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Most sandy soils used for cropping in south-western Australia are now deficient in potassium (K) due to removal of K from soil in hay and grain, and profitable grain yield responses to applied fertiliser K are commonly obtained for spring wheat (Triticum aestivum L.) and canola (oilseed rape, Brassica napus L.). However, there are only limited data comparing the K requirements of these 2 species in the region. In a glasshouse experiment we compared the K requirements of wheat (cv. Westonia), conventional canola cv. Outback (cultivars of canola not produced by classical breeding techniques to be tolerant of specific herbicides), triazine-tolerant (TT) canola cvv. Pinnacle and Surpass 501, and imidazolinone-tolerant (IT) canola cv. Surpass 603. The following measures were used: yield of 54-day-old dried shoots and seed (grain) without added K, applied K required to produce 90% of the maximum yield of shoots and grain, K required to attain a K concentration in shoots of 30�g/kg, and K required to achieve a K content in shoots (K concentration multiplied by yield) of 40�mg/pot. We also determined for each species and cultivar the concentration of K in dried shoots that was related to 90% of the maximum grain yield, to estimate critical concentration in shoots below which K deficiency was likely to reduce grain production. All 4 canola cultivars produced similar results. Both canola and wheat produced negligible shoot yields and no grain when no K was applied. For each species and cultivar the amount of applied K required to produce 90% of the maximum yield was similar for shoots and grain, and was ~121�mg�K/pot for the 4 canola cultivars and 102�mg�K/pot for wheat, so ~19% more K was required for canola than for wheat. For each amount of K applied, the concentration of K in shoots was greater for canola than for wheat. The amount of applied K required to attain a K concentration of 30�g�K/kg in shoots was ~96�mg�K/pot for canola and 142�mg�K/pot for wheat, so ~48% more K was required by wheat than by canola. The amount of K applied required to achieve a K content of 40�mg�K/pot in shoots was ~46�mg�K/pot for canola and 53�mg�K/pot for wheat, so ~13% more applied K was required by wheat than by canola. The data suggest that canola roots were better able to obtain K from soil than wheat roots, but wheat used the K taken up more effectively than canola to produce shoots and grain. The concentration of K in dried shoots of 54-day-old plants that was related to 90% of the maximum dried shoot yield or grain was ~32�g/kg for canola and ~23�g/kg for wheat. |
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