ISSN 1330-7142
UDK = 631.422
SOIL BULK DENSITY AS RELATED TO SOIL PARTICLE SIZE DISTRIBUTION
AND ORGANIC MATTER CONTENT
T. Aşkin (1), N. Özdemir (2)
Original scientific paper
Izvorni znanstveni članak
SUMMARY
Soil bulk density is a dynamic property that varies with the soil structural conditions. The relationships
between some soil physical and chemical properties such as, clay content (C), silt content (Si), sand content
(S), very fine sand content (Vfs) and organic matter content (OMC) with soil bulk density (ρb) were studied
using path analysis on 77 surface soil samples (0-20 cm). Soil bulk density showed positive relationships with
S and Vfs and negative relationships with Si, C and OMC. It was determined that the direct effects of some
soil properties on ρb were in the following order; S>C>Si>OMC>Vfs. On the other hand, the indirect effects
of soil particle size distribution varied among soil bulk densities. The indirect effects of the soil particle size
distribution generally occured through sand content. Sand content was the most effective soil property that
affected bulk density in soils.
Key-words: soil bulk density, clay content, silt content, sand content, organic matter content
INTRODUCTION
Soil bulk density is defined as the ratio of oven-dried mass weight to its bulk volume depends on the
soil particles densities such as sand, silt, clay and organic matter and their packing arrangement. Bulk
density values are required for converting gravimetric soil water content to volumetric and to calculate
soil porosity which is the amount pore space in the soil (Blake and Hartge, 1986). Researchers often
need a bulk density value to use in models, characterize field conditions, or convert to volumetric
measurements (Reinsch and Grossman, 1995). Soil bulk density is a basic soil property influenced by
some soil physical and chemical properties. Bulk density is a dynamic property that varies with the
structural condition of the soil. This condition can be altered by cultivation, trampling by animals,
agricultural machinery, weather, i.e. raindrop impact (Arshad et al., 1996). Knowledge of soil bulk
density is essential for soil management, and information on the soil bulk density of soils is important
in soil compaction and structure degradation as well as in the planning of modern farming techniques.
If both, bulk density and particle density are known, the total porosity can be calculated by using these
values (Hillel, 1982). Soil bulk density should be used as an indicator of soil quality parameter. Akgül
and Özdemir (1996) studies on relationships between soil bulk density and some soil properties
explained that these constants can be estimated by means of developed regression models. A unit
increases in organic matter and clay content caused a relatively larger decrease in soil bulk density. A
soil system can be thought as a network of soil properties. Path analysis may be used to investigate the
relationships among these soil properties. The path diagram gives a picture of network of relations
among the characters, as quantitative evaluation is possible from the data (Wright, 1968).
The objective of this study was to determine relationships between soil particle size distribution and
organic matter content and soil bulk density by using path analysis.
___________________________
(1) Ph.D Tayfun Aşkin, Associate Professor - Karadeniz Technical University, Faculty of Agriculture,
Department of Soil Science, 52200, Ordul, Turkey; (2) Ph.D. Natullah Özdemir, Full Professor – Faculty of
Agriculture, Department of Soil Science, 55139, Samsun, Turkey
MATERIAL AND METHODS
Samples of seventy-seven surface soils (0-20 cm depth) were taken from grassland in Samsun district
in Turkey. This area has a brown forestry soil. Annual mean of precipitation is 670.4 mm and mean
temperature is 14.2 0C (Anonymous, 2002). Bulk soil samples were air dried and then crushed to pass
through a 2 mm sieve.
Soil organic matter content was measured by a modified Walkley-Black method (Nelson and
Sommers, 1982); soil particle size distribution was determined by the hydrometer method (Gee and
Bauder, 1979); lime content was measured by Scheibler Calcimeter (Soil Survey Staff, 1993); soil pH
was measured by using a 1:2.5 (w/v) soil-water ratio by pH-meter with glass electrode (Black, 1965).
Bulk density was determined by means of the clod method (Blake and Hartge, 1986).
The soil bulk density was selected as dependent variables to determine statistical relationships
between soil particle size distribution and organic matter content (C, Si, S, Vfs, and OMC) and soil
bulk density. Also, direct and indirect effects of the variables were determined with path analysis
(Wright, 1968), using TARIST computer package program.
