Potassium in the soil - Soil Essentials: Managing Your Farm's Primary Asset - page 37

How much fertiliser should be used?
Potassium is often applied as part of a complete fertiliser mixture containing
nitrogen, phosphorus and potassium (N, P, K), but may be applied as potassium
alone using potassium chloride or potassium sulphate, or in combination with
nitrogen as potassium nitrate (Table 7, and Figure 12 on p. 105).

Table 7 How much fertiliser is needed?

(Source: Soil Sense C-06)

Product %K (total) Milligrams of

potassium
per kilogram
of product

100 kg/ha raises Skene potassium in
micrograms per gram (µg/g) by:

Sand Loam Clay

Muriate of
potash
(potassium
chloride)

50 500 33.4 38.5 41.7

Sulphate of
potash
(potassium
sulphate)

41.5 415 27.7 32.92 34.6

Nitrate of
potash
(potassium
nitrate)

38.0 380 25.4 29.4 31.7

Excessive fertiliser application will increase your costs without improving
production.

Worked example
To raise the available potassium levels of a sandy soil from low to moderate using
sulphate of potash.
From Table 6: the required level of potassium = 200 (using the higher figure in
the 120-200 range).
Current level of potassium = 50, so, potassium needed = 200 - 50 = 150 µg/g of
soil.
From Table 7: 100 kg/ha of sulphate of potash raises Skene potassium by 27.7
µg/g, so, sulphate of potash required = 150 ÷ 27.7 × 100 (kg/ha) = 5.4 × 100 = 540
kilograms of sulphate of potash needed per hectare.