Effect of tillage and crop rotations on pore size distribution

and soil hydraulic conductivity in sandy clay loam soil

of the Indian Himalayas

Ranjan Bhattacharyya *, Ved Prakash, S. Kundu, H.S. Gupta

Vivekananda Institute of Hill Agriculture (Indian Council of Agricultural Research),

Almora 263 601, Uttaranchal, India

Received 18 August 2004; received in revised form 17 January 2005; accepted 7 February 2005

www.elsevier.com/locate/still

Soil & Tillage Research 86 (2006) 129140AbstractTillage management can affect crop growth by altering the pore size distribution, pore geometry and hydraulic properties of

soil. In the present communication, the effect of different tillage management viz., conventional tillage (CT), minimum tillage

(MT) and zero-tillage (ZT) and different crop rotations viz. [(soybeanwheat (SW), soybeanlentil (SL) and soybeanpea (S

P)] on pore size distribution and soil hydraulic conductivities [saturated hydraulic conductivity (Ksat) and unsaturated hydraulic

conductivity {k(h)}] of a sandy clay loam soil was studied after 4 years prior to the experiment. Soil cores were collected after 4

year of the experiment at an interval of 75 mm up to 300 mm soil depth for measuring soil bulk density, soil water retention

constant (b), pore size distribution, Ksat and k(h). Nine pressure levels (from 2 to 1500 kPa) were used to calculate pore size

distribution and k(h). It was observed that b values at all the studied soil depths were higher under ZT than those observed under

CT irrespective of the crop rotations. The values of soil bulk density observed under ZT were higher in 075 mm soil depth in all

the crop rotations. But, among the crop rotations, soils under SP and SL rotations showed relatively lower bulk density values

than SW rotation. Average values of the volume fraction of total porosity with pores 150 mm in diameter (pores draining freely with gravity) were 0.124, 0.096 and 0.095 m3 m3 under CT, MT and ZT; and0.110, 0.104 and 0.101 m3 m3 under SW, SL and SP, respectively. Saturated hydraulic conductivity values in all the studied

soil depths were significantly greater under ZT than those under CT (range from 300 to 344 mm day1). The observed k(h)

values at 075 mm soil depth under ZTwere significantly higher than those computed under CTat all the suction levels, except at

10, 100 and 400 kPa suction. Among the crop rotations, SP rotation recorded significantly higher k(h) values than thoseunder SWand SL rotations up to 40 kPa suction. The interaction effects of tillage and crop rotations affecting the k(h) values* Corresponding author. Tel.: +91 5962 241 005; fax: +91 5962 231 539.

E-mail address: ranjan_vpkas@yahoo.com (R. Bhattacharyya).

0167-1987/$ see front matter # 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.still.2005.02.018

R. Bhattacharyya et al. / Soil & Tillage Research 86 (2006) 129140130were found significant at all the soil water suctions. Both SL and SP rotations resulted in better soil water retention and

transmission properties under ZT.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Conservation tillage; Soil water retention; Pore size distribution; Saturated and unsaturated hydraulic conductivity; Soybean based

cropping system1. Introduction

Soil moisture conservation is a critical issue in

rainfed farming in sub-temperate regions of the Indian

Himalayas. Conservation tillage management systems

(zero-tillage and minimum tillage) are effective means

in reducing water loss from the soil and improving soil

moisture regime (Hatfield and Stewart, 1994). Soil

pore geometry (pore size, shape and distribution) and

soil structure are affected by tillage management and

influence soil water storage and transmission (Azooz

et al., 1996). Overall, tillage effects on soil physical

properties are uncertain and variable. For example,

some researchers have found no or negative effect of

tillage on soil water transmission characteristics (Obi

and Nnabude, 1988; Heard et al., 1988), while others

found beneficial effects of zero-tillage (ZT) on soil

water retention properties than conventional tillage

(CT) (Blevins et al., 1971; Datiri and Lowery, 1991).

