3.8 Trace Elements for Pastures

Role in plant

There are seven trace elements required for pasture growth. The most common deficiencies in pastures are copper, zinc, manganese and molybdenum. Plants use trace elements in very small amounts in their enzyme systems for plant growth. Trace elements are essential for plant growth, but because only small amounts are required deficiency symptoms often only occur sporadically.

Copper

Highly immobile in the soil.
Up to 98% of soil copper can be unavailable.
Most copper is bound in complexes in the soil. Copper "tie up" is highest on soils with high organic matter eg peats.
Copper deficiency is most commonly seen in sandy soils with low organic matter levels.
Copper availability is reduced in light textured alkaline soils eg coastal calcareous sands.
Copper application to the soil has a long residual benefit.
Copper symptoms are rarely seen in vegetative growth, but first indications may be low seed production.
In livestock, high animal intake of iron, molybdenum and/or sulphur can lead to copper deficiency in the animals.
Low blood copper levels in stock should be followed up with a pasture sampling to determine copper and molybdenum levels of the pasture.

Zinc

Zinc is highly immobile in the soil and in the plant.
Fertiliser containing zinc must be placed near the root or on the leaves.
Zinc application to the soil has useful residual benefits.
Zinc is readily locked up in high pH soils (pH > 8.0).
Liming will reduce soil available zinc.
Zinc deficiency is rare in permanent pastures on acid soils.

Manganese

Availability of manganese will depend on breakdown of insoluble forms by soil organisms and soil chemical processes. This is strongly influenced by weather conditions and soil biological activity. As a result, the manganese status of pastures can change dramatically in a short time.
Manganese availability is reduced by high pH, soil carbonates and low organic matter.
Deficiencies are most common in late winter/early spring on sandy soils where limestone is present, but can also occur on acid sand over clay soils - especially if waterlogging in winter has restricted root growth and if the soil surface dries out quickly in early spring.
Liming soils can induce a manganese deficiency, as can limestone dust drift onto paddocks adjacent to roads.
Manganese availability greatly increases as pH drops - manganese can become toxic to plants if pH (CaCl2) falls below 4.3.

Molybdenum

Molybdenum is essential for the nitrogen nutrition of plants. This applies particularly to legumes eg sub clover and white clover, although molybdenum responses have been also been recorded in cereals and sunflowers grown on acid sandy soils.
Molybdenum is held on clay particles (especially those containing iron) and in organic matter but is mobile in the soil. (Molybdenum is the most mobile trace element in high rainfall soils).
Molybdenum deficiency is most likely to occur on acid sands or acid soils containing ironstone. Soil pH is the major factor affecting availability of molybdenum to plants. Often a lime application is enough to overcome molybdenum deficiency.
Molybdenum deficiencies will not occur on soils containing carbonates. However, molybdenum should only be applied if a deficiency is confirmed following a plant tissue test as some low pH soils can contain high levels of molybdenum.
Extreme caution should be used if applying molybdenum to pastures as excessive plant levels will induce copper deficiency in livestock.

Losses of trace elements

Trace element losses from pastures are very low once the initial deficiency is overcome when pastures are sown into virgin country. The only exception is copper on peat soils where regular repeat applications are required. Rather, the availability of trace elements can vary according to seasonal conditions and changes in soil pH as discussed earlier.

Identification of trace element status

Visual symptoms of trace element deficiencies are hard to detect. Soil tests are of little value unless they have been calibrated against a plant response. Developing soil tests for trace elements has been frustrated by very low levels in the soil ie hard to detect large variations in availability during the growing season Low removal levels in pasture and livestock produce Low requirement by plants Soil tests have been developed using either DTPA or EDTA as method of extracting plant available trace elements from the soil. However, the relationship between these tests and plant responses under Australian conditions is vague. At the best, they are broad indicators of soil trace element status, rather than indicating a plant deficiency/toxicity.

On the other hand, leaf analysis can be used to indicate adequacy or deficiency and a wide range of trace element plant tissue tests have been calibrated under Australian conditions. These tests are readily available from reputable laboratories and cover all plant trace element deficiencies likely to be seen in South Australian pastures. As most pastures are legume based, sampling the legume component of the pasture in late winter is the best method of establishing trace element status.

Livestock have a higher requirement for some trace elements than pastures and may also exhibit trace element deficiencies before pastures. Blood testing livestock can be a useful way of establishing your trace element status - especially for copper.

How much to apply?

Copper.

1-2 kg/ha Cu applied to the soil in autumn will correct a deficiency . A foliar spray of 75-100g/ha can be applied if plants are deficient during the growing season, but there is little residual benefit.

Zinc.

2 kg/ha Zn mixed thoroughly into the soil in autumn will correct a deficiency. A foliar spray of 250-350g/ha can be applied if plants are deficient during the growing season, but there is little residual benefit.

Manganese.

Manganese is readily locked up in the soil so soil applications are often insufficient to overcome a deficiency. In pastures, 1 kg/ha Mn can be sprayed once deficiency symptoms are noticed

Molybdenum.

25g/ha applied to the soil in autumn will correct a deficiency. Beware of using foliar molybdenum on pastures as this may induce a copper deficiency in livestock. If the copper status of grazed pastures is doubtful then apply copper with the molybdenum in autumn.

3.8 Trace Elements for Pastures

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