Minerals are present in the soil in sufficient amounts. Do plants need to adjust the type of solutes that reach the xylem? Which molecules help to adjust this? How do plants regulate the type and quantity of solutes that reach the xylem?

(N/A) $\Rightarrow$ Yes,plants adjust the quantity and types of solutes that reach the xylem. The transport proteins present in the plasma membrane of endodermal cells act as control points to adjust this.
$\Rightarrow$ Mineral ions in the soil are often present at lower concentrations than in the root cells. Therefore,they cannot move passively into the root hair cells.
$\Rightarrow$ Mineral salts are transported to the xylem through both active and passive transport. Once they reach the xylem,they are transported to the required parts of the plant via the transpiration stream.
$\Rightarrow$ Regulation of solutes:
$(i)$ The endodermis has many transport proteins in its plasma membrane. These proteins allow some solutes to cross the membrane but not others.
$(ii)$ The transport proteins are control points where a plant adjusts the quantity and types of solutes that reach the xylem.
$\Rightarrow$ Transport of specific minerals:
$(i)$ Nitrogen travels primarily as inorganic ions ($NO_2^-$ and $NO_3^-$),but it is also transported as amino acids and related organic compounds.
$(ii)$ Phosphorus and sulfur are often transported as organic compounds.
$(iii)$ Minerals are frequently remobilized from older,senescing parts of the plant to younger,developing parts. For example,minerals from leaves that are about to fall are transferred to new leaves.