The Calcium in high concentrations is toxic for the plant, therefore the plants have some mechanisms to deal with it, and Calcium often precipitates into plant tissues as Ca-oxalate crystals. I don't know the relationship of Calcium with nitrate ions, but in my studies I found a strong relationship of calcium with Sr, Ba and Ga across the xilem. For more details you can check it here (pag. 70):

https://www.researchgate.net/publication/299486353_Environmental_signals_in_radial_growth_stable_isotope_variations_and_nutrient_concentration_of_trees_from_different_forest_ecosystems_in_southern_Ecuador 

Thesis Environmental signals in radial growth, stable isotope varia...

Hi Maher Souguir

In xylem a mass flow exist and therefore the ratio of concentration reflects also ratio of flows. I studied xylem and phloem transport in Ricinus over several years (see as review: Peuke A.D. (2010) Correlations in concentrations, xylem and phloem flows, and partitioning of elements and ions in intact plants. A summary and statistical re-evaluation of modelling experiments in Ricinus communis. Journal of Experimental Botany 61, 635-655.).

Indeed [Ca] is lower as nitrate or K, but higher as Mg, Cl etc. In phloem saps Ca is relative low and it is assumed that Ca is less transported in phloem. As a consequence Ca accumulates in (older) leaves, since it cannot be recycled by the phloem.

In principal transport saps must be electrically neutral, therefore a balance exists with counter ions and the sum of all charges should be +/- zero.

I would not say that Ca is assimilated in plants, perhaps accumulated. Ions like nitrate, sulphate, CO2 are assimilated.

Andreas

As you can see from the two preceding answers, Ca is transported in the xylem.  Think about it, how else can it get transported from the roots to the tops?  In contrast, Ca is not very mobile in the phloem.