Evaluation of the potentialities of different soil types to yield
response of soybean under deficit irrigation
Shakil Uddin AHMED1 , Masateru SENGE2 , Kengo ITO2 and John Tawiah ADOMAKO2

Abstract: An experiment was conducted in a vinyl houseat Gifu University, Japan, from June to November 2007
to evaluate the potentialities of the three soil types undervarious water deficit conditions to yield of soybean. The
soil type was the first factor with three different soil types,
comprising of clay loam, sandy clay loam, and sandy loam
soils, classified as Inceptisol, Ultisol, and Andisol, respectively. Water deficit (D) was the second factor with fourlevels including D1 (0 – 25 %), D2 (25 – 50 %), D3 (50 –75 %) and D4 (75 – 100 %) water deficits of total available
water (TAW).
The crop water requirement (CWR) of soybean in thethree soil types significantly decreased with the increasing water deficit levels, and the highest was in Inceptisol, followed by Ultisol and then Andisol under all water deficit levels. Grain yield of soybean per unit area in
Inceptisol was the highest, followed by Ultisol and thenAndisol under all water deficit levels. The values of yieldefficiency (YE), indicating the grain yield per unit CWR,
was strongly influenced by water deficit level, and the maximum YE occurred at the water deficit level D3 (50 – 75 %)in all the three soil types. However, there were no significant differences at 5 % level among the maximum valuesof YE in the three soil types. The lowest yield response
factor (Ky ), indicating the relative yield loss to relative water deficit, was seen in Inceptisol (Ky = 0.42), followed by
Ultisol (Ky = 0.64) and then Andisol (Ky = 0.87) under thewater stress lower than 50 – 75 % of TAW. These results
indicate that deficit irrigation in Inceptisol contained thefinest soil texture is the most effective for economic water
usage among the three soil types under the water deficit
lower than 50 – 75 % of TAW (D3 ).
Key Words : deficit irrigation, soybean, crop water requirement, yield efficiency, yield response factor

1. Introduction
Soybean is economically an important crop withwidespread consumption and utilization in vegetable oiland veterinary industries. Soybean crop seldom attains its
full yield potential because of limitations on physiological
1 Graduate School of Agriculture, Gifu University, Yanagido 1-1, Gifu

City, Japan.
2 Faculty of Applied Biological Science, Gifu University, Yanagido 1-1,
Gifu City, Japan. Corresponding author: Masateru Senge, 岐阜大学応用生物科学部
2009 年 7 月 21 日受稿 2009 年 9 月 24 日受理
土壌の物理性 113 号, 13–19 (2009)

processes imposed by environmental stresses. It is true that
insects and diseases sometimes drastically reduce yields,
but in the long run, such reductions are small comparedwith those caused by unfavorable weather. Shortage ofavailable water is one of the most significant environmental stresses that cause yield reductions in a wide range ofcrops including soybean (Frederick and Hesketh, 1994).
Water resources in many areas of the world are limited but their demand is increasing. Irrigation agriculture is under economic and political pressure to improvethe efficiency with which water is used. Efficient use ofwater resources depends on reducing water losses, which
can be minimized through use of new irrigation techniquessuch as irrigation programs with deficient evapotranspiration. Demand for evapotranspiration can be reduced either
through agronomic measures or use of deficit irrigation
programs. The main approach in deficit irrigation practiceis to increase crop water use efficiency by partially supplying the irrigation requirement and allowing water stress toplanned plant with the least impact on crop yield. Deficit
irrigation management requires optimizing the degree ofplant stress within the restriction of available water.
Different soil types are known to influence the total
available water to plants and, therefore, the time when cropwater stress develops during a period of drying. This effect is incorporated in irrigation scheduling systems basedon water balance estimates. However, it is generally assumed that the rate of transpiration from a crop with full
canopy development and adequate water is controlled only
by atmospheric condition and by physical and physiological properties of the canopy with soil type having little orno effect. There are few experiments on evapotraspirationfrom crop species grown on different soil types. Also, thereports of soil type effects on total water use by a wellwatered crop exist, but the cause of water deficit effects is
not known.
Therefore, the research was conducted with the objective to evaluate the potentialities of different soil types toyield response of soybean under water deficit conditions.
This research can be used to determine water saving irrigation schedules for different soil types to ensure optimum
production of soybean.

