Sugarcane

Potassium is vital to sugarcane production because it improves yield, disease resistance and lodging resistance.

Nutrient Removal

Potassium is removed from the soil through the cane stalk and through soil leaching.  The potassium removed in cane stalk can range from 80-140kg potassium/ha, with a similar amount contained in the tops and leaves.  As much as 280kg of potassium per hectare is contained in the green crop immediately prior to harvest.

Where cane is harvested green, most of the potassium in the tops and leaves is returned to the soil in the trash and quickly returns to the soil.  However, if cane is burned, around 70% of the potassium contained in the trash can be lost from the field and deposited as ash elsewhere.

Potassium Soil Testing for Cane

Two separate soil tests are used to make potassium recommendations for sugarcane.  The exchangeable potassium pool is extracted with a pH neutral salt.  Fixed potassium is removed by digesting with nitric acid. This allows an assessment of the amount of potassium available by slow release over the lengthy cane growing season.  Both the exchangeable and nitric acid extractable potassium are used to determine soil potassium supply and potassium fertiliser requirement.

Potassium for Yield and Quality

High nitrogen applications will depress the yield and quality of sugar cane if the potassium supply is not adequate. (See table.)

The effect of potassium supply on sugarcane yield and sugar content is shown in this table.

Low available soil potassium can lead to erratic germination of cane sets.  Long term deficiency affects the growing tips resulting in a fan appearance.

During fallows and the early period of plant cane growth, some potassium will become available from trash and from slow release soil supplies.  Ratoon cane also accumulates potassium more rapidly than plant cane. For these reasons ratoon cane may require more applied potassium than plant cane.

Excessive potassium applications can have a negative impact on manufacturing quality of sugarcane by raising its ash content. High ash content in juice can interfere in the manufacture of raw sugar.  Therefore it is important to match potassium supply with crop potassium requirement for yield.

Potassium Sources

Most potassium for cane is applied as MOP, although alternative sources are also available.

Dunder, a liquid waste product from alcohol distilleries, is available in some cane-growing areas and is a good source of recycled nutrients. If dunder is used, some MOP may also be necessary to reach the required application rate of potassium. (See table.)

The nutrient content of several potassium fertiliser sources used in cane production is seen in this table.

In the Burdekin Delta in Queensland, sugarcane crops are irrigated from underground water which contains potassium. In some instances this is sufficient to meet crop requirements.

When applying MOP at planting, care must be taken to ensure the fertiliser is not placed too close to cane sets, because it can cause fertiliser burn.  Banding beside the row and below the surface is the best method of application.

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