The adsorption of copper by various soil materials and intact
soils was examined at equilibrium solution copper concentrations
within the range found in natural soil solutions. The distribution
of copper between solution and solid phases was measured by means
of labelling with radioactive copper-6 4. In the majority of soils
specific adsorption of copper by oxides and organic materials is
considered likely to control solution copper concentrations.
Although the adsorbed copper is strongly held, experiments have shown
that a large proportion of it remains isotopically exchangeable and
can still therefore be regarded as 'plant available'. Solution
concentrations of copper were relatively unaffected by both the
background concentration of major cations and by changes in pE within
the ionic strength and pH range found in normal agricultural soils.
The relative abundance of different forms of copper in pasture
and arable soils was compared by means of a fractionation scheme.
The principle difference found was that the pasture soils contained
higher concentrations of soluble copper complexes. A specific ion
electrode was used successfully to differentiate between ionic and
complexed copper in simple systems but suffered from interference
by other ions when used with natural soil solutions.
Soil samples, both with and without added copper were incubated
moist at a temperature of 20°G for a period of ten months. Duplicates were stored dry. Regular analysis showed that the amount of
copper extractable with EBTA decreased with time, especially in the
moist soils. Fixation of copper by soil oxides appeared to be
responsible. Additional changes were observed in both calcium
chloride extraetable and isotopically exchangeable copper which were
not reflected by changes in the amount of copper extracted by EDTA.
Plant uptake studies with red clover under glass demonstrated
that significant increases in plant copper content can be achieved
by adding copper to a variety of soils. The size of the response
varied considerably from soil to soil but no clear relationship was
observed between the copper content of the controls or the response
to copper additions and individual soil properties.
A second glasshouse trial, using barley, clover and ryegrass,
indicated that there appears to be no advantage in using Cu-EDTA
rather than CuSO₄ as a copper fertilizer. Clover responded to copper
additions more readily than ryegrass and both plants responded far
better when the copper fertilizer was mixed into the soil rather
than applied to the surface.
The existence of interactions between copper and other trace
elements was also demonstrated in pot trials. However, the nature
and importance of such interactions seems to depend on the particular
conditions under which an experiment is carried out.