Vol.2, No.1, 1-8 (2011) Agricultural Sciences
doi:10.4236/as.2011.21001
Copyright © 2011 SciRes. Openly accessible at http:// www.scirp.org/journal/AS/
Quantitative and qualitative changes of humus in whole
soils and their particle size fractions as influenced by
different levels of compost application
Thu Ha Nguyen, Haruo Shindo*
Faculty of Agriculture, Yamaguchi University, Yamaguchi, Japan; *Corresponding Author: shindo@yamaguchi-u.ac.jp
Received 11 November 2010; revised 24 December 2010; accepted 7 January 2011
ABSTRACT
Effect of long-term application (ca. 30 years) of
compost at different levels on humus composi-
tion of whole soils and their particle size frac-
tions in a field subjected mainly to double
cropping (barley and paddy rice) was investi-
gated. Soil samples were collected from three
plots of different types of management: (a) F
plot, only chemical fertilizers containing N, P
and K were applied; (b) F+LC plot, both chemi-
cal fertilizers and a low level of compost were
applied; (c) F+HC plot, both chemical fertilizers
and a high level of compost were applied (the
amount of compost applied in the F+HC plot
was three times larger than that applied in the
F+LC plot). Each soil sample was divided into
coarse sand- (CSA), medium sand-(MSA) and
fine sand-(FSA) sized aggregate, silt-sized ag-
gregate (SIA) and clay-sized aggregate (CLA)
fractions by wet-sieving and sedimentation. In
addition, the CSA and MSA fractions were sub-
divided into “mineral particles” (MP) and “de-
cayed plants” (DP) by a density fractionation.
Humus composition was influenced depending
upon the level of compost applied. The applica-
tion induced an increase in the amounts of total
humus (TH), humic acid (HA) and fulvic acid (FA)
in the whole soil and many size fractions, par-
ticularly, SIA fraction. The increase was re-
markable in the F+HC plot. In the CSA and MSA
fractions, the amounts of TH, HA and FA were
much larger in the CSA- and MSA-DP fractions
than in the CSA- and MSA-MP fractions. The
amounts of TH, HA and FA in the SIA fraction
were larger than those in the CLA fraction for
the F+HC and F+LC plots, and the reverse was
true for the F plot. On the other hand, the de-
grees of humification of humic acids in whole
soils and many size fractions, particularly SIA
fraction, decrease d by com post appli cation. The
decrease w as markedly in the F+HC plot. These
findings suggest that the SIA fraction play an
important role in the quantitative and qualitative
changes of humus, including HA and FA, as in-
fluenced by a long-term compost application.
Keywords: Paddy and Upland Fields; Straw-Cow
Dung Compost; Humic and Fulvic Acids;
Humification
1. INTRODUCTION
A variety of organic amendments such as compost,
farmyard manure, plant residues, food processing
wastes and sewage sludge is applied to various agri-
culture soils. Many researchers have reported beneficial
effects of these amendments on the physical, chemical
and biological properties, and fertility of soils in upland
and paddy fields. However, the role of the amendments
in the soils of double cropped fields (upland and paddy
crops) has received little attention. Thus, the authors
began to investigate the effect of continuous compost
application on various properties of soils in a field
subjected to long-term double cropping (paddy rice and
barley). The results obtained are summarized as follows:
Compost application increased (1) the activities of or-
ganic C, N and P decomposing enzymes, (2) the micro-
bial biomass N content and the hyphal length, (3) the
degree of water-stable macroaggregation (>0.25 mm)
and (4) the contents of total humus (TH) including hu-
mic (HA) and fulvic (FA) acids, organic C, total N,
hydrolysable carbohydrates and active Al [1-3].
Humus is one of the most important constituents of
soils and it affects various soil properties as well as
global C cycle. Regarding the effects of organic amend-
ments on humus composition, we can find some papers.
For examples, Roppongi et al. [4] reported that the in-
T. H. Nguyen et al. / Agricultural Sciences 2 (2011) 1-8
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2
corporation of compost into an alluvial soil induced an
increase in the amounts of TH, HA and FA and a de-
crease of humification degree of humic acids (HAs).
Similar results were obtained even when cattle manure
was applied to an Ando soil or a Yellow soil [5,6].
