Page 95 - Plant-Based Remediation Processes

84 M. Mleczek et al.

available resources, referring to the amounts of macro- and microelements which
can be absorbed from the soil solution and, partially, also from the sorptive complex
or some poorly soluble salts. The content of available forms in soils is determined
most frequently using chemical laboratory methods. They are based on the prepa-
ration of extracts with the assistance of special, most often buffer, solutions and
determination of a given component using spectrometric, colorimetric, or other
methods. Next, the content of the specific macro- or microelement is compared with
the boundary numbers which allow determination of the appropriate class of soil
nutrient availability. Apart from many advantages, the basic disadvantage of these
methods is lack of possibility of selection of universal extraction parameters (kind
of solution, suitable proportions, extraction time, etc.) in different soils. Moreover,
plants differ with respect to the structure of their root systems as well as different
capabilities of nutrient absorption during various phases of development. In addi-
tion, also the prevailing soil conditions (moisture content, temperature, reaction,
etc.) may affect this process.
For many centuries, for plant production purposes man used almost exclusively
natural soil resources only partially supplemented by replenishment of the uptaken
constituents in the form of natural and organic fertilisers. Hence, a definite state of
equilibrium was maintained in soil resulting, on the one hand, from low levels of
yields and, on the other, from a small mass of the components taking part in a
traditional farming cycle (field–byre–field). Steady population increase made it
necessary to intensify plant production through the application of new
agrotechnical systems. This upset natural soil resources and made it unavoidable
to introduce their supplementation by mineral fertilisation. These changes, in many
instances, resulted in the occurrence of shortages or excess of soil nutrient
constituents following the intensification of agricultural activities. In many places,
the above problems were additionally aggravated by anthropogenisation of the
environment (industrialisation, emissions of metal-bearing dusts, etc.). Moreover,
plants can absorb some constituents in excessive quantities. This refers, in particu-
lar, to biogens such as nitrogen and sulphur. This often leads to the accumulation in
plant tissues of amounts of these elements toxic to humans. This refers also to many
microelements (boron, copper, zinc, manganese, molybdenum, and cobalt) which
can be absorbed by plants in excessive quantities following abundant fertilisation
by easily soluble compounds of the above-mentioned chemical elements.
Furthermore, it should be remembered that different compounds found in soils
interact with one another, leading to the activation of some chemical elements and
enhancement of their availability to plants. The phenomenon is known as syner-
gism. Reverse phenomena, namely antagonisms, causing changes of easily avail-
able forms into ones which become unavailable to plants, also take place.
Fertilisation with magnesium provides a good example of the above-mentioned
phenomena as the increase of its content in soil increases zinc and manganese
uptake but, simultaneously, limits absorption of potassium, calcium, and copper
(Czuba 1996). Calcium is a cation exhibiting strongly antagonistic action in
relation to other cations as well as to some anions. Liming, by changing soil
reaction, abolishes a harmful effect of high quantities of aluminium and