Potassium in the soil - Soil Essentials: Managing Your Farm's Primary Asset - page 36

Potassium in the soil
Most soils contain large amounts of total potassium, but only a small proportion is
readily available to the plant. As plants remove available potassium over time,
previously unavailable potassium is modified by soil chemical reactions to become
available to the plant. This is a slow process, however, and if plant products are
removed from the paddock available potassium will have to be added to maintain
short-term production levels.
The potassium to magnesium ratio in the soil is satisfactory if there is less
potassium than magnesium.

Soil tests for potassium
Soils are usually tested for plant available potassium by the Colwell or Skene
methods. Results are quoted in micrograms per gram (µg/g) or milligrams per
kilogram (µg/kg). Some soils such as the Riverine plains of northern Victoria show
little response to potassium applications, even though soil tests indicate low
available potassium levels, because the soil tests do not measure the 'unavailable'
potassium in the soil (that may later become available).
Potassium deficiency can inhibit growth and plants may have small roots and
leaves. Plants become spindly and need more water for respiration. Symptoms may
show as scorching along leaf margins and small and shrivelled seed and fruit.

What do the soil test results mean?
The Colwell or Skene results give a guide to the likely responses by crops or
pastures to an application of potassium fertiliser (Table 6). Plant needs vary with
soil types, with the lowest need in sandy to sandy loam soils, and the highest need
in clays, laterites and peats.

Table 6 Potassium levels in soil (micrograms per gram): Colwell or Skene methods of testing

K, potassium. (Source: Soil Sense C-06)

Skene K levels
(0-10 cm µg/g)

Pastures and crops Horticultural

Level of K
indicated

Sands Loams Clays Sands Loams Clays

Low <50 <80 <120 <80 <140 <150

Marginal 50 -120 80 -160 120 -180 80 -150 140 -190 150 -210

Moderate 120-200 160-250 180-300 150-230 190-280 210-330

High >200 >250 >300 >230 >280 >330