R Y SZ A R D W A LC ZA K , B A R B A R A W IT K O W SK A -W A L C Z A K
EFFECT OF WETTING-DRYING CYCLES ON THE AGGREGATION OF SOIL
In stitu te o f A grop h ysics, P o lish A ca d em y of S cien ces, L u b lin
One of the soil structu re features is aggregation and the factor causing its changes is the mechanical cultivation of soil [3, 5]. The soil aggregation is changing in time the direction and rate of its changes being dependent on environm ental factors, among which the most significant role play m oisture and tem perature [1, 2]. The aim of the present work was to determ ine the effect of soil m oisture changes on its aggregation. The knowledge of this effect makes, namely, possible to predict stabilization of the aggregation of soil under n atural conditions after carrying out agronomy measures as well as enables to determ ine the preparation way of aggregated soil samples for determ ination of hydrophysical characteristics under laboratory conditions.
M A T E R IA L A N D M ETH O DS OF IN V E ST IG A T IO N S
The object of investigations was chernozem developed from loess, the samples of which were taken from the arable layer. Upon drying up soil at the laboratory to the air-d ry state, its aggregation was determ ined by the standard sieve m ethod (Table 1). Then cylinders w ith the capacity of 100 cm8 were filled up w ith aggregates of the fractions separated of 0.25-0.5, 0.5-1, 1-3, 3-5 and 5-10 cm, condensing them at use of a vibrator, to ensure a unifor density. Upon filling up cylinders w ith aggregates, they were weighed up and subject to subsequent w et ting-drying cycles. The samples were w etted capillarly w ith w ater for 24 hours in a therm ostatic room w ith the tem perature of 2 0± 1°C and then dried up at the tem perature of 40°C in a drier w ith the air exchange to the air-dry state. The hum idity of samples after w etting w ith w ater am ounted to 0.25-0.5 — 59.7°/o п Л п “ 8, to 0.5-1 mm — 58.9% п Л п “*, to 1-3 mm — 59.5% m 8m “ 3, to 3-5 mm — 55.7% m 3m ~3 and to
a b 1 в 1 Bae ic p r o p e r t i e s o f chernozom dove lo p e d from l o e s s
O rg a ni c m a t t e r c o n t e n t % Ac t u a l s p e c i f i c m a t t e r gem“ 1 S p e c i f i c a r e a * 2g P e r c e n t a g e o f e l e m e n t a r y p a r t i c l e s a f t e r t h e i r s i z e mm i n d i a н2о n2 1-0 . 1 0 . 1 - 0 . 0 5 0 . 0 5 - 0 . 0 2 0.02-0.002 ^ 0 .002 2 . 1 1 2. 5385 49 15 3. 5 6 . 5 47. 5 28. 5 14 P e r c e n ta g e o f a g g r e g a t e s a f t e r t h e i r s i s я - mm i n d i a < 0 . 2 5 0 . 2 5 - 0 . 5 0 . 5 - 1 15 19 29 22
5-10 mm — 56.0% m 3m~8. The investigations were carried out during the subsequent 15 w etting-drying cycles, w hereas after every cycle 5 samples were chosen at random and th eir aggregation and density w ere determ ined (Fig. 1—6).
IN V E ST IG A T IO N R ESU L TS
The investigation m ethod assumed enables to determ ine amounts of w aterproof aggregates of the fractions investigated as well as amounts of decomposition products of initial aggregates, and thus the aggregation of soil samples and their density after the subsequent w etting-drying cycle.
Under the effect of w ater 55% of aggregates of the fraction of 0.25-0.5 mm disintegrated as early as during the first w etting-drying cycle (Fig. 1). During the second cycle a fu rth er disintegration of about
R ys. 1. A gg reg a tio n a fter su b seq u en t w e ttin g -d r y in g c y cle s n of an in itia l sam ple w ith a g g reg a tes of 0.25-0.5 m m in dia
5% of aggregates of this fraction took place, w hereas in fu rth er cycles no destructive action of w ater was observed. Beginning from the 3rd cycle w aterproof aggregates of the fraction investigated am ounted to 40% and the aggregates of > 0.5 mm in dia — to 2°/o. Such state m ain tained till the end of the experim ent.
