, (c) ridge top-placed PFBA, and (d) ridgeshoulder-placed DFBA tracers on DOY 217 at Treynor.

Table 3. Fraction of tracer recovered in soil cores taken at thepoint of tracer application divided by total tracer recovered inall soil cores taken on that date.

Location

Boone

Treynor

Samplingdate

DOY170190218241176197217

Br~(furrow)

0.990.870.580.490.990.980.88

TFMBAf(shoulder)

1.000.990.910.951.001.000.99

PFBAf(ridge)

0.961.000.980.991.001.000.99

DFBAf(shoulder)

1.000.980.850.550.970.990.98

t TFMBA = o-(trifluoromethyl)benzoate; PFBA = pentafluorobenzoate;DFBA = 2,6-difluorobenzoate.

Little lateral spreading of tracers was observed formost dates at either Boone or Treynor. Table 3 lists thefraction of tracer mass recovered in soil cores takendirectly at the point of tracer application divided by thetotal tracer mass recovered in all soil cores taken oneach sampling date. On DOY 170 at Boone, 96% ormore of the entire mass of each tracer was recovereddirectly below where it was applied. However, all tracershad spread at measurable concentrations laterally withinthe surface soil layer (Fig. 4). This spreading may havebeen accentuated by differential soil wetting and dryingcaused by ridge configuration and residue distribution(Radke et al., 1993; Bargar et al., 1999). At later sam-pling dates at Boone, more than 90% of the mass ofthe PFBA applied to the ridge top and the TFMBAapplied to the ridge shoulder was recovered directlybelow where it was applied. By DOY 241, DFBA ap-plied to the ridge shoulder had only 55% of its massrecovered directly below where it was applied; however,this lower recovery appeared more owing to the massmovement of tracer towards the centerline of the ridgerather than to lateral spreading (Fig. 5d). Conversely,Br~ showed increased lateral spreading over time. Only=50% of the total Br~ mass recovered was recoveredin cores taken from the furrows on DOY 241 becauseof a more uniform distribution of Br~ across the soilprofile (Fig. 5a). At Treynor, lateral spreading of eachtracer was measurable within the surface soil layer onall sampling dates. Only on DOY 217 and for furrow-applied Br~ did any tracer have <97% of its mass recov-ered directly below where it was applied (Table 3).

SUMMARYTracer applied by point injector 100 mm below fur-

rows showed more leaching under natural rainfall thantracers applied below ridge shoulders and tops. Thisincreased movement was most likely driven by thegreater infiltration of precipitation within the furrowsvs. the ridges (Clay et al., 1992) and it agrees with ob-served changes in soil water content observed by Bargaret al. (1999) at these sites. Lateral movement of ridgetop- and shoulder-placed tracers was much less thanvertical movement, which agrees with the results ofHamlett et al. (1990) for solute movement under singleartificial rain events.

Differential solute movement within a ridge-tilled sys-

JAYNES & SWAN: SOLUTE MOVEMENT IN UNCROPPED RIDGE-TILLED SOIL 269

tern implies that chemical leaching can be reduced byapplication method and placement. Mobile solutes suchas nitrate will tend to leach less when applied in ridgetops or shoulders as opposed to furrows. Less leachingwould maintain the nitrate near the growing seedlingand reduce the likelihood of nitrate movement to under-lying groundwater. This benefit notwithstanding, themethod of fertilizer application must not result in theformation of secondary microrelief features that in-crease local infiltration, or the benefits of the ridge andfurrow configuration may be negated or reversed (Clayet al., 1994a, 1994b).

While these studies were conducted under naturalrainfall, they were conducted in the absence of a growingcrop. A crop could modify these results by changingthe water redistribution pattern within the soil by rootuptake and differential shading of the soil surface (Tim-lin et al., 1992). A plant canopy could also change theinfiltration pattern across the ridge and furrow configu-ration by either increasing the fraction of rain fallingon the ridges vs. the furrows (Parkin and Codling, 1990;Dowdy et al., 1993) or by increasing the permeabilityof the ridges (Prieksat et al., 1994). Root distributionsaffect the amount and location of solute uptake andthus may change the leaching pattern as a function ofsolute placement found in this study. While additionalresearch is needed to determine the effect of a cropon chemical leaching, the results found here showedreduced leaching of ridge-injected chemicals under nat-ural rainfall.

ACKNOWLEDGMENTSWe wish to express our appreciation to E. Behn for provid-

ing access to his fields, to D. Joplin and J. Theis for assistingin the field aspects of this study, to P. Rogers for performingthe tracer analysis, and to J. Benjamin, D. Clay, and D. Timlinfor helpful suggestions on earlier versions of this manuscript.