Many researchers have reported that saturated

hydraulic conductivity (Ksat) and unsaturated hydrau-

lic conductivity [k(h)] were significantly and posi-

tively affected by ZTowing to either greater continuity

of pores (Benjamin, 1993) or to water flow through a

very few large pores (Allmaras et al., 1977) or more

depth (Ehlers, 1977). The inconsistent results of soil

physical and hydraulic properties under different

tillage systems may be related to the transitory nature

of soil structure after tillage, site history, initial and

final water content, the time of sampling and the extent

of soil disturbances (Azooz and Arshad, 1996).

In a long-term study, Dao (1996) reported that no-

till soil had lower bulk density than that under

conventionally tilled soil. On the other hand, Roseberg

and McCoy (1992) found that CT increased total

porosity of the soil, but the macropores (effective

pores) decreased in number, stability and continuity

compared with no-till soil.

Macropores are responsible for the effective

porosity of the soil. Effective porosity has beenrelated to saturated hydraulic conductivity (Ahuja

et al., 1989). However, it also reflects the percentage of

total pores that are open to infiltration during a rain

event. The volume fraction of total porosity with pores

150 mm in diameter are effective pores for drainageof water freely with gravity (Azooz et al., 1996).

Under dry soil, transmission of water across a matric

pressure gradient occurs more rapidly through small

than large pores. Soil water storage and transmission

can, therefore, be manipulated with alteration of pore

size distribution through different tillage management

practices.

Information on potential changes in soil water

storage and transmission properties due to tillage

management and crop rotation is scanty. Therefore,

the present study was undertaken with the objective

to assess the effect of conservation tillage and

different crop rotations on soil pore size distribution

and water transmission properties under rainfed

production system. We determined water retention,

pore size distribution, Ksat and k(h) on a sandy clay

loam soil that was subjected to continuous conven-

tional tillage (CT), minimum tillage (MT) and zero-

tillage (ZT) for 4 years under soybeanwheat (SW),

soybeanlentil (SL) and soybeanpea (SP) rota-

tions.2. Materials and methods

2.1. Site details

The experiment was initiated in 1999 on a sandy

clay loam soil (pH 5.9, oxidizable soil organic carbon

9.2 g kg1, alkaline permanganate oxidizable N

280.5 kg ha1, 0.5 M NaHCO3 extractable P

20.1 kg ha1 and 1N NH40Ac K 96.2 kg ha1) at

R. Bhattacharyya et al. / Soil & Tillage Research 86 (2006) 129140 131the experimental farm of the institute located at

298360N, 798400E, and 1250 m above mean sea level).The sandy clay loam soil contained 21.8% clay, 19.7%

silt and 58.5% sand at 0150 mm and 19.3% clay,

20.9% silt and 59.8% sand at 150300 mm depth. The

climate of the region is sub-temperate. The average

daily maximum and minimum air temperatures ranged

from 31.7 and 20.6 8C in June and 17.8 and 1.1 8C inJanuary. The mean annual rainfall is 1058 mm.

Approximately, 70% of the total precipitation occurs

during the rainy season (JuneSeptember).

2.2. The experiment

The experiment was laid out in split-plot design with

three tillage management practices (zero, minimum and

conventional) in main plots (9 m 3 m size) and threesequential cropping [soybean (Glycinemax (L.) Merr.)

wheat (Triticum aestivum L. Emend. Flori and Paol),

soybeanlentil (Lens culinaris Medicus) and soybean

field pea (Pisum sativum L. Sensu Lato) in sub-plots

(3 m 3 m size) with three replications. The growingperiod of soybean was from June to October and that for

lentil and pea was from November to 1st week of April

and for wheat from November to 1st week of May.

Under ZT, every year the seeds were sown in the

furrows with the help of a hand pulled furrow opener.