14

土壌の物理性第 113 号

(2009)

Particledensity

Texturaldensity(g cm−3 )(m3 m−3 )

Solid
phase

1.06
1.37
1.07

0.60
0.48
0.57

0.40
0.52
0.43

Table 1 Physical properties of three soil types.
Three phase distribution

Soil texture
Soil
types
Inceptisol
Ultisol
Andisol

Sand

Silt

Clay

Texturalclass(g cm−3 )clay loam
sandy clay loam
sandy loam

2.61
2.65
2.49

(g g−1 )(g g−1 )(g g−1 )

0.40
0.59
0.63

0.27
0.18
0.20

0.33
0.23
0.17

Total
porosity(m3 m−3 )

Water
phase
θFC (34.7 kPa)

0.39
0.32
0.35

Air
phase
0.21
0.16
0.22

Table 2 Moisture properties of three soil types.
Soil Types
Inceptisol
Ultisol
Andisol

Field CapacityθFC (31 kPa)(m3 m−3 )
0.390
0.323
0.350

Wilting PointθPWP (1553 kPa)(m3 m−3 )
0.228
0.182
0.185

2. Materials and Methods
This research was conducted in a vinyl house with open
surrounding sides, located in the experimental farm of Gifu
University, Japan, from June to November 2007. A factorial experiment was arranged in randomized block design
with three replications.
The first factor was the soil property with three different soil types namely; Inceptisol, Ultisol and Andisol. The
physical properties of the three soil types are shown in Tables 1 and 2. The Inceptisol, which was taken from a paddy
field in Yanagido farm of Gifu University, was clay loam.
It had the highest fine particles consisting of clay and silt,
and the highest water content at field capacity and wilting
point. The Ultisol also taken from Yanagido farm of Gifu
University, was sandy clay loam with the highest bulk density and the lowest total available moisture. The Andisol,taken from Minokamo farm of Gifu University, was sandy
loam, locally named as “Kurobku soil” with the highest
total available moisture.
The second factor was the water stress (D) with four levels of water deficit treatments imposed as D1 (0 – 25 %),D2 (25 – 50 %), D3 (50 – 75 %) and D4 (75 – 100 %)of available water deficit. The water deficit level D2 (25– 50 %), for example, meant that the available water wasmaintained between 25 % and 50 % of total available water(TAW) throughout the growing season. When the maximum allowable depletion of available water got close to 50% of TAW, water was applied to restore the available water
to the deficit level of 25 % of TAW. TAW is defined as thewater content between field capacity (θFC ) and permanent
wilting point (θPWP ).
Plastic pots of 10 liters volume and 23.8 cm diameter
were filled with 10 kg air-dried soil and then five soybean
seeds were planted in each pot. The soybean cultivar used
was Glycinemax L. Merrill. One week later, the seedlings

Total Available MoistureθFC − θPWP
(m3 m−3 )
0.162
0.141
0.165

were thinned to only 2 seedlings, and maintained until theend of the growth period. Basal fertilizers were appliedjust once during the seedling stage at the rate of 20 N, 180
P2 O5 and 100 K2 O kg ha−1 . The period of irrigation lasted
for 22 weeks from June 9 to November 9. The average temperature was 22.4◦ C and the relative humidity was 67.5 %for the duration of the experiment. Data were analyzed using Tukey’s multiple comparison test (p < 0.05) (Nagata
and Yoshida, 1997). The PC software ‘Excel Statistics’(Version 5.0, Esumi Co. Lts., Japan) was used for the calculations.
Agronomic variables evaluated in this research werecrop water requirement (CWR, g/pot), oven dry (at 65◦ C
for 96 h) weight of total biomass including roots (TDB,g/pot) and air-dried grain yield of soybean (Y, g/pot),
and leaf area index (LAI, m2 m−2 ). LAI was measured
at 84 DAS according to Fehr and Cavines (1977) using a
portable leaf area meter (Model LI 3000A; LI-COR Inc.
Lincoln, NE, USA) from each pot. Crop water requirement(CWR, g/pot) was calculated from the evapotranspiration
during the irrigated period of soybean according to Allenet al. (1998). Daily evapotranspiration (ET) was measured
by weighing the pot every day.

