Effect of rate and timing of K on three crops

In a glasshouse experiment, wheat, lupin, and canola were grown on a yellow sandy earth from a field site where the K had never been applied. The Colwell soil extractable K was 26 mg/kg, a soil test level expected to be deficient for the growth and yields of these three crops. Several levels of K, as sulphate of potash, were applied to give a range of K additions from deficient to an adequate supply of K for dry weight of shoots and grain. Fertilizer K was applied at 4 growth stages from 2 to 3 leaves to booting in cereal or bud formation in canola and lupin.

The late application was ineffective to correct severe K deficiency in wheat. To achieve about half the maximum shoot weight at time 3, about 4.5 times more K needed to be applied. Similarly for grain yield (g/pot), to achieve 2 g/pot about 5 times more K needed to be applied. The maximum grain yield was halved by delaying the correction of K deficiency. For lupins, late application was also ineffective to correct severe K deficiency but the effects were not as drastic as for wheat. For grain yield (g/pot), to achieve 2.5 g/pot about 7 times for time 3 and 15 times for time 4 more K needed to be applied. The maximum grain yield was 75% of maximum by delaying the correction of K deficiency to application time 4. Likewise for canola, the late application turned out to be ineffective to correct severe K deficiency. However, the effects were similar to wheat but less drastic than for lupin. To achieve 1.0 g/pot, about 7 times more K was required at time 3 and 15 times for time 4 more K needed to be applied.

The maximum grain yield was about halved by delaying the correction of K deficiency to application time 4. Many of the lower K application rates produced no grain. So, in terms of K responsiveness, for K applied at 2-3 leaf stage, reducing rates from 100 kg K/ha to 50 kg K/ha gave greater (about 20%) yield loss in wheat and canola than in lupin (5% yield loss). ANZ-07