Whereas, under MT sowing was done after a single

tillage operation by spade (up to 15 cm soil depth) and

under CT sowing was done following two tillage

operations (up to 15 cm soil depth) made by spade at a 7

days interval. The details of the sowing and harvesting

time of the crops and the detail of the weather

parameters during the period of the experimentation are

given in Table 1. Before sowing of soybean and winter

crops, weeds were controlled with the application of

gramaxone (1,10-dimethyl 1-4,40-bipyridylium) at1.0 kg a.i. ha1 under zero-tillage. Weeds in MT and

CT systems were controlled by pre-emergence spray of

alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxy-

methyl)acetamide] at 2.0 kg a.i. ha1 in soybean

followed by one hand weeding at 45 days after sowing

except in zero-tillage. Similarly, during winter season

weeds were controlled in CT and MT systems by

spraying isoproturon [3,-(4-isopropylphenyl)1,1-di-

methyl urea] at 1.0 kg a.i. ha1 at 35 days after sowing

in wheat and pendimethalin [N-(1-ethylpropyl)-3,4-

dimethyl-2,6-dinitrobenzenamine] at 1.0 kg a.i. ha1(as pre-emergence) in field pea and lentil followed by

one hand weeding as and when required except in zero-

tillage. Soybean variety VL Soya 2 was sown in 1st

fortnight of June whereas, winter crops viz., wheat cv.

VL Gehun 616, field pea cv. VL Matar 1 and lentil

cv. VL Masoor 4, were sown during 2nd fortnight of

October in each year of the study. Recommended doses

of fertilizers, which were applied to different crops

were, 20 kg N + 34.9 kg P + 33.3 kg K ha1 to soy-

bean, 60 kg N + 13.1 kg P + 16.7 kg K ha1 to wheat,

20 kg N + 17.5 kg P + 33.3 kg K ha1 to field pea

and 20 kg N + 17.5 kg P + 16.7 kg K ha1 to lentil.

Full amount of N, P and K in pulses and half amount of

N along with full amount of P and K in wheat were

applied at the time of sowing. The remaining half

amount of N was top-dressed in wheat after winter rains

in February.

At maturity the above ground portion of all the

crops were harvested leaving 5 cm stubbles in the

field.

2.3. Soil physical and chemical analysis

Initial soil samples (015 cm) were collected and

analyzed for pH by pH meter in 1:2.5 soil: water

suspension (Jackson, 1973), organic carbon by the

method of Walkley and Black (1934), available N by

standard procedure using a FOSS Tecator (Model

2200), available P following the method of Olsen et al.

(1954) and available K by 1 N NH4OAc using a flame

photometer (Jackson, 1973).

In 2003, after the harvest of wheat crop, triplicate

undisturbed soil cores were collected to a depth of

300 mm in 75 mm increments with a core sampler.

Bulk density was determined from oven-dried core

mass divided by the core volume. For determining the

soil water desorption characteristics at 0 to 10 kPasuction range, we followed the method of Kooistra

et al. (1984) with some modifications. A closed porous

cup was used for measurement of outflow from a

single sample as a function of soil water matric

potential. A soil core was saturated by capillary rise on

the porous fritted disc of the Buchner funnel. Starting

from saturation, water outflow during successive Cdecrements (2, 4 and 10 kPa) was measured in aburette connected to the Buchner funnel. Water

retention was determined successively at 10, 20,30, 40 100, 400 and 1500 kPa using a

R. Bhattacharyya et al. / Soil & Tillage Research 86 (2006) 129140132

Table 1

Weather parameters of the site during the experimental period and crop calendar

Crops and year Rainfall

(mm)

Average maximum

temperature (8C)Average minimum

temperature (8C)Average

evaporation

(mm day1)