3. Results and Discussions
3.1 Crop water requirement (CWR) and water
stress coefficients (K s )Table 3 shows that crop water requirement (CWR) in
each soil type significantly decreased with the increasingwater deficit levels imposed. Furthermore, when the CWR
under each of the corresponding water deficit levels wascompared among the three soil types, the following trend
was observed; Inceptisol > Ultisol > Andisol in that order. Therefore, the above CWR relationship indicates thatplants can undergo water stress quickly in Andisol whichis coarse textured (sandy loam) soil, whereas plants in the論文：Evaluation of the potentialities of different soil types to yield response of soybean under deficit irrigation 15
Table 3 The effect of water deficit level on crop water requirement (CWR), leaf area index (LAI), total dry biomass, grain yield, water
use efficiency (WUE) and yield efficiency (YE) of soybean under different soil types.
Soil Types

Water Deficit
Level
(%)
D1 ( 0 – 25)
D2 (25 – 50)

Inceptisol

D3 (50 – 75)
D4 (75 – 100)
D1 ( 0 – 25)
D2 (25 – 50)

Ultisol
D3 (50 – 75)
D4 (75 – 100)
D1 ( 0 – 25)
D2 (25- 50)
Andisol
D3 (50 – 75)
D4 (75 – 100)

CWR
(g/pot)①
116,504 Aa
92,949 Ab76,794 Ac60,608 A
d90,034 B
a77,877 Bb63,442 B
c51,980 B
d88,883 Ba68,961 Cb
54,355 C
c40,919 Cd

LAI(m2 m−2 )
②
5.1 A
a
4.7 A
b
4.1 A
c
3.1 A
d
4.6 B
a
4.2 B
b
3.3 B
c
2.3 B
d
4.4 B
a
3.5 C
b
2.5 C
c
0.8 C
d

Total dry biomass(g/pot)
③
214 A
a
176 A
b
146 A
c
121 A
d
172 B
a
151 B
b
124 B
c
106 B
d
160 C
a
128 C
b
102 C
c
74 C
d

Grain yield(g/pot)④
31.8 Aa29.5 Ab
26.7 Ac
18.8 Ad27.4 Ba25.4 Bb
21.4 Bc
14.8 Bd26.6 Ba21.5 Cb
17.5 C
c
6.7 C
d

WUE
(g g−1 )⑤=③/①
0.00184 ABc0.00189 Ab0.00190 Aa
0.00200 Aa
0.00191 Ab
0.00194 Ab
0.00195 Ab
0.00204 Aa
0.00180 Ba0.00186 Aa0.00188 Aa0.00181 Aa

YE(g g−1 )⑥=④/①
0.000273 B
c0.000317 A
b0.000348 A
a0.000310 Ab
0.000304 Ab
0.000326 A
a0.000337 A
a0.000285 B
c0.000299 A
a0.000312 Aa
0.000332 A
a0.000164 Cb

Means followed by different small letters (a – d) in the same column in each soil types under different water deficit levels are significantly different
according to Tukey’s multiple comparison test (p < 0.05). .
Means followed by different capital letters (A – C) vertically at same water deficit level among the three soil types are significantly different according
to Tukey’s multiple comparison test (p < 0.05).