When organic amendments were applied to soils, the
sizes of the amendments become smaller and the
amounts of mineral particles adhering and/or being
associated with them increase with the progression of
transformation and degradation by abiotic and biotic
reactions. Although these processes are very compli-
cated, the amendments-derived organic matter may be
distributed into soil fractions with different particle
sizes under the soil conditions. Organic matter of recent
plant origin in soils is considered to be preferentially
recovered in the sand-sized fraction, whereas more
microbially processed material can be found in the
silt-sized and clay-sized fractions [7]. Accordingly,
particle size fractionation of soil may provide an effec-
tive approach to clarify the fate and behavior of organic
matter in soil ecosystem. Leinweber and Reuter [8]
found that compost had clearly greater effects than
fresh farmyard manure and straw plus chemical fertil-
izers for the enrichment of organic C and N in the par-
ticle size fractions. Aoyama and Kumakura [5] divided
manure-treated soil into two fractions, particulate or-
ganic matter (>53 µm) and mineral-associated organic
matter (<53 µm) fractions. They found that manure
application increased significantly the amount of par-
ticulate organic matter, while it did not affect the
amount of mineral-associated organic matter. Further-
more, the analysis of the HAs revealed that manure
application led to a decrease in the degree of humifica-
tion and an accumulation of high molecular weight
components. However, the effects of amendments on
humus composition in various particle size fractions of
soils and the relationship between the amount of
amendment and humus composition need to be further
studied. The objective of the present study was to in-
vestigate the effects of different levels of compost on
the quantitative and qualitative changes of humus in
whole soils and various particle size fractions, using
soil samples which were taken from three plots (only
chemical fertilizer, chemical fertilizer plus a low level
of compost and chemical fertilizer plus a high level of
compost plots) in the field described above.
2. MATERIALS AND METHODS
2.1. Field Experiment
The field experiments with different types of man-
agement were established in 1975 at Yamaguchi Prefec-
ture Agricultural Experimental Station, Yamaguchi, Ja-
pan. The soil at this site was classified as Gray Lowland
soil. Three plots (200 m2 each) of the field experiments
were selected: (a) F plot, only chemical fertilizers con-
taining N, P, and K were applied; (b) F+LC plot, both
chemical fertilizers and a low level of compost were
applied; (c) F+HC plot, both chemical fertilizers and a
high level of compost were applied (the amount of
compost applied in the F+HC plot was three times larger
than that applied in the F+LC plot). Ta bl e 1 shows the
amounts of fertilizer and compost applied in the soils of
three different plots. The same plots were used as paddy
fields for rice in summer and as upland fields for barley
in winter until June 2001. The application rate of N,
P2O5 and K2O for each crop was 100 kg ha-1. After har-
vest (June and November), rice straw-cow dung compost
was applied at two different levels of 5 and 15 Mg ha-1.
However, since June 2001, these plots were used only as
paddy fields and the amounts of fertilizer and compost
applied were reduced by half. In April 2007, to obtain an
average soil sample in each plot, soils were taken from
the plow layer (0-15 cm) of five sites across each of the
three plots and mixed well. The soils were air-dried,
gently crushed and then passed through a 2-mm mesh
sieve. These sieved samples were used for analytical
determinations and particle size fractionation.
2.2. Particle Size Fractionation
Particle size fractionation of the soils was conducted
by the physical fractionation method described in Tanaka
and Shindo [9]. Soil-water suspension was divided into
coarse sand-sized aggregate (212-2,000 μm, CSA) and
medium sand-sized aggregate (53-212 μm, MSA) frac-
tions successively, using a 212 and 53 μm mesh sieve.
Clay-sized aggregate (>2 µm, CLA) and silt-sized ag-
gregate (2-20 µm, SIA) fractions in the suspension
which passed through the sieves were collected succes-
sively by repeating dispersion and sedimentation, and
then concentrated. After removal of the CLA and SIA
fractions, the remained particle was recovered as fine-
sand aggregate fraction (20-53 µm, FSA). Furthermore,
the CSA and MSA fractions were subdivided into “de-
Table 1. Amounts of fertilizers and compost applied in three
different plots.
