J. Jpn. Soc. Soil Phys.
土壌の物理性No. 134, p.5 ∼ 13 (2016)

Long-term temporal changes in micromorphology ofcattle manure compost pellets mixed with soils
Hideo KUBOTERA1,2 , Tsuyoshi YAMANE1 , Yasuyuki WAKIYAMA1 and Yusuke ARAKAWA1,3

Abstract: Compost pellets (PC) are different from nonpelletized composts in various properties including supply
and retention of nutrients and production of greenhousegases because of the micromorphology of PC. Therefore, it
is important to micro-morphologically characterize PC for
its proper utilization. We investigated long-term changes in
the micromorphology of cattle manure PC after its application in a soil. The PC grains were applied in a three-types
soil core containing a Lowland soil, a Yellow soil, and an
Andosol. The soil cores were buried in a field plot of its
soil type. After 1, 6, and 18 months, the soil core wassliced into thin sections after solidifying with a polyester
resin for the cross-sectional observation. Decompositionof the compost was also investigated using a glass-fiber
filter bag method. Results from the investigation were as
follows. 1) Weight residual ratio and carbon residual ratio of PC decreased rapidly in the first 6 months, and afterthat the decomposition became slower. 2) With the decomposition of PC, their shape was preserved, whereas theirvolume decreased. Estimated volume residual ratio of PC
was 28 % for Lowland soil, 43 % for Yellow soil, and 39 %for Andosol after 18 months. Voids corresponding to thevolume loss of PC by decomposition was formed aroundeach PC grain. 3) PC mixed with the Yellow soil werecross-sectionally divided into curved segments by cracks
whereas this separation was not observed in the Lowland
soil and Andosol. 4) Internal microstructure of PC did notshow a clear temporal change from 1 month to 18 months
in any of the examined soils. Microstructure type was massive microstructure to subangular blocky microstructure,with poor separation of peds.
Key Words : compost pellets, decomposition, micromorphology, thin sections, burial experiment

1. Introduction
Compost pellets (hereafter PC: pelletized compost) are
1 Kyushu Okinawa Agricultural Research Center,

NARO. 2421 Suya,
Koshi, Kumamoto, 861-1192, Japan.
2 Present address: Central Region Agricultural Research Center, NARO.
2-1-18 Kannondai, Tsukuba, Ibaraki, 305-8666, Japan.
3 Present address: Kyushu Okinawa Agricultural Research Center,
Miyakonojo Research Station, NARO. 6651-2 Yokoichicho, Miyakonojo,
Miyazaki, 885-0091, Japan, Corresponding Author: Hideo KUBOTERA2016 年 5 月 9 日受稿 2016 年 9 月 24 日受理

obtained by the compression of compost using a disk pelleter or extruder machine after pulverization and moisture content adjustment. PC has various advantages overregular compost of easy handling, homogeneous nutrients, longer-term storage, and lower transportation costs(Yakushido, 2002), and therefore, is becoming increasingly popular among farmers. Although constituents ofcomposts are concentrated as the moisture decreases withthe pelletization, concentration of major components suchas total nitrogen, phosphate, and potassium on dry matter
base, is similar to that of the compost before it is pelletized(Kano et al., 1997; Hara, 2005; Araki et al., 2007). However, PC is different from non-pelletized compost with respect to properties such as nitrogen mineralization (Araki
et al., 2007), phosphate efficiency (Arakawa, 2012), retention and leaching of inorganic ions (Haraguchi et al.,2008), and production of greenhouse gases (Inoue andShibukawa, 2008; Yamane and Yamada, 2009; Yamane etal., 2011). These differences can be ascribed to the morphological characteristics of PC, especially the internal microstructure that can be quite different from regular compost. Morphological characteristics of PC can affect various physical and chemical phenomena such as retention
and leaching of ions, redox status, and microbial activities.
Therefore, studying the morphology of PC is important toinvestigate unique properties of PC and further improve effective application methods. However, there are limited
studies how the micromorphology of PC changes after applying PC in a field.
We developed a technique for observing the micromorphology of PC, which employs the thin section method
used in soil micromorphology studies. With this technique,
we investigated internal micromorphology of cattle manure compost PC that were collected from an Andosol field
at 1 to 31 days after application (Kubotera et al., 2009). As
a next step, we further applied our thin section method tothe cattle manure PC that was packed in polyvinyl chloride pipes with three representative cultivated soils of the
Kyushu Okinawa region, after long-term (1 to 18 months)
burial treatment as reported in this paper. Our objectives
in the current research were, 1) long-term investigation, 2)土壌の物理性

