Page 98 - Plant-Based Remediation Processes

5 Impact of Metal/Metalloid-Contaminated Areas on Plant Growth 87

Table 5.2 Optimal soil 1
Crop plant Bulk density (g mL )
density for some species of
crop plants (Grzebisz 1996) Potato 1.0–1.2
Sugar beet 1.1–1.2
Winter wheat, light soil 1.4–1.6
Winter wheat, heavy soil 1.2–1.4
Barley, light soil 1.3–1.6
Barley, heavy soil 1.3–1.5
Oats 1.2–1.5
Maize 1.3–1.4
Flax 1.2–1.4

Soil density determines another soil property known as porosity, which
expresses the total amount of all free spaces in a soil volume unit. In natural
conditions, these spaces are occupied by water (liquid phase) and air (gaseous
phase). The value of this trait in the surface layer of mineral soils can range from
30 to 75 %, but most frequently it assumes the value of 45–50 %. This feature is
very unpredictable during the year and it also depends on soil origin and texture.
Sometimes, minimal and maximal porosities are distinguished (Rzasa and
˛
Owczarzak 2004) and the interval between maximal and minimal porosity can be
very wide. In general, soils developed from loesses are characterised by low and
uniform (30–40 %) porosity values, while compact alluvial soils as well as
rendzinas and clayey formations are distinguished by high (40–50 %) porosities.
Boulder formations exhibit more uniform porosity intervals ranging from 35 to
40 % (Rzasa and Owczarzak 2004). Apart from general porosity, differential
˛
porosity is a very important soil characteristic expressing the proportions of pores
of specific diameters. Different pore size boundaries are assumed, but as a rule,
pores with the size of 8.5 μM are referred to as macropores, those with diameters
ranging from 8.5 to 0.2 μM as mesopores, and those with sizes less than 0.2 μMas
micropores. Optimal conditions for crop plant growth and development occur when
macropores are filled with soil air, while meso- and micropores are saturated with
water. Boundaries between liquid and gaseous phases are very labile and can
assume different values during the vegetation period, accelerating or slowing
down production of plant biomass.
Soil texture and porosity values exert a significant influence on the soil property
referred to as soil structure. It expresses the size, shape, and degree of aggregation
of the soil solid phase. For the growth and development of crop plants, aggregate
structures (first and foremost, spheroidal ones) are most favourable and those
having separate granular and coherent (massive) structures are much less advanta-
geous (Brady 1990). The development of aggregates is strongly affected by the
following factors:
• Wetting and drying
• Freezing and defreezing
• Physical action of roots and soil fauna