Potassium is one of the major nutrients required by the plant in large amounts, (about 1.5 to 2.0% of plant weight). Many of the lighter soils in WA are now under 100ppm. Given time they will drop to levels where production will be adversely affected. Where levels are below 60ppm, most cereals and pastures will lose production due to potash deficiency.

Potassium is a Cation, so is held reasonably firmly in the soil. It binds to clay particles and organic matter. For this reason it can accumulate in clay layers below the surface (as in duplex soils). In this situation surface levels will generally not fall below 60 to 80ppm as the Potassium is brought to the surface by plants. When surface levels fall below 60ppm it will mean that there is no ready reserve in the sub-soil. In light soils where a majority of the Potassium is held by the Organic Matter, an erosion event will lead to large losses of plant available Potassium.

Only small amounts of potassium are removed in grain (see Nutrient Removal). The majority of the Potassium is held in the leaves and stems. A wheat crop which yields 3.0t/ha would have a mature weight of around 10.0t/ha (roots, leaves, stem and grain). At 2.0% Potassium, the crop has to access 200kg/ha of K, an equivalent to 400kg/ha of Muriate of Potash.

Only 12 to 15kg/ha would be removed in the grain. The rest is available to be returned to the soil, removed by livestock or lost through stubble burning or erosion. (burning of stubble will lose about 5kg/tonne of K in the smoke. Bigger losses will occur if the ash is removed by wind. Rain will wash potash from stubble or ash into the ground, and will only be lost by soil erosion.) When soil sampling much of the Potassium may be still in the plant at the time of sampling. It will be leached into the soil with rainfall. Marginal levels in November may be adequate in May.

If removing large amounts of plant material, as in hay and silage production, some grazing systems and chaff carts behind headers, removal of Potassium can be large. Replacement of Potassium is essential, using Muriate or Sulphate of Potash.

Function in the Plant

Potassium is involved in most of the biological processes of the plant. Potassium is not metabolized and forms only weak complexes in which it is readily exchangeable. This allows it a high degree of mobility. Some of the more important roles in the plant include:

  • Vital for Photosynthesis. When Potassium is deficient, photosynthesis declines and respiration increases. This effect will lower the plants carbohydrate supply
  • Protein Synthesis
  • Breakdown of carbohydrates - a process which provides energy for growth
  • Translocation (movement) of metal ions such as Iron (Fe).
  • Helps control ionic balance.
  • Helps the plant resist disease.
  • Important to fruit and grain formation.
  • Important in withstanding
    • drought
    • cold
    • Frost
  • Osmoregulation: The process of opening and closing the stomata (pores in the surface of the leaf) to control the amount of water lost to the atmosphere. In potassium deficient situations the stomata may be slow to close and may only partially open. This increases the stress from drought.
  • A large effect on the quality of grain protein, efficiency of nitrogen uptake, baking quality and yield (Seed weight, seed numbers)

Table 1: Effect of Potassium on Grain size and Screenings.

Deficiencies

Potassium is a mobile nutrient in the plant. When deficiencies occur, the plant will remove Potassium from the older leaves and pump it into the younger leaves. Therefore the symptoms appear on the older leaves first. Symptoms begin as a speckling along the leaf, spreading quickly to the tip and the margin. Complete senescence (death) of the older leaf may occur.

Soil Tests

Table 2: Generalised guide to soil test levels and Potassium requirements.

      Pasture
Soil Test Level
(K - ppm or mg/kg)
Cereals Lupins
Canola*
Pulses
Low Rainfall Medium Rainfall High Rainfall
>120 0 0 0 0 0#
80 - 120 P.T. 0 0 10-20 20-30
60 - 80 15-20 0 10-20 20-30 30-40
40 - 60 20-25 P.T. 20-30 30-50 40-60
<40 25-35 20-25 30-50 40-60 50-60

P.T. = Plant Test. Levels are marginal, but may not respond to applied K.

*CANOLA: does not appear to respond to low Potassium levels. However data is very scarce. To help the following years crop, and to cover all eventualities, it is suggested that up to 50kg/ha Muriate of Potash be applied pre - sowing.

# BALANSA: Balansa clover is a vigorous growing clover often used for hay making. Observations from the Great Southern and South Coast regions indicates that Balansa is more sensitive to potash deficiency than other pasture legumes. If soil tests indicate levels around the 120 to 150ppm then use of potash would be a wise precaution.

Leaching

One of the myths about potassium is that it leaches rapidly. In fact, Potassium is a cation and binds quite strongly to the soil particles. Leaching may be a problem in the high rainfall, deep sands. However in the stronger soils, lower rainfall areas where the soil has a reasonable clay or gravel content, or the duplex sand over clay/gravel, then leaching is not a major concern.

In waterlogged clays, or soils where surface erosion is likely to be a problem, then delayed or split applications may be beneficial.

Lime and Potash

Applying lime to soils low to marginal in Potassium status (up to 140ppm) can have a detrimental effect on the availability of Potassium to the plant. This is particularly prevalent in Lupins. The application of lime applies large amounts of Calcium. Calcium and Potassium appear to be absorbed through the same part of the root, so the plant will absorb the nutrient in abundance. The other possibility is that the Calcium (also a cation) will replace the Potassium on the soil colloid, releasing the Potassium for ready leaching.

Products to Supply Potash

Table 3: Products containing Potassium

  Analysis
Product K N P S
Muriate of Potash 50.0     1.75
Sulphate of Potash 41.5     17.0
Pasture Potash 11 32.5   6.4 4.0
Pasture Potash 32 28.5   9.2 5.4
Pasture Potash 21 16.5   6.1 7.6
Pasture Potash 31 12.5   6.8 8.4
Pasture Potash 41 10.0   7.3 8.9
Pasture Potash 51 8.5   7.6 9.2
SuperPasture Potash 11 30.0   5.5 4.6
SuperPasture Potash 32 25.0   6.8 5.6
SuperPasture Potash 21 21.0   7.9 6.4
SuperPasture Potash 31 18.0   8.7 7.0
SuperPasture Potash 41 13.5   10.0 7.9
SuperPasture Potash 51 12.0   10.4 8.1
Super Potash 1:1 25.0   4.6 5.9
Super Potash 3:2 20.0   5.5 6.9
Super Potash 2:1 16.5   6.1 7.6
Super Potash 3:1 12.5   6.8 6.5
Super Potash 4:1 10.0   7.3 8.9
Super Potash 5:1 8.5   7.6 9.2
VIGOUR 12.0 10.0 12.0 5.0
GUSTO 14.0 10.0 12.0 7.5
DAIRY 12.5 14.5 8.0 9.3
Dairy TE 12.5 11.8 8.0 12.2
Hay 16.5 13.3 3.0 13.8
Hay Special 16.5 18.4 2.6 9.4
Grass Boost 17.0 13.8   16.0

See the full range of products containing potassium in the analysis section.