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第 134 号

(2016)

Table 1 Chemical composition of the pelletized compost (PC) and non-pelletized compost(NPC) used in the experiment.
Type

Total
carbonTotal
nitrogen

P2 O5

K2 O

CaO

MgO

g kg−1 , dry matter base
Tested compost
PC
352.9
NPC
369.5

20.4
20.2

23.1
22.5

38.5
38.6

12.1
11

12.2
11.4

23 ± 17(n = 401)

24 ± 13(n = 401)

30 ± 26(n = 357)

10 ± 5(n = 357)

a Cattle manure compost in Japan

NPC

353 ± 94(n = 314)

19 ± 7(n = 400)a Data from Yamaguchi et al. (2000) is shown. Average ± SD

studying the morphology of PC for different soil types, and3) observation of PC and soils under undisturbed natural
condition.

2. Materials and methods
2.1 PC for experiment
We prepared PC for the experiment from cattle (primarily milk cows) manure compost produced in a composting
plant in Yamaga City, Kumamoto Prefecture. The chemical composition of the PC and its parent material, nonpelletized compost (hereafter NPC), are shown in Table
1. Total carbon and total nitrogen contents were measured
by the dry combustion method using a Vario EL, Elementar. Phosphorus content was measured using the molybdate yellow method, while potassium content was measured using the flame emission method. Calcium and magnesium were measured by ICP-AES after digestion withnitric-perchloric acid. As per the data from literature (Yamaguchi et al., 2000), this PC was similar in chemical composition to the commonly available Japanese cattle manurecompost. In addition, the chemical composition of PC and
NPC were similar.
The compost was air-dried in a greenhouse and pulverized, followed by addition of water to yield an approxi-

Fig. 1 Preparation of core samples with PC.

mate water content of 0.27 kg kg−1 , before pelletization
was done using a disk pelleter with holes of 5 mm in diameter. After the pelletizing, the PC was air-dried again in agreenhouse until about 0.10 kg kg−1 water content. Shapeof the PC after the air-drying was columnar, and its size
was 4.8 mm in diameter and 7.4 mm in height (average of
30 grains). Average weight was 0.159 g per grain.
2.2 Field burial experiment
The experiment was conducted with three representative
cultivated soils of the Kyushu Okinawa region: a Lowland
Paddy soil (hereafter “Lowland soil”), a Yellow soil, andan Andosol. We collected topsoil samples of Lowland soil,
Yellow soil, and Andosol from experimental fields of Saga
Prefectural Agriculture Research Center in Saga City, Nagasaki Agricultural and Forestry Technical Development
Center in Isahaya City, and Kyushu Okinawa Agricultural
Research Center in Koshi City, respectively. The soils were
air-dried and passed through a 4 mm mesh sieve, and theircore samples were prepared as follows.
Polyvinyl chloride pipes of 2.5 cm bore diameter were
cut into approximately 2 cm long pieces, and a piece ofroot restriction sheet was pasted onto one opening of eachpipe piece to serve as the bottom cover. Root restriction
sheet is a polyester cloth that can keep the sample from
intrusion of roots, worms and insects although the air and論文：Long-term temporal changes in micromorphology of cattle manure compost pellets mixed with soils

7

water can permeate. These pipes were then half-filled with
one of the soils, and 5 grains of PC were laid sideways on
the soil (Fig. 1). Next, the pipes were filled with additional soil and pasted with another piece of root restriction
sheet as the top cover. Hereafter, we refer to these as “coresamples.”

enclosed in an envelope, and C0 is the initial carbon content(g kg−1 ) per dry sample of PC and NPC. W1 and C1 are
the dry weight and carbon content of whole contents in an
envelope of control sample without PC and NPC, and W2
and C2 are those of a sample with PC and NPC.

