infiltration in response to water quality, tillage, and gypsum

6
Infiltration in Response to Water Quality, Tillage, and Gypsum R. L. Baumhardt,* C. W. Wendt, and J. Moore ABSTRACT Maintaining adequate infiltration into cropped soils is a continuing problem in the Trans-Pecos region of Texas due to the accumulation of electrolytes from irrigation water. The objectives of this study were to determine the effect of (i) number of years of irrigation, (ii) applied water quality, (iii) tillage, and (iv) surface-applied powdered gypsum on field infiltration rate and amount. Infiltration rate into a Hoban silt loam (fine-silty, mixed, thermic Ustollic Calciorthid) at Pecos, TX, was measured with a rainfall simulator in field experiments with dif- ferent irrigation amounts and qualities of applied water treatments or tillage and powdered-gypsum treatments. Electrical conductivities from the soil surface (0-50 mm) increased 300% after the first year's irri- gation and remained relatively constant with continued irrigation. Infiltration rate and amount increased when the soil salinity and sod- icily decreased or the salinity of the applied water increased. Tillage increased infiltration in the order of plow-disk > double-disk > com- pacted = control. The application of powdered gypsum to the soil surface increased infiltration amount by an average of 38% in tilled or compacted soil, where the surfaces had been disturbed. Powdered gypsum did not affect the infiltration amount where the soil surface was undisturbed. These data indicate that (i) sufficient electrolytes from irrigation water will accumulate in one growingseason to reduce infiltration and (ii) field applications of powdered gypsum will increase the infiltration into tilled or disturbed soils. I RRIGATION is REQUIRED for crop production in the Trans-Pecos region of Texas because of low grow- ing-season precipitation, which averages only 150 mm (Texas Water Development Board, 1968). Transpir- ation requirements for crop production ranges from 450 to 650 mm, leaving a need for irrigation amounts of 300 to 500 mm. Most water available for irrigation has a high EC^ that results in a very high salinity hazard, and a SAR W that causes a moderate Na hazard. Salts from the irrigation water accumulate in the soil profile (Longenecker et al., 1969) and cause soil dis- persion and surface seal development during rainfall or irrigation, thus, decreasing infiltration rate and amount. Field evaluation of infiltration rate and amount with rain or irrigation water as a function of irrigation quantities is needed under drop-impact conditions to identify when a reduction due to salt accumulation occurs. Many studies have documented the effect of water quality on infiltration rate and amount under labora- tory conditions. For example, Oster and Schroer (1979) showed that the ponded infiltration rate into undis- turbed soil columns was controlled by the chemical properties of the infiltrating water through its effects on the soil surface rather than by the chemical prop- erties of the soil. Agassi et al. (1981) attributed de- creased infiltration rate of simulated rain to greater soil surface dispersion of the prepared columns when R.L. Baumhardt and C.W. Wendt, Texas Agric. Exp. Stn., Route 3, Box 219, Lubbock, TX 79401; and J. Moore, Texas Agric. Exp. Stn., Box 1549, Pecos, TX 79772. Contribution from the Texas Agric. Exp. Stn. Received 21 Jan. 1991. * Corresponding author. Published in Soil Sci. Soc. Am. J. 56:261-266 (1992). the EC^, decreased or the soil exchangeable Na per- centage increased. Rapid seal formation due to chem- ical dispersion of the soil was reduced by greater EC^,. Soil amendments for increasing the EQ, and re- ducing rapid seal formation have been evaluated. Keren and Shainberg (1981) reported that soil surface amendments of PDG or PG, a by-product of fertilizer production, were moderately water soluble. These amendments increased infiltration rate in a laboratory study by increasing the EC^ of the infiltrating water. The effectiveness of PG in increasing infiltration rate and amount as the rate of surface-applied PG in- creased was shown under laboratory conditions by Agassi et al. (1986) using two soils having an ex- changeable Na content of 2.5%. Results from some field studies indicate that gypsum increases the infil- tration rate and amount in both cultivated and crusted soils (Hadas and Frenkel, 1982; Agassi et al., 1990; Freebairn et al., 1988). Surface-applied gypsum, however, resulted in no difference in runoff from a field watershed (Freebairn et al., 1988) and was con- cluded to be a short-lived treatment. In summary, in- creasing the EC,, of infiltrating water with surface- applied PG or PDG reduces chemical dispersion and seal development in soil column experiments; how- ever, not all field applications resulted in increased infiltration rate and amount. The effectiveness of PG or PDG in increasing in- filtration by reducing seal development may depend on the soil physical condition as affected by tillage. Published results on the effect of gypsum on infiltra- tion into different tillage treatments on saline soils were not found. Therefore, the objectives of this study were to (i) quantify infiltration of water with different qualities, EQ,,, into soil following different depths of irrigation, and (ii) evaluate the affects of PDG on in- filtration of water into soil having different tillage re- gimes (including compaction). MATERIALS AND METHODS Two experiments were conducted to measure infiltration rate and amount into a Hoban silt loam (Soil Survey Staff, 1975) at the Texas Agricultural Experiment Station, Pecos (31°20'N, 103°30'W). In the first experiment, the effects of accumulating salts from irrigation water and the quality of the applied water on the infiltration rate and amount were measured. In the second experiment, the effects of tillage and PDG on infiltration rate and amount were evaluated. In both experiments, soil surface EC e were measured on soil samples collected from areas adjacent to individual in- filtration plots, according to the methods of Rhoades and Miyamoto (1990). Surface soil samples were also collected from six sites within the same irrigation or tillage treatment and combined for composite determinations of the soil SARg and texture. The soil SAR,., also from saturated-paste ex- Abbreviations: EQ,,, electrical conductivity of applied water; SAR^, sodium adsorption ratio of applied water; PDG, powdered gypsum; PG, phosphogypsum; EC e , electrical conductivity from saturated-paste extracts; SAR 5 , soil sodium adsorption ratio; T p , time to ponding; RI, rainfall intensity; ANOVA, analysis of vas- riance; PET, potential evapotranspiration; CV, coefficient of var- iation. 261