RESULTS AND DISCUSSION
Soil Properties
Some descriptive statistical results for some soil physical and chemical properties are given in Table 1.
Table 1. Descriptive statistics for some soil physical and chemical properties of soil samples (n=77)
Tablica 1. Deskriptivna statistika za neka fizikalna i kemijskih svojstva uzoraka tla (n=77)
Soil properties – Svojstva tla
Mean
Prosjek
25.7
8.9
27.4
46.9
7.1
Min.
Max.
Sand – Pjeskovito (S), %
14.8
45.0
Very fine sand – Vrlo fino pjeskovito (Vfs), %
2.2
12.4
Silt – Muljevito (Si), %
22.1
32.7
Clay – Glinasto (C), %
32.0
55.9
pH (1:2.5 soil: water suspension – tlo: vodena suspenzija)
6.1
7.7
Electrical conductivity – Električna provodljivost (EC),
0.21
0.09
0.46
dS.m-1
Lime content – Sadržaj vapnenca (LC), %
2.35
0.00
15.94
Organic matter content – Sadržaj organske tvari (OMC),
4.65
2.67
7.27
%
1.57
1.35
1.70
Bulk density - Volumna gustoća (ρb), g cm-3
Sd; standard deviation – standardna devijacija, Se = standard error – standardna greška
Sd
Se
5.59
1.60
1.96
4.69
0.44
0.640
0.180
0.220
0.530
0.050
0.06
0.007
3.59
0.410
0.86
0.100
0.07
0.009
The results can be summarized as; soil samples have mostly fine in texture, neutral in pH, high in
organic matter (average of 4.65 %), low in lime content (average of 2.35 %), and alkaline problem free
(ESP<15 %) (Soil Survey Staff, 1993) (Table 1).
Relationships Between Some Soil Properties and Soil Bulk Density
Correlation coefficients between soil particle size distribution and soil organic matter content and the
soil bulk density are given with direct and indirect effects of the variables on the bulk density in Table
2.
According to Table 2, sand content showed significant positive relations with the soil bulk density at
p<0.05. Direct effect of sand content on soil bulk density was found to be higher than that of the other
soil properties. Also, the soil properties had higher indirect effects through sand content on the soil
bulk density. It indicates that sand content was the most important soil property that affected bulk
density in soils. Indirect effects of sand content through the other soil properties on soil bulk density
were as follows; Vfs (0.05 %), Si (10.72 %), C (38.78 %) and OMC (0.24 %). On the other hand the
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direct effect of clay content on bulk density (43.82 %) was higher than the other fractions. The clay
content and organic matter content gave significant negative relations with soil bulk density. Soil bulk
density decreases with increasing organic carbon concentration (Bauer and Black, 1992). Also Gupta
and Larson (1979), proposed a model for predicting the dry bulk density of soil. The particle size
distribution and organic matter content are used in the model. Organic matter content showed the very
significant negative correlation (r=-0.59**) with the soil bulk density (Table 2). The clay content had
higher direct effects on soil bulk density after sand content. The silt content was adversely related with
soil bulk density as non significantly. Also, the silt content had higher indirect effects through sand
content (49.44 %) on the soil bulk density (Table 2). Behavior of particle size distribution of Si in soils
might be the similar to clay fractions of soil texture. Therefore increasing of silt content may decrease
soil bulk density. Thus, silt content did not give highly significant correlate with soil bulk density
according to clay content. Statistically non significant relation was determined between the very fine
sand content of soil and soil bulk density. Wagner et al. (1994) suggested using soil texture parameters
along with organic carbon content values to estimate soil bulk density.