Three types of soils without air-drying were packed
about 15 cm deep in square plots (1 m × 1 m) that wereseparated using concrete boards up to a depth of 1 m, in theexperimental field of Kyushu Okinawa Agricultural Research Center, Koshi City. Core samples of each soil wereburied vertically at a depth of 5 cm on May 25, 2011. The
plots were kept bare with manual weeding. Two core samples from each soil plot were collected periodically. The
samples collected at 1 month, 6 months, and 18 months
after the burial were used for micromorphology observations.

Thin sections were prepared following Kubotera et al.
(2009) as follows. A collected core sample was air-dried
and placed in a paper cup. Polyester resin mixed with benzoyl peroxide dissolved in acetone was poured in the cup,and the core sample was saturated with it by de-aerating
in an airtight container using a vacuum pump. Next, thecup was placed in a drying oven and the temperature was
gradually raised from 45 ◦ C to 65 ◦ C in order to solidify
the resin. The solidified block of soil was cut out fromthe pipe using a jigsaw and a cutting machine (Maruto Instrument MC-110) equipped with a rotary blade. A crosssectional cut was then made in the solidified soil block at
the mid-height using the rotary blade. The cutting position was particularly crucial for the experiment. Because,
if it was not proper, some PC grains would be lost from thethin section. Furthermore, the cutting plane needed to belocated close to the central axes of PC grains, because themicromorphology might be different at the center and at
the edge of PC. Therefore, we compared the condition of
the two pieces, and the one that had the cut plane near thecentral axis of the PC was selected for further analyses. If
the condition of both cut pieces was not good, we prepareda new solidified sample using another core sample fromthe first step of resin impregnation. The cut plane was polished using the Carborundum, a silicon carbide abrasive
compound. Different grain sizes of the compound, # 100,
# 300, # 600, and # 1000, were used in the ascending order (from coarse to fine). The polished plane was pasted
on a slide glass using epoxy adhesive, and it was cut using
the rotary blade to obtain samples of 1 mm thickness orless. The samples were polished repeatedly using the compound, until the internal voids and solids parts of the pellets
became clearly observable and the solid parts turned fromdark green to dark orange in color, when observed under amicroscope.

In addition to the core sample burial experiment, we conducted a burial experiment using the glass-fiber filter paper bag method (Saito, 1997), in order to investigate thedecomposition of composts. We took around 4 g of PC
samples, and weighed them precisely. And then we mixed
them with 15 g of air-dried soils. The mixture was enwrapped using an envelope of glass-fiber filter paper to prevent contamination of samples with outer soils. The filter
paper is resistant to microbial decomposition and therefore
is preservable in soils for a long period. This envelope wasthen covered with a root restriction sheet in order to prevent the intrusion of worms and insects. Samples that contained the NPC instead of PC, and control samples without any compost, were also prepared in the same manner
for the comparison of decomposition rates. Samples wereburied in the plots of each soil on March 30, 2011, andcollected periodically at 1, 3, 6, 12, and 18 months after
the burial. Weight residual ratio and carbon residual ratio
were measured by the method of Saito (1997) as follows.
The collected samples were air-dried, weighed and pulverized finely. Then the moisture content was measured byweighing a portion of the sample before and after the ovendrying at 105 ◦ C. Total carbon content of the samples wasmeasured by the dry combustion method using a Vario EL,
Elementarr. Weight residual ratio and carbon residual ratio
were calculated as follows:

