In studies of mineral nutrition of plants, we often use systems in which we can precisely manipulate concentrations of mineral elements. Typically, this would be a hydroponic system in which plants are grown in solution(s) containing set concentrations of mineral elements. It is relatively easy to change the concentration of a particular mineral element, although any changes made have to be carefully planned and designed. It is recommended to use a programme which helps to determine required changes to retain as similar conditions as possible in the different treatments. One of such example is GEOCHEM-EZ, which is “a multi-purpose chemical speciation program, used in plant nutrition and in soil and environmental chemistry research to perform equilibrium speciation computations, allowing the user to estimate solution ion activities and to consider simple complexes and solid phases. Programs of this type allow the user to estimate the interactions between metals and ligands and to calculate the free activities of the ions of interest. In doing so, the scientist can make a solution in which requisite conditions are satisfied and the design is intelligent.” 
For my experiments into root exudates (ie the bulk of all the macromolecules or mineral elements released into the soil by roots) I’ve build a simple hydroponic system in which I’m growing the selected of Brassica oleracea genotypes in solutions with “normal” phosphorus concentration or “low” phosphorus concentration in combination with “normal” zinc concentration or “low” zinc concentration.
I’ll be collecting exudates in two days and then I’ll perform chemical profiling to determine the changes in the composition of the exudates as affected by treatments and as affected by genotypes. I hope to see important differences between genotypes which enable the increased uptake of zinc even in the presence of high phosphorus.
 Shaff JE, Schultz BA, Craft EJ, Clark EJ, Kochian LV (2010) GEOCHEM-EZ: a chemical speciation program with greater power and flexibility. Plant and Soil 330, 207–214. DOI 10.1007/s11104-009-0193-9