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| As the name implies, only very small amounts of trace elements are needed
for healthy plant growth. Sufficient is normally available in the soil or in recycled soil
organic matter but this is not always so. For specific crops and particularly where the
soil is abnormally acid or alkaline, trace element deficiencies may occur. These can be
corrected by adding whichever nutrient the crop is short of. Leaf colour and size are
useful in detecting deficiencies. |
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| Most crops grow best between pH 6 and 7 (7 is neutral, below 7 is acid and
above 7 is alkaline). Adding lime will bring acid soils up to the ideal pH. However, there
is no practical field-scale treatment for soils which are too alkaline but the pH will
slowly reduce with time. |
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| Where only one trace element is required, the use of a trace element
‘cocktail’ i.e. a mixture of several trace elements, is to be discouraged and is
a waste of money. In addition, the particular trace element that is required may not be
supplied in adequate amounts by the ‘cocktail’. |
| Grass : Under Irish conditions, responses in pasture growth to trace
element application are rare. The importance of trace element levels in pastures
therefore, relates principally to animals, and not to plant requirement. The trace
elements which are associated directly with clinical deficiency in animals are cobalt,
copper, iodine, manganese, selenium and zinc. Molybdenum is important because at high
levels in the herbage it reduces the availability of copper for absorption. Soil analysis
is a better indicator of availability than herbage analysis for cobalt and iodine whereas
herbage analysis is more reliable for copper, manganese, seleniumand zinc. |
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| The soils giving rise to low cobalt pastures are typically those formed
from granite and sandstones but even soils rich in cobalt can support low cobalt pastures
if the total manganese content of the soil is also high. Hence, when determining the level
of cobalt in a soil it is also necessary to analyse for total manganese in order to know
whether cobalt can be applied to the soil or if it must be fed directly to the animals. If
the manganese is high (>600 mg 1-1) the cobalt should be fed directly. |
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| Soils derived from sandstones, granites and peat soils are likely to have
low Cu levels. The amount of Cu normally absorbed from food by the ruminant is less than
10%. Its absorption is also subject to interference by molybdenum, sulphur and to a lesser
extent iron and zinc. Another factor which can be involved is dietary soil. It is known
that grazing animals can ingest large amounts of soil which will reduce Cu availability.
Obviously the higher the stocking rate the greater the soil intake. |
| Soil Cu tests, although useful, are not capable of being used as
definitive diagnostic tests owing to the complexity of the problem of Cu availability and
utilisation by the ruminant. It is better to use herbage Cu levels together with Mo and S
values. |
| In heavy textured soils, Cu is not considered deficient unless the EDTA
soil extractable copper level falls below 1.0mg1-1, whereas in light textured soils it may
be low up to 2.5mg1-1. On peat soils a level of 5mg 1-1 maybe necessary. |
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