ῐ
J. Jpn. Soc. Soil Phys.
῏῍ῌῐῑ῎
No. 22, p. /-ῌ0* ῍,**+῎E#ect of Roots on Formation of Internal Fissures in
Clayey Paddy Soil during Desiccation
Shuichiro YOSHIDA and Kazuhide ADACHINational Agricultural Research Center, NARO Inada +ῌ,ῌ+, Joetsu, Niigata, 3.-ῌ*+3-, Japan
Abstrct

Internal ﬁssures in clayey soil are important for tillage e$ciency. The roots reinforce thesoil and absorb soil moisture, resulting in shrinkage and fractures in the massive soil structure.
The objective of this study is to clarify the inﬂuence of roots on the prevention of wide cracks
and the formation of small ﬁssures during desiccation. Soil samples were taken from thesurface layer of inter-row spaces of a clayey paddy three weeks after the release of ponding
water in the summer +332. In half of the sampling sites, the stems and leaves of the rice plants
were cut away just before the release of the ponding water, in order to compare the e#ects ofthe two resulting water absorption conditions. After measurement of the volume fractions of
the solid, liquid, and air phases, the oven-dried samples were soaked in water and then scoured
with tap water in a sieve. The remaining roots were dried and weighed. The relationships
among volumetric air ratio, moisture ratio, and root density were statistically analyzed, withthe volumetric air ratio being regarded as the ratio of newly formed ﬁssures due to desiccation.
Positive direct correlations were observed between root density and air ratio for all plots. In
the plots with transpiration, since the moisture ratio correlated with root density and air ratio,a large proportion of the variances in air ratio explained by moisture ratio and root density
were shared. These results lead to the following conclusion : Rice plant roots inhibit theformation of wide cracks and induce small ﬁssures in clayey well-puddled soil during desiccation. With transpiration, these e#ects are attributed to the compound functions of roots :absorption of water and soil reinforcement. However, even without transpiration, soil reinforcement by roots uniquely contributes to the development of small ﬁssures.
Key words : Soil shrinkage, Root density, Internal ﬁssure, Soil reinforcement, Absorption ofsoil moisture+.

Introduction

Internal micro-cracks in soil are practicallyimportant for tillage e$ciency, because soil’s
tensile strength depends on the number andextent of ﬁssures from which fractures would
begin. Utomo and Dexter (+32+) proposed thatthe friability of soil clods can be expressed by
the linear relationship between the logarithmof aggregate volume and the logarithm of tensile strength. The slope of the relationship is ameasure of the dispersion of the strengths of
micro-cracks and ﬂaws. Major processes in theformation of aggregates in soils are desiccation, shear failure, and biological activity(Wilding and Hallmark +32.). Micro-cracks insoil are also considered to be formed by the
combination of such factors or by previous
tillage.
Desiccation causes soil shrinkage and brings
the particles closer together, but if the movement of soil particles is constrained, tensile῏54ΐῐ῎῔῕ῑ ῒ 22 ῏ ῌ,**+῍stress is generated between the particles,
resulting in tensile failure. The shrinkage
characteristic of soil depends not only on theproperties of the soil, but also on the rate ofdesiccation or substances that hinder the homogeneous movement of soil particles. Rootsin soil exert large e#ects on soil shrinkage
behavior, because they increase the rate ofdesiccation and reinforce the soil. Mitchell andvan Genuchten (+33-) clariﬁed the e#ects ofroots on an in situ soil shrinkage rate. Afterobserving greater shrinkage volume in fallow
ﬁelds than in wheat and alfalfa ﬁelds, they
concluded that soil’s shrinkage characteristic
may change, depending on the cropping condition.
Waldron et al. (+32-) measured the shear resistance of soil reinforced by alfalfa and pineroots, and observed a signiﬁcant increase compared to the soils without roots. Waldron andDakessian (+32+) measured the mechanical properties of roots, and related them to increased
soil shear resistance. Their analysis showedthat the elastic behavior of roots enhances resistance against the shear stress. Thus, it isnaturally deduced that roots also increase thetensile strength of soil, and inhibit homogeneous shrinkage.
The e#ects of roots on soil’s shrinkage behavior include not only reinforcement but also
water absorption. It should be noted that roots
reinforce the soil against the tensile stressgenerated by the depletion of water, which theroots themselves cause. Yoshida et al. (+331)
pointed out that the introduction of wintercrops into paddy ﬁelds enhances the macropores in soil in spring, when the crops begin togrow again. Pillai and McGarry (+333) compared the e#ect of crops on the structural
repair of compacted soil. They concluded thatlablab [Lablab purpureus (L.) Sweet] and mungbean [Vigna radiata (L.) R. Wilezek] lead to therapid repair of soil structures due to the great
cyclic variation in wetting and drying induced
by a large evapotranspiration rate. Thesestudies suggest that the absorption of water byroots may also contribute to the formation ofsoil structure because roots inhomogeneously
distribute suction and tensile stress, possibly
inducing micro-cracks.
The present study examines the relationshipbetween the internal ﬁssures and root densitywith respect to water content, aiming to discriminate between the two di#erent e#ects ofroots on soil shrinkage : water absorption andsoil reinforcement. Such discrimination ispractically useful to estimate the di#erence between the e#ects of living and remaining roots
on soil structure during desiccation.
,.

