soil - Lublin

swelling and shrinking processes result in fractures in which the value of the angle
of internal friction determines the deviation from a 90° angle.
In newly formed aggregates, the number of contact points depend on the range
of moisture potential and on the distribution of particle sizes as well as on their
mobility (i.e., state of dispersion, flocculation, and cementation). Soil shrinkage,
including crack formation, increases bulk density of aggregates. The increase in
bulk density with the initial watering and drying of the soil permits the aggregates
to withstand structural collapse. The increase of the strength of single aggregates is
further enhanced by a more pronounced particle rearrangement, if the soil is nearly
saturated with water increasing the mobility of clay particles due to dispersion and
greater menisci forces of water. (Horn and Dexter, 1989). Drying causes enhanced
cohesion by capillary forces. Consequently, in order to carry the same soil load the
bulk density of aggregates and thus the number of contact points decreases. With
increasing intensity of drying of the moulded soil, its ability to perform reversible
volume changes decreases. In wetter soils, the smaller proportion of residual to
normal shrinkage (i.e., the greater reversibility during swelling) causes more intensive
particle mobility and rearrangement in order to reach a state of minimum free
energy. Although swelling may lead to partial expansion of contracted particles
following rewetting of aggregated soils, a complete disaggregation is not possible
if there is no additional input of kinetic energy, as has been demonstrated also by
the puddling or kneading of rice soils (Horn, 1976). Thus, aggregate strength will
depend on (i) capillary forces, (ii) intensity of shrinkage (normal/residual), (iii)
number of swelling and shrinkage cycles, (iv) mineral particle mobility (i.e., rearrangement
of particles to reach the status of lowest free energy), and (v) bonding
energy between particles and/or between aggregates, or in the bulk soil.
HYDRAULIC ASPECTS
A. Water Retention Curve
Aggregation due to swelling and shrinking is affected by hydraulic properties
of the soil. With an increase in the number of drying cycles the total porosity first
decreases. Later it may increase again (Horn and Dexter, 1989). The volume of fine
pores (i.e., the volumetric water content at pF > 4,2) is enhanced by decreasing drying
intensity. In addition, the amount of water available to plants (i.e., water content
at pF 1,8-4.2) is reduced with more intensive soil drying. Only at more negative
water potentials is the air entry value exceeded depending how wet the soil had
been kept. The latter effect is determined by the correspondingly steep slope of the
pF/water content curve at pF 1.8.
B. Darcy Law: Hydraulic Conductivity
Given a laminar flow and a homogeneous pore system, the water flux in soils
can be described and quantified by the Darcian law. Generally, the values of the
hydraulic gradient vary only by half an order of magnitude depending on water
potential, grain, and pore size distribution. If the soil-plant interaction is also taken
into account, values of hydraulic gradients of up to 9 kPa ⋅ m -1 can be calculated
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