Soil potassium—crop response calibration relationships and criteria for field crops grown in Australia Ross F. Brennan A,C and Michael J. Bell B A Department of Agriculture and Food Western Australia, 444 Albany Highway, Albany, WA 6330, Australia. B Queensland Alliance for Agriculture and Food Innovation, University of Queensland, PO Box 23, Kingaroy, Qld 4610, Australia. C Corresponding author. Email: ross.brennan@agric.wa.gov.au Abstract. The Better Fertiliser Decision for Crops (BFDC) National Database holds historic data for 356 potassium (K) fertiliser rate experiments (431 treatment series) for different rain-fed grain crops and soil types across Australia. Bicarbonate- extractable K (Colwell soil-test K) is the most extensively used soil test reported in the database. Data are available for several crop species grown on a range of soil types from all states except Tasmania. Species represented and number of treatment series in the database are: wheat (Triticum aestivum L.), 254; barley (Hordeum vulgare L.), 5; canola (Brassica napus L.), 130; lupin (Lupinus angustifolius L.), 32; sunflower (Helianthus annuus L.), 10; sorghum (Sorghum bicolor L.), 5; and faba bean (Vicia faba L.), 2. About 77% of the available soil-test K (STK) data on wheat, canola, and lupin are from Western Australia. The usual sampling depth of 0–10 cm is recorded for all treatment series within the database, while 68% of experiments have STK information from other soil horizons down the profile, usually in 10-cm increments. The BFDC Interrogator, a comprehensive data search and calibration support tool developed for use with the BFDC National Database, was used to examine STK–yield relationships for each crop across Australia, with more detailed analysis by state/region and then by soil type if data were available. The BFDC Interrogator was used to determine a critical STK concentration to achieve 90% of the maximum relative yield (90%RY) for each crop species, with a critical range (determined by the 70% confidence limit for the 90%RY) also reported. The STK for 90%RY for wheat was 40–41 mg/kg on Tenosols and Chromosols, ~49 mg/kg on Kandosols, and ~64 mg/kg on Brown Ferrosols. There was some evidence of critical values increasing with increasing crop yield and on soils with no acidity constraints to root growth, with effects presumably driven by increased crop K demand. The STK for 90%RY for canola, grown mainly on Tenosols, was similar to that for wheat, ranging from 43 to 46 mg K/kg, but for lupin, also grown mainly on Tenosols, the STK for 90%RY was a relatively low ~25 mg K/kg. Data for sunflower were limited and the STK for 90%RY was poorly defined. A comparison of critical STK concentrations for different crops grown on Tenosols suggested that critical ranges for 90%RY of lupin (22–27 mg K/kg) were significantly lower than that for wheat (32–52 mg K/kg) and canola (44–49 mg K/kg). Critical STK values were not determined for sorghum and faba bean. Additional keywords: bicarbonate-extractable K, canola, lupin, soil test K, sunflower, wheat. Received 4 January 2013, accepted 11 April 2013, published online 22 August 2013 Introduction Potassium (K) is an essential plant nutrient required in large quantities, second to nitrogen, by crop and pasture plants (Syer 1998). Potassium fertilisers have been applied to pastures, mainly subterranean clover (Trifolium subterraneum L.), grown on K- deficient soils in most Australian states for many years, e.g. New South Wales (NSW) (Waring 1956), South Australia (SA) (Tiver and Manett 1958), Tasmania (Paton 1950; Piper and De Vries 1960), Victoria (Newman 1948), and Western Australia (WA) (Rossiter 1952; Fitzpatrick and Dunne 1956). In Queensland, K fertiliser has been applied to cotton (Gossypium hirsutum L.) (Empire Cotton Growing Corporation 1950), sugarcane (Saccharum officinarum L.) (Vallance 1950), and tropical grasses (Bryan and Sharpe 1965) for many years. However, for broad-scale crops such as wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), canola (Brassica napus L.), lupin (Lupinus angustifolius L.), sunflower (Helianthus annuus L.), sorghum (Sorghum bicolor L.), and faba bean (Vicia faba L.), K fertiliser has not been needed until much more recently. When first cleared for agriculture, most soils in Australia had adequate indigenous soil K for crop and pasture production. However, soon after establishment of pastures based on subterranean clover on sandy soils in the >800-mm annual average rainfall areas of WA, areas of K deficiency became evident and regular applications of fertiliser K were required (Cox 1980). The deficiency of K was due to leaching from soil by rain, removal in silage and hay, and redistribution by grazing animals. In the early 1990s, K deficiency of wheat became apparent for sandy duplex soils in the Great Southern region Journal compilation Ó CSIRO 2013 www.publish.csiro.au/journals/cp CSIRO PUBLISHING Crop & Pasture Science, 2013, 64, 514–522 http://dx.doi.org/10.1071/CP13006