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Page 1: Infiltration in Response to Water Quality, Tillage, and Gypsum

Infiltration in Response to Water Quality, Tillage, and GypsumR. L. Baumhardt,* C. W. Wendt, and J. Moore

ABSTRACTMaintaining adequate infiltration into cropped soils is a continuing

problem in the Trans-Pecos region of Texas due to the accumulationof electrolytes from irrigation water. The objectives of this study wereto determine the effect of (i) number of years of irrigation, (ii) appliedwater quality, (iii) tillage, and (iv) surface-applied powdered gypsumon field infiltration rate and amount. Infiltration rate into a Hobansilt loam (fine-silty, mixed, thermic Ustollic Calciorthid) at Pecos, TX,was measured with a rainfall simulator in field experiments with dif-ferent irrigation amounts and qualities of applied water treatments ortillage and powdered-gypsum treatments. Electrical conductivities fromthe soil surface (0-50 mm) increased 300% after the first year's irri-gation and remained relatively constant with continued irrigation.Infiltration rate and amount increased when the soil salinity and sod-icily decreased or the salinity of the applied water increased. Tillageincreased infiltration in the order of plow-disk > double-disk > com-pacted = control. The application of powdered gypsum to the soilsurface increased infiltration amount by an average of 38% in tilledor compacted soil, where the surfaces had been disturbed. Powderedgypsum did not affect the infiltration amount where the soil surfacewas undisturbed. These data indicate that (i) sufficient electrolytesfrom irrigation water will accumulate in one growing season to reduceinfiltration and (ii) field applications of powdered gypsum will increasethe infiltration into tilled or disturbed soils.

IRRIGATION is REQUIRED for crop production in theTrans-Pecos region of Texas because of low grow-

ing-season precipitation, which averages only 150 mm(Texas Water Development Board, 1968). Transpir-ation requirements for crop production ranges from450 to 650 mm, leaving a need for irrigation amountsof 300 to 500 mm. Most water available for irrigationhas a high EC^ that results in a very high salinityhazard, and a SARW that causes a moderate Na hazard.Salts from the irrigation water accumulate in the soilprofile (Longenecker et al., 1969) and cause soil dis-persion and surface seal development during rainfallor irrigation, thus, decreasing infiltration rate andamount. Field evaluation of infiltration rate and amountwith rain or irrigation water as a function of irrigationquantities is needed under drop-impact conditions toidentify when a reduction due to salt accumulationoccurs.

