Page 99 - Plant-Based Remediation Processes

88 M. Mleczek et al.

• Impact of decomposing organic matter as well as mucus excreted by micro-
organisms and other living soil organisms
• Modifying impact of absorbed cations
• Soil cultivation
Quantitative and qualitative composition of the soil solid phase, through its
porosity and structure, exerts a decisive influence on soil water retention
capabilities. In this regard, the ability of soil to retain water available for plants,
i.e. water held in soil mesopores, plays an exceptional role. In the overwhelming
majority, it is capillary water whose quantities fluctuate between field water capac-
ity (pF 2.0–2.5) and the point of permanent plant wilting (pF 4.2). It occurs as soil
solution and moves from thicker to thinner films. The rate of plant root growth
depends on the amount of water available to plants. In general, this growth is very
fast, which allows plants good supply of water without greater participation of
capillary forces (Buckman and Brady 1969). With respect to soil chemical
properties, it is organic matter that exerts the strongest impact on plant growth
and development. Its content in the surface layers of mineral soils can range from
several decimal fractions to about 5 % (in soils in Poland, 2.0–2.5 %). Humus
compounds constitute an important storage house of nutrients, primarily of nitrogen
and phosphorus, which are liberated into the soil environment during the process of
mineralisation. In addition, as the main constituent of the sorptive complex, the
above-mentioned compounds retain many macro- and microelements which find
their way into the soil solution as a result of exchangeable sorption and are utilised
2+
3+
3+
2+
by plants. In the course of reactions with metal ions (Fe ,Al ,Ca , and Mg ),
humus compounds can form both simple and chelate complex compounds
(Stevenson 1985; Ulrich 1983). In the case of strongly acid soils, humus plays a
significant role in the detoxification of aluminium ions. Experiments conducted by
Bloom et al. (1979) (after Bednarek et al. 2004) demonstrated that the addition of
even small quantities of organic matter to a strongly acidified soil reduced the toxic
effect of aluminium. This was also corroborated by investigations carried out by
Mys ´ko ´w(1984), who reported that in soil containing 1 % humus, already 1 mg Al
100 g 1 soil resulted in a distinct yield reduction. In soils which contained about
5–6 % humus, a negative impact of aluminium was observed only at concentrations
of 15 mg Al 100 g 1 soil.
According to Zaujec (2007), there is a rectilinear dependence between soil humus
content and many physical soil properties, and, consequently, humus indirectly
creates better conditions for plant growth and development. The increase of soil
humus content by 0.1 % enhances water capacity by 0.5–0.6 gravimetric percentage,
1
sorptive capacity by 0.7 cmol(+) kg , and pore volume by about 1 %. Furthermore,
it exerts a significant impact on peptide bonds by reducing their toxicity and
accelerating sensitivity to biodegradation (Stevenson 1985). Papers were also
published emphasising a direct impact of humic compounds on soil microorganisms
and plants (Flaig 1975; Tołpa 1982). Certain fractions of these compounds can
penetrate into plant roots creating reduction–oxidation (redox) systems inside cells
and modifying their metabolism. Additionally, humus exerts a protective influence