Agriculture Reference
In-Depth Information
elements may interact to inhibit the uptake of others (antagonism)‚ or to promote it
(synergism). Beneficial elements are mineral elements which will stimulate growth but
are not essential or are essential only for certain plant species or under certain conditions.
A number of elements taken up by the roots and subsequently translocated throughout
the plant serve no known structural or biochemical function within the plant. Among these
are included such elements as selenium which are required by animals and constitute the
major source of supply to the higher members of biological communities. These and
other elements may also be taken up‚ even though some are actively inimical to the health
of living organisms‚ either through their radioactivity or because of their capacity to
inhibit natural processes at the functional level of the cell‚ organ or the whole organism.
A range of elements may also be absorbed through the leaf surfaces. Elements deposited
on these surfaces diffuse through the cuticle and may be actively absorbed by the cells
and transported elsewhere in the plant. This capability is used to alleviate deficiencies of
certain trace and other elements in some tree and other crops; however‚ it also provides
a pathway for pollutants such as lead to enter the leaves.
In an ecological sense‚ the nutrition of plants (and the animals dependent upon them)
is concerned with rather more than the events taking place at the surfaces where nutrients
are absorbed. Three separate and complex stages occur‚ namely‚ release of the nutrients
from their sources‚ transport to the roots and uptake by the plant (Chapin‚ 1988).
Each step potentially involves interactions between the inorganic components of the soil
and organic processes or products. Also‚ plants are not passive absorbers of nutrient and
other elements and may actively modify their root distributions in response to those of
certain elements or alter the environments around their roots in ways that modify element
uptake. Further‚ most higher plants enter into symbiotic associations with representatives of
one or more groups of micro-organisms that aid or effect nutrient uptake (Chapter IV.3.1.2.2).
Plants form the productive nutritional base on which animals ultimately depend‚
whatever their trophic level. Through its effects on plant productivity‚ the uptake of
nutrient and other elements by plants is of prime importance since it eventually affects
the entire biological community‚ including the higher order predators and the saprovores.
3.1.1
CLASSIFICATION OF THE NUTRIENT ELEMENTS
In addition to the nutrient and non-essential elements‚ the major components of
higher plant tissues are carbon and oxygen‚ each of which comprise
ca.
45 % on a dry
weight basis. Table I.14 lists the elements known to be required by higher plants and
animals‚ the ionic forms in which they are most commonly taken up from the soil by
plants together with some 'typical' concentrations in the tissues of unspecialised plants.
In addition‚ for the elements needed by plants‚ the functional groups that they belong to
(see below) are also indicated (Mengel and Kirkby‚ 1987). The concentrations presented
in Table I.14 are indicative only and macronutrient tissue concentrations required
for highest production in crop plants are generally greater than those presented
(Marschner‚ 1995). Table I.15 presents the ranges of concentrations for six macronutrient
elements found in plant tissues sampled from a variety of natural environments.
