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? 

Yes, plants need to adjust the type and quantity of solutes that reach the xylem. The transport proteins of endodermal cell help in maintaining and adjusting solute movement. As the minerals are present in soil as charged particles with a very low concentration as compared of roots, they, all cannot be complately passively transported across cell membranes of root hairs.
Thus, minerals are transported both by active and passive processes, to the xylem. Upon reaching xylem, they are further transported, i.e., upwards to sinks through transpiration stream. At the sink regions mineral ions are unloaded through diffusion and active uptake by receptor cells.
Some of the mineral ions moving frequently through xylem are 
(i) Nitrogen travels in plants as inorganic ions ${\mathrm{NO}}_2$ and ${\mathrm{NO}}_3$ but much of the nitrogen moves in the form of amino acids and related organic compounds.
(ii) Sulphur and Phosphorus small amount of these two nutrients are carried in organic forms.
fiii) Mineral ions are frequently remobilised particularly from older senescing parts. Older dying leaves export much of their mineral content to younger leaves. Similarly, before leaf fall in decidous plants, minerals are removed to other parts.

Elements most readily mobilised are phosphorus, sulphur, nitrogen and potassium. Some elements that are structural components like calcium are not remobilised.