Many studies have documented the effect of waterquality on infiltration rate and amount under labora-tory conditions. For example, Oster and Schroer (1979)showed that the ponded infiltration rate into undis-turbed soil columns was controlled by the chemicalproperties of the infiltrating water through its effectson the soil surface rather than by the chemical prop-erties of the soil. Agassi et al. (1981) attributed de-creased infiltration rate of simulated rain to greatersoil surface dispersion of the prepared columns when

R.L. Baumhardt and C.W. Wendt, Texas Agric. Exp. Stn., Route3, Box 219, Lubbock, TX 79401; and J. Moore, Texas Agric.Exp. Stn., Box 1549, Pecos, TX 79772. Contribution from theTexas Agric. Exp. Stn. Received 21 Jan. 1991. * Correspondingauthor.Published in Soil Sci. Soc. Am. J. 56:261-266 (1992).

the EC ,̂ decreased or the soil exchangeable Na per-centage increased. Rapid seal formation due to chem-ical dispersion of the soil was reduced by greater EC ,̂.

Soil amendments for increasing the EQ, and re-ducing rapid seal formation have been evaluated. Kerenand Shainberg (1981) reported that soil surfaceamendments of PDG or PG, a by-product of fertilizerproduction, were moderately water soluble. Theseamendments increased infiltration rate in a laboratorystudy by increasing the EC^ of the infiltrating water.The effectiveness of PG in increasing infiltration rateand amount as the rate of surface-applied PG in-creased was shown under laboratory conditions byAgassi et al. (1986) using two soils having an ex-changeable Na content of 2.5%. Results from somefield studies indicate that gypsum increases the infil-tration rate and amount in both cultivated and crustedsoils (Hadas and Frenkel, 1982; Agassi et al., 1990;Freebairn et al., 1988). Surface-applied gypsum,however, resulted in no difference in runoff from afield watershed (Freebairn et al., 1988) and was con-cluded to be a short-lived treatment. In summary, in-creasing the EC,, of infiltrating water with surface-applied PG or PDG reduces chemical dispersion andseal development in soil column experiments; how-ever, not all field applications resulted in increasedinfiltration rate and amount.

The effectiveness of PG or PDG in increasing in-filtration by reducing seal development may dependon the soil physical condition as affected by tillage.Published results on the effect of gypsum on infiltra-tion into different tillage treatments on saline soilswere not found. Therefore, the objectives of this studywere to (i) quantify infiltration of water with differentqualities, EQ,,, into soil following different depths ofirrigation, and (ii) evaluate the affects of PDG on in-filtration of water into soil having different tillage re-gimes (including compaction).

MATERIALS AND METHODSTwo experiments were conducted to measure infiltration

rate and amount into a Hoban silt loam (Soil Survey Staff,1975) at the Texas Agricultural Experiment Station, Pecos(31°20'N, 103°30'W). In the first experiment, the effectsof accumulating salts from irrigation water and the qualityof the applied water on the infiltration rate and amount weremeasured. In the second experiment, the effects of tillageand PDG on infiltration rate and amount were evaluated.In both experiments, soil surface ECe were measured onsoil samples collected from areas adjacent to individual in-filtration plots, according to the methods of Rhoades andMiyamoto (1990). Surface soil samples were also collectedfrom six sites within the same irrigation or tillage treatmentand combined for composite determinations of the soil SARgand texture. The soil SAR,., also from saturated-paste ex-Abbreviations: EQ,,, electrical conductivity of applied water;SAR^, sodium adsorption ratio of applied water; PDG, powderedgypsum; PG, phosphogypsum; ECe, electrical conductivity fromsaturated-paste extracts; SAR5, soil sodium adsorption ratio; Tp,time to ponding; RI, rainfall intensity; ANOVA, analysis of vas-riance; PET, potential evapotranspiration; CV, coefficient of var-iation.

261

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262 SOIL SCI. SOC. AM. J., VOL. 56, JANUARY-FEBRUARY 1992

tracts, was calculated from ion concentrations measuredusing standard spectrometry methods (Baker and Suhr, 1986).Soil texture was measured by the hydrometer method ofGee and Bauder (1986). Surface (0.0-75.0-mm) soil den-sity was measured from six sites within the same irrigationor tillage treatment by the core method of Blake and Hartge(1986).

