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

Since plants absorb minerals, like potassium, calcium, sulfur, etc. in their ionic form, like K⁺, NO₃^-, SO₄^2-, Ca²⁺, they need to absorb them actively. Since these ions are charged, they have difficulty while passing through the plasma membrane. Hence, plants need to expend energy in order to transport them. For this, they use ATP and membrane transporter proteins. These transporters transport ions across cell membrane with an accompanying ion (H⁺) so as to maintain electrochemical gradient. As a result (partly), these transporters are highly selective for the ions they carry. For example, a K⁺ transporter will not transport SO₄^2- or another ion. See this answer to see how transporters achieve this selectivity.

The above part explains how ions are transported in plants. Now, lets come to the main question i.e. why some ions are transported faster than others. Well, there is no definitive answer to this (at least none that I know of). But I can think of two possible reasons for this (using only the examples of K⁺, Ca²⁺, NO₃^- and SO₄^2-):

• Magnitude of Charge: one fact here is that both K⁺ and NO₃^- have a charge of $\pm$1, whereas Ca²⁺ and SO₄^2- have a charge of $\pm$2. So what? This makes their transport a bit more difficult. For example, to transport K⁺ across the membrane, the transporter would antiport 1 H⁺ for 1 K⁺, whereas a similar antiporter would transport 2 H⁺ out to bring 1 Ca²⁺ in. Obviously, this does not explain the difference between ions with same charge, which the following point seems to do.

• Requirement: one of the most important points to keep in mind here is the difference in requirements of different ions by the plant. This is why plant macronutrients (i.e. required in large quantities) have been divided into primary macronutrients (N, P, K) and secondary macronutrients (all others, including Ca, S, Na). As you see, nitrogen and potassium are mostly the limiting factors of crop production. This is why plants require them in large quantities, and hence try to absorb them as fast as possible. This, partly again, explains why K⁺ and NO₃^- (the only artificial nitrogen source other than NH₄⁺) are absorbed faster by plants while Ca²⁺ and SO₄^2- are absorbed slowly.

References:

• Mineral Absorption - Wikipedia

• Secondary Plant Nutrients: Calcium, Magnesium, and Sulfur - Mississippi State University