Agriculture Reference
In-Depth Information
In field conditions‚ the elements Cu‚ Ni and Zn are the micronutrients most commonly
implicated in toxicity. In glasshouse experiments‚ yields of maize (
Zea mays
) grown in
certain soils formed from ultramafic parent materials (oxisols) in New Caledonia were
negatively correlated with shoot Ni concentrations (
r
= 0.78). In addition‚ several crop
plants grown on lowland soils under reducing conditions had higher Ni concentrations
than those in well drained control soils (L'Huillier and Edighoffer‚ 1996).
In addition to growth impairment‚ plants exposed to either extreme may exhibit such
characteristic symptoms as leaf deformation‚ discolouration or necrosis. Manganese is an
example of an essential element that is deficient in some soils although‚ under reducing
conditions‚ it may also be taken up in sufficient amounts to produce concentrations toxic
to certain plants. In manganese-deficient plants‚ a chlorosis or yellowing occurs between
the veins of the leaves while toxic concentrations lead to an uneven distribution of
chlorophyll in the leaves and with brown spots mostly comprising precipitated
manganese oxides (Mengel and Kirkby‚ 1987).
Because of the basic biochemical similarities of all living organisms‚ most elements
required for the nutrition of higher plants are also needed by the lower plants and the
animals. There are‚ however‚ some exceptions to this. Selenium is required by animals
but not by plants and‚ conversely‚ boron is essential to the nutrition of higher plants but
is not known to be required by animals. Cobalt is needed by animals‚ the bacteria and
actinomycetes involved in nitrogen fixation and some fungi but perhaps not by higher
plants. Nutrient supply is rarely optimal in nature and one or more of the nutrients listed
above probably limit the growth and productivity of organisms in most situations.
Positive correlations occur between the concentrations of nutrient elements at the levels
of both the tissues of higher plants and the microbial biomass. Garten (1976) has shown
that‚ in terrestrial plants at least‚ foliar concentrations of nitrogen and phosphorus are
highly correlated‚ as are those of magnesium and calcium. Singh
et al.
(1991) report
correlations exceeding 0.85 among carbon‚ nitrogen and phosphorus concentrations in
the microbial biomass of savanna soils. For the above reasons‚ correlations may also be
expected to occur between a wider range of essential elements.