Weight residual ratio(%) =Carbon residual ratio(%) =W2 −W1× 100
W0

W2 ×C2 −W1 ×C1× 100
W0 ×C02.3 Preparation of thin sections

As stated above, in case the quality of in-process orcompleted thin sections was not good, we discarded themand prepared new thin sections from another core sample.
However, in some cases, the second thin section also had aproblem. For example, Lowland soil 1 month thin section
shown in Fig. 4 contained only 4 grains and the PC in leftedge was too thin by overpolishing. In such cases, observation and morphological analysis was conducted only forthe remaining, good-condition grains.
2.4 Micromorphology investigation

Where W0 is the dry weight (g) of PC and NPC that was

Micromorphology observation in low magnification was

8土壌の物理性第 134 号

(2016)

Fig. 2 Weight residual ratio of PC and NPC measured using glass-fiber filter paper bag method.
Error bars were added in order to show the SD (n = 2). However, all error bars were hidden
under the markers because SD was smaller than 4 % in all samples.

Fig. 3 Carbon residual ratio of PC and NPC measured by glass-fiber filter paper bag method.
Error bars were added in order to show the SD (n = 2). However, most of the error bars were
hidden under the markers because SD was smaller than 7 % in many samples.

conducted using scanned images of the thin sections. The
voids that were formed around the PC after a long burial
period were measured as follows. We printed the imageof a thin section, cut out the pellets and the voids withscissors, and measured its weight (a grams). The voids
around the pellets were then cut away, and the weight ofthe cutouts was measured (b grams). b/a was the ratioof pellet area to the total area of pellets and surrounding
voids. Volume is proportional to the cross-sectional arearaised to the three second power, therefore, we considered(b/a)3/2 as the volume ratio of a pellet to the total space
including the surrounding void. In this paper, we refer tothe value of 100 × (b/a)3/2 as “volume residual ratio” (%).
Micromorphology observation in high magnification
was done using photographs obtained by a digital camera attached to a microscope. We took a photograph ofsize 1.97 mm × 1.58 mm at the center of PC, for all
5 grains within each core sample. As stated above, if
some PC grains were lost or in bad condition, five photos were taken for the remaining good-conditioned grains.
We measured the area ratio of voids in PC using a freeware
named “lia 32 for Windows 95”, developed by Yamamoto
K. (http://www.agr.nagoya-u.ac.jp/~shinkan/LIA32/index-

e.html) as follows. We opened the image file for eachphoto using this software and collected the color data from50 points among voids and 50 points among solids. Basedon these color data, the software classified all the 3871488
pixels in one image, as either a void or a solid. Ratio of thenumber of void pixels or solid pixels to the total number ofpixels (3871488) was assumed to be the area ratio of eachphase.

3. Results and discussion
3.1 Decomposition rate of compost
Weight residual ratio and carbon residual ratio are shownin Fig. 2 and Fig. 3, respectively. Weight residual ratios at
1, 3, and 6 months after the burial were 89 % – 98 %, 87 %– 94 %, and 66 % – 80 %, respectively. The temporal decrease became slower after 6 months, and the weight residual ratio of PC at 18 months was 60 % in Lowland soil, 62% in Yellow soil and 59 % in Andosol. The residual ratio
for NPC was 56 % in Lowland soil, 62 % in Yellow soil
and 50 % in Andosol. Although differences among thetypes of soils and compost were not clear, NPC in Andosol
at 6 months (66 %), 12 months (56 %), and 18 months (50論文：Long-term temporal changes in micromorphology of cattle manure compost pellets mixed with soils

9Fig. 4 Micromorphology of whole core samples.

