3.7 Sulphur For Pastures

Role in Plant

Sulphur (S) is essential for plant growth and is absorbed by the plant in the sulphate form. S is involved in cell development and in some amino acids, as well as an essential element for nitrogen fixation and chlorophyll formation. Plants with adequate S nutrition contain between 0.2 and 0.35% S.

S Cycle

When applied to the soil as sulphate, fertiliser S forms part of the inorganic S. Elemental fertiliser S requires an oxidation process to form the available S. The fate of this inorganic pool of S can be seen in the S cycle.

The major losses of S from the pasture system are due to animal removal, leaching and fixation into organic S. At any one time over 95% of the S is in the organic form. Therefore, those processes that favour mineralisation will increase S supply to the pasture.

Figure 1:
Potassium requirements by enterprise

Identification of S Status

S status of a pasture can be ascertained by:

• Visual symptoms

• Fertiliser strips

• Soil tests

• Plant tests

Visual symptoms

S deficiency is more likely to be seen in legumes than grasses. Plants that are S deficient tend to be light green to pale yellow in colour.

Fertiliser strips

S deficiency can be identified with test strips, but S deficiency is seasonal and it is difficult to predict an S response due to changing weather conditions.

Soil tests

Soil tests for S have been experimented with over the years, with limited success. However, recent work has produced an S test (KCl-40) that measures the inorganic and part of the organic S. A typical calibration is shown. A critical range of 5 to 8mg/kg extractable S is suggested

Figure 2
Extractable S - critical levels for pasture production

The interpretation of soil S tests needs to take into account soil type and the weather conditions at time of sampling, ie values will be lower in sandy soils that are dry.

Figure 3:
Critical S concentrations in pasture tissue

Figure 4:
Sulphur applications to maintain adequate S nutrition (Kg/haS)

Plant tests

Plant tests for S are very reliable if young tissue is collected. A range of critical concentrations for a range of pasture species is shown in Figure 3.

Values are for plants actively growing. Critical concentrations for whole shoots will decrease rapidly as flowering and maturity approaches.

How much S to apply

In most pasture situations, the S has been added with the phosphorus as a form of by-product. However, with the move to high analysis products, S inputs have been reduced in some instances. Generally S will be required for pastures growing on sands through to loams. The rate of S applied will depend on the stocking rate, texture and amount of S in the soil. Removal figures for S are listed in Figure 5.

Figure 5:
Removal of S by product

As a guide to the S rate required to maintain adequate S nutrition, figures can be taken from Figure 3.

Product

S can be directly applied to the soil as gypsum (CaSO4), but is more likely to be added with the phosphorus fertiliser either in the sulphate or elemental S form. Trial work suggests that, with adequate rainfall, either form is suitable.

3.7 Sulphur For Pastures

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