finer textured Inceptisol (clay loam) have ample time to adjust to low soil water matric pressure, and may remain unaffected by water stress. Tables 1 and 2 support the above
explanation.
Fig. 1 shows that CWR linearly correlated with leaf areaindex (LAI) without differences among the three soil types.
Based on this result, it can be said that among all the agronomic factors, LAI as a growth indicator was the most sensitive in the control of evapotranspiration rate. This result
agrees with Setiyono et al. (2008) who found the same
phenomenon that transpiration is directly controlled by leaf
area index.

can be used for deficit irrigation scheduling. Ks describe
the effect of water stress on crop transpiration (Allen et al.
1998). Assuming that the evapotranspiration at D1 (0 – 25%) occurred under the ideal condition for plant growth inwhich the soil water content is near the field capacity, theactual evapotranspiration (ETa ) at D1 is crop evapotranspiration (ETc ), which means the evapotranspiration of plantunder standard conditions (Allen et al., 1998). Water stresscoefficient (Ks ) is calculated as the ratio between the actual
evapotranspiration (ETa ) at each water deficit level and thecrop evapotranspiration (ETc ). The ratio of water deple-

According to Allen et al. (1998), evapotranspiration under water stress condition is referred to as the adjustment
evapotranspiration (ETcadj , mm d−1 ) which can be calculated by the following equation.
ETcadj = Ks ETc(1)where ETc (mm d−1 ) is the crop evapotranspiration understandard condition, Ks is water stress coefficient (no dimension).
The value of Ks is important for estimating ETcadj , andFig. 1 The relation between crop water requirement (CWR) andleaf area index (LAI).

16

土壌の物理性tion to the total available water in the root zone, referred
to as “p”, is an indicator of the water deficit level. For
example, the average value of “p” under the water management of D2 (25 – 50 %) treatment is calculated as “p”= (0.25 + 0.50)/2 = 0.38.
Fig. 2 shows that the Ks value decreased linearly with
the increase of available water deficit level “p”. It indicatesthat the Ks value (water stress coefficients) in Andisol ismore sensitive to water deficit than the other two soil types.
This is mainly due to the coarse textured nature of Andisol(Table 1).

第 113 号

(2009)

photosynthesis.
Yield efficiency (YE, g g−1 ) is defined as the ratio of
grain yield (Y, g/pot) to crop water requirement (CWR,
g/pot). There was an effect of water deficit (D) on the YE
value in the three soil types at 5 % significant level (Table3). Table 3 shows that the YE value slightly increased with

3.2 Total dry biomass (TDB) and water use efficiency (WUE)
Table 3 shows that total dry biomass (TDB, g/pot) of
soybean in each of the three soil types significantly decreased with the increase of water deficit levels. Furthermore, the TDB of soybean in the three soil types undereach of the corresponding water deficit levels, significantly
decreased in the order of Inceptisol > Ultisol > Andisol.
The TDB of the three soil types linearly correlated withCWR under the water deficit levels (Fig. 3). This result
indicated that the decrease in total dry biomass was dueto the considerable reduction in plant growth and canopy
structure caused by the water stress conditions. This phenomenon agrees with Shao et al. (2008) who found that thebiomass of soybean plant was reduced by the water stress
imposed.
Water use efficiency (WUE, g g−1 ) is defined as the ratio of total dry biomass (TDB, g/pot) to the crop waterrequirement (CWR, g/pot). Table 3 shows that the WUE
value slightly increased with the increase of water deficit
level, except water deficit level D4 in Andisol. Consequently, the highest WUE value was obtained at the water
deficit level D4 in Ultisol and Inceptisol, while the highest
WUE in Andisol was at water deficit level D3 . However,there was no difference at 5 % significant level among the
WUE values of the same water deficit level from D2 to D4
in the three soil types. This result indicated that there waslittle influence of the soil types on WUE value at the samewater deficit level.

Fig. 2 The effect of available water deficit (p) on water stresscoefficient (Ks ).

Fig. 3 The relation between total dry biomass (TDB) and cropwater requirement (CWR).