Much more resistan t to the w ater action were aggregates of 0.5-1 mm in dia (Fig. 2). The w ater action resulted in a gradual disintegration
F ig. 2. A g g reg a tio n a fter su b seq u e n t w e ttin g -d r y in g c y cle s n of an in itia l sam ple w ith a g g reg a tes of 0.5-1 m m in dia
of a certain per cent of the aggregates investigated between the 1st and the 5th w etting-drying cycle. Beginning from the 5th cycle the am ount of w aterproof aggregates of the initial fraction did not undergo any changes and am ounted to 71'%>. D estruction and structure-form ing processes caused by the w ater action resulted in this case in the form ation of about 1% of aggregates of 1-3 mm, 10% — of 0.25-0.5 mm and about 19% — of <0.25 mm in dia. The changes of the aggregation of soil samples were observed betw een the 1st and the 10th w etting-drying cycle.
In case of aggregates of the fraction of 1-3 mm in dia destructive action of w ater was observed between the 1st and the 12th w etting- dying cycle. It caused a gradual decrease of the am ount of aggregates of the initial fraction down to 55% (Fig. 3). The w ater effect on aggre gates of the fraction of 1-3 mm in dia resulted in the form ation of 1-3% of aggregates of > 3 mm, of 18% of those 0.5-1 mm and of 19% of those of < 0.25 mm in dia. Form ation of aggreates of the diam eter higher than 3 mm took place druing the 1st and the 2nd w etting-drying
cycle, of rem aining ones — betw een the 1st and the 12th cycle.
The w ater effect on aggregates of the fraction of 3-5 mm led to a disintegration of about 70% of aggregates, during the first two w etting- drying cycles (Fig. 4), w hereas in the next cycles a gradual slight
decre-Fig. 3. A g g reg a tio n a fter su b seq u en t w e ttin g -d r y in g c y cle s n of an in itia l sam ple w ith a g g reg a tes of 1-3 m m in dia
Fig. 4. A ggregation a fter su b seq uent w e ttin g -d r y in g cy cles n of an in itial sam ple w ith a g g reg a tes of 3-5 m m in dia
ase of the am ount of aggregates of the initial fraction in soil samples took place, which finished after the 10th cycle, when the am ount of aggregates of 3-5 mm in dia was 14%. The highest am ount — 41% of aggregates of 1-3 mm in dia was formed in consequence of disintegration jof the initial fraction. Between the first and the ninth cycle 20% of aggregates of <0.25 mm, 15% of those of 0.5-1 mm and 8% of those of 0.25-0.5 mm i dia were formed gradually. The structure-form ing effect of w ater was observed after the first w etting-drying cycle, when 5% of aggregates of > 5 mm in dia were formed. However, after the fo urth cycle a decrease of their am ount down to 2% took place and m aintained till the end of the experim ent.
80% of aggregates of the fraction of 5-10 mm in dia gradually disintegrated under the w ater effect between the first and the tw elfth w etting-drying cycle (Fig. 5). During the first six w etting-drying cycles
Fig. 5. A g g reg a tio n a fter su b seq u e n t w e ttin g -d r y in g cy c le s n of an in itia l sam p le w ith a g g reg a tes of 5-10 m m in dia
21% of particles of <0.25 mm, 8% of aggregates of 0.5-1 mm and 12% of those of 0.25-0.5 mm in dia were formed. Beginning from the 6th cycle their am ount did not undergo any changes. D ifferent was the percentage of aggregates of 3-5 mm and 1-3 mm in dia. Initially the am ount of aggregates of 3-5 mm increased to 10% (lst-7 th cycle) and afterw ards it decreased down to 6% (llth -1 2 th cycle) and m aintained at th a t level till the end of the experim ent. On the other hand, the percentage of fraction of 1-3 mm in dia increased since the 1st cycle at a much higher ra te than of all rem aining fractions of aggregates, and only after the 12th w etting-drying cycle no changes in the am ount of aggregates of th a t fraction were observed. Aggregates of the fraction of 1-3 mm in dia constituted a m ain product of disintegration of aggre gated of the fraction of 5-10 mm in dia, as they am ounted to 25%. During the investigations no fraction of aggregates of > 1 0 mm in dia was found.