N P2O5 K
2O Compost
Plot
(kg ha-1) (Mg ha-1)
Fa 100 100 100 0
F+LCb 100 100 100 5
F+HCc 100 100 100 15
a. only chemical fertilizers containing N, P and K were applied; b. chemical
fertilizers plus a low level of compost were applied; c. chemical fertilizers
plus a high level of compost were applied.
T. H. Nguyen et al. / Agricultural Sciences 2 (2011) 1-8
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cayed plants” (DP) and “mineral particles” (MP) by a
density fractionation (decantation) in water. All the frac-
tions obtained were freeze-dried, weighed and then used
for the determination of humus composition.
2.3. Humus Composition
Humus composition was analyzed according to the
method described in Kumada [10]. Humus was extracted
with a mixture of 0.1 mol L-1 NaOH + 0.05 mol L-1
Na4P2O7 (1:1). Then, the extract was separated into HA
and FA by addition of H2SO4. The amounts of TH, HA
and FA were determined by the KMnO4 oxidation me-
thod. In the present study, 1 mL of 0.02 mol L-1 KMnO4
consumed was calculated as corresponding to 0.48 mg C
[11]. The degree of humification (darkening) of HA was
determined using color coefficient ( log K) and relative
color intensity (RF) values, where the log K is the lo-
garithm of the ratio of the absorbance of HA at 400 nm
to that at 600 nm; the RF represents the absorbance of
humic acid at 600 nm multiplied by 1,000 and then di-
vided by the number of milliliters of 0.02 mol L-1
KMnO4 consumed by 30 mL of HA solution. Humus
composition was analyzed in duplicate at least and the
average values obtained were given.
3. RESULTS AND DIS CUSSION
3.1. Distribution of Mass Weight in Particle
Size Fractions
The recoveries of mass weight by the physical frac-
tionation used were 97.0 for the F plot, 96.0 for the
F+LC plot and 100.9% for the F+HC plot. Thus, the
percentage distribution of mass weight in the particle
size fractions was corrected to a total of 100% (Table 2).
The percentage distribution of mass weight in each par-
ticle size fraction ranged from 11.9 to 32.2% in the F
plot, from 10.9 to 36.2% in the F+LC plot and from 9.3
to 34.0% in the F+HC. In all plots, the distribution val-
ues increased in the order of CLA < FSA < MSA < SIA
< CSA fractions. These results indicate that in the field
experiments, the distribution of mass weight was not
greatly affected by compost application. As expected, in
the CSA and MSA fractions, the distribution values of
the MP were much higher than those of the DP.
It is well known that the digestion of soil samples by
hot H2O2 can decompose soil organic matter. Compari-
son of percentage distribution of mass weight in the par-
ticle size fractions before and after H2O2 treatment re-
vealed that the distribution values in the 2-20 and
210-2,000 µm fractions were higher in the former sys-
tem than in the latter system, and the reverse was true for
the <2 and 20-210 µm fractions (data was not shown).
These findings indicate that highly recalcitrant organic-
inorganic complexes, which are resistant to water-sieving,
exist in the soils studied. Thus, in the present study, par-
ticle size fractions obtained were designated as aggre-
gate fractions.
The aggregation of soil particles may be promoted
when paddy field is changed to upland field.
3.2. Humus Composition of Whole Soils
The humus composition of whole soils in the F, F+LC
and F+HC plots is shown in Table 3. The amounts of the
TH, HA and FA in the whole soils ranged from 13.9 to
26.0, from 4.65 to 11.6 and from 4.02 to 6.76 g C kg-1,
Table 2. Percentage distribution (%) of mass weight in particle size fractions.
CSAb MSAc FSAd SIAe CLAf Total
Plota MPg DPg Sum MP DP Sum
F 31.1 1.1 32.2 19.7 0.9 20.6 14.1 21.3 11.9 100
F+LC 33.2 3.0 36.2 15.1 1.5 16.6 13.4 23.0 10.9 100
F+HC 32.7 1.3 34.0 17.1 1.7 18.8 12.6 25.3 9.3 100
a. see Table 1, b. coarse sand-sized aggregate (212-2,000 µm) fraction, c. medium sand-sized aggregate (53-212 µm) fraction, d. fine
sand-sized aggregate (20-53 µm) fraction, e. silt-sized aggregate (2-20 µm) fraction, f. clay-sized aggregate (< 2 µm) fraction, g. MP and
DP stand for “Mineral particles” and “Decayed plants”, respectively.