Materials and Methods

The experimental paddy ﬁeld used for thisstudy is located at the Hokuriku Agricultural
Experiment Station in Niigata Prefecture,
Japan. It has been utilized as an irrigatedpaddy for more than ,* years. The soil isclayey, montomorillonitic, mesic, typic Epiaquepts, and contains -2ῌ clay, -1ῌ silt, and,/ῌ sand. The experiment was performed in+332. The rice seedlings were transplanted inrows by hand from sca#olds that had been set
up around and across the sample site in order
to avoid disturbing the surface of the soil. The
row spacings were set as -* cm or 0* cm. The
ponding water was maintained on the surfaceof the ﬁeld until ++ August +332. In half of thesampling sites, the stems and leaves of the rice
plants were cut away just before the release of
the ponding water, in order to compare thee#ect of water absorption on soil shrinkage
behavior. In these sites, only the roots, whichhardly absorb water, remained in the soil.
Sampling of the soil was performed on +* and++ September. Undisturbed core samples, eachwith a diameter of /* mm and a thickness of ,/mm, were taken from the surface layer in theinter-row space, avoiding the wide visible
cracks. The distances from the plants werereferred to in determining the sampling sites,
since samples with large deviations in root
density were desirable for the analysis. Aftermeasuring volume fractions in the solid, liquid,῎῍ : E#ect of roots on formation of internal ﬁssures in clayey paddy soil during desiccation55

and air phases, the oven-dried samples were
soaked in water for a day and then scoured
with tap water in a sieve with a *..,-mm mesh.
The remaining roots were picked up, dried for,. hr. in an oven at 2/ῌ, then weighed.
-.

