Iʻm not sure about measuring the flow rates, but you could do either a mass balance (Ca lost from solution) or total calcium in the plants at different stages of growth (if the plants are small, which I assume they are if you are growing in hydroponics).

The answer mentioned by Mr. Lincoln may have a point, you can measure the Ca lost from the solution and observe the plants vegetative parts especially the young leaves and also the roots because Ca have important role in these parts. If the young leaves are deformed and necrotic then there is Ca deficiency hence there is a little or no Ca uptake same goes to its roots. Now if your plants are fine then do some measurements of Ca lost from the solution and that may be the Ca uptake of your plants.

Did you try to use radioactive tracer (45Ca2+)? It works quite well on cell suspensions (see the attached file). In your condition, the volume of the culture medium is maybe to high for developping such approach ?

David

Calcium uptake in plants can be measured following hydroponic culture. For this single plant can be grown under recycled hydroponic system. The amount of water evaporation or loss should be recorded. Nutrient solution sampling for analysis can be done weekly or even daily basis. Uptake amount of Ca+2 in the once used nutrient solution can be determined using Atomic Absorption Spectrophotometer (methods for determining Ca+ in the attachment).