The researchers studied the distribution of different forms of potassium in black soils in maize-growing areas. They found that the available potassium was higher in the surface layer compared to the sub-surface layer. Water-soluble and exchangeable potassium decreased from the surface to sub-surface layers. Non-exchangeable potassium was higher in the sub-surface layer. Overall, the total potassium content in black soils ranged from 1.45% to 2.15% in the sub-surface layer.