In plants why are potassium and nitrate ions absorbed faster than any others?

Question

Quoting my A-level biology textbook: >$\\ce{K^+, NO3^-}$ are absorbed by plants faster than any other ions. Whereas, $\\ce{Ca^2+, SO4^2-}$ are absorbed with the slowest speed. What is the reason? My reasoning: I first though the size of ions would matter in this case. Obviously, $\\ce{K^+}$ is smaller than $\\ce{Ca^2+}$, then it 'seemed to me' that smaller ions would be absorbed faster. The same reasoning can be applied to $\\ce{NO3^-, SO4^2-}$. But, there are smaller ions than $\\ce{K^+}$ which are absorbed slower eg. $\\ce{Na+}$. So, obviously size only does not matter here. There are other factors.

Answer 1

The transport of ions involves an exchange of ions (transporter-antiporter mechanism). Since K+ and NO3- have a magnitude of a single charge, they can be easily exchanged with H+ ion. The same mechaism will require two ions to antiport for ions such as Ca2+ or SO4 2-. So the magnitude of just one charge on the ions is what is causing the faster absorption.