Date of

sowing

Date of harvesting

Soybean,

1999

538.3 28.5 18.0 3.3 2 June 1999 3 October 1999

Soybean,

2000

870.5 27.9 17.9 3.0 1 June 2000 3 October 2000

Soybean,

2001

464.0 29.3 17.8 3.4 6 June 2001 5 October 2001

Soybean,

2002

643.1 28.8 17.1 3.0 5 June 2002 6 October 2002

Wheat,

19992000

307.6 22.6 4.7 2.3 10 October 1999 3 May 2000

Wheat,

20002001

130.1 23.6 5.3 2.2 9 October 2000 4 May 2001

Wheat,

20012002

238.4 23.4 4.2 2.2 12 October 2001 7 May 2002

Wheat,

20022003

353.3 23.3 4.2 2.5 12 October 2002 8 May 2003

Lentil and pea,

19992000

250.4 21.4 3.8 2.0 22 October 1999

and 22 October 1999

16 April 2000

and 25 April 2000

Lentil and pea,

20002001

98.0 22.8 3.7 2.0 20 October 2000

and 20 October 2000

15 April 2001

and 26 April 2001

Lentil and pea,

20012002

182.1 22.4 3.5 2.4 23 October 2001 and

23 October 2001

18 April 2002

and 28 April 2002

Lentil and pea,

20022003

306.3 22.4 3.4 2.2 25 October 2002 and

25 October 2002

19 April 2003

and 27 April 2003pressure plate apparatus for each replication of each

treatment. After taking the wet mass at the final

potential (1500 kPa), the saturation water content(assumed to represent 0 kPa suction) of soil was

determined. Water retention data were fitted to the

equation:

C Ceu=usatb (1)where C is the matric pressure, Ce the intercept (also

termed as the air entry point), u the volumetric watercontent at a given matric pressure, usat the volumetric

water content at saturation, and b is an empirically

derived constant describing the slope of the relation-

ship between matric pressure and relative saturation

(u/usat).

Soil pore size distribution data was computed from

the soil water retention data for C range of 2 to400 kPa, using the theoretical relation between soilwater characteristic and distribution of pore sizes

(Vomocil, 1965). Equivalent pore diameter (EPD) of a

given matric pressure was estimated according to the

following expression that relates the suction applied toa water column as a function of the capillary radii (the

capillary rise equation):

EPD 4s cosa=rgh (2)

where s is the surface tension of water; cos a the

cosinus of the angle a displayed by the water menis-

cus; r the water specific weight; g the gravity accel-

eration and h the matric pressure. At 22 8C the value ofs become 0.07357 kg s2 and a = 0, the capillary rise

equation can be reduced to the following expression

(Marshall and Holmes, 1988):

EPD 300=h (3)

where the equivalent pore diameter (EPD) of the

smallest pore (mm) drained at water suction of h (kPa).Pore size distribution was presented as pore volume

occurring within a given size interval per unit soil

(total) volume. The volume fraction of total porosity

with pores 150 mm in diameter wasdefined as pores draining freely with gravity.

R. Bhattacharyya et al. / Soil & Tillage Research 86 (2006) 129140 133Saturated hydraulic conductivity (Ksat) values to a

depth of 300 mm in 75 mm depth increments were

determined in the laboratory using Darcys law by

constant head method (Klute and Dirksen, 1986).

Unsaturated hydraulic conductivity [k(h)] values were

calculated from water retention and saturated hydraulic

conductivity using the following equation (Campbell,

1974):

kh Ksatu=usat2b3 (4)where Ksat is the saturated hydraulic conductivity.

Analysis of variance was used to determine tillage

effects on bulk density, water retention constant (b),

volume fraction of pores, Ksat and k(h) at each matric

suction for each depth of soil (Gomez and Gomez,

1984). All the soil physical parameters were analyzed

as a split-plot model (tillage as main effect, cropping

system as split-plot effect). Tillage and cropping

system means were separated with an LSD at

P 0.05.3. Results and discussion

3.1. Soil bulk density

Soil bulk density was significantly higher under ZT

at the soil surface (075 mm) compared with tilled

(MT and CT) soil (Table 2), whereas, it was

significantly lower under MT at 225300 mm soil

depth than that of CT. There were no significant

variations in soil bulk density values due to tillage

management at other two studied soil depths. But there

was a significant increase in bulk density under SW

rotation over other cropping systems at the surface soil

layers (075 and 75150 mm depth). With increase in

soil depths, soil bulk density values also increased and

the highest bulk density value (1.40 Mg m3) was

observed at 225300 mm soil depth under CT and ZT

systems with SW and SP crop rotations. At that soil

depth soil bulk density under SW rotation was higher

than that of SL rotation and was at par with that of S

P rotation. The significantly higher value of soil bulk

density under ZT at the surface soil layer (075 mm)

might be due to non-disturbance of soil matrix that

resulted in less total porosity compared to tilled plots.