%) showed relatively lower values in comparison with theother samples (73 % – 80 % at 6 months, 65 % – 72 % at
12 months, and 56 % – 62 % at 18 months).
Carbon residual ratio also showed a rapid decrease in thefirst 6 months, followed by a slower decrease in subsequent
periods. In the first 3 months, the ratio was higher for NPC
samples than for the PC samples. However, it decreased
rapidly and became 4 % – 7 % smaller for NPC than for
PC at 6 months or later. As for the soil types, Andosol had
lower values than for Lowland soil and Yellow soil in thelater periods and carbon residual ratio of Andosol at 6, 12,
and 18 months was 53 % – 59 %, 50 % – 56 %, and 38 % –46 %, respectively, whereas Lowland soil and Yellow soil
had values in the range of 62 % – 75 %, 61 % – 71 %, and
45 % – 52 %, respectively.
Carbon residual ratio showed a slightly larger decrease
between 12 and 18 months than between 6 and 12 months,
which was probably due to the differences in soil temperature across these periods. Average air temperature between 6 and 12 months (October 2011 to March 2012) was
10.7 ◦ C in Kumamoto City (Japan Meteorological Agency,2012), whereas that between 12 and 18 months (April to
September 2012) was 23.5 ◦ C. Therefore, decompositionof compost in the latter period is expected to be more rapid
than in the former period.
As seen from the trends for weight and carbon residual ratio, compost decomposition occurred rapidly in thefirst 3 months and became slower thereafter. Residual ra-

tio of weight and carbon for PC after 6 months was slightly
higher than that of NPC, and Andosol samples showed
lower values in comparison with Lowland soil and Yellow
soil.
3.2 Micromorphology of thin sections
Micromorphology of thin sections under low magnification is shown in Fig. 4. Coarse voids formed between the
PC surface and soil had a tendency to increase in size overtime. As shown in Fig. 5, the outer rim of the voids (i.e.,

Fig. 5 Similarity in the shape of PC surface andsurrounding voids. Red line and yellow line respectively
indicate the outer rim and inner rim of the voids between
the PC and soil. (Lowland soil 18 months sample, lower
left grain).

10

土壌の物理性第 134 号

(2016)

minant coefficient of linear regression was 0.789 (n = 9).
This result indicated that the volume of PC in the fields
decreases over time along with the decrease in weight due
to decomposition, and the loss of volume causes the formation of voids between PC surface and the surrounding
soil.

Fig. 6 Residual ratios of weight and volume of PC.
Volume residual ratio ( %) = 100 × (b/a)3/2
a: area of PC and the voids around PC in photo.
b: area of PC in photo.

the boundary between voids and soil, indicated by a redline in Fig. 5) showed shapes similar to the inner rim (i.e.,the boundary between PC and voids, the yellow line in Fig.
5). As a result, the shapes of PC and the voids surrounding
it were nearly the same. This fact suggested that the decomposition of PC in soils occurred from the outer rim, andthe loss of outer layers of PC had left the voids with shapes
similar to the initial PC. Relationship between the weightresidual ratio and the volume residual ratio (defined in thematerials and methods section) is shown in Fig. 6. Both theratios were closely related and the relationship was similar
across Lowland soil, Yellow soil, and Andosol. The deter-

Fig. 7 Internal micromorphology of PC.

Deformation of PC with time depended on soil type. In
the case of Lowland soil, the void formation at the outer
rim was a primary change, while transformation of PC, formation of internal cracks, or change in the surface smoothness of PC were not notable. In Yellow soil, cross-sectional
cracks formed and the PC became partially divided. In addition, the divided PC parts were curved and arch-shaped at
18 months. Andosol showed morphological characteristics
similar to Lowland soil. Unlike Lowland soil, however, thesurface of PC showed roughness at 18 months.
3.3 Internal micromorphology of PC
We selected 1 representative photo from 5 micrographsof PC in each thin section, and showed them in Fig.7.
These photos were selected according to the following criteria: 1) having a non-extreme (neither the largest nor thesmallest) area ratio of voids among the five photos, and 2)
having a micromorphology that seemed representative of
the five photos. Terms of soil micromorphology description (Stoops, 2003) are used henceforth, following the example of Kubotera et al. (2009).
Although the microstructure of PC was not uniform
across the grains and the observed areas, solid phase wasdominant in all photos and area of voids was generally
small. Dominant types of voids were zigzag planes (e.g.,seen in the right half of Lowland soil, 1 month photo)