Table 2. Path analysis results on soil bulk density and relationships with some soil properties
Tablica 2. Path analiza rezultata volumne gustoće tla i odnos između nekih svojstava tla
Soil properties
Svojstva tla
S
Vfs
Si
C
OMC
**
p<0.01; *p<0.05
Direct
effect
r
Direktni
učinak, %
0.25*
50.21
0.04
6.42
-0.16
29.30
-0.23*
43.82
-0.59**
23.14
Indirect effect - Indirektni učinak, %
S
Vfs
Si
C
OMC
39.53
49.44
50.41
37.18
0.05
0.27
0.01
1.64
10.72
43.08
5.68
27.98
38.78
3.18
20.17
10.06
0.24
7.79
0.82
0.08
-
CONCLUSION
Soil bulk density gave the significant positive correlation with S and negative correlation with C at
p<0.05. Also, bulk density achieved the significant negative correlation with OM at p<0.05. Sand and
clay contents showed higher direct effects on soil bulk density. Indirect effect of the soil properties
were generally occured through sand and clay content. Sand content was found to be the most
effective soil fraction that influenced soil bulk density. It was determined that the direct effects of
some soil particle size distribution and organic matter content on ρb were in the following order;
S>C>Si>OMC>Vfs. It was suggested that the sand fraction of soils should also be assessed in soil
management.
REFERENCES
1. Akgül, M., Özdemir, N. (1996): Regression models for predicting bulk density form measured soil
properties. Tr. J. Of Agriculture and Forestry, 20:407-413.
2. Anonymous (2002): Samsun Meteorology Bulletin Reports. Samsun, Turkey.
3. Arshad, M.A., Lowery, B., Grossman, B. (1996): Physical tests for monitoring soil quality. In:
J.W. Doran and A.J. Jones(eds.) Methods for assessing soil quality, Soil Sci. Soc. Am. Spec. Publ.
49:123-142, SSSA, Madison, WI, USA.
4. Bauer, A., Black, A.L.(1992): Organic carbon effects on available water capacity of three soil
textural groups. Soil Sci. Soc. Am. J., 56:248-254.
5. Black, C.A. (1965): Methods of Soil Analysis. Part 1, American Society of Agronomy, No 9.
6. Blake, G.R., Hartge, K.H. (1986): Bulk Density. Methods of Soil Analysis, Part 1, Soil Sci. Soc.
Am., 363-376, Madison, WI, USA.
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7. Gee, G.W., Bauder, J.W. (1979): Particle size analysis by hydrometer: A simplified method for
routine textural analysis and a sensitivity test of measured parameters. Soil Sci. Soc. Am. J.,
43:1004-1007.
8. Gupta, S.C., Larson, W.E. (1979): A model for predicting packing density of soils using particle
size distribution. Soil Sci. Soc. Am. J. 43:758-764.
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London.
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11. Reinsch, T.G., Grossman, R.B. (1995): A method to predict bulk density of tilled Ap horizons.
Soil & Tillage Research, 34:95-104.
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POVEZANOST VOLUMNE GUSTOĆE TLA S DISTRIBUCIJOM VELIČINE ČESTICA TLA
I SADRŽAJEM ORGANSKE TVARI
SAŽETAK
Volumna gustoća tla je dinamičko svojstvo koje varira sa stanjem strukture tla. U radu je istražena
povezanost nekih fizikalnih i kemijskih svojstava tla, kao što su sadržaji gline (C), mulja (Si), pijeska (S), vrlo
finog pijeska (Vfs) i organske tvari (OMC), s volumnom gustoćom tla (ρb) pomoću path analize na 77
površinskih uzoraka tla (0-20 cm). Volumna gustoća tla pokazala je da postoji pozitivna korelacija sa S i Vfs i
negativna korelacija sa Si, C i OMC. Utvrđeno je da su direktni učinci nekih svojstava tla na ρb bili sljedeći:
S>C>Si>OMC>Vfs. Nasuprot tome, indirektni učinci distribucije veličine čestica tla varirali su kod volumne
gustoće tla. Indirektni učinci distribucije veličine čestica uglavnom su se javljali ovisno o sadržaju pijeska .
Sadržaj pijeska bilo je najučinkovitije svojstvo tla koje je utjecalo na volumnu gustoću tala.
Ključne riječi: gustoća tla, sadržaj gline, sadržaj mulja, sadržaj pijeska, sadržaj organske tvari
(Received on 10 April 2003; accepted on 1 September 2003 - Primljeno 10. travnja 2003.; prihvaćeno
1. rujna 2003.)
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