effect of tillage and crop rotations on pore size distribution and soil hydraulic conductivity in...

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) 129–140

Abstract

Tillage 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. [(soybean–wheat (S–W), soybean–lentil (S–L) and soybean–pea (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 0–75 mm soil depth in all

the crop rotations. But, among the crop rotations, soils under S–P and S–L rotations showed relatively lower bulk density values

than S–W rotation. Average values of the volume fraction of total porosity with pores <7.5 mm in diameter (effective pores for

retaining plant available water) were 0.557, 0.636 and 0.628 m3 m�3 under CT, MT and ZT; and 0.592, 0.610 and 0.626 m3 m�3

under S–W, S–L and S–P, respectively. In contrast, the 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 m�3 under CT, MT and ZT; and

0.110, 0.104 and 0.101 m3 m�3 under S–W, S–L and S–P, 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 day�1). The observed k(h)

values at 0–75 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, S–P rotation recorded significantly higher k(h) values than those

under S–Wand S–L 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: [email protected] (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) 129–140130

were found significant at all the soil water suctions. Both S–L and S–P 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 system

1. 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 been

related 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

<7.5 mm in diameter are classified as effective pores

for retaining plant available water. In contrast, the

volume fractions of total porosity with pores

>150 mm in diameter are effective pores for drainage

of 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 soybean–wheat (S–W),

soybean–lentil (S–L) and soybean–pea (S–P) 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 kg�1, alkaline permanganate oxidizable – N

280.5 kg ha�1, 0.5 M NaHCO3 extractable – P

20.1 kg ha�1 and 1N NH40Ac – K 96.2 kg ha�1) at

R. Bhattacharyya et al. / Soil & Tillage Research 86 (2006) 129–140 131

the 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 0–150 mm and 19.3% clay,

20.9% silt and 59.8% sand at 150–300 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 in

January. The mean annual rainfall is 1058 mm.

Approximately, 70% of the total precipitation occurs

during the rainy season (June–September).

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 three

sequential cropping [soybean (Glycinemax (L.) Merr.)–

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

soybean–lentil (Lens culinaris Medicus) and soybean–

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

(3 m � 3 m size) with three replications. The growing

period 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) at

1.0 kg a.i. ha�1 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. ha�1 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. ha�1 at 35 days after sowing

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

dimethyl-2,6-dinitrobenzenamine] at 1.0 kg a.i. ha�1

(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 ha�1 to soy-

bean, 60 kg N + 13.1 kg P + 16.7 kg K ha�1 to wheat,

20 kg N + 17.5 kg P + 33.3 kg K ha�1 to field pea

and 20 kg N + 17.5 kg P + 16.7 kg K ha�1 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 (0–15 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 kPa

suction 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 a

burette connected to the Buchner funnel. Water

retention was determined successively at �10, �20,

�30, �40 �100, �400 and �1500 kPa using a


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 day�1)

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,

1999–2000

307.6 22.6 4.7 2.3 10 October 1999 3 May 2000

Wheat,

2000–2001

130.1 23.6 5.3 2.2 9 October 2000 4 May 2001

Wheat,

2001–2002

238.4 23.4 4.2 2.2 12 October 2001 7 May 2002

Wheat,

2002–2003

353.3 23.3 4.2 2.5 12 October 2002 8 May 2003

Lentil and pea,

1999–2000

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,

2000–2001

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,

2001–2002

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,

2002–2003

306.3 22.4 3.4 2.2 25 October 2002 and

25 October 2002

19 April 2003

and 27 April 2003

pressure 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 ¼ Ceðu=usatÞ�b (1)

where C is the matric pressure, Ce the intercept (also

termed as the air entry point), u the volumetric water

content 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 to

�400 kPa, using the theoretical relation between soil

water 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 to

a water column as a function of the capillary radii (the

capillary rise equation):

EPD ¼ 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 of

s become 0.07357 kg s�2 and a = 0, the capillary rise

equation can be reduced to the following expression

(Marshall and Holmes, 1988):

EPD ¼ 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 <7.5 mm in diameter was defined as

effective pores for retaining plant available water,

whereas that with pores >150 mm in diameter was

defined as pores draining freely with gravity.