The results obtained prove th a t the subjection of m ono-aggregate soil samples to subsequent w etting-drying cycles affects the aggregation changes of the m aterial investigated. These changes occurred only till a definite mom ent — subsequent w etting-drying cycle, after surpassing of which an equilibrium between the processes of form ation and dis integration of soil aggregates was established. The intensity and course of disintegration processes and the point since which a stabilization of soil aggregation occurred, formed also differently for particular fractions of the aggregates. The quickest stabilization of the aggregation was observed for aggregates of the fraction of 0.25-0.5 mm in dia (the 3rd w etting-drying cycle), whereas aggregates of the fraction of 0.5-1 mm and 3-5 mm in dia did not show any aggregation changes since the 10th cycle, and aggregates of the fraction of 1-3 mm and 5-10 mm in dia — since the 12th w etting-drying cycle. The strongest effect of w ater
on aggregates of all the fractions investigated was observed during the 1st w etting-drying cycle. The action of w ater caused, namely, dis integration of 14% of aggregates of the fraction of 0.5-1 mm, 16% of aggregates of the fraction of 1-3 mm, 31% of those of the fraction of 3-5 mm. The next w etting-drying cycles did not cause such considerable disintegration of aggregates ; it could be observed p articularly distinctly for the fraction of 0.25-0.5 mm and 0.5-1 mm in dia, which after the 3rd and the 5th w etting-drying cycle showed an unchanged am ount of w aterproof aggregates aof the initial size. The disintegration of the rem aining fractions investigated occurred gradually during fu rth er cycles, and only betw en the 10th and the 12th cycle a stabilization of the am ount of w aterproof aggregates of the initial size was observed. The results obtained prove also th a t the fractions of aggregates of < 3 mm are more resistant to the w ater action than those of > 3 mm in dia. k\t the same time it is to stress th at the disintegration of aggregates of < 3 mm led to the form ation m ainly of aggregates of < 0.25 mm in dia, whereas the disintegration of aggregates of > 3 mm resulted in the form ation m ainly of aggregates of 1-3 mm in dia. On the other hand, products of the disintegration of aggregate fractions investigated, except for the fraction of 5-10 mm occurring in the lest amounts, constitute aggregates of 0.25-0.5 mm in dia. Such behaviour of the aggregates investigated under the action of w ater can be explained by the fact th at connections between grains and microaggregates, from which sm all aggregates are built, are much more resistant to the action of w ater than connections between grains, m icroaggregates and sm aller aggregates forming aggregates of greater size [4, 6, 7].
Sim ultaneously w ith investigations on the aggregation changes of samples and the w aterproofness of soil aggregates, investigations on sample density changes under the action of w ater, which can occur in consequence of changes in the volume of samples along w ith changes of their aggregation and of leaching of clayey particles, were carried out. The data obtained prove th a t the w etting-drying cycles resulted in density changes of the samples analyzed, m anifesting themselves in a decrease of the density of aggregates of < 3 mm and in an increase of those of > 3 mm in dia. The density changes occurred, first of all, in the first w etting-drying cycle (Fig. 6). It is to stress th a t an insigni ficant swelling up of samples containing aggregates of < 1 mm in di-л and a decrease of the volume of samples containing aggregates of > 3 mm was observed. Moreover, during the subsequent w etting-drying cyclers an insignificant leaching of the finest soil particles of samples containing all the aggregates investigated. This leaching was the most intensive during the 1st and the 2nd w etting-drying cycle, w hereas after the 1 0 th -llth cycle no soil particles were found on filter paper. The cause of reduction the value of samples containing aggregates of > 3 mm in
Fig. 6. C h an ges in d en sity of soil sam p les under th e e ffe c t of w a ter in p articu lar w e ttin g -d r y in g cy cle s n
dia was probably swelling up of samples and leaching of soil particles, whereas in case of samples containing aggregates of < 3 mm in dia, a main factor causing the sample density increase was change of the sample volume (sedimentation), which could constitute the effect of decreasing weight of samples caused by leaching.