Table 3. Humus composition of whole soils.
Total humus Humic acid Fulvic acid
Plota
(g C kg-1 whole soil) Amount
(g C kg-1 whole soil) log Kb RFc Typed(g C kg-1 whole soil)
F 13.9 4.65 0.696 40 B 4.02
F+LC 17.6 7.04 0.738 36 Rp 4.91
F+HC 26.0 11.6 0.807 28 Rp 6.76
a. see Tabl e 1, b. the logarithm of the ratio of the absorbance of humic acid at 400 nm to that at 600 nm, c. the absorbance of humic acid at
600 nm multiplied by 1,000 and then divided by the number of milliliters of 0.02 mol L-1 KMnO4 consumed by 30 mL of humic acid solu-
tion, d. classification diagram by Kumada [10].
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respectively, and these amounts were in the order of F <
F + LC < F + HC plots. The amounts of FA and HA in
the F + HC plot were 1.4 and 1.6 times larger than those
in the F + LC plot, respectively. In the F + LC and espe-
cially F + HC plot, the amount of HA was much larger
than that of FA.
According to Kumada [10], the degrees of humifica-
tion of soil HAs become higher as log K value de-
creases and RF value increases. In the present study, the
log K values of the HAs increased from 0.696 for the
F plot to 0.807 for the F + HC plot. In contrast, the RF
values of the HAs decreased from 40 for the F plot to 28
for the F + HC plot. The HAs in the F, F + LC and F +
HC plots belonged to Types B, Rp and Rp, respectively
[10], and the degree of humification of the HA was the
lowest in the F + HC plot.
In the present study, by continuous compost applica-
tion, the amounts of TH, HA and FA increased and the
humification degrees of the HAs decreased. The increase
and decrease occurred markedly in the F + HC plot. The
reasons are proposed as follows: (1) the growth of crops
is promoted to a larger extent in the F + HC plot than in
the F + LC and F plots, because larger amount of com-
post was applied in the former plot. Consequently, larger
amounts of plant residues such as roots and stubbles,
which can be transformed to humus, remain in the F +
HC plot than in the other plots. (2) The degrees of humi-
fication of the HAs in the compost as well as in fresh
plant residues are very low [10]. (3) Part of indigenous
soil HA with a high degree of humification is decom-
posed by microorganisms, due to priming effect [12].
The decomposition is accelerated to a larger degree in
the F + HC plot than in the other plots.
3.3. Humus Composition of Particle Size
Fractions
In the present study, the amounts of the TH (Table 4),
HA (Table 5) and FA (Table 6) in the particle size frac-
tions were expressed as g C kg-1 fraction and as g C kg-1
whole soil. Furthermore, percentage distribution of the
TH, HA and FA in each size fraction were calculated and
shown in Figures 1-3.
The amount of the TH per fraction (g C kg-1 fraction,
Ta bl e 4 ) ranged from 1.00 to 246 in the F, from 1.68 to
154 in the F + LC and from 1.13 to 222 in the F + HC
plots. In all plots, the amount of the TH in the CLA frac-
tion exceeded that in the SIA or FSA fraction. The amounts
of the TH in the CSA- and MSA-DP fractions were
Table 4. Amount of total humus (TH) in particle size fractions.
Fb F+LCb F+HCb F F+LC F+HC
Particle size
fractiona (g C kg-1 fraction) (g C kg-1 whole soil)
CSA MP 1.28 1.68 3.17 0.398 0.558 1.04
DP 246 154 222 2.71 4.62 2.89
MSA MP 1.00 N.Dc 1.13 0.198 N.D 0.192
DP 187 140 185 1.68 2.10 3.15
FSA 1.52 2.46 3.72 0.214 0.330 0.469
SIA 18.3 27.1 27.9 3.90 6.23 7.06
CLA 39.9 46.1 60.2 4.75 5.02 5.61
Total 13.9 18.9 20.4
a. see Table 2; b. see Table 1; c. not determined.
Table 5. Amounts of humic acids (HAs) in particle size fractions.
