Relationship between soil water retention model parameters and structure stability

Abstract

Studying and modeling the effects of soil properties and management on soil structure, a basic property of soil health, is vital for the development of effective soil and water conservation practices. Contribution of soil intrinsic properties and extrinsic conditions on structure stability was inferred in quantitative terms from changes in the water retention curves at near saturation (low matric potential, 0-50 cm, macropores > 60 µm) that were obtained by the high energy moisture characteristic (HEMC) method. The S-shaped water retention curves were characterized by the modified van Genuchten model that provided: (i) the model parameters α and n, which represent the location of the inflection point and the steepness of the water retention curve, respectively; and (ii) the soil structure index, SI=VDP/MS, where VDP is the volume of drainable pores, and MS is the modal suction. Model parameters, claculated y soil-HEMC model, were related to soil properties and hence soil water retention properties were linked to measured field characteristics. Soil SI increased exponentially with increase in α and decrease in n, while the relationship between SI and α/n was linear. An improved description of the water retention and its link to pore- and apparent aggregate size distribution, by using the model parameters α and n, can assist in (i) the selection of management practices for obtaining the most suitable type of soil structure depending on the desired soil function, and (ii) apply model parameters in soil-crop models for predicting the effects of changes in soil structure on soil hydraulic properties, erosion potential and crop yield.

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International Congress on “Soil Sciences in International Years of Soils”
Relationship between soil water retention model parameters
and structure stability
Amrakh Mamedov1*, G.J.Levy 2, I.Ekberli3, C.Gülser 3, I.Gümüş4,and Ü.Çetin 4
1 Institute of Soil Science & Agrochemistry, and Institute of Botany, ANAS, Baku,
Azerbaijan
1 Institute of Soil, Water and Environmental Sciences, ARO, The Volcani center, Bet
Dagan, Israel
3Ondokuz Mayis University, Faculty of Agriculture, Department of Soil Science &
Plant Nutrition, Samsun, Turkey
4Selcuk University, Faculty of Agriculture, Department of Soil Science and Plant
Nutrition, Konya, Turkey
*Corresponding author: amrakh03@yahoo.com
Key words: aggregate stability, structure stability, pore size, water retention, stability
index.
Introduction
Soil structure is a basic property of soil fertility, quality and hence soil health. The
formation of soil aggregates and structure is the result of biotic and abiotic factors and
their interaction. It is important for understanding the influence of structural condition
on the rhizosphere water-nutrient regime and crop yield, and also surface runoff
generation and soil erosion. Thus, studying the effects of soil properties and
management practices on soil structure is vital for the development of effective soil
and water conservation, and predictive modeling tools in order to avoid risks of soil
deterioration. Tillage, soil compaction, crop rotation and amendment application can
alter pore size distribution (PSD), and subsequently affect physical and chemical
properties of soils, and nutrients availability. Furthermore, plant growth associated
with activities of soil biota interacts with environmental variables such as dry-wet and
freeze-thaw cycles to modify soil structure. The ability to study soil structure
dynamics and affecting mechanisms thereon are complicated by the (i) magnitude of
temporal variability which in itself is affected considerably by the spatial location and
growing season, (ii) effects of management practices, and (iii) difficulties involved in
relating results from laboratory measurements to real field behavior (Kay and Angers,
2002; Strudley et al, 2008; Mamedov and Levy, 2013).
The difficulty to quantify the impact of soil properties and conditions, coupled with
management practices, on soil structure stability, either by empirical or by conceptual
models, has been widely recognized. Characterization of soil aggregate stability has
commonly been used to portray structure stability, although aggregates are not
necessarily a suitable proxy of soil structure. This complexity is also associated with
a variety of physical and physicochemical mechanisms involved in soil aggregates
breakdown by water (Levy and Mamedov, 2013). Several aggregate stability
methods, utilizing diverse primary breakdown mechanisms (e.g. wet sieving, drop

