1. The iron of ferric tartrate soon becomes converted into insoluble compounds when added to nutrient solutions, and the change is more rapid in solutions of higher PH. 2. The introduction of carbon dioxide lowers the PH of slightly acid, neutral, or alkaline solutions, but does not increase the solubility of iron compounds which they contain. 3. The addition of certain organic compounds to an alkaline nutrient solution increases the amount of soluble iron in the solution.This fact may be of significance in maintaining an adequate supply of soluble iron in solutions for the growth of plants.