論文：Long-term temporal changes in micromorphology of cattle manure compost pellets mixed with soils

11

ogy of cattle manure PC that was collected from an Andosol field between 1 to 31 days after application. The
results showed that the solid phase was separated by various degrees and formed peds of crumbs and granules at
4 days after application. Dominant microstructure types
were subangular blocky, while partly crumb and granularmicrostructures were also observed. Area ratio of voids
was 14.0 %, 14.7 %, and 18.0 % at 1, 4, and 31 days after
application, respectively.

Fig. 8 Temporal changes in the ratio of internal voidsin PC. Error bars indicate SD (n = 5). Horizontal positionof each plot was slightly changed from the accurate value(1, 6, 18 month) in order to make error bars visible.

and curved planes (e.g., seen in Yellow soil, 6 months
photo). In addition, vughs (irregular-shaped and not interconnected voids) with a round or oval shape were also observed (e.g., in Andosol, 18 months photo). Packing voids
that were interconnected and caused a separation of peds
were scarce. A coarse void in the left edge of Yellow soil,
18 months photo (Fig. 7), was a part of the cracks that divided PC. The type of microstructure was mainly massive
microstructure, and subangular blocky microstructure, inwhich the peds were poorly separated. Area ratio of thevoids at 1 month was 7.0 %, 7.9 %, and 11.6 % in Lowland soil, Yellow soil, and Andosol, respectively (Fig. 8).
Temporal change in the voids ratio showed no clear tendency in Lowland soil and Andosol. On the other hand,the voids ratio increased to 14.2 % at 6 months and to 25.0
% at the end of 18 months in Yellow soil. This was notdue to an increase in planes or vughs in the solid phase,
but was instead caused by coarse cracks, as seen in the 18
months photo (Fig. 7). The internal micromorphology of
PC between 1 and 18 months after the burial showed noclear tendency with either the soil type or the time, except for an increase of voids volume in Yellow soil, which
was related to the generation of coarse cracks in PC. We
propose two hypotheses on the reason why micromorphology of PC was similar in the three examined soils: 1) decomposition rate of compost was not remarkably different
among the examined soils as shown in the grass-fiber filter
paper bag experiment, and 2) interior portion of PC was
isolated from the soils, especially after the coarse voids
were formed around PC grains, and therefore the internal
micromorphology was not subject to the influence of outer
soil, as assumed by Hara et al. (2003). However, thesehypotheses should be examined by further study.
Kubotera et al. (2009) described the micromorphol-

Our current study documented the micromorphology of
PC after a burial period longer than that of Kubotera etal. (2009). Although microstructure type showed a resemblance to the data recorded by Kubotera et al. (2009),
the separation of peds was poor, and partial formation of
crumb or granular microstructures was not observed. Area
ratio of voids after 1 month was also smaller in comparison with Kubotera et al. (2009). This difference might berelated to the burial conditions, i.e., Kubotera et al. (2009)
applied PC to a field with a rotary cultivator, whereas we
enclosed PC and soils in pipes and buried them without
any physical disturbance.

4. Conclusion
We obtained some results concerning the morphologicalchanges in PC after its application to soils from a burial
experiment for 18 months and micromorphology observations using thin sections. The outer shape and internal
microstructure of PC were preserved, suggesting that theproperties of PC related with micromorphology such as
the microbial habitats could be long-term persistent in asoil. At the same time, voids between the PC and soils
corresponding to the volume loss and initial shape of PC
were formed. These voids might influence the water andair permeability of soils, especially when the PC grains areinterconnected with tubular voids formed after the deathand decomposition of plant roots that have a tendency togather around PC (Arakawa et al., 2012). In addition,
cracks formed in PC observed in the Yellow soil might alsosimilarly contribute flows in a soil. Further studies regarding function and sustainability of these voids, influence ofsoil types on deformation of PC, and long-term changes insoil water and air permeability after for the PC application
should be conducted.