Infiltration MeasurementsField infiltration was measured with a sprinkling infiltro-

rrieter using well (pH = 7.5, EQ. = 4.0 dS m-1, SAR^= 3.97 [mmol L,-1]1'2) or reverse-osmosis (pH = 7.9, EC ,̂= 0.12 dS m-1, SAR,, = 0.50 [mmol L,-1]1'2) water, asdescribed by Baumhardt and Wendt (1988). Briefly, waterwas applied at 50 mm h~* at an impact energy of 22 Jmm-1 m~2 for 1 h, using a rotating-disk-type rainfall sim-ulator (Morin et al., 1967). Because rainstorms in this re-gion are intense and of short duration, a 50 mm h-1

application intensity, which approximates the average rainintensity for a 15-min period (Frederick et al., 1977), wasused. The soil (target area) was contained within a frame1 m wide by 1.2 m long by 0.2 m tall that had been pressedinto the soil 50 mm. Infiltration rate was determined as thedifference between the application rate and the measuredrunoff rate. Estimates of Tp and the reduction of infiltrationrate were determined using nonlinear regression methods(SAS Institute, 1985) to fit infiltration-rate data with anequation of the form:

ft ~ if) exp(aRIf) [1]where i, is the infiltration rate (mm h-1) at time t (h), if isthe final infiltration rate (mm h"1), it is the initial infiltra-tion rate (mm h-1), a (mm-1) is a parameter that describesthe rate of reduction in infiltration as a function of theamount of water applied, and RI is the rainstorm intensity(mm h-1) (Morin and Benyamini, 1977). Tp was calculatedby solving Eq. [1] for the time when the infiltration rateand RI were identical and runoff began.

Experiment 1. Irrigation and Water QualityThe effects of irrigation and water quality on infiltration

rate and amount were determined on a 12-ha field that hadbeen out of production for 15 y. The field was divided intofour equal 3-ha sections where irrigated cotton (Gossypiumhirsutum L.) production was resumed, beginning with thespring of 1985. Therefore, sections had been furrow irri-gated for 0, 1, 2, or 3 y, with total irrigation applicationsof 0, 884, 1559, and 2550 mm, respectively, of well water.Irrigation depths were calculated from the measured waterapplication amount metered at the well. Normal tillage withineach cropped section included moldboard plowing to a depthof 0.3 m and several disk and leveling operations to breakup large aggregates and prepare a suitable seedbed. Theinitial soil ECe was determined by depth at four locationswithin the entire study area and after each cropping season(1986-1988) at six locations within the section irrigated for3 y. These soil samples were collected using 50-mm-diam.cores to a depth of 0.9 m in 0.15-m increments. Infiltrationrate and amount were measured within each irrigated sec-tion using well or reverse-osmosis water in lieu of rain-water. Infiltration rate and amount data from triplicate randomapplications of water-quality treatments were analyzed usinga split-plot design within an irrigation main-plot treatment(ANOVA) (SAS Institute, 1985).

Experiment 2. Tillage and GypsumThe second experiment was conducted in a separate 50

by 100 m area of a field that had been idle for «15 y. The

area was divided into four equal 6-m-wide strips for tillagetreatments, with intervening 6-m-wide alleys. Tillage-treat-ment effects on infiltration rate into soil were comparedafter different degrees of disturbance (or compaction ) in-cluding (i) moldboard plowing to a depth of 0.3 m plusdisking to 0.15 m, (ii) double-disk plowing to 0.15 m (iii)scraping plus compaction (i.e., road maintainer plus roadcompacter applying 1.2-MPa pressure), and (iv) undis-turbed. Powdered gypsum (88% CaSO4-2H2O) was appliedat rates of 0 or 6 Mg ha-1 to randomly assigned subplotswithin each main-plot tillage treatment before measuringinfiltration rate and amount. In addition to the 0 and 6 MgPDG ha-1 treatments, infiltration rate and amount weremeasured on a 3 Mg PDG ha- a treatment applied to a plow+ disk-tilled soil in order to estimate the minimum gypsumrate needed to increase infiltration. Infiltration was mea-sured using reverse-osmosis water for each gypsum andtillage treatment combination because of the rapid reductionin infiltration rate in high ECe soils measured in the firstexperiment. These data were analyzed according to a split-plot design with three replicates, using ANOVA methods(SAS Institute, 1985).