Saturated hydraulic conductivity (Ksat) values to a

depth of 300 mm in 75 mm depth increments were

determined in the laboratory using Darcy’s 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):

kðhÞ ¼ Ksatðu=usatÞ2bþ3 (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 (0–75 mm) compared with tilled

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

significantly lower under MT at 225–300 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 S–W

rotation over other cropping systems at the surface soil

layers (0–75 and 75–150 mm depth). With increase in

soil depths, soil bulk density values also increased and

the highest bulk density value (1.40 Mg m�3) was

observed at 225–300 mm soil depth under CT and ZT

systems with S–W and S–P crop rotations. At that soil

depth soil bulk density under S–W rotation was higher

than that of S–L 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 (0–75 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 were

non-significant at all the soil depths, except at 150–

225 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 (0–75 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 S–W, S–L and

S–P rotations. The long (>200 days) lag time between

the 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 0–75 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 0–75 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 (0–75, 75–150, 150–225 and 225–

R.Bhatta

charyya

etal./S

oil&

Tilla

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rch86(2006)129–140

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 m�3) Saturated hydraulic conductivity

(mm day�1)

Soil water retention constant (b)b

0–75

mm

75–150

mm

150–225

mm

225–300

mm

0–75

mm

75–150

mm

150–225

mm

225–300

mm

0–75

mm

75–150

mm

150–225

mm

225–300

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

S–W 1.36 1.35 1.39 1.40 368 348 319 307 4.4 4.4 4.2 4.3

S–L 1.33 1.35 1.39 1.39 377 348 320 312 4.5 4.4 4.3 4.2

S–P 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 NS

a CT, conventional tillage; MT, minimum tillage; ZT, zero-tillage; S–W, soybean–wheat; S–L, soybean–lentil; S–P, soybean–pea.b Water retention constant (b) from the equation C = Ce(u/usat)

�b; NS, non-significant.


300 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 Ksat

under reduced tillage systems (ZT and MT). Saturated

hydraulic conductivity is highly dependent upon the

size, continuity and arrangement of pores. Greater Ksat

in 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 (150–30 mm diameter pores) was lower

under 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).

Soybean–wheat cropping resulted in significantly

lower Ksat than those of other systems. At 0–75 mm

soil depth, the average Ksat value under S–L rotation

was significantly higher than those under S–W and S–

P rotations and at 75–150 mm soil depth Ksat under S–

P rotation was significantly greater than those under

S–W and S–L rotations. In contrast, at 225–300 mm

soil depth, Ksat value under S–P 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 S–L and S–P rotations than that

under S–W of CT management. The greater Ksat

values 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 0–75 to

75–150 mm and from 75–150 to 150–225 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

0–75 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 depends

primarily upon the pore size distribution and the

capillary effect (Hillel, 1998). Water retention at

<�100 kPa suction is affected by texture and the

specific surface of the soil material. The greater soil

water retention in plots under ZT might have been due

to higher soil organic carbon content.

The majority of the pores within each tillage and

cropping system were <7.5 mm in diameter at all the

four depths (Figs. 1 and 2). The volume fraction of

total porosity with pores <7.5 mm in diameter was

0.06 m3 m�3 greater under MT than under CT and

0.04 m3 m�3 greater under ZT than under CT at 0–

75 mm soil depth. In the surface soil layer (0–75 mm)

the volume fraction of total porosity with pore

diameters from 7.5 to 150 mm occupied 0.01 and

0.04 m3 m�3 less volume under ZT and MT than that

under CT. There was a significant (P = 0.05) effect of

reduced tillage systems on the above stated reduction

in volume fraction of total porosity with pore

diameters <7.5 mm except at 0.02–0.75 mm pore size

range at 0–75 mm soil depth. The volume fraction of

total porosity with pore diameters >150 mm was

significantly higher in S–W system than S–P at 0–

75 mm soil and was significantly higher in S–W

rotation than S–L at 75–150 mm soil depth. Other than

those, there were no definite trends in pore size

distribution though the individual effects were

significant at different pore size classes (Fig. 1) at

the upper two soil layers. The interaction effects of

volume fraction of total porosity were also significant


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

system management at 0–75 and 75–150 mm soil depth. CT, con-

ventional tillage; MT, minimum tillage; ZT, zero-tillage; S–W,

soybean–wheat; S–L, soybean–lentil; S–P, soybean–pea. 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 150–225 and 225–300 mm soil depth. CT,

conventional tillage; MT, minimum tillage; ZT, zero-tillage; S–W,

soybean–wheat; S–L, soybean–lentil; S–P, soybean–pea. LSD




at the upper soil layers, except at <0.02 mm pore

diameter of 0–75 mm soil layer. The effect of reduced

tillage management was to reduce the volume fraction

of large pores and to increase the volume fraction of

small pores relative to CT management.

We used the criteria of Greenland (1981) for

describing soil pores in this study. The volume of

pores drained >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 as

storage pores. Transmission pores are important both

in soil–plant–water relationships and in maintaining

good soil structure, and the storage pores are important

for holding water needed for plants and microorgan-

isms (Pagliai et al., 1995). The 150–30 mm portion of

transmission porosity (300–30 mm EPD or pore space

drained 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 150–30 mm portion of transmission

porosity of 0–300 mm soil accounted for 9.4, 8.7

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


Fig. 3. Calculated unsaturated hydraulic conductivity as a function

of matric pressure as affected by tillage and cropping system

management at 0–75 and 75–150 mm soil depth using Eq. (4).