Fb F+LCb F+HCb F F+LC F+HC
Particle size
fractiona (g C kg-1 fraction) (g C kg-1 whole soil)
CSA MP
N.A.c N.A. N.A. N.A. N.A. N.A.
DP
111 70.7 118 1.22 2.21 1.53
MSA MP
N.A. N.A. N.A. N.A. N.A. N.A.
DP
61.4 126 112 0.553 1.89 1.90
FSA
N.A. N.A. N.A. N.A. N.A. N.A.
SIA
7.71 11.9 21.3 1.64 2.74 5.39
CLA
14.8 15.0 26.7 1.76 1.64 2.48
Total
5.17 8.48 11.3
a. see Table 2; b. see Table 1; c. not applicable because the amount of humic acid is very small.
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Table 6. Amounts of fulvic acids (FAs) in particle size fractions.
F
b F+LCb F+HCb F F+LC F+HC
Particle size
fractiona (g C kg-1 fraction) (g C kg-1 whole soil)
CSA MP N.A.c N.A. N.A. N.A. N.A. N.A.
DP 73.6 48.3 55.7 0.810 1.450 0.724
MSA MP N.A. N.A. N.A. N.A. N.A. N.A.
DP 17.6 28.2 42.8 0.158 0.423 0.728
FSA N.A. N.A. N.A. N.A. N.A. N.A.
SIA 5.32 8.68 11.5 1.13 2.00 2.91
CLA 17.2 18.9 22.6 2.05 2.06 2.10
Total 4.15 5.93 6.46
a. see Table 2; b. see Table 1; c. not applicable because the amount of fulvic acid is very small.
0
10
20
30
40
50
60
70
80
90
100
FF+LC F+HC
Percentage distributi on (%)
CS A-MP
CS A-DP
MSA-MP
MSA-DP
FSA
SIA
CL A
Figure 1. Percentage distribution of total humus (TH) in
particle size fractions. See Table 1 for F, F + LC and F + HC.
See Table 2 for CSA, MSA, FSA, SIA, CLA, MP and DP.
0
10
20
30
40
50
60
70
80
90
100
FF+LCF+HC
Percentage distributi on (%)
CS A-DP
MSA-DP
SIA
CL A
Figure 2. Percentage distribution of humic acids (HAs) in
particle size fractions. See Table 1 for F, F + LC and F +
HC. See Table 2 for CSA, MSA, SIA, CLA and DP.
about 70-190 times larger than those in the CSA- and
MSA-MP fractions.When the amount of the TH was
calculated as g C kg-1whole soil (Tab le 4), the amount
reached 4.75 for the F, 6.23 for the F + LC and 7.06 for
the F + HC plots. In almost fractions, the amount of the
TH was in the order of F < F + LC < F + HC plots. Fur-
thermore, in the F + LC and especially F+HC plots,
0
10
20
30
40
50
60
70
80
90
100
FF+LCF+HC
Percentage distribution (%)
CSA-DP
MSA-DP
SIA
CLA
Figure 3. Percentage distribution of fulvic acids (FAs)
in particle size fractions. See Table 1 for F, F + LC and
F + HC. See Table 2 for CSA, MSA, SIA, CLA and DP.
the amount of the TH in the SIA fraction exceeded that
in the CLA fraction, while the reverse was true for the F
plot.
Percentage distribution of the TH in each fraction
(Figure 1) ranged from 1.43 to 34.4 for the F, from 1.75
to 33.0 for the F + LC and from 1.19 to 34.6% for the F
+ HC plots. In all plots, about 60% of the TH was occu-
pied by the SIA and CLA fractions, and the other 30%
existed in the CSA- and MSA-DP fractions. In the F +
LC and F + HC plots, the distribution value was higher
in the SIA fraction (33.0 and 34.6%) than in the CLA
fraction (26.6 and 27.4%), while the reverse was true for
the F plot (28.2 and 34.4%).