Results and Discussion

Figure + provides a typical shrinkage characteristic curve, representing decreasing soil void
ratio with decreasing soil moisture ratio. The
structure of the well-puddled paddy soil shows
little stability. Most of the pores are plastic
and shrink easily within the stress range induced by low suction. Thus the entry of air
into stable macro-pores, usually observed instructured soils (a), is negligible for puddled
clay soils (b, c). In the normal shrinkage range,
cracks or small internal ﬁssures are easily produced in the in situ puddled soils (b), while inthe conventional shrinkage tests in the laboratory, puddled samples packed in the testing
dishes exhibit volume decreases that arealmost equal to the water losses (c). This isbecause the horizontal movement of soil is notallowed in the ﬁelds but is allowed in the laboratory test. The formation of cracks or manysmall ﬁssures is the only process in which theincreasing horizontal stress is released. At ahigher suction range, soil exhibits residual
shrinkage or zero shrinkage. These shrinkage
regimes are relatively less important for theformation of structures, since the entry of air
into pores precedes the deformation of pores.
In soils reinforced by many roots, crack formation tends to be inhibited and instead smallﬁssures are induced. To compare the e#ect ofroots on such ﬁssures induced by desiccation,
we used the volumetric ratio of air with respect
to the volume of a solid, assuming that all the
air-ﬁlled pores are newly made by fractures.
This hypothesis seems to be reasonable because the puddled pasty clayey soil never initially has large stable pores subject to air entry,
due to low suction. The pores between larger
clods would be ﬁlled with small particles
during the sedimentation process after puddl-

Fig. +

Typical shrinkage characteristic curvesfor (a) structured non-swelling soils, (b)well-puddled swelling soils in situ, and(c) well-puddled swelling soils in conventional shrinkage tests.

ing.
Fig. , (a) through (c) show the relationships
among root density, volumetric moisture ratio(Vw/Vs), and volumetric air ratio (Va/Vs) for
the samples in the plots with transpiration.
According to the shrinkage characteristic observed in the disturbed samples of the sameﬁeld, the maximum and minimum moisture
ratios of ,., and +.- shown in Fig. , correspond
approximately to suction levels of + kPa and+** kPa, respectively. All the samples are
therefore considered to be within the normal
shrinkage range. Negative signiﬁcant correlations are observed between volumetric moisture ratio and volumetric air ratio (a) and between root density and volumetric moisture
ratio (c) ; and a positive signiﬁcant correlation
is observed between root density and volumetric air ratio (b). The negative correlation between moisture ratio and air ratio, shown in (a),can be naturally accepted by regarding typicalshrinkage characteristics discussed in the preceding section : the increase in air ratio, representing the formation of internal ﬁssures, is
induced by a decrease in the moisture ratio, in
other words, ‘desiccation’. On the other hand,56  22,**+

Fig. -

Concept of shared and unique varianceexplained by two inter-correlated factors.

the positive correlation between root density
and air ratio does not necessarily demonstratethat roots mechanically reinforce the soil
resulting in the formation of ﬁssures by shrinkage, because roots simultaneously reduce themoisture ratio, resulting in the observed increase in the air ratio. Thus we should
delineate the e#ect of root density on air ratio
by calculating the unique and shared variances
between these independent variables. The concept is summarized in Fig. - after Hair et al.
+332. The total variance in the volumetric air
ratio consists of the variance uniquely explained by moisture ratio (S+ ), the variance
uniquely explained by root density (S, ), thevariance explained jointly by both factors(S+,), and the variance not explained by thesefactors, i.e., error (SE). The procedure of calculation is provided in the Appendix. The results
are shown in Table +. All these variances arecalculated for the standardized data : the total
variance in the air ratio is equal to one, and theFig. ,

Relations between (a) volumetric moisture ratio (Vw/Vs) and volumetric air
ratio (Va/Vs), (b) root density andvolumetric air ratio (Va/Vs), and (c)root density and volumetric moisture
ratio (Vw/Vs) for the samples in theplots with transpiration.
*, ** : Signiﬁcant at the *.*/ and *.*+
probability levels, respectively.

