Understanding soil potassium (K) availability is critical for developing K fertilizer recommendations, especially in regions with soil K rundown due to negative K balances. Five typical WA soils with varied K supply were selected to study the contribution of different K pools to plant uptake under successive cropping. The soil extractable K concentrations decreased sharply after the first cropping cycle, with the decline slowing in subsequent cycles. The contribution of nonexchangeable K varied in soils, but overall, it was limited, up to 45 mg/kg. Different crops were used to compare K uptake in soils with low and high K levels. In soils with high extractable K (138 mg/kg), wheat and canola had similar K uptake, while lupin’s uptake was about 50% of that. In soils with low extractable K (30 mg/kg), wheat showed severe K deficiencies, while lupin showed no apparent K deficiency, resulting in 78% higher K uptake by lupin than by wheat. Compared to low K soil, the increased K uptake by lupin in high K soil was primarily due to higher extractable K, as similar amounts of nonexchangeable K (10 mg/kg) were taken in both high and low K soils. In contrast, the increased K uptake by wheat in high K soil resulted from both higher extractable K and nonexchangeable K, with nonexchangeable K contributing 4 mg/kg in low K soil and 35 mg/kg in high K soil. This difference can be attributed to the restricted root growth of wheat under K deficiency. Overall, the studies demonstrated that crop K uptake was primarily dependent on soil exchangeable K, with limited contribution from nonexchangeable K. Under K deficiency, reduced root growth further hindered the uptake of K from the nonexchangeable pool, making soils highly susceptible to K depletion due to negative K balances from plant uptake and leaching.