R EC A PIT U LA T IO N
Results of the investigations on changes in aggregation of mono aggregate soil samples, w aterproofness of the aggregates investigated and changes of density under the effect of w ater in particular w etting- drying cycles allow to state as follows :
— w ater action in particular w etting-drying cycles results in changes of aggregation and density of initially monoaggregate soil samples,
— destructive and structure-form ing action of w ater manifests itself most intensively during the first w etting-drying cycles, whereas the next cycles cause less changes of the param eters investigated,
— for every fraction of aggregates there is a characteristic subsequent w etting-drying cycle, after which no effect of w ater on the param eters investigated in observed.
Moreover, it seems th a t the m ethod as presented above could be applied in investigations on aggregation of arable layer of soil after perform ed agronomy measures, provided th a t this layer should have a relatively stable aggregation as well as a preparation way of soil with stabilized aggregation for investigations of hydrophysical characteristics of soil, requiring constant physical param eters of the object during the perform ance of the respective m easurem ents.
REFEREN CES
a g gregation of so ils cu ltiv ed w ith a c tiv e im p lem en ts. Pol. Journ. S o il S ei. 8, 1976, 2, 27.
[2] D e с h n i к I., T а г к i e w i с z S. : C hanges of agrop h ysical properties under th e e ffe c t of a ctiv e tools. P robl. A grofiz. 18, 1975.
[3] D o m ż a ł H. : C on d en sation of solid p h a se and its role in th e fo rm a tio n of w a ter and air in d ices of soils ([Doctor’s th esis]. A R L u b lin , 1977.
[4] E m e r s o n W. W. : T he stru ctu re of so il crum bs. Jour. S o il S ei. 10, 1959. [5] F i n n e y J. B. : M ech a n iza tio n and so il stru ctu re. A g ricu ltu re 78, 1971, 1. [6] T о к a j J. : Q uan titative m icroscop ico-ch em ical ex am in ation s of soil aggregates
as stru ctu ral e lem en ts. R ocz. gleb ozn . 17, 1967, 283.
[7] W a l c z a k R., W i t k o w s k a В. : D eterm in a tio n of w a terp ro o fn ess of d if fe r e n t fra ctio n s of soil aggregates. Rocz. gleb ozn . 25, 1974, 2, 275.
R. WALCZAK, В. W ITKOW SKA-W ALCZAK
W PŁYW C YK LI N A W ILŻ A N I A -O S U S Z A N IA N A SK Ł A D A G R EC A TO W Y G L EBY
Z akład A g ro fizy k i P A N w L u b lin ie
S t r e s z c z e n i e
P rzed sta w io n o w y n ik i badań d o ty czą cy ch w p ły w u d zia ła n ia w o d y w cy k la ch n a w ilż a n ia -o su sz a n ia na sk ład a g reg a to w y gleb y, w od ood p orn ość a g reg a tó w g le b ow ych i gęstość. W raz ze sp ad k iem lub w zro stem w ilg o tn o ści w sz y stk ie badane p a ram etry u leg a ją zm ianom , zw ła szcza podczas p ierw szy ch c y k li n a w ilż a n ia -o su szania. W ykazano, iż dla każd ej fra k cji a g reg a tó w is tn ie je ch a ra k tery sty czn y k o le jn y cyk l n a w ilż a n ia -o su sz a n ia , po k tórym n ie stw ierd zo n o w p ły w u d zia ła n ia w o d y na badane p aram etry.
Dr R y s z a r d Walczak, Z a k ł a d A g r o f i z y k i P A N