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test, application of ultrasonic energy, etc.), are used for establishing an index of soil
structure, which makes comparison between treatments difficult. A recent method, the
modified high energy moisture characteristic (HEMC) method (Pierson and Mulla,
1989; Levy and Mamedov, 2002) is sensitive and capable of detecting even small
changes in aggregate and structure stability of a range of soils from arid and humid
zones (e.g. review paper by Mamedov and Levy, 2013).
Resistance of soil structure to changes induced by management practices largely
depends on soil genesis and properties (Mamedov et al., 2010). Structure and
aggregate stability can be inferred from changes in the soil water retention curve even
at the low end of the matric potential (e.g., as employed by the HEMC method).
Changes in the model parameters used to describe the water retention curve are
considered to be related to changes in the PSD, and therefore in aggregate (particle)
size distribution; thus, they may characterize the contribution of aggregates size to
soil structure condition. If model parameters can be related to measured soil
properties, then soil water retention curves can be derived also from easily measured
field data (Lipiec et al., 2007; Mamedov and Levy, 2013). The objectives of the
current study were to (i) characterize structure stability indices of semi-arid soils
using the HEMC method, and (ii) examine the relationship between these soil
structure stability indices and the water retention model parameters.
Materials and Methods
An arry of samples from semi-arid cultivated soil (Azerbaijan, Israel, Turkey, USA)
varying in type and exyrinsic conditions were used: (i) three soil varying in texture
and treated with two biosolid amanedments (composted manure and sweage sludge);
(ii) long term cutivated soils (~100 samples) varying in texture from loamy sand to
clay; (iii) clay soil treated with fresh and composted poultry litter, and zeolite under
corn production, and (iv) loam soil (rhizosphere and bulk) used under various wheat
types. Soil water retention, modified van Genuchten model parameters and structure
stability indices were determined using a modified version of the HEMC method and
soil-HEMC model (Pierson and Mulla, 1989; Levy and Mamedov, 2002; Mamedov
and Levy, 2013). In this method, 15 g macroaggregates (0.5-1 mm) are placed in a
funnel with a fritted disc (pore size 20-40 µm) which is wetted from the bottom in a
controlled manner (slow ~2 or fast ~100 mm h-1), with a peristaltic pump, and then a
water retention curve at high energies of matric potential (0 to 50 cm H2O),
corresponding to drainable pores (> 60 µm), is performed using small steps (1-2 cm).
Soil structure stability is expressed in terms of a structural index (SI) defined as the
ratio of volume of drainable pores (VDP) to modal suction (MS). Soil-HEMC model
(Mamedov and Levy, 2013), which enables an accurate fit of the water retention
curves (ψ, 0 to 50 cm) for a wide variety of soils (R2>0.99), was used to calculate
structural indices (VDP, MS) and model parameters (α and n) by the following
equations (Pierson and Mulla, 1989):
 
CBψAψψ) (α1 )θ(θθ θ2
1)(1/n
n
rsr  
[1]
   
B2Aψ)ψ) (αψ(1n/ψ) 1)(αn1(ψ) (α1)θ(θdψdθnn
1)(1/n
n
rs  
[2]

International Congress on “Soil Sciences in International Years of Soils”
where, θr and θs are the residual and saturated water content, respectively; α (cm-1)
and n represent the location of the inflection point and the steepness of the S-shaped
water retention curve; A, B and C are the coefficients.
Results and Discussion
Changes in soil structure following aggregate breakdown by wetting, generally,
results in the shift of the wetting curve to the left resulting from formation of a larger
number of aggregates or particles of smaller sizes (leading to smaller inter-aggregate
pores) than the original ones. Semi-arid soils, known to have weak aggregates, were
found to be sensitive to aggregate breakdown by fast wetting (i.e., slaking); with the
effect being more pronounced in the soil with low clay content (Fig.1).
0
10
20
30
40
50
60
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Water content, θ (g/g)
Matric potential, - ψ (cm)
Loam WR 2 mm/h
Loam WR 100 mm/h
Clay WR 2 mm/h
Clay WR 100 mm/h
0
10
20
30
40
50
60
0.2 0.3 0.4 0.5 0.6 0.7 0.8
Water content, θ (g/g)
Matric potential, - ψ (cm)
Control
Manure compost
Sewage sludge
Fig. 1. Soil water retentiaon as affected by
texture and wetting rate (WR)
Fig. 2. Loam soil water retentiaon as affected
by composted manure and sewage sludge
application
0
10
20
30
40
50
60
0.4 0.5 0.6 0.7 0.8 0.9
Water content, θ (g/g)
Matric potential, - ψ (cm)
Control
Bulk soil
Rhizosphere soil
0
10
20
30
40
50
60
0.2 0.3 0.4 0.5 0.6 0.7 0.8
Water content, θ (g/g)
Matric potential, - ψ (cm)
Control
Zeolite
Poultry litter + Zeolite
Fig. 3. Loam soil (rhizosphere and bulk
soil) water retentiaon as affected by
cropping
Fig. 4. Clay soil water retentiaon as
affected by poultry litter and zeolite
treatments
Differences in the water retention curves between the less stable and stable soil
aggregates (originating from differences in soil type or extrinsic conditions) were
mostly in the matric potential range of 0 to -12 cm, and smaller in the range of 12 to -