RESULTS AND DISCUSSIONIrrigation and Water Quality

The soil textures determined in each of the fourirrigation-treatment sections were silt loam with anaverage sand, silt, and clay content of 194, 637, and169 g kg-1, respectively. The soil bulk density, aftertillage, was 1.04 Mg m-3 to a depth of 0.15 m anddid not vary across irrigation treatments. The SARSand ECe of the nonirrigated surface soil, 0 to 50-mmzone, was lower than the irrigated treatments eitherbecause salts were not preisent before the 15-yr idleperiod or because of leaching by rainfall. The SARSincreased significantly from 6.3 (mmol L-1)1/2 to 15.4(mmol L~1)*/2 following the first irrigation season, buttended to decline as the number of irrigation seasonsincreased; that is, SARS was 10.4 and 9.3 (mmolL- 1)1/2 after ^Q an(j mree irrigation seasons. Sodiumwas being replaced by Ca or Mg ions from the irri-gation water and soil. The surface soil ECe increasedfrom 1.10 to 4.02 dS m"1 after 1 yr of irrigation and,compared with the first year of irrigation, was notsignificantly different from the ECe of 3.25 and 3.41after two and three irrigation seasons. Soil surfacesalinity and sodicity increased during the first irriga-tion season and, after a small decline, remained rel-atively constant during subsequent irrigation seasons.

The initial ECe (year = 0) and the ECe after 1, 2,and 3 yr of irrigation are plotted as a function of soildepth in Fig. 1. Except for the surface samples, ECevalues were highly variable (CV = 27%). The initialprofile ECe increased from 1.10 dS m-1 soil at thesurface and 2.81 dS m-1 at 0.15 m to between 7.0and 8.0 dS m-1 below 0.3 m. The amount of saltapplied to an irrigated soil increases with irrigationamount, which is especially true when using the highECw local well water (Longenecker et al., 1969).Therefore, after 1 yr of irrigation (amount = 880 mm)electrolytes increased the soil electrical conductivity>300% in the surface and =200% at the 0.15 m depth.Electrolyte concentrations decreased below 0.3 m, thusreducing the ECe, during the first year of irrigation.The soil surface ECe remained relatively constant 3.3dS m-1 (275% of the initial ECe) after 2 and 3 yr of

Page 3: Infiltration in Response to Water Quality, Tillage, and Gypsum

BAUMHARDT ET AL: INFILTRATION RESPONSE TO WATER QUALITY, TILLAGE, AND GYPSUM 263

ELECTRICAL CONDUCTIVITY, dS rrf1

8 10

YEAROYEAR 1YEAR 2YEAR 3

n CROPD RAIN

LEACHINGIRRIGATION

Fig. 1. Electrical conductivity (EC.) by depth for the Hobansilt loam after irrigation for zero, one, two, and three growingseasons. Total irrigation depths were 0.0, 884, 1560, and2550 mn.

irrigation (amount = 0.675 and 0.991 m, respec-tively) but a reduction in ECe from the 0.15 and 0.30— m depth was observed in areas irrigated 2 yr due togreater rainfall in Year 2 (0.505 m) compared withYear 1 (0.279 m) or Year 3 (0.343 m). The ECe below0.45 m decreased as the number of years of irrigationincreased.

To estimate leaching in the irrigated soils, the depthof crop and leaching water demands were calculatedand are shown with annual rainfall and irrigation depthsin Fig. 2. Potential evapotranspiration was calculated(R.J. Lascano, 1991, personal communication) andcrop water use was estimated as a fraction of the PETbased on cotton water consumption (Stockton et al.,1967). The crop water used during 1985 to 1987 was500 to 600 mm or approximately one-half of the com-bined rain plus irrigation depth. Approximately 900mm of irrigation was applied during the 1985 and1987 growing season, compared with 600 mm duringthe 1986 growing season. Annual rain was greatestduring 1986. The depth of irrigation and rainwater toleach the soil, based on the ECe of the soil profile andcalculated according to Kamphorst and Bolt (1978),was greatest in 1987 and similar for 1986 and 1985.Combined crop water use and leaching requirementwas approximately equal to the depth of rain plus ir-rigation water during all years, showing that adequatewater was applied to remove excess salts, resulting inno increase in the ECe after the first year.

Infiltration rates of well or reverse osmosis waterfor soil irrigated 0, 1, 2, or 3 seasons are plotted inFig. 3. Infiltration rate declined more rapidly and toa lower value when using reverse-osmosis than wellwater because of the more dispersive nature of low-EC^ water that contributes to seal formation (Agassiet al., 1981). For example, the infiltration rate after50 mm of applied well water was =17 mm h"1 for

1987

1986

1985

500 1000

DEPTH OF WATER, mm

1500

Fig. 2. Crop water use estimated from potential eva-potranspiration and the calculated leaching demand (upperbars in annual pairs) compared with the measured annualrain and seasonal irrigation depth (lower bars in annualpairs).