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

W, soybean–wheat; S–L, soybean–lentil; S–P, soybean–pea. LSD




systems and 9.0, 9.0 and 8.7% of the total porosity in

S–W, S–L and S–P rotations, respectively. The pores

with diameters between 0.1 and 15 mm are assumed to

retain more plant available water than the larger pores

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

storage porosity (30–0.2 mm EPD) accounted for 58.4,

60.0 and 55.8% of the total porosity under ZT, MT and

CT systems, respectively, at 0–150 mm soil depth and

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

MT and CT systems, respectively, at 150–300 mm soil

layer. In contrast, the volume fraction of total porosity

of pores >150 mm in diameter (pores draining freely

with gravity) averaged 0.124, 0.096 and 0.095 m3 m�3

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 0–75 mm

soil depth (Fig. 3). Soybean–pea cropping resulted in

significantly higher k(h) at 0–75 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 at

�400 kPa suction. Significantly higher k(h) values

were obtained under S–P rotation from �2 to �30 kPa

suction than those under S–L and S–W rotations. Again,

the interaction effects of tillage and cropping systems

on k(h) were significant at all the matric pressures.

Unsaturated hydraulic conductivity under ZT–SP

system was significantly greater than k(h) values under

MT–SW, MT–SL and CT–SW and CT–SL systems at

�2 and �4 kPa suctions. At greater suctions (�10 to

�100 kPa), k(h) values under MT–SP system was

higher 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

75–150 mm soil depth (Fig. 3). Higher k(h) values

were obtained under S–L cropping than those

computed under other systems. The interaction effects

were found to be significant on k(h) values at �4 to

�40 kPa suction. With increasing soil depth the k(h)

values reduced, irrespective of the management


Fig. 4. Calculated unsaturated hydraulic conductivity as a function

of matric pressure as affected by tillage and cropping system

management at 0–75 and 75–150 mm soil depth using Eq. (4).

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

W, soybean–wheat; S–L, soybean–lentil; S–P, soybean–pea. LSD




systems. Average k(h) under ZT was significantly

higher than that under MT and CT up to �10 kPa

suction ranges. At �20 to �400 kPa suction range,

conservation tillage systems had greater k(h) values

than those under CT. At 150–225 mm soil depth, k(h)

under S–P rotation was significantly higher than that

of other cropping systems at all suction levels except

at �2 and �10 kPa (Fig. 4). There were significant

interaction effects on k(h) values at all the suction

levels, except at �30 kPa suction. A close analysis of

observed k(h) data at 225–300 mm soil layer revealed

the same trend of significantly higher k(h) values

under reduced tillage systems (Fig. 4). At �2 to

�40 kPa suction, k(h) under S–P system had higher

values than those under S–W and S–L 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 was

greater 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 S–P rotation than in S–L and S–W

rotations were probably due to greater volume fraction

of pores <7.5 mm in diameter, which, in turn, resulted

faster transmission of water across a matric pressure

gradient than larger pores (Azooz et al., 1996). Few

differences in k(h) values were observed between

tillage regimes at matric pressures greater than

�40 kPa (Fig. 4), especially at sub-surface soil layers.

This observation was a result of more large pores and

lower b value under CT than those under ZT. These

opposing effects on k(h) suggest that in zero-tilled

plots, water transmission through small pores con-

tributes a great deal under both near-saturated and dry

conditions in this type of soil (Azooz et al., 1996).

The larger difference in k(h) in different tillage and

cropping systems also suggests that the pores under

reduced tillage and S–P/S–L rotation may be more

continuous than under CT and S–W system, resulting

in greater water movement with ZT. Tillage can

disrupt pore continuity, especially within the tilled

zone and between the tilled and untilled zone (Kay,

1990). Our calculated results of greater k(h) under ZT

and MT systems than that under CT at different soil

layers are consistent with the findings of Azooz et al.

(1996). But the observed greater k(h) values under S–P

and S–L rotations than those under S–W at 0–300 mm

soil depth in sandy clay loam soils have rarely been

found previously.

4. Conclusions

These findings indicate that conservation tillage

may be more desirable than conventional tillage in

terms of water flow, both saturated and unsaturated. In


this 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. Soybean–lentil or soybean–

pea cropping system managed with conservation

tillage in this area may be better suited to tolerate

water scarcity than soybean–wheat 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-

rated conditions.

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