SOIL ORGANISMS - Soil Ecology - page 243

elements, carbon, hydrogen, oxygen and nitrogen comprise most of the biomass.
Sulphur, phosphorus, potassium, calcium, magnesium and occasionally silicon are other
important components and chloride and sodium, to a lesser extent. Their concentrations
are of the order of mg that of the minor or trace elements is of (Chapter I.3.1).
Plants absorb water and nutrients selectively and their mineral compositions are thus
different from those of the soil solutions from which they derive their nutrients. Each
plant species has its own specific composition in terms of its concentrations and balance
of nutrient elements (see, e.g., Callot et al., 1982).
Thus, it may be possible to define for plants the botanical equivalent of the animal
dietary regimes, listing the nutrients used and the amounts of each needed to sustain
growth. A further characterisation of nutrient resources may also be envisaged using
Tillman's (1982) system of classification for pairs of resources. Depending on the shape
of the nutritional isoclines observed, resources may be recognised as substitutable,
complementary, antagonistic or essential (see Chapter I, Figure I.30). However, the data
necessary to define such hypothetical nutritional regimes are still largely lacking.

3.2.1.2 Internal and external factors of nutrient assimilation.

Nutrient absorption by roots depends on:
(i) The availability of nutrients in solution in the surrounding soil;
(ii) The relative mobility of ions, and,
(iii) The capacity of the rooting system to absorb these elements.

Availability of nutrients in soil solutions
Callot et al., (1982) calculated that 200 kg of potassium per ha, ( i.e., about five times the
stock of exchangeable K in a current temperate brown soil, inceptisol) in solution would
only be in contact with 45 % of solid particles of an arable soil. As nutrient solutions are
generally much less concentrated and discontinuous, an adequate nutrient supply implies
appropriate levels of nutrient transport through diffusion or mass flow. Nutrient supply
to the plant actually depends on the mobility of ions and root density (Figure III.24) and
Bowen (1984) states that 'the limiting factor in uptake of ions moving to the root by
diffusion is not usually the absorption ability of the root but the movement of ions to
the root'. This is especially true for non-mobile ions such as phosphorus, whereas the
absorption of mobile ions such as nitrates may be limited in some circumstances by
the ability of the root to assimilate this element (Scholes et al., 1993).
Thus, all the factors which affect diffusion and mass flow such as texture, structure
and soil moisture will determine the accessibility of nutrients to roots. Drying especially
affects diffusion and massflow by increasing the tortuosity of the water film. At poten-
tials less than -0.2 MPa, soil water retreats to voids less than 1.5 in diameter which

are inaccessible to root hairs (Clarke and Barley, 1968).
In some cases, roots may dissolve specific nutrients (especially nitrogen) through
interactions with micro-organisms that have been triggered by the input of high-energy
exudates (see Section IV.3.2.1.1) (Elliott et al., 1979; Clarholm, 1983; Sallih and
Bottner, 1988; Billès et al., 1990).