3.5 Phosphorus

Role in Plant

Phosphorus (P) is essential for pasture growth and has a major role in promoting early root growth. It also improves the plants resistance to cold and moisture stress. Most growing tissue, with adequate P nutrition, contains between 0.2 and 0.4% P.

P Cycle

Within the pasture system fertiliser P moves into the soil as inorganic P. This P has a number of fates as shown: (figure 1)

Figure 1:
P cycle in a pasture

Click for larger image.

The major "losses of P in the pasture/soil system are due to tie up in organic matter, fixing in unavailable forms e.g. Al, Fe and Ca phosphates, leaching in sandy soils and removal in animal or plant products.

Identification of P Status

The need for P in a pasture system can be ascertained by:

• Visual symptoms

• Fertiliser strips

• Soil testing

• Plant testing

Visual symptoms

P deficiency shows as a dark blue green to purple red colourations. These symptoms will only appear in plants grown in extremely deficient soils.

Fertiliser strips

Applying a range of P rates can give a good guide to P status. However, management and accurate measurements of yield differences are generally difficult to obtain by a farmer.

Soil tests

By far the most used method of determining the P status is soil testing. Critical soil extractable P tests have been obtained for pastures using mowing trials. A typical calibration for pasture is shown:

Figure 2:
Extractable Colwell P Soil Test

Figure 3:
Critical extractable Colwell soil P values for pastures are:

It is suggested that these values may be higher under heavy grazing pressure.

Plant tests

Plant tests provide a guide to the plants nutritional status. Traditionally plant testing for P in pastures has not been used to a large extent. However, reliable P tests are available and the critical concentrations are:

Figure 4: 
Critical concentrations in pasture tissue

Note: Values are for plants collected prior to flowering and when plants are vegetative and actively growing.

How much P to apply

A number of factors will determine the rate of P to apply to a pasture. These include:

• P status of soil

• Enterprise and stocking rate

• Soil type

• Pasture quality

The removal of nutrients from a grazing system are:

Figure 5:
Removal of P by product

The P rate required to maintain the P status of a soil and provide adequate nutrition, can be obtained from the following table:

Figure 6:
P rate required to maintain the P status of soil

i.e. Pasture on a sandy loam soil with an extractable P of 16mg/kg running 12 DSE would require 12 x 1.1 kgP = 13 kg/P.

Product

• The P product used will depend on:
  Price per unit of P.

• Rate of sulfur required (and other nutrients).

• Soil factors (pH, leaching potential etc.)

• Seasonal conditions

3.5 Phosphorus

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