Unless precautions are taken to keep the Fe of the culture medium in solution, results obtained by varying the pu will not represent the true effect of this factor on growth. Fe is precipitated at the lower H-ion concentrations, the Fe of certain salts is less likely to be precipitated than that of others, and certain salts of organic acids tend to keep Fe in solution. In general, ferric citrate seems to be the most favorable source of Fe. In addition to chemical precipitation, it is also possible for the Fe to be removed by adsorption on an amorphous precipitate such as calcium phosphate As this precipitate is frequently formed when nutrient solutions are made alkaline, this may account for the discordant results in the literature as to the availability of certain forms of Fe. By omitting Ca from the culture solution, Fe can be maintained in a form available for growth in alkaline solutions by the addition of sodium citrate. In such solutions, the maximum growth of Chlorella occurred at pn 7.5 The alkaline limit for growth has not been established as yet In investigating the availability of Fe at varying pn values, changes in the pn of the solution during the course of an experiment should be taken into account. This is especially important in unbuffered solutions. The differential absorption of the ions of ammonium salts may cause a marked increase in the H-ion concentration, which increases the solubility of Fe. In strongly buffered solutions, as used in these experiments, this effect is slight.