@ Sharma, because pH influenced the variability of extractable K. I think in black soil of central India, the soil pH ranged between 7.2-8.5, therefore available K is significantly positively correlated whereas in acidic soil the available K is negatively correlated with soil pH.

Well,

Black soil means the existence of organic matter, which may originate to plant depend on fertilizers of K- type.

Regards

Dr J.C.Tarafdar has given appropriate explanations and due to this only this type of relationships exists.

I agree with J. C. Tarafdar 's answer! Indeed, if a soil has a pH(KCl) of less than 4,8 it can be regarded as being acidic.

https://dergipark.org.tr/tr/download/article-file/565838