The interaction effects of tillage cropping werenon-significant at all the soil depths, except at 150225 mm soil layer (all the interaction means were

calculated and not shown).

Trends in soil bulk density are generally considered a

rough approximation of soil structural changes (Liebig

et al., 2004). Several studies have reported higher bulk

density under ZTat the soil surface compared with tilled

soil (Wu et al., 1992; Hill, 1990; Klute, 1982). Tillage

loosens the soil and decreases soil macroporosity (Hill

et al., 1985; Vazquez et al., 1991). Significantly lower

core (075 mm) soil bulk density with CT system could

be due to the incorporation of crop residues by tillage to

the surface soil depth. There appeared to be very little

differences in bulk density values among the CT, MT

and ZT fields and among the plots under SW, SL and

SP rotations. The long (>200 days) lag time betweenthe most recent tillage event and soil sampling might

have contributed to the similar bulk density values

among the CT, MT and ZT fields. Our results are of

close conformity with other researchers on the

Canadian prairies who have reported slight or no

differences in bulk density values between CT and ZT

(Chang and Lindwall, 1989; Azooz et al., 1996).

3.2. Soil water retention constant (b)

At a given matric pressure, soil under ZT retained

significantly more water than soil under MT and CT,

irrespective of the crop rotation at 075 and 75

150 mm soil depths, suggesting significant rearrange-

ment of pores near the soil surface (Table 2). Soil

water retention constant (b) was the highest under ZT

at the surface soil layer. The b values under ZT were

significantly higher than the corresponding values

under CT at all the soil depths. In contrast, the

differences in b values due to crop rotations were not

significant at different soil depths except at 150

225 mm. The interaction effects on b were also non-

significant at all the soil depths.

Greater water retention in the 075 mm soil depth

under ZT than under CT was also observed in a silt

loam and sandy loam soil by Azooz et al. (1996). The

greater the b value, the greater was the water retention

across a range of matric pressures.

3.3. Saturated hydraulic conductivity (Ksat)

Saturated hydraulic conductivity (core) of the

studied four depths (075, 75150, 150225 and 225

R.Bhatta

charyya

etal./S

oil&

Tilla

geResea

rch86(2006)129140

13

4

Table 2

Soil bulk density, saturated hydraulic conductivity and soil water retention constant (b) as affected by tillage management and cropping system to a depth of 300 mm

Treatmentsa Soil bulk density (Mg m3) Saturated hydraulic conductivity

(mm day1)

Soil water retention constant (b)b

075

mm

75150

mm

150225

mm

225300

mm

075

mm

75150

mm

150225

mm

225300

mm

075

mm

75150

mm

150225

mm

225300

mm

Tillage

CT 1.34 1.35 1.39 1.40 344 315 308 300 4.2 4.3 4.1 4.2

MT 1.34 1.34 1.38 1.39 370 364 313 306 4.5 4.3 4.3 4.3

ZT 1.35 1.35 1.38 1.40 393 372 331 331 4.6 4.6 4.5 4.4

LSD (P = 0.05), tillage 0.007 NS NS 0.006 7.6 2.4 9.3 19.9 0.15 0.25 0.18 0.19

Cropping system

SW 1.36 1.35 1.39 1.40 368 348 319 307 4.4 4.4 4.2 4.3

SL 1.33 1.35 1.39 1.39 377 348 320 312 4.5 4.4 4.3 4.2

SP 1.34 1.34 1.38 1.40 361 354 314 318 4.4 4.4 4.3 4.3

LSD (P = 0.05), cropping system 0.012 0.008 NS 0.008 8.3 5.2 8.6 7.7 NS NS 0.08 NS

LSD (P = 0.05), tillage cropping system NS NS 0.011 NS 14.5 8.9 14.8 13.3 NS NS NS NSa CT, conventional tillage; MT, minimum tillage; ZT, zero-tillage; SW, soybeanwheat; SL, soybeanlentil; SP, soybeanpea.b Water retention constant (b) from the equation C = Ce(u/usat)

b; NS, non-significant.