Fig. 4 The relation between the grain yield (Y) and crop waterrequirement (CWR).

3.3 Grain yield and yield efficiency (YE)The grain yield of soybean in the three soil types decreased with the increase of water deficit levels (Table 3).
Similar to TDB, the grain yield of soybean in the three soil
types under each of the corresponding water deficit levels,
significantly decreased in the order of Inceptisol > Ultisol> Andisol. Figs. 4 and 5 shows that the grain yield ofsoybean was strongly influenced by both CWR and LAI
among the three soil types. These results indicated that the
reduction in CWR by water stress caused the decrease ofsoil water uptake with soluble nutrients and consequently
the decrease of soybean grain yield through reduction in

Fig. 5 The relation between the grain yield (Y) and leaf areaindex (LAI).

論文：Evaluation of the potentialities of different soil types to yield response of soybean under deficit irrigation 17
Table 4 The effect of water deficit level on water stress coefficient (Ks ) and yield response factor (Ky ) of soybean under different soil
types.
Soil
Types
Inceptisol

Ultisol

Andisol

Water deficit
level (%)
D1 ( 0 – 25)
D2 (25 – 50)
D3 (50 – 75)
D4 (75 – 100)
D1 ( 0 – 25)
D2 (25 – 50)
D3 (50 – 75)
D4 (75 – 100)
D1 ( 0 – 25)
D2 (25 – 50)
D3 (50 – 75)
D4 (75 – 100)Yield
(g/pot)
31.8
29.5
26.7
18.8
27.4
25.4
21.4
14.8
26.6
21.5
17.5
6.7

ETa
(g/pot)116,504
92,94976,79460,60890,03477,87763,442
51,98088,88368,96154,35540,919

Ks =

ETa
ETm

1
0.80
0.66
0.52
1
0.86
0.70
0.58
1
0.78
0.61
0.46

1−

Ya
Ym

0
0.07
0.16
0.41
0
0.07
0.22
0.46
0
0.19
0.34
0.75

1−

ETa
ETm

0
0.20
0.34
0.48
0
0.14
0.30
0.42
0
0.22
0.39
0.54

Ky
0.36
0.47
0.85
0.54
0.74
1.09
0.86
0.88
1.39

Ya : actual yield, Ym : maximum yield = yield under no water stress condition (D1 ), ETa : actual evapotranspiration,
ETm : maximum evapotranspiration = evapotranspiration under no water stress condition (D1 )).

the increase of water deficit level from D1 to D3 . However,there was no significant difference at 5 % level among theYE values of the three soil types at the water deficit levelD3 . These results indicated that soil moisture and aeration
at the water deficit level D3 were the most appropriate for
maximizing the YE value, and the maximum values of YE
were slightly influenced by the soil types. On the other
hand, significant differences appeared among the YE values at the water deficit levels D1 and D4 of the three soil
types. The smallest YE value at full irrigation (D1 ) was
shown in Inceptisol, which contained the finest soil texture, probably due to lack of aeration in the soil. On theother hand, the smallest YE value at the deficit irrigation
which controlled soil moisture near the wilting point (D4 )was in Andisol with the coarsest soil texture probably due
to excessive water stress.
3.4 Yield response factor (K y )According to Doorenboss and Kassam (1979), in orderto quantify the effect of water stress, it is necessary to derive the relationship between relative yield decrease andrelative evapotranspiration deficit given by the following
equation.
(
)
ETa
Ya
= Ky × 1 −
1−
Ym
ETm