ῐ῏ : E#ect of roots on formation of internal ﬁssures in clayey paddy soil during desiccation

Table +

57

Direct correlation coe$cient between the variables and composition of variances ofstandardized Va/Vs (With Transpiration)

R : Direct correlation
coe$cient-* cm row
spacing
Variances ofstandarized Va/Vs

R : Direct correlation
coe$cient

0* cm row
spacing
Variances ofstandardized Va/Vs

Volumetric moisture
ratio (Vw/Vs)

Root
density῍*42,/1***41,1**

Volumetric moisture
ratio (Vw/Vs)ῌ῍*42/13**(S+) : Unique varianceexplained by Vw/Vs*4+/.0(S,) : Unique varianceexplained by root density*4**+-(S+῎,) : Shared variance*4/,1,(S+῎,) : Total varianceexplained by these factors*402-*Air ratio (Va/Vs)

Volumetric moisture
ratio (Vw/Vs)

Root
density῍*40-12**41113**

Volumetric moisture
ratio (Vw/Vs)ῌ῍*42/+-**(S+) : Unique varianceexplained by Vw/Vs*4**,,(S,) : Unique varianceexplained by root density*4,**.

(S+῎,) : Shared variance*4.*.1(S+῎,) : Total varianceexplained by these factors*40*1-Air ratio (Va/Vs)*, ** : Signiﬁcant at the *.*/ and *.*+ probability levels, respectively.

total variance explained by factors (S+῎,) is
identical to the coe$cient of determination (R,)
after the multivariate analysis. It should be
noted that the shared variance is substantial
compared to the unique variances of thefactors. Therefore, while the roots absorb soil
moisture, the compound function of roots :water absorption and reinforcement, is exerted
on the increase in internal ﬁssures during
shrinking. The variances not explained bythese factors may include experimental errors
and e#ects of unknown variation in propertiesof the soil or the roots.
Fig. . (a) through (c) show the relationships
between the factors for the samples in the plots
without transpiration. No correlations wereobserved between moisture ratio and air ratio ;however, positive signiﬁcant correlations be-

tween root density and air ratio were observed.
Although the moisture ratio is slightly correlated with root density in the 0* cm row spacing plot, this might be due to the di#erence indrainage e$ciency between the row and therow-middle, and has no signiﬁcant meaning.
Thus we regard that no correlations exist between the root density and the moisture ratio
in the following discussion. The signiﬁcant
positive correlations between root density andair ratio without the e#ect of moisture ratio
prove that the mechanical reinforcement of theroots induces small ﬁssures after desiccationeven when the roots do not absorb soil moisture. In other words, during desiccation due
solely to evaporation, roots mechanicallyinduce internal ﬁssures in soil by meshing andﬁxing the soil particles together, against the

58ΐῐ῎῔῕ῑ ῒ 22 ῏ ῌ,**+῍

fracturing forces generated by shrinkage.
The e#ect of root density on the increase in
air ratio is considerably less in the plots without transpiration than in the plots with transpiration, according to the slope of the relationship shown in Fig. , (b) and Fig. . (b). Therefore, the e#ect of roots on the development offriable structures of soil is better exerted
during the period when rice plants activelytranspire. However, these ﬁndings may also
aid in controlling the structure of clayey soils
after harvesting rice. If the roots remain until
the next spring and become enmeshed in thesoil, they can contribute to the creation of ﬁssures as the soil dries. The clayey paddy soil
just after rice harvesting is so sticky that large
clods would be produced if tillage were performed then. Such clods would shrink with
few internal fractures the next spring, becoming extremely hard with poor friability. If no
mechanical procedures are performed until thenext spring, many ﬁssures can be naturally
produced as the soil dries.
The increase in the volume of minute ﬁssures depresses the development of wide
cracks. These ﬁndings therefore suggest thatroot density controls the distribution of thetotal shrinkage volume to the wide cracks andsmall ﬁssures. Root density varies with respect
to the distance from the plant. The regions inwhich roots extend sparsely are subject towide cracks, while the regions near the plants
are not. Soil reinforcement by roots also
a#ects the cracking patterns between the rows
in this manner.

Fig. .

Relations between (a) volumetric moisture ratio (Vw/Vs) and volumetric air
ratio (Va/Vs), (b) root density andvolumetric air ratio (Va/Vs), and (c)root density and volumetric moisture
ratio (Vw/Vs) for the samples in theplots without transpiration.
*, ** : Signiﬁcant at the *.*/ and *.*+
probability levels, respectively.