R. Bhattacharyya et al. / Soil & Tillage Research 86 (2006) 129140 135300 mm) were always in the order ZT > MT > CT(Table 2), with the difference between ZT and CT

always significant (P = 0.05). The effect of ZTand MT

management was to reduce the volume fraction of

large pores and increase the volume fraction of small

pores with better pore continuity relative to CT

management, which ultimately resulted in higher Ksatunder reduced tillage systems (ZT and MT). Saturated

hydraulic conductivity is highly dependent upon the

size, continuity and arrangement of pores. Greater Ksatin tilled soils was an indication of better pore

continuity, as the proportion of larger pores was

comparatively less. Though the part of the transmis-

sion porosity (15030 mm diameter pores) was lowerunder reduced tillage systems, a greater pore

continuity (possibly as a result of minimal soil

disturbances) was indicated by higher Ksat (Ehlers,

1977; Benjamin, 1993). Greater content of water

stable aggregates in the reduced tillage system

probably also contributed to its higher Ksat (Singh

et al., 1994). Ehlers (1975), Blevins et al. (1983),

among others, also measured a greater Ksat under no-

till system. In contrary, Carter and Kunelius (1986)

and Heard et al. (1988) found a reduced Ksat under no-

till. Although Ksat can be extremely variable, it is

possible that the higher Ksat values for the ZT field

might have been partially due to the burrows of the

endogenic earthworms (Joschko et al., 1992).

Soybeanwheat cropping resulted in significantly

lower Ksat than those of other systems. At 075 mm

soil depth, the average Ksat value under SL rotation

was significantly higher than those under SW and S

P rotations and at 75150 mm soil depth Ksat under S

P rotation was significantly greater than those under

SW and SL rotations. In contrast, at 225300 mm

soil depth, Ksat value under SP rotation was the

highest. At all the studied soil layers, the interaction

effects of tillage and cropping were significant. The

Ksat values of reduced tillage systems were signifi-

cantly greater under SL and SP rotations than that

under SW of CT management. The greater Ksatvalues at the surface soil layer under leguminous

cropping system might be due to presence of bio-

channels as a result of more microbial activities and

greater content of soil organic carbon (measured by

us) at the surface soil layers. That favourable soil

environment might have resulted in better pore

continuity.There was a sharp reduction in Ksat from 075 to

75150 mm and from 75150 to 150225 mm soil

layers, irrespective of the management (tillage and

cropping) systems. In the first two layers, there were

significant differences in Ksat values under minimum

and conventionally tilled plots. A much larger Ksat in

075 mm soil layer than those in the lower layers

might be due to more vigorous macro-faunal activity

and higher pore continuity in the surface layer (Singh

et al., 1996).

3.4. Porosity and pore size distribution

At any given water potential, soil under ZT field

retained a greater amount of water compared to the CT

field. This trend suggested a soil structural improve-

ment under the reduced tillage condition as the amount

of water retained at >100 kPa suction dependsprimarily upon the pore size distribution and the

capillary effect (Hillel, 1998). Water retention at

R. Bhattacharyya et al. / Soil & Tillage Research 86 (2006) 129140136

Fig. 1. Pore size distribution as affected by tillage and cropping

system management at 075 and 75150 mm soil depth. CT, con-

ventional tillage; MT, minimum tillage; ZT, zero-tillage; SW,

soybeanwheat; SL, soybeanlentil; SP, soybeanpea. LSD

(0.05) values for the means of tillage, cropping and tillage crop-cropping interactions at each data point are indicated by +,

and signs, respectively.

