Moderate amounts of KH2PO4 (0.5 to 0.7 millimols) caused iron chlorosis of seedlings in solutions less acid than pH 5.5-6.0. The trouble could be avoided by omitting the phosphorus entirely and adding it separately after 2-4 days. Even with this modification the Knop type of solution (Hoagland, 2) caused persistent chlorosis in soybeans and broccoli. In the commonly used Shive solution (R5-C2) a large excess of KH2PO4 holds the pH of the solution between 4.5 and 5.0 and chlorosis is moderate to slight. The Shive solution contains 36 times as much phosphorus as the Hoagland and 6-week-old plants growing in it absorbed 15 times as much to make a slightly poorer growth. The addition of as little as 0.125 g per liter of NH4NO3 resulted in decidedly acid solutions in every experiment with nine plant species representing seven families. Initial pH's of 5.25 in an unbuffered solution fell to minimums of 3.9 to 2.9 and then rose, to pH 5.0 or 6.0 with large plants. The low pH's were shown to be due to a preferential absorption of NH+4 ion and the later rise to the slower absorption of the NO-3 ion. The use of NH4NO3 prevented iron chlorosis with some plants, notably soybeans, but resulted in acidities which were injurious to young corn and to cotton. In work done in this series but not described above, the "X" solution containing NH4NO3 gave exceptional growth of young coffee (Coffea arabica), but was toxic to older plants. It is probable that iron chlorosis has more effect on solution culture results than any other single factor, and frequently than all other factors. Iron absorption from cultures is reduced by phosphorus, probably by H2PO-4 ions especially, at pH's of about 6.0 or higher. The use of Ca3(PO4)2 reduces the trouble as does the use of enough KH2PO4 to maintain a pH below 5.5. In tank culture H2SO4 may be used more cheaply to accomplish the same result. In miscellaneous work with the Knop type of solution chlorosis can be reduced or prevented by omitting phosphorus from the solution and adding it separately 2-4 days later after iron has been absorbed.