Aoyama [13] and Aoyama and Kumakura [5] found
that soil organic C accumulated to a larger amount in the
fraction of larger than 53 µm than in the fraction of less
than 53 µm, when manure (160 and 320 Mg ha-1 y-1) was
applied continuously to Kuriyagawa Ando soil for 20
years. A similar result was obtained for Hirosaki Ando
soil [14]. However, in the case of a Gray Lowland soil of
present study, the amount of the TH accumulated was
larger in the SIA and CLA fractions of less than 20 µm
than in the other fractions. In the study, compost was
applied at the level of 30 Mg ha-1 y-1 or less for about 30
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years. Furthermore, in Fujisaka Ando soil where com-
post was incorporated continuously at the level of 11 and
34 Mg ha-1 y
-1 for 43 years, the distribution pattern of
soilorganic C in size fractions was similar to that in the
present study [13]. Accordingly, it is assumed that parti-
cle size fraction, which can accumulate a large amount
of organic matter after the application of organic amend-
ments, changes depending upon annual application
amount.
The amount of the HA per fraction (g C kg-1 fraction,
Table 5), except for the CSA- and MSA-MP fractions
and the FSA fraction (not calculated because the amount
of HA was very small), ranged from 7.71 to 111 for the F,
from 11.9 to 126 for the F + LC and from 21.3 to 118 for
the F + HC plots. In all plots, the amount of the HA in
the CLA fraction exceeded that in the SIA fraction. As
expected, the amounts of the HAs in the CSA- and
MSA-DP fractions were very large.
The amount of the HA per whole soil (g C kg-1 whole
soil, Table 5) reached 1.76 for the F, 2.74 for the F + LC
and 5.39 for the F + HC plots. In almost fractions, the
amount of the HA was the highest in the F + HC, fol-
lowed by the F + LC and F plots. Furthermore, in the F +
LC and especially F + HC plots, the amount of the HA
was larger in the SIA fraction than in the CLA fraction
and CSA- and MSA-DP fractions. In contrast, in the F
plot, the amount of HA was larger in the CLA fraction
than in the SIA fraction and CSA- and MSA-DP frac-
tions.
Percentage distribution of the HA in each fraction
(Figure 2) varied from 10.7 to 34.0 for the F, from 19.5
to 32.7 for the F+LC and from 13.5 to 47.7% for the
F+HC plots. The distribution value was higher in the
SIA fraction (32.7 and 47.7%) than in the CLA fraction
(19.5 and 21.9%) for the F+LC and F+HC plots and the
reverse was true for the F plot (31.7 and 34.0%). In the
CSA- and MSA-DP fractions, the distribution value
ranged from 10.7 to 25.3%.
The amount of the FA per fraction (g C kg-1 fraction,
Table 6), except for the CSA- and MSA-MP fractions
and the FSA fraction (not calculated because the amount
of FA was very small), varied from 5.32 to 73.6 for the F,
from 8.68 to 48.3 for the F + LC and from 11.5 to 55.7
for the F + HC plots. In all plots, the amount of the FA in
the CLA fraction exceeded that in the SIA fraction. The
amounts of the FAs in the CSA- and MSA-DP fractions
of three plots varied from 17.6 to 73.6.
The maximum amount of the FA per whole soil (g C
kg-1 whole soil, Table 6) was 2.05 for the F, 2.06 for the
F + LC and 2.91 for the F + HC plots. In almost frac-
tions, the amount of the FA was in the order of F < F +
LC < F + HC plots. In the F + HC plot, the amount of
the FA was larger in the SIA fraction than in the CLA
fraction, while the reverse was true for the F and F + LC
plots.
Percentage distribution of the FA in each fraction
(Figure 3) varied from 3.51 to 59.1 for the F, from 8.62
to 42.0 for the F + LC and from 2.58 to 49.4% for the F
+ HC plots. In the F + HC plot, the distribution value of
the FA was larger in the SIA fraction (49.4%) than in the
CLA fraction (35.7%), while the reverse was true for the
F (32.8 and 59.1%) and F + LC (40.5 and 42.0%) plots.