fer a greater yield loss than the crop with a lower Ky
value (Moutonnet, 2000). The Ky values for most crops
are derived on the assumption that the relationship between relative yield (Ya /Ym ) and relative evapotranspiration (ETa /ETm ) is linear and valid for water deficit of up
to about 50 percent or 1 − ETa /ETm = 0.5 (Kirda et al.,1999). According to a report by Doorenboss and Kassam(1979), the Ky of soybean under water deficit for the wholegrowing period was found to be 0.85.
The Ky values of soybean in the three soil types, calculated by using the above equation (2), are shown in Table 4. The smallest Ky value was in Inceptisol, followed
by Ultisol and then Andisol under all water deficit levels.
Deficit irrigation in Inceptisol was effective (Ky < 1.0) for
economic water usage under all water deficit levels. On theother hand, the deficit irrigation in both Ultisol and Andisol
was effective (Ky < 1.0) under the water deficits lower than
50 – 75 % of TAW (D3 ).
The relative yield (1 − Ya /Ym ) linearly decreased withthe relative water deficit (1 − ETa /ETm ) up to the D3 water
deficit levels (50 – 75 % of TAW ) and thereafter, greatly(2)

where 1 − Ya /Ym : relative yield decrease, Ya : actual
yield, Ym : maximum yield (under no stress condition),
1 − ETa /ETm : relative evapotranspiration decrease, Ky :yield response factor, ETa : actual evapotranspiration, andETm : maximum evapotranspiration
Under conditions of limited water distributed equally
over the total growing season, involving crops with different Ky values, the crop with higher Ky value will suf-

Fig. 6 Yield response factor (Ky ) for water deficit of soybean
under three soil types..

18

土壌の物理性decreased form D3 to D4 water deficit level among thethree soil types, and the mean value of Ky was 0.41, 0.62,
and 0.87 in Inceptisol, Ultisol, and Andisol, respectively(Fig. 6). However, the above results indicated that the Ky
values of soybean were strongly influenced by soil physical properties, especially soil texture. The response ofwater stress to soybean grain yield was the smallest in finetextured soil like Inceptisol (Ky = 0.41), and was the greatest in coarse-textured soil like Andisol (Ky = 0.87). It canbe concluded from these results that the effect of deficit irrigation for saving irrigation water was great in Inceptisol
with fine soil texture, followed by Ultisol with medium soil
texture, and then Andisol with coarse soil texture.
3.5 Optimum deficit irrigation
The highest grain yield of soybean per unit area was produced under the full irrigation (D1 ) in all the three soil
types. The highest grain yield of soybean (Y, g/pot) at
full irrigation was obtained in Inceptisol (Y = 31.8 g/pot),
followed by Ultisol (Y = 27.4 g/pot), and then Andisol (Y= 26.6 g/pot). On the other hand, the optimum grain yieldof soybean with the highest yield efficiency (YE) was obtained by the deficit irrigation, in which water deficit level
was maintained at 50 – 75 % of TAW (D3 ). The water
stress coefficient (Ks ) at D3 was 0.66, 0.70, and 0.61 in Inceptisol, Ultisol, and Andisol, respectively. The YE value
at water deficit level D3 was 1.27 times as much as that under the full irrigation (D1 ) in the Inceptisol, and 1.11 times
those of both Ultisol and Andisol. It was observed that thegrain yield of soybean per unit area under deficit irrigation at 50 – 75 % of TAW (D3 ) was reduced by 16.0, 21.9,
and 34.2 %, but could conserve 21.6, 9.8 and 9.9 % of irrigation water to produce the same yield compared to thefull irrigation (D1 ) in the Inceptisol, Ultisol and Andisol,
respectively.

4. Conclusions
The present study indicates that the decrease of CWR
by water stress resulted in a decrease of LAI, TDB anda subsequent decrease in soybean grain yield with significant differences among the three soil types. The soybean
plants in Inceptisol could absorb and transport more watersoluble nutrients from soil through the roots with a subse-

第 113 号

(2009)

quently higher grain yield due to its fine-textured properties that could retain much more water than the other twosoil types.
Yield efficiency (YE) values indicated that soil moistureand soil aeration at the water deficit level 50 – 75 % of
TAW (D3 ) were the most appropriate for maximizing theYE values in the three soil types, and the maximum valuesof YE were slightly influenced by the three soil types.
The lowest yield response factor Ky under the water
stress below 50 – 75 % of TAW was in Inceptisol (0.42),
followed by Ultisol (0.64) and then Andisol (0.87). These
results suggest that deficit irrigation in Inceptisol (clayloam) provided the most effective economic water usage
among the three soil types, followed by Ultisol (sandy
clay loam) and then Andisol (sandy loam) under the water deficit level lower than 50 – 75 % of TAW (D3 ).