Fig. 2. Pore size distribution as affected by tillage and cropping

system management at 150225 and 225300 mm soil depth. CT,

conventional tillage; MT, minimum tillage; ZT, zero-tillage; SW,

soybeanwheat; SL, soybeanlentil; SP, soybeanpea. LSD

(0.05) values for the means of tillage, cropping and tillage crop-cropping interactions at each data point are indicated by +, and signs, respectively.at the upper soil layers, except at 10 kPa are combination of transmis-sion pores and macropores through which water

moves freely under gravity. The volume of pores

drained between 10 and 1500 kPa is termed asstorage pores. Transmission pores are important both

in soilplantwater relationships and in maintaininggood soil structure, and the storage pores are important

for holding water needed for plants and microorgan-

isms (Pagliai et al., 1995). The 15030 mm portion oftransmission porosity (30030 mm EPD or pore spacedrained when suction is reduced from 1 to 10 kPa)was greater in the CT management than the reduced

tillage systems. Douglas et al. (1980) and Singh et al.

(1996) also observed less volume of transmission

pores under direct drilling and no-till soil, respec-

tively. The 15030 mm portion of transmissionporosity of 0300 mm soil accounted for 9.4, 8.7

and 8.6% of the total porosity in the CT, MT and ZT

R. Bhattacharyya et al. / Soil & Tillage Research 86 (2006) 129140 137

Fig. 3. Calculated unsaturated hydraulic conductivity as a function

of matric pressure as affected by tillage and cropping system

management at 075 and 75150 mm soil depth using Eq. (4).

CT, conventional tillage; MT, minimum tillage; ZT, zero-tillage; S

W, soybeanwheat; SL, soybeanlentil; SP, soybeanpea. LSD

(0.05) values for the means of tillage, cropping and tillage crop-cropping interactions at each data point are indicated by +, and signs, respectively.systems and 9.0, 9.0 and 8.7% of the total porosity in

SW, SL and SP rotations, respectively. The pores

with diameters between 0.1 and 15 mm are assumed toretain more plant available water than the larger pores

(Hill et al., 1985). The amount of plant available water

storage porosity (300.2 mm EPD) accounted for 58.4,60.0 and 55.8% of the total porosity under ZT, MT and

CT systems, respectively, at 0150 mm soil depth and

58.5, 58.7 and 56.3% of the total porosity under ZT,

MT and CT systems, respectively, at 150300 mm soil

layer. In contrast, the volume fraction of total porosity

of pores >150 mm in diameter (pores draining freelywith gravity) averaged 0.124, 0.096 and 0.095 m3 m3

under CT, MT and ZT systems, respectively.

Tillage resulted in the distribution of soil porosity

with time, and soil under ZT had a larger proportion of

water-filled pores than did conventionally tilled soil.

This might be due to better soil aggregation under ZT

system (Shukla et al., 2003). Although the soil of the

ZT system had higher bulk density in the surface layer

and lower total porosity and less macropore volume, it

probably had limited effect on soil water recharge and

drainage because of a higher amount of residue on the

soil surface. The macropores were more continuous

under ZT plots probably because of more soil fauna,

preceding crop root channels, and minimum dis-

turbances (Zachamann et al., 1987). These continuous

macropores partially compensate for reduction in total

macroporosity of the soils under ZT plots. Tilled soils

had higher tillage-induced macropore volume in the

topsoil, but these might not be well connected to the

subsoil macropore (Hussain et al., 1998).

3.5. Unsaturated hydraulic conductivity [k(h)]

Greater unsaturated hydraulic conductivity was

computed for the soils under ZT than those under

MT and CT at 2 and 4 kPa suctions in the 075 mmsoil depth (Fig. 3). Soybeanpea cropping resulted in

significantly higher k(h) at 075 mm soil depth than

those under other systems up to 40 kPa suction.Unsaturated hydraulic conductivity values were higher

under the soils of MT plots than those under CTand ZT

plots at higher suction range (>10 kPa), except at400 kPa suction. Significantly higher k(h) valueswere obtained under SP rotation from 2 to 30 kPasuction than those under SL and SW rotations. Again,

the interaction effects of tillage and cropping systemson k(h) were significant at all the matric pressures.