YEAR1o RO WATER— FITTED• WELL WATER- - FITTED

E

ii

0 10 20 30 40 0 10 20 30 40 50

WATER APPLIED, mm

Fig. 3. Infiltration rate over depth of applied well and reverse-osmosis water for a Hoban silt loam after zero, one, two,and three irrigation seasons.

the nonirrigated and 10 mm h-1 for the irrigated treat-ments, compared with =2 mm h-1 after 30 to 40 mmof applied reverse-osmosis water, whether irrigated ornot. As the infiltration rate declined more rapidly, thea of Eq. [1] averaged across all treatments becamemore negative (300%), as shown by a = —0.0006mm"1 for well water, compared with a = —0.00184mm"1 for reverse osmosis water. Infiltration rate de-clined more slowly, despite the water applied, in non-irrigated treatments (a —0.0008 mm-1) than intreatments irrigated for 1 through 3 yr (a = — 0.0014mm-1). The Tp, i.e., when the infiltration rate andrainstorm intensity are identical, represents the timewhen infiltration is first limited by the soil and can beused in modeling infiltration (Baumhardt et al., 1990).

Page 4: Infiltration in Response to Water Quality, Tillage, and Gypsum

264 SOIL SCI. SOC. AM. J., VOL. 56, JANUARY-FEBRUARY 1992

Table 2. Hoban silt loam electrical conductivity (EC,.),, sodiumadsorption ratio (SARJ, and particle-size distribution withintillage treatment sites for the 0- to 50-mm depth.

YEARS OF IRRIGATIONFig. 4. Cumulative infiltration after 1 h (50 mm) of applying

well and reverse-osmosis water into a Hoban silt loam afterzero, one, two, and three irrigation seasons.

Table 1. Analysis of variance (ANOVA) and covariance (Cov.ANOVA) for irrigation and water effects on infiltrationamount. Electrical conductivity (ECe) was used as thecontinuous parameter covariant with infiltration amount.

Source ofvariationtEC.Irrig. (I)Error (a)Water (W)I x WError

df

38138

ANOVAmeansquare

0.6140.173

14.5110.1470.019

Cov. ANOVA

P

<0.01

<0.010.01

df138136

meansquare0.0020.0250.019

12.5570.1050.021

P0.780.35

<0.010.04

t Irrig. = irrigation treatment, Water = water quality.

On the average, T^ with well water was twice thatwith reverse-osmosis water. No significant differencein Tp was observed among irrigation treatments ex-posed to reverse-osmosis water. When using well water,the Tp for soil with no previous irrigation was 0.35 h,compared with 0.177, 0.207, and 0.225 h for the 1-,2-, and 3-yr irrigation treatments, respectively.

Cumulative infiltration of well water was signifi-cantly greater than reverse-osmosis water, regardlessof the previous irrigation history (Fig. 4). Cumulativeinfiltration within well and reverse-osmosis water ap-plication treatments were greatest for the unirrigatedsoil and increased with increasing years of irrigationbecause the soil ECe and SARS decreased with increas-ing years of irrigation. The analysis of variance (Table1) indicates that cumulative infiltration declined sig-nificantly as the applied water EC«, declined (reverse-osmosis water less than well water) or the number ofyears of irrigation changed (decreasing ECe and SARSwith continued irrigation). Increased infiltration withcontinued irrigation indicates that the increasing soilsalinity became high enough to counter, in part, thesodic conditions. The effects of soil ECe and SARSinteracted with water EC, making infiltration differ-ences less apparent when the water applied had a lowEC. A covariance analysis of these data to segregate

Tillagetreatment

Plow-diskDouble-diskCompactedUndisturbedCompositeMeant

Particle size

EC.tdSm-1

1420 a750 b510 b490 b

SAR,mmol/L"2

16.1014.2011.307.50

12.30 ± 3.80

Sand

238240266224

242 ± 18

Silt-gkg-1 —

595607578678

615 ± 44

Clay

167153180169

167 ± 11t Triplicate EC, determinations were taken near infiltration measurements.Means followed by the same letter are not significantly different (P = 0.05)according to Duncans multiple-range test.$ Composite mean values were deteimined from combined soil samplescollected within treatment sections ± one standard deviation.

the effect of the easily measured initial soil ECe wasperformed (Table 1). Even though soil sodicity variedsimilarly to ECe, when accounting for ECe effects inthis manner, variation in infiltration amount due toirrigation treatments was reduced and only water-qual-ity treatments caused significant (P = 0.01) differ-ences in infiltration amount. Both the soil and thewater EC act to govern infiltration under field con-ditions; however, soil ECe was less correlated to in-filtration with the reverse-osmosis water than with thewell-water treatments. Runoff during a natural high-intensity, short-duration raiin would develop rapidlyfrom irrigated soils.

