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82 M. Mleczek et al.

found in soils in different forms, and different classifications can be found in the
literature on this subject. The most frequent forms include (Filipek 2002):

• Active forms—constituents are found as ions or chelated molecules in soil
solution
• Mobile forms—chemical elements as ions adsorbed by the soil sorptive complex
• Reserve forms—constituents as ions adsorbed in a non-exchangeable manner
and forming part of crystalline structures of soil minerals
There is a definite dynamic equilibrium between all these forms, and quantitative
and qualitative changes between individual chemical elements in the soil are shaped
by physical, chemical, and biological processes taking place in the environment.
Bearing in mind plant nutrient requirements, the following three groups of
constituents of plant mineral nutrition are distinguished (Grzebisz 1996):
• Building—carbon, hydrogen, and oxygen
• Macroelements—nitrogen, phosphorus, potassium, calcium, magnesium, and
sulphur
• Microelements—iron, manganese, zinc, copper, boron, and molybdenum

Plants absorb nutrients from the soil solution mainly in the form of cations or
anions. In addition, part of the constituents can be taken up in the form of chelates
(many microelements) as well as organic molecules (e.g. in the form of urea,
amino acids). A continuous exchange takes place between soil solution and the
soil sorptive complex made up of mineral and organic colloids. This exchange,
in soil science known as exchangeable sorption, refers primarily to cations, because
soil colloid particles are in the majority negatively charged. Therefore, high
concentrations of cations occur in the direct vicinity of the negatively charged
surface of colloids at very low (close to zero) anion concentrations. This area is
sometimes referred to as the Stern layer (Mengel and Kirkby 2001). Moving away
from the colloid surface, numbers of cations decline, whereas those of anions
increase slightly. This stratum, frequently called the Gouy-Chapman layer, is not
very thick (5–10 nm) and passes into the soil solution, in which quantities of cations
and anions are similar. A constant exchange also takes place between soil solution
ions and the Gouy-Chapman stratum, and therefore, constituents taken up by plants
from the soil solution are supplemented by diffused ions from the above-described
double Gouy-Chapman stratum. Due to the fact that ions have different valences,
the exchange occurs in a stoichiometric system, e.g. one magnesium cation is
replaced by two cations of potassium.
Approximately 20 chemical elements are essential for appropriate plant growth
and development. Some of them are absorbed in greater quantities and are called
macroelements, of which the most important include N, P, K, Ca, Mg, and S. Other
elements taken up by plants in small amounts are known as microelements (e.g. Fe,
Mn, Cu, B, Zn, Mo, and Co). A synthetic collation of the most important plant
nutritional components occurring in agroecosystem soils is presented in Table 5.1.
From the point of view of biomass production, soil nutrient resources are most
frequently regarded as total resources, in the majority inaccessible to plants; and