῔ΐ : E#ect of roots on formation of internal ﬁssures in clayey paddy soil during desiccation..

Conclusion

The roots of rice plants inhibit the formationof wide cracks and induce small ﬁssures inclayey well-puddled soil during desiccation.
This e#ect of roots on soil is exerted even when
the roots do not absorb soil moisture. Thisﬁnding is important in regard to the physicalfunction of the roots that remain after crops
are harvested.
Acknowledgment
We thank Dr. Noburo Haraguchi for his
advice on statistical analysis. We also thankMr. Yoichi Wakai and Mr. Yuji Yokoyama of
the ﬁeld management section in our institutefor their experimental assistance and technicaladvice.
References
Hair, Jr., J.F., Anderson, R.E., Tatham, R.L., andBlack, W.C. (+332) : Multivariate data analysis,
Prentice-hall Internatinal, Inc., p. +3*ῒ+3+.
Mitchell, A.R. and van Genuchten, M. Th. (+33-) :Flood irrigation of cracked soil, Soil Sci. Soc.

59

Am. J., /1 : .3*ῒ.31.
Pillai, U.P. and McGarry, D. (+333) : Structure repairof a compacted vertisol with wet-dry cycles
and crops, Soil Sci. Soc. Am. J., 0- : ,*+ῒ,+*.
Utomo, W.H. and Dexter, A.R. (+32+) : Soil Friability, J. Soil Sci., -, : ,*-ῒ,+-.
Waldron, L. J. and Dakessian, S. (+32+) : Soil reinforcement by roots : calculation of increased
soil shear resistance from root properties, Soil
Sci., +-, : .,1ῒ.-/.
Waldron, L. J., Dakessian, S., and Nemson, J.A.
(+32-) : Shear resistance enhancement of +.,,ῒ
meter diameter soil cross sections by pine andalfalfa roots, Soil Sci. Soc. Am. J., .1 : 3ῒ+..
Wilding, L.P. and Hallmark, C.T. (+32.) : Development of structural and microfabric properties
in shrinking and swelling clays. In Proceedings ISSS Symposium on water and solute
movement in heavy clay soils. J.Bouma and P.
A.C.Raats (eds.) ILRI Publ. No. -1, Wageningen,the Netherlands, p. +ῒ,,.
Yoshida. S., Itoh, K. and Adachi, K. (+331) : Improvement of drainage in clayey rotational
ﬁeld by introducing winter crops after conversion from paddy, Soil Phys. Cond. Plant
Growth, Jpn., 10 : -ῒ+,. (in Japanese with English abstract)

Appendix (after Hair et al. +332)
The analyst can determine the shared andunique variance for inter-correlated independent variables using direct correlation coe$cients. Part correlation of Y, X+, given X, canbe calculated as :rY, X+ (X,)ῐ

rY, X+῎(rY, X,῏rX+, X,)
ῌ+῎rX+, X,,ῌwhere ra, b denotes a direct correlation coe$cient between variables a and b ; and Y denotes the dependent variable. X+ and X,
denote independent variables, inter-correlatedwith each other. The unique standardized variance predicted by X + is equal to the square of
the part correlation :S+ῐr,
Y, X+ (X,)῍

The unique variance explained by X, is also
written as :S,ῐrYῌX, (X+),῎

The amount of shared variance can be conﬁrmed by subtracting the standardized uniquevariance from the square of the direct correlation coe$cient between Y and Xi :S+ῒ,ῐrY, X+,῎S+orS+ῒ,ῐrY, X,,῎S,

῏ῐBoth equations lead to the same result. The
summation of these variances yields total variances explained by these independent variables :S+ῑ,ῐS+῍S,῍S+ῒ,ῑThis value exactly corresponds to the coe$cient of determination R, after the performanceof multivariate analysis.

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