C - Lublin

w/w ratios, moistened with distilled water to 25% w/w and subjected to cyclic
changes of temperature. One cycle consisted of one-week treatment at -20°C
following by one-week treatment at 30°C. The material for investigations was
taken after 6 th , 12 th and 18 th week.
Water retention vs. moisture dependencies (pF curves) were measured using
laboratory set LAB 012 produced by Soil Moisture Equipment in a range of soil
water potential from 0 Jm -3 (pF 0) to 1,5⋅10 6 Jm -3 (pF 4,2). Prior to the
measurements the studied samples were placed in stainless steel cylinders (1.8 cm
radius and 1 cm height) and subjected to several cycles of 48h wetting (capillary
rise) and 48h (40°C) drying to stabilize the structure i.e. until differences in sample
bulk densities were insignificant. Changes in granulometric composition and bulk
density of the soil and mixtures were measured, as well.
From the above dependencies, the quantity of gravitational water, plant
available water and non-accessible water were estimated. Quantity of large pores
(ϕ>18,5 µm), medium pores (18,5µm>ϕ>0,2 µm) and small pores (ϕ>0,2 µm)
were estimated also.
RESULTS AND DISCUSSION
Figure 1 shows dependencies of moisture on water potential (pF-curves) for
initial samples and after cyclic changes in temperature.
First cycle of freezing-defreezing (6 weeks) lead to significant increase in
water retention for all samples. During next cycles the water retention decreases
consecutively reaching after third cycle (18 weeks) lower values than the initial
samples. The above phenomena suggest changes of soil structure during freezingthawing
periods. Initially the compact structure of artificially prepared soil and
mixtures is obtained. This structure markedly loosened after first 3 cycles of
freezing and thawing indicating that most probably larger soil aggregates were
formed. These aggregates may brake during further cycles and the soil structure
becomes more stabilized.
After third cycle when the structure seems to be most stabilized, the water
retention is higher in all mixtures than in the soil itself. This is due to higher water
capacity at low pF values i.e. in the range of coarser pores. However, in higher pF
range (smaller pores), the water retention seems to be smaller in the mixtures than
in the soil. This may be connected with an increase in silt and loam fractions,
resulting most probably from dispersion of coarser rubble fractions by water during
cyclic changes of temperature.
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