Potassium (K)

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Potassium is an essential plant nutrient that is commonly in short supply in Western Australian soils. This nutrient is critical for plant growth as it is involved in most of the biological processes of plants. It’s classified as a macronutrient because plants take up large quantities of potassium throughout their life cycle. As a result, limited supply of potassium can limit crop yields.

As demonstrated in this nutrient removal table, only small amounts of potassium are removed in grain. The majority of potassium is held in the leaves and stems.

Example of grain removal

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 potassium (equivalent to 400kg/ha of muriate of potash). Out of this potassium, 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. Greater losses will occur if the ash is removed by wind, rain or soil erosion. When soil sampling, much of the potassium may be still in the plant at the time of sampling as 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.

Role of potassium in plants

  • Vital for photosynthesis and protein synthesis.
  • Helps to breakdown carbohydrates to provide energy for growth.
  • Assists in movement of metal ions such as iron (Fe).
  • Helps control ionic balance.
  • Helps the plant resist disease and withstand drought, cold and frost.
  • Essential for fruit and grain formation.
  • Aids in 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.
  • Affects the quality of grain protein, efficiency of nitrogen uptake, baking quality and yield (seed weight and numbers).

Potassium deficiency

Potassium is a mobile nutrient within the plant. When deficiencies occur, the plant will remove potassium from the older leaves and pump it into the younger leaves, therefore the symptoms will appear on older leaves first. Plants lacking potassium will have speckling along the leaves, spreading quickly to the tip and the margin. Complete senescence (death) of the older leaf may occur.

Soil Tests

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
25-35
20-25
30-50
40-60
50-60
Product
Autumn Application
Winter Application
After 4 weeks
After 7 weeks
After 4 weeks
After 7 weeks
Amsul
3.7
6.9
1.0
2.5
Urea
59.9
58.4
36.8
38.9
Cations
Anions
Chemical Symbol & Charge
Name
Chemical Symbol & Charge
Name
K+
Potassium
NO3-
Nitrate
NH4+
Ammonium
Cl-
Chlorine
Na+
Sodium
SO4--
Sulphate
Cu++
Copper
HPO4- -
Phosphate
Zn++
Zinc
BO3----
Boron
Mn++
Manganese
MoO4- -
Molybdenum
Ca++
Calcium
Mg++
Magnesium
Fe++
Iron
N
P
K
S
Cu
Zn
Mn
Ca
Mg
Clover Pasture
25-30
3.5
16-20
2-3
0.005
0.02
0.40
0.4
1.2

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 high rainfall, deep sands. However, in lower rainfall areas where the soil has a reasonable clay or gravel content, or in duplex sand on top of clay/gravel, 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 with lower potassium levels (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.

To find out more about preventing or correcting a potassium deficiency, speak to your local Area Manager.

Product
% N
Urea
46.0
MAXam/Amsul
21.0
UreaPlus
37.1
NitroPlus
33.4

Compounds

Product
%Ammonium-N
DAPSZC
16.4
MAPSZC
10.6
AllStar
13.0
Vigour
5.0
Product
N
P
K
S
Ca
Mg
Cu
Zn
Mn
kg per tonne
grams per tonne
Wheat
23
3
4
1.4
0.33
0.93
5
29
40
Barley
20
2.9
4.4
1.1
0.3
1.08
3
15
11
Oats
16
3
4
1.5
0.5
1.0
3
17
40
Canola
40
6.5
9.2
9.8
4.1
4.0
4
40
40
Lupins
51
3.8
8.8
3.1
1.7
1.7
5
30
60
Chickpeas
34
3.8
8.9
1.8
1.1
1.2
7
38
34
Faba Beans
39
3.8
9.8
1.4
1.1
1.0
10
28
30
Field Peas
37
4.0
8.2
2.0
0.7
1.2
5
35
14
Hay
20
2.0
25
2.0
0.5
1.1
5
20
40
Milk
5.7
0.95
1.4
0.3
1.2
0.12
Greasy Wool
170
0.26
15.8
28.5
1.2
0.3
Sheep - Live
34
7.0
2.3
4.0
14.4
0.4

Stubble

Product
N
P
K
S
Ca
Mg
Cu
Zn
Mn
kg per tonne
grams per tonne
Wheat
17
1.8
42
2.7
Canola
18
2.4
70
4.8
Lupins
17
0.6
26
2.7