Which potassium fertilizer contains the lowest percentage of K2O?

The choice of potassium fertilizer that contains the lowest percentage of K2O is potassium thiosulfate (K2S2O3). This potassium source typically has a lower potassium oxide (K2O) content compared to the other options listed. In terms of chemical composition, K2S2O3 provides potassium along with sulfur, but its potassium content, when expressed as K2O, is less than that found in KCl, K2SO4, and KNO3.

When comparing the potassium content in potassium chloride (KCl), potassium sulfate (K2SO4), and potassium nitrate (KNO3), each of these fertilizers has a significantly higher percentage of K2O: KCl is widely known for having a high K2O concentration due to its straightforward composition, while K2SO4 is valued for both potassium and sulfur, and KNO3 provides both potassium and nitrogen, maintaining substantial K2O percentages as well.

Thus, potassium thiosulfate is distinguished by providing potassium in a formulation that results in a lower percentage of K2O, making it the correct answer for this question.