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Soil potassium decline in western Australian agricultural soils

F.H. Ripper, B.J. Leach and M.G. Browne

CSBP and Farmers Ltd, 40 The Esplanade, Perth WA 6000

Soils low in potassium (K) are widespread in agricultural regions of WA (1). With a net loss of K by removal in products a decline in average soil K levels is expected. More than 40,000 farmers' soil samples analysed at CSBP's Soil Fertility Service Laboratory each year should provide evidence of long-term change. Around 20 % of these samples have indicated a likely response to potassium application on legume pasture.

Methods

Soil samples (each made up of twelve 10 x 2 cm cores) were collected at two sites and analysed for bicarbonate extractable potassium. At Darkan, WA (560 mm annual rainfall) samples were taken from eight marked sites on medium to coarse textured soils in three paddocks over 13 years. At Brookton, WA (600 mm annual rainfall) nine paired samples were collected on medium to coarse textured soil along a 1.5 km border between 30-year-old farmland and virgin bush.

Results and discussion

While Soil Fertility Service K levels are yet to show a trend, sampling at Darkan and Brookton highlight a decline in soil K (Figs 1 and 2).

Soil K decline at Darkan was accompanied by reduced clover content and pasture production. As soil K declines to less than about 60 ppm the clover content of pasture falls as it becomes less competitive against grasses which have a lower K requirement (1). Where clover content has declined to an extent that re-seeding is necessary for restoring productive pasture, application of K fertiliser alone has given large increases in dry matter (DM) and seed production (Table 1). Sometimes production responses have commenced in the year after application due to increased plant density from increased seed set in the first year.

Table 1 Pasture DM production and clover seed-set responses to Muriate of Potash at Broomehill WA 1989.

Reference

Cox, W.J. 1973. Western Australia Department of Agriculture, Bulletin 3915.

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