Dairy and beef pastures in the high (>800 mm annual average) rainfall areas
of south-western Australia, based on subterranean clover
(Trifolium subterraneum) and annual ryegrass
(Lolium rigidum), grow on acidic to neutral deep (>40
cm) sands, up to 40 cm sand over loam or clay, or where loam or clay occur at
the surface. Potassium deficiency is common, particularly for the sandy soils,
requiring regular applications of fertiliser potassium for profitable pasture
production. A large study was undertaken to assess 6 soil-test procedures, and
tissue testing of dried herbage, as predictors of when fertiliser potassium
was required for these pastures. The 100 field experiments, each conducted for
1 year, measured dried-herbage production separately for clover and ryegrass
in response to applied fertiliser potassium (potassium chloride).
Significant (P<0.05) increases in yield to applied
potassium (yield response) were obtained in 42 experiments for clover and 6
experiments for ryegrass, indicating that grass roots were more able to access
potassium from the soil than clover roots. When percentage of the maximum
(relative) yield was related to soil-test potassium values for the top 10 cm
of soil, the best relationships were obtained for the exchangeable (1
mol/L NH4Cl) and Colwell (0.5 mol/L
NaHCO3-extracted) soil-test procedures for potassium.
Both procedures accounted for about 42% of the variation for clover,
15% for ryegrass, and 32% for clover + grass. The Colwell
procedure for the top 10 cm of soil is now the standard soil-test method for
potassium used in Western Australia. No increases in clover yields to applied
potassium were obtained for Colwell potassium at >100 mg/kg soil. There
was always a clover-yield increase to applied potassium for Colwell potassium
at <30 mg/kg soil. Corresponding potassium concentrations for ryegrass
were >50 and <30 mg/kg soil. At potassium concentrations
30–100 mg/kg soil for clover and 30–50 mg/kg soil for
ryegrass, the Colwell procedure did not reliably predict yield response,
because from nil to large yield responses to applied potassium occurred. The
Colwell procedure appears to extract the most labile potassium in the soil,
including soluble potassium in soil solution and potassium balancing negative
charge sites on soil constituents. In some soils, Colwell potassium was low
indicating deficiency, yet plant roots may have accessed potassum deeper in
the soil profile. Where the Colwell procedure does not reliably predict soil
potassium status, tissue testing may help. The relationship between relative
yield and tissue-test potassium varied markedly for different harvests in each
year of the experiments, and for different experiments. For clover, the
concentration of potassium in dried herbage that was related to 90% of
the maximum, potassium non-limiting yield (critical potassium) was at the
concentration of about 15 g/kg dried herbage for plants up to 8 weeks old,
and at <10 g/kg dried herbage for plants older than 10–12 weeks.
For ryegrass, there were insufficient data to provide reliable estimates of
critical potassium.
How Closely Is Potassium Mass Balance Related to Soil Test Changes?