As shown in Figures 1-3, it is clear that CSA- and
MSA-DP fractions contribute to the accumulation of
humus, although the percentage distribution of TH, HA
and FA in both DP fractions were smaller than that in the
SIA and CLA fractions in most cases. Comparison of the
distribution values of TH, HA and FA between the SIA
and CLA fractions indicated that the values in the SIA
fraction were mostly higher than those in the CLA frac-
tion for the F + HC and F + LC plots and the reverse was
true for the F plot. In the physical fractionation of the
present study, after the CLA fraction was collected by
sedimentation, the SIA and then FSA fractions were ob-
tained. Since the quantitative distribution of TH, HA and
FA in the FSA fraction was very small (Figures 1-3)
compared to the SIA and CLA fractions, it is reasonable
to consider that free (not connected with minerals, a low
density) and fine decayed plants are recovered in the
CLA fraction. Accordingly, in most cases of the F + HC
and F + LC plots, higher distribution values of TH, HA
and FA in the SIA fraction compared to the CLA fraction
may be because organic matter including decayed plants
were connected to the minerals (including aggregated
clay-sized minerals) in the SIA fraction and preserved
there as stable aggregates. The distribution values of TH,
HA and FA in the SIA fraction were higher in the F +
HC plot than in the F + LC plot, indicating that stable
aggregates might be produced to a larger extent in the
former plot compared to the latter plot.
According to Kumada [10], the degrees of humifica-
tion (darkening) of HAs become higher as log K value
decreases and RF value increases. The log K and RF
values and types of HAs obtained from the CSA-DP,
MSA-DP, SIA and CLA fractions of three plots were
investigated (Figure 4). The analysis of HA was not
conducted for the CSA-MP, MSA-MP and FSA fractions,
because the amount of HA was very small. The RF val-
ues ranged from 14 to 52 for the F plot, from 18 to 51
for the F + LC plot and from 15 to 37 for the F + HC
plot, and increased with decreasing particle size in these
plots. On the other hand, the log K values varied from
0.638 to 0.940 for the F plot, from 0.667 to 0.867 for the
F + LC plot and from 0.719 to 0.939 for the F + HC plot,
and decreased with decreasing particle size in the plots.
From the log K and RF values, the types of HAs were
examined [9]. The HAs in the CSA- and MSA-DP frac-
T. H. Nguyen et al. / Agricultural Sciences 2 (2011) 1-8
Copyright © 2011 SciRes. Openly accessi ble at http://www.scirp.org/journal/AS/
7
0
10
20
30
40
50
60
0.60.70.80.91.01.1
log K
RF
B
P
Rp
F+HC
,
CLA
F+HC
,
SIA
F+HC
,
MSA-DP
F+HC
,
CSA-DP
F+LC, CLA
F+LC
,
SIA
F+LC
,
MSA-DP
F+LC
,
CSA-DP
F
,
MS A-DP
F, SI
A
F
,
CLA
F
,
CSA-DP
Figure 4. RF and log K values of humic acids in particle
size fractions. See Table 1 for F, F + LC and F + HC. See
Table 2 for CSA, MSA, SIA, CLA and DP. Rp, B and P in
the figure indicate the types of humic acids.
tions of the F, F + LC and F + HC plots belonged to
Type Rp with a low degree of humification (Figure 4),
and the HAs in the SIA fractions of the F + LC and F +
HC plots and the HA in the CLA fraction of the F + HC
plot belonged to Type Rp. On the other hand, the HAs in
the SIA and CLA fractions of the F plot and in the CLA
fraction of the F + LC plot were Type B with a medium
degree of humification. Larger amount of compost ap-
plication decreased remarkably the humification degree
of HA.
Since the sum of the percentage distribution of HA in
the SIA and CLA fractions of three plots was between
52.2 and 69.6% (Figure 2), these fractions affected
largely on the amount and type of the HAs in the whole
soils (Table 3 and Figure 4).
4. CONCLUSION
Continuous compost application to the field subjected
to long-term double cropping (ca. 30 years) affected the
quantitative and qualitative changes of humus depending
upon the application amount. The application at a high
level (F + HC plot) increased remarkably the amounts of
TH, HA and FA in the whole soil and many particle size
fractions, particularly, SIA fraction. On the other hand,
the degree of humification of HA decreased markedly by
the application. The changes at a low level (F + LC)
were limited. The SIA fraction merit close attention as an
important reservoir of soil organic C, including HA and FA.
5. ACKNOWLEDGEMENTS
The authors are grateful to the members of the Soil Fertility and
Conservation Division, Yamaguchi Prefecture Experimental Station,
Yamaguchi, Japan for supplying the soil samples.
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