Unsaturated hydraulic conductivity under ZTSP

system was significantly greater than k(h) values under

MTSW, MTSL and CTSW and CTSL systems at

2 and 4 kPa suctions. At greater suctions (10 to100 kPa), k(h) values under MTSP system washigher than those observed under other systems.

With higher suction values (from 4 kPa onwards),k(h) values under reduced tillage systems were

significantly higher than those under CT system at

75150 mm soil depth (Fig. 3). Higher k(h) values

were obtained under SL cropping than those

computed under other systems. The interaction effects

were found to be significant on k(h) values at 4 to40 kPa suction. With increasing soil depth the k(h)values reduced, irrespective of the management

R. Bhattacharyya et al. / Soil & Tillage Research 86 (2006) 129140138

Fig. 4. Calculated unsaturated hydraulic conductivity as a function

of matric pressure as affected by tillage and cropping system

management at 075 and 75150 mm soil depth using Eq. (4).

CT, conventional tillage; MT, minimum tillage; ZT, zero-tillage; S

W, soybeanwheat; SL, soybeanlentil; SP, soybeanpea. LSD

(0.05) values for the means of tillage, cropping and tillage crop-cropping interactions at each data point are indicated by +,

and signs, respectively.systems. Average k(h) under ZT was significantly

higher than that under MT and CT up to 10 kPasuction ranges. At 20 to 400 kPa suction range,conservation tillage systems had greater k(h) values

than those under CT. At 150225 mm soil depth, k(h)

under SP rotation was significantly higher than that

of other cropping systems at all suction levels except

at 2 and 10 kPa (Fig. 4). There were significantinteraction effects on k(h) values at all the suction

levels, except at 30 kPa suction. A close analysis ofobserved k(h) data at 225300 mm soil layer revealed

the same trend of significantly higher k(h) values

under reduced tillage systems (Fig. 4). At 2 to40 kPa suction, k(h) under SP system had highervalues than those under SW and SL systems. The

tillage cropping interaction effects were also sig-nificant at all the matric pressure levels.Most researchers have reported a significant tillage

effect on unsaturated hydraulic conductivity, but they

found that it was dependent on water potential range

(Allmaras et al., 1977), depth (Ehlers, 1977), soil type

and layering (Datiri and Lowery, 1991) and crop

rotation (Benjamin, 1993). Higher k(h) under ZT was

attributed to the destruction of macroporosity in the

tilled soil (Ehlers, 1977). Datiri and Lowery (1991)

found that tillage and soil layering both affected k(h).

Azooz et al. (1996) found that k(h) at 40 kPa wasgreater for ZT than that for CT, and they explained that

this trend was due to more continuous pores under ZT.

Higher k(h) values under ZT and MT systems than

under CT and in SP rotation than in SL and SW

rotations were probably due to greater volume fraction

of pores

R. Bhattacharyya et al. / Soil & Tillage Research 86 (2006) 129140 139this sub-temperate climate of the Indian Himalayas, a

sandy clay loam soil can effectively be managed with

conservation tillage to increase water storage and

transmission properties. Soybeanlentil or soybean

pea cropping system managed with conservation

tillage in this area may be better suited to tolerate

water scarcity than soybeanwheat rotation managed

with conventional tillage. The superiority of the above

mentioned system is due to more effective water

storage within numerically more numbers of fine pores

and better rearrangement of pore size classes for faster

water transmission both under saturated and unsatu-

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Effect of tillage and crop rotations on pore size distribution and soil hydraulic conductivity in sandy clay loam soil of the Indian HimalayasIntroductionMaterials and methodsSite detailsThe experimentSoil physical and chemical analysis

Results and discussionSoil bulk densitySoil water retention constant (b)Saturated hydraulic conductivity (Ksat)Porosity and pore size distributionUnsaturated hydraulic conductivity [k(h)]

ConclusionsReferences