Potassium (K) is one of the essential nutrients in plants and has a number of roles in various plant systems including photosynthesis, enzyme activation, stomatal control and transporting plant sugars. Potassium is commonly found in plants at levels above all other macro nutrients except carbon, oxygen, hydrogen and occasionally nitrogen.
Throughout Queensland’s cropping regions there has been a gradual decline in soil potassium levels due to crop removal of potassium and low fertiliser application rates. In particular, grain growers on red ferrosol soils in the inland Burnett region have increasingly encountered potassium deficiency over the last 10 years, due to the lower available reserves in these soils. The problem is also becoming increasingly evident on medium-heavy cracking clay soils. Cotton, legumes and hay baling/silage systems have had a particularly high impact on potassium reserves in some soils.
Crops may vary in their response to potassium fertiliser application and, in winter cereals, responses are generally low unless large deficiencies are present. However, while significant soil potassium reserves still exist in many Queensland cropping soils, particularly the heavier alluvial and cracking clay soils, it is important to maintain soil reserves by replacing the potassium removed in harvested products. If potassium is allowed to be depleted to such an extent that crop productivity is affected, heavy and very costly fertiliser applications will be required..
Soil and plant tissue analyses give insight into the availability of potassium in the soil. Growers should not rely on soil testing alone as potassium concentrations may be very variable throughout the paddock, making representative sampling difficult. Ideally, growers should use soil test results in conjunction with plant tissue testing and checks for visual symptoms of deficiency to determine paddock potassium requirements (figures 1 & 2).
Figure 1: Potassium deficiency can appear as stripes in the crop due to uneven distribution of potassium. (Photo S Loss, CSBP)
Figure 2: Potassium deficiency symptoms in wheat (plant on right).
Because potassium is not highly mobile, nutrient levels need to be adequate in both the topsoil (0–10 cm), to support initial seedling growth, and the subsoil (10–30 cm), to meet plant requirements as the crop grows.
Analysis of whole tops will determine whether a deficiency exists but doesn’t define a potassium requirement. Tissue testing is useful for identifying potassium issues for the coming seasons, but is generally too late to be useful in the current season.
Potassium lost through product removal should be replaced once paddocks reach a responsive situation. Requirements for each crop differ, and this must be accounted for when budgeting potassium requirements for the coming season (table 1).
Table 1: Potassium (K) removal per tonne of produce.
Annual K removal (kg)
Pasture legumes are particularly susceptible to, and can be affected by, potassium deficiency when cereal yields remain unaffected. Unless plant symptoms are recognised, or soil or tissue testing done, the first signs of potassium deficiency in a paddock may be poor growth and a gradual disappearance of the pasture legume component.
Potassium is highly mobile in the phloem and can be moved to newer leaves if the nutrient is in short supply, with deficiency symptoms appearing first on older leaves. General symptoms initially include a light green to yellow colour of the older leaves. Marginal scorch of the edges and tips of these leaves follows, often resulting in senescence. As the severity increases, this condition progresses towards the top of the plant. These characteristic symptoms of potassium deficiency can often be mistaken for leaf diseases such as yellow spot and Septoria nodorum blotch in wheat or brown leaf spot in lupins. Other symptoms include slow plant growth, weak stems and lodging, high screenings levels in the harvested grain and reduced disease resistance.
Potassium fertilisers can be side-banded at planting, drilled into the fallow pre-plant, or broadcast and cultivated in fallow prior to planting. However, potassium can affect germination, so application with seed is not recommended.
Surface applications of potassium may not be sufficient to supply crop requirements if deeper layers are low in potassium and the surface soil dries out. Subsoil deficiencies can be a particular issue at direct drill sites, where mixing via cultivation is absent and nutrients become concentrated in surface layers. Where subsoil deficiencies are expected to occur, deep application of potassium may be required to maintain soil productivity.
Muriate of potash (MOP—KCl; 49.5% K) is the cheapest form of potassium and is applied by top dressing either before seeding or up five weeks after seeding. Sowing MOP directly with the seed can significantly reduce crop germination and establishment, with rates of MOP higher than 30 kg/ha (22 cm row spacing) affecting germination significantly.
The development of sulphate of potash (SOP) is a less damaging form of potassium and can be drilled with seed. This product is significantly more expensive than MOP per unit of potassium.
White J (2000) Potassium in agriculture: Australia and New Zealand, Agrow Australia, Sydney
The National Soil Quality Monitoring Program is being funded by the Grains Research and Development Corporation, as part of the second Soil Biology Initiative.
The participating organisations accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.