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50 cm corresponding to differences in macro-, meso-, and micropores (>250; 60-250
m) (Figs. 1-4). Aggregates’ breakdown is reflected by a decrease in SI and α or α/n
and by an increase in n. The SI of soil increased exponentially with increase in water
retention model parameters α and a decrease in n (Figs. 5 and 6). Furthermore the
relationship between the SI and α/n could be considered as linear, yet of different
properies (Figs. 7-9). In the coarser textured loam, aggregate resistance to slaking was
attained mainly by the presence of the coarser fraction of organic matter (e.g., plant
roots and fungal hyphae), whereas in the clay soil it was obtained by the high clay
content. The rhizosphere soil is directly influenced by the root, plant residues,
root secretions and symbiotic associated microorganisms (e.g. mycorrhizal fungi).
Poultry litter and zeolite application increased or enhanced soil water retention;
treating soil with composted manure and sewage sludge coated bridge between soil
clay platelets, and analogous to the effect of hydrophobic humic substances, they
improved structure stability and aggregate resistance to slaking by water (Mamedov
and Levy, 2013).
R2 = 0.79
(P<0.05)
0.00
0.01
0.02
0.03
0.04
0.02 0.04 0.06 0.08 0.10
a (1/cm)
SI (1/cm)
Clay
Loam
Sandy
R2 = 0.61
(P<0.05)
0.00
0.01
0.02
0.03
0.04
510 15 20 25
n
SI (1/cm)
Clay
Loam
Sandy
Fig. 5. SI as a function of α for soils treated with
composted manure and sewage sludge
Fig. 6. SI as a function of n for soils
treated with composted manure and
sewage sludge
R2 = 0.80
(P<0.05)
0.00
0.01
0.02
0.03
0.04
0 0.005 0.01 0.015
a/n (1/cm)
SI (1/cm)
Clay
Loam
Sandy
R2 = 0.92
P<0.05
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.000 0.005 0.010 0.015
a/n (1/cm)
SI (1/cm)
Fig. 7. SI as a function of α/n for soils treated
with composted manure and sewage sludge
Fig. 8. SI as a function of α/n for long
term cultivated soils varying in texture
from sandy to clay

International Congress on “Soil Sciences in International Years of Soils”
R2 = 0.7
(P<0.05)
R2 = 0.87
(P<0.05)
0.00
0.01
0.02
0.03
0 0.005 0.01
a/n (1/cm)
SI (1/cm)
bulk and rhizosphere
poultry litter and zeolite
Linear (poultry litter
and zeolite)
Fig. 9. SI as a function of α/n for (▲) loam
used under wheat varieties () clay treated
with poultry litter and zeolite
Conclusion
The paper presents results obtained from studies, in which the HEMC method
was employed to characterize soil structure stability in terms of changes in macro
pore size distribution obtained from water retention curves at near saturation (ψ, 0 to -
50 cm). Research data reporting a wide range of changes in PSD, and structure
stability indices of semiarid soils widely varying in intrinsic properties and
management histories were used and relationship between SI and model parameters α
and n were established. The results indicate that pore and aggregate size distribution
of cultivated semi-arid soils can be strongly influenced by agricultural management,
but the resilience of the structure largely depends on soil type and properties. It is
postulated that description of the water retention by soil-HEMC model and linking
model parameters to soil structural index and thus pore- and aggregate size
distribution, may help to select proper management practices for obtaining the most
suitable type of aggregation depending on the desired soil function or soil type.
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