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 xylem ?

Explanation

Yes the plant adjusts the quantity and types of solutes that reach the xylem. The protein present in cells of endodermis helps for this type of adjustment
Compared to roots, mineral ions present in soil are of low concentration. They all do not completely pass through cell wall of root hair.
This way mineral salts are transported till xylem through active transport and passive transport. After reaching to xylem they move to required parts in upward direction. Where mineral ions enter into symport through diffusion and become active by receptor cell.
Some mineral ions flow fast in xylem :
(i) Nitrogen moves in plant in the form of inorganic ions `NO_2` and `NO_3`. But more Nitrogen flows in the form of Amino acid them organic compounds.
(ii) Sulphur, phosphorus These both elements move in organic form in very small quantity.
(iii) From older organs of plants mineral ions are translocated fast. Minerals are transformed into new leaves from old falling leaves. In shed loving plants, minerals take place in other plants before falling of leaves.