Our results show that infiltration processes mea-sured in the field are dependent on the soil salinityand sodicity and the salinity of the applied water. Di-rect measurements of surface seals or crusts were notmade; however, decreased Infiltration of low ECw orSAR ,̂ reverse-osmosis water, compared with high EC ,̂or SAR,,, well water, is consistent with seal formationin response to chemical dispersion reported by Agassiet al. (1981). These data suggest that infiltration maybe increased by increasing the EC ,̂ of applied waterusing, for example, moderately soluble PDG appliedto the soil surface, as suggested by Keren and Shain-berg (1981).

Tillage and GypsumSoil surface texture and SARS from composite sam-

ples taken within the tillage-treatment sections are givenin Table 2. The soil texture in each tillage-treatmentsection was silt loam and did not vary among treat-ment sections. The ECe of the plow-disk surface soilwas significantly higher than the ECe of the other til-lage treatments. The soil surface ECe and the com-posite SARS values decreased as the depth of tillagedecreased. Since the surface ECe of soil measured inthe intervening alleys between tillage treatments was0.75 dS m-1, or approximately the same as for thecompacted and undisturbed soils, the plow-disk (depth= 0.3 m) and double-disk (depth = 0.15 m) tillagetreatments mixed salts from the underlying soil withthe surface, resulting in the higher soil ECe and SARSvalues. The relatively small difference in tillage depthalmost doubled the ECe for plow-disk (2.22 dS m-1),compared with double-disk (1.17 dS m-1) tillage. Soil

Page 5: Infiltration in Response to Water Quality, Tillage, and Gypsum

BAUMHARDT ET AL: INFILTRATION RESPONSE TO WATER QUALITY, TILLAGE, AND GYPSUM 265

bulk density of the undisturbed and compacted soilswere 1.31 and 1.30 Mg m-3 to a depth of 0.10 m,compared with soil bulk densities of 0.94 Mg m~3 toa depth of 0.3 m for the plow-disk and 1.04 Mg m~3

to a depth of 0.15 m for the double-disk tilled soils.Infiltration rates into soil treated with surface-ap-

plied PDG at rates of 0 and 6 Mg ha"1 are plotted forthe different tillage treatments as a function of theamount of water applied in Fig. 5. Powdered gypsumincreased infiltration rate in the plow-disk, double-disk, and compacted soils; however, no apparent gyp-sum effect was measured in the undisturbed soil. Re-gardless of gypsum treatment, infiltration rate decreasedmore rapidly as the degree or amount of tillage de-creased, because tillage increases soil saturated hy-draulic conductivity and infiltration rate (Klute, 1982).The infiltration rate after 50 mm of applied water was=20 mm h"1 for the plow-disk and 10 mm h-1 forthe double-disk treatments, compared with 7 mm h-1

for the scraped-compacted and undisturbed treat-ments, when PDG was applied. When PDG was notused, the relatively consistent steady infiltration rateof 7 mm h-1 was reached after 50, 40, 30 and 20 mmof applied water for the plow-disk, double-disk, com-pacted, and undisturbed tillage treatments, respec-tively. The values of a (Eq. [1]) in PDG-treated soilswas —0.00076 mm"1 for the tilled soil, comparedwith - 0.0027 and - 0.0046 mm-1 for the compactedand undisturbed soils, respectively, indicating that in-filtration rate declined more rapidly as the degree oramount of tillage decreased. When PDG was not ap-plied, a values were higher for the plow-disk (— 0.0005mm-1) and double-disk ( — 0.0014 mm"1) soils andsimilar for the compacted ( — 0.0024 mm-1) and un-disturbed ( — 0.004 mm"1) soils. The Tp, estimatedwith Eq. [1], was similar for all tillage treatmentswhen PDG was not applied; however, when PDG wasapplied, Tp increased for all tillage treatments, result-ing in the following order: plow-disk (0.309 h), dou-ble-disk (0.206 h), compacted (0.163 h), andundisturbed (0.103 h).

