3.6 Potassium for Pastures

Role in Plant

Potassium (K) is an essential macronutrient for plants, being required for photosynthesis, cell formation and cation-anion balance. Most pasture plants contain high amounts of K i.e. 1 to 4%.

K cycle

The uptake of K by plants is almost entirely from the soil solution. Plant factors that influence the uptake of K are:

• Plant age

• Transpiration rate

• Root distribution

• Plant morphology

• Soil factors that influence K uptake are:

• Those influencing K diffusion i.e. water content, temperature.

• Soil buffering capacity.

The movement of K in the soil can be illustrated by the following cycle:

Figure 1:
Soil K Cycle

Identification of K status

K status can be ascertained by:

• Visual symptoms

• Fertiliser strips

• Soil tests

• Plant tests

Visual symptoms

K deficiency in legumes appears in the older leaves and is characterised by a white spotting of the leaves a bronze colouration and dying of tissue from the edges. Deficiencies are more likely in legumes than grasses.

Fertiliser strips

Fertiliser strips are quite a useful tool to diagnose K deficiency but they require careful management.

Figure 2:
Colwell K- Critical values for pasture production.

Soil tests

Soil tests for K have been developed for pasture species on a range of soil types. Within SA a good calibration has been developed for the Colwell K test as seen in the following diagram. Critical values of between 80 and 120 mg/kg extractable K have been proposed.

Plant tests

Plant tests for K in pasture plants are very reliable but as with other nutrients young tissue needs to be analysed.

Figure 3:
Critical concentrations of K in pasture tissue.

Ratios of K/Ca + Mg in the plant have been used to identify grass tetany prone pastures. Values greater than 2.2 will predict pastures that may cause problems.

How much K to apply

Pasture systems that lead to large dry matter removals can lead to K deficiency.

Figure 4:
Removal of K by product

Taking these figures into account along with soil K concentration the following table gives a guide to K requirements.

Figure 5:
Potassium requirements by enterprise/

Product

K is generally supplied to pastures as muriate of potash (50%K) as a stand-alone fertiliser or blends with phosphorus and sulfur products.

3.6 Potassium for Pastures

[ Back ] [ Next ]