References
Allen, R.G., Pereira, L.S., Raes, D. and Smith, M. (1998): Cropevapotranspiration, guidelines for computing crop water requirements. Irrigation and drainage paper no.56. p.301, FAO,
Rome.
Doorenbos, J and Kassam, A.H. (1979): Yield response to water.
pp.1–57, FAO, Rome.
Frederick, J.R., and Hesketh, J.D. (1994): Genetic improvement
in soybean, physiological attributes. In Slaver, G.A. (Ed), Genetic improvement of field crops. pp.237–286, Marcel Deker,
New York.
Shao, H.B., Chu, L.-Y., Juleel, C.A., and Zhao, C.-X. (2008):
Water-deficit stress-induced anatomical changes in higher
plants. C. R. Bilogies, 331: 215–225.
Kirda, C., Moutonnet, P., Hera, C. and Nielsen, D.R. (1999): Crop
yields response to deficit irrigation. pp.21–38, Kluwer Academic Publisher, Dordrecht.
Moutonnet, P. (2000): Deficit Irrigation Practice. Water reports
no.22. FAO, Rome.
Nagata, Y. and Yoshida. M. (1997): Basis of statistical multiple
comparison tests (in Japanese). 187p, Scientist Co., Tokyo.
Setiyono, T.D., Weiss, A., Specht, J. E., Cassman, K. G. andDobermann, A. (2008): Leaf area index simulation in soybean
grown under near-optimal conditions. Field Crops Research,
10: 10–16.

論文：Evaluation of the potentialities of different soil types to yield response of soybean under deficit irrigation 19

要旨

土壌特性の違いが節水灌漑下におけるダイズ収量への影響を評価するために，2007 年 6 月から 11 月にかけて岐阜大学内のビニールハウス内で，次の二因子で三反復の栽培実験を実施した．第一因子は土壌特性の相違であり，Inceptisol（clay loam）, Ultisol（sandy clay），Andisol（sandy loam）の３つの土
壌型に分かれる．第二因子は土壌水分管理の方法であり，圃場容水量からの土壌水分欠損量（D）によって処理した．すなわち，土壌水分欠損量が総容易有効水分量の 0∼25 % (D1 )，25∼50 % (D2 )，50∼75% (D3 )，75∼100 % (D4 ) の 4 レベルの試験区を設けた．
ダイズの総消費水量は，3 種類の土壌型とも土壌水分欠損レベルが増加するにつれて減少し，同じ
土壌水分欠損レベルでは，Inceptisol の総消費水量が最大となり，次いで Ultisol，Andisol の順に大きい．単位面積当たりの穀物収量も，同じ土壌水分欠損レベルに対して Inceptisol が最大となり，次いでUltisol，Andisol の順に大きい．収穫効率（単位消費水量当たりの収穫量）は，土壌水分欠損レベルによって強く影響され，その値は 3 種の土壌型とも D3 レベルで最大となった．しかし、収穫効率の最大値は 3 種の土壌間で有意な差が見られなかった．収量反応係数（Ky ：消費水量減少量に対する収穫減少量の比）は，土壌水分欠損量が総容易有効水分量の 50∼75 % （D3 レベル）以下の場合は，Inceptisol(Ky = 0.42) で最小となり，次いで Ultisol (Ky = 0.64)， Andisol (Ky ＝ 0.87) の順に小さくなる．以上のことから，３種の土壌型の中で，土壌組成が最も細かい Inceptisol における節水灌漑が経済的な水利用の観点から最も有効であることが明らかになった．
キーワード：節水灌漑，ダイズ，水利用効率，収穫効率，収量反応係数