Plants do need to adjust the type and quantity of solutes that reach the xylem. The transport of proteins in endodermal cells help in maintaining and adjusting solute movement. The minerals are present in soil as charged particles with a very low concentration compared to that of roots, and thus cannot be completely transported passively across the cell membranes of roots hairs.
Minerals are thus transported both by active and passive processes, to the xylem. Upon reaching xylem, they are further transported, upwards to sinks through transpiration stream. At the sink regions mineral ions are unloaded through diffusion and active uptake by receptor cells. The mineral ions moving frequently through xylem include.
(i) Sulphur and Phosphorus in small amounts are carried in organic forms.
(ii) 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.
(iii) 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 deciduous plants, minerals are removed to other parts.
The most readily mobilised elements are phosphorus, sulphur, nitrogen and potassium. Structural components elements like calcium are not remobilised.