Acknowledgements
Soil samples were provided by Norio Ootsuka (Saga
Prefectural Agriculture Research Center) and Masashi Fujiyama (Nagasaki Agricultural and Forestry Technical Development Center). Akihiro Tanaka, Yusuke Asai, Yuichi
Takakura, Hiroyuki Ito, Masaki Noguchi, Kimiyo Mat-

土壌の物理性

12

suoka, and Sayoko Hirayama (Kyushu Okinawa Agricultural Research Center (KARC), NARO) assisted in the
preparation and analysis of samples, and the maintenanceof field plots. Takashi Kusaba (KARC, NARO) gave us
valuable support of various kinds during the overall study
period. Shoko Miyazaki (Nihon University) helped us
in void measurement. We sincerely thank all of them.
This study was financially supported by a research project“Development of mitigation and adaptation techniques toglobal warming in the sectors of agriculture, forestry, andfisheries” sanctioned by Ministry of Agriculture, Forestry
and Fisheries. Budget from research projects 210a3,
151a3, and 151a1 of NARO was also used.

第 134 号

(2016)English summary)Inoue, K. and Shibukawa, H. (2008): Nitrous oxide emission fromupland soils applied pelletized poultry manure compost. Jpn.
J. Soil Sci. Plant Nutr., 79: 181–182.(in Japanese)
Japan Meteorological Agency (2012): Amedas, Climatic datain the past. Available at http://www.data.jma.go.jp/obd/stats/
etrn/index.php (confirmed at 19. Apr. 2016)
Kano, M., Toyoshima, K., Ohashi, H., Masuda, T., Tanaka,
A., Fukutome, Y. and Fujitomo, E. (1997): Experiments on
blending and granulating livestock feces compost. Bull. Aichi.
Agric. Res. Cent., 29: 299–304. (in Japanese with English
summary)
Kubotera, H., Yamane, T., Wakiyama, Y. and Arakawa, Y. (2009):
Micromorphological observation of pelletized cattle compost

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要旨

堆肥ペレット（PC）は，養分の保持供給特性や温室効果ガス発生などの様々な特性が非成型の堆肥と異なっており，それは微細形態に起因すると考えられるが，PC の微細形態に関する知見は少ない．そのため，PC の微細形態特性の解明は適正な利用を行う上で重要である．そこで，圃場に施用した PC の分解に伴う微細形態の経時的変化を調査するため，牛ふん堆肥の PC を九州沖縄地域の代表的な農耕地土壌である低地土，黄色土，黒ボク土と共に塩化ビニール管に封入し，圃場に埋設して，1 月，6 月，18 月
後の微細形態を樹脂固化薄片法で観察した．また分解速度をガラス繊維ろ紙埋設法で調査した．その結果，1) PC の重量残存率と炭素残存率は最初の 6 月でそれぞれ 73 % – 80 % と 58 % – 75 % まで減少し，以後は分解速度が低下して，18 月後にはそれぞれ 59 % – 62 % と 46 % – 52 % になった．2) 分解に伴い PC は最初の形と相似形を保ったまま体積が減少した．18 月後の体積残存率は低地土で 28 %，黄色
土で 43 %，黒ボク土で 39 % となった．体積の減少に対応する量の孔隙が PC の周囲に形成された．3)
黄色土と混合した PC はクラックによって輪切り方向に離断され，曲がった断片になったのに対し，低地土と黒ボク土ではこの離断は見られなかった．4) PC 内部の微細形態は全ての土壌において埋設 1 ∼

18 月の間では明瞭な経時変化を示さず，微細構造型は全試験期間，壁状構造（massive microstructure）
ないしペッドの分離が悪い亜角塊状構造（subangular blocky microstructure）だった．
キーワード：堆肥ペレット，分解，微細形態，薄片，埋設試験13