Cumulative infiltration is compared among all til-lage and PDG treatments in Fig. 6. Again, as thedegree of disturbance by tillage increased, the cu-mulative infiltration increased, resulting in a treatmentranking of plow-disk > double-disk > compacted =control. Cumulative infiltration into soil treated withgypsum was significantly greater than untreated ex-cept for the control. A relative increase in infiltrationamount of 38 ± 4% was obtained with gypsum aslong as the soil was disturbed. Powdered gypsum ef-fects on infiltration were determined for plow-disk soilusing an additional gypsum rate of 3 Mg ha"1. Cu-mulative infiltration for plow-disk tilled soil exposedto a 50 mm h"1 water application intensity for 1 hwas 26, 29, and 36 mm for gypsum application ratesof 0, 3, and 6 Mg ha"1 or an average increase of 1.6mm for each tonne of applied gypsum per hectare. Aminimum rate of 5 Mg ha"1 of gypsum is recom-mended since 3 Mg ha-1 rate was within one standarddeviation of the 0 Mg ha"1 rate.

Analysis of variance of the observed cumulativeinfiltration (data not shown) identified significant dif-ferences in response to the tillage and PDG treat-ments; that is, infiltration increased with tillage andthe application of PDG (even on a compacted soil).A significant interaction between tillage and PDG in-dicates that the increase in cumulative infiltration dueto PDG effects depended on tillage, resulting in noincrease in infiltration when PDG was applied to anundisturbed soil. Recalling that the higher soil ECe ofirrigation treatments accounted for reductions of in-filtration in the first experiment, ECe values were in-cluded as a covariant because of the significantdifferences in soil ECe corresponding to tillage. Thecovariance analysis, which segregates the effect of theinitial soil ECe on infiltration, resulted in little differ-ence from the ANOVA. The tendency of tillage todecrease infiltration by mixing higher ECe subsoil withthe surface, was offset by the tendency of tillage toincrease infiltration by reducing soil bulk density.

80

60

- 40.c

20

60

40

20

SCRAPED-COMPACTED UNDISTURBEDo NO GYPSUM

- - - FITTED• GYPSUM

—— FITTED

-*-!-,0 10 20 30 40 0 10 20 30 40 50

WATER APPLIED, mm

Fig. 5. Tillage and powdered-gypsum effects on infiltrationrate over depth of applied reverse-osmosis water into a Hobansilt loam. Powdered gypsum was applied at 0 to 6 Mg ha"1

rates after plow-disk, double-disk, compaction, orundisturbed soil treatments.

PLOW DISK COMPACTED CONTROL

TILLAGE TREATMENTFig. 6. Tillage and powdered-gypsum effects on cumulative

infiltration after 1 h (50 mm) of applying reverse-osmosiswater into a Hoban silt loam. Powdered gypsum was appliedat 0 or 6 Mg ha-1 rates after plow-disk, double-disk,compaction, or undisturbed soil treatments.

Page 6: Infiltration in Response to Water Quality, Tillage, and Gypsum

266 SOIL SCI. SOC. AM. J., VOL. 56, JANUARY-FEBRUARY 1992

Our measurements show that gypsum increased theinfiltration amount to a similar degree regardless oftillage or compaction treatment, but not when the soilwas undisturbed. The increase in infiltration for thedisturbed soil is consistent with laboratory results ofKeren and Shainberg (1981) and Agassi et al. (1981)and field studies by Agassi et al. (1990), because thesoils were not previously crusted. Gypsum did notincrease the infiltration rate or amount in the undis-turbed soil due to an interaction with tillage, probablybecause of existing crusts that were broken by tillageor compaction treatments. These data suggest that theeffect of surface-applied PDG to increase infiltrationwould be short lived, as noted by Freebairn et al.(1988). The effect of tillage to mix higher ECe subsoilwith the surface soil, thus enabling more rapid sealformation and reducing infiltration, was offset by re-duced soil density, which increased infiltration.

CONCLUSIONSThe accumulation of salts from irrigation water with

a moderate Na and very high salinity hazard into a siltloam soil reduced infiltration after irrigating for only1 y. Continued irrigation tended to increase infiltrationby increasing the soil salinity level, thus counteringthe soil sodicity effects on infiltration. Rainfall andexcess irrigation water prevented salt accumulation atthe surface and a progressive reduction in the lowerprofile, 0.3 to 0.9 m, as irrigation continued through3 y. Compared with high ECw well water, low EC ,̂water reduced infiltration; however, PDG can be usedto increase infiltration of low ECe water in tilled soils.Infiltration into compacted soils with PDG was onlyslightly greater when no PDG was applied and un-changed by the PDG treatment in undisturbed soils.Tillage increased infiltration of reverse osmosis waterand interacted with gypsum to increase infiltration.