Calibrachoa is considered an iron inefficient plant that commonly has problems with iron deficiency. Iron becomes less available to the plant as substrate pH increases. Deficiency is most commonly the result of high substrate pH, rather than undersupply of the nutrient. Monitoring and controlling substrate pH will prevent the majority of nutritional problems during crop production. Avoiding these problems will increase crop quality. The composition of fertilizer can cause substrate pH to decrease or increase and the direction of the shift is controlled primarily by the form of nitrogen. Nitrogen is the most important pH controlling ion because it is the only element required by plants that can be supplied as both a positive cation (ammonium: NH4+) or a negative anion (nitrate: NO3-) and accounts for more than half of the nutrient ions taken up by plant roots. Fertilizers high in NH4+ have an acidifying effect and cause substrate pH to decrease and the opposite is true for fertilizers high in NO3-. When NH4+ (or other positive cations) is taken up by the plant a positive charge enters the root. Plants must remain electrochemically neutral and thus the root secretes a positively charged H+, which reduces the pH (Fig. 2). When NO3- (or other negative anions) is absorbed, the root balance the negative charge by absorbing H+. As more NO3- is absorbed, more H+ is removed from the soil solution and the substrate pH increases (Fig. 3). The purpose of this experiment was to determine the ammonium to nitrate ratio that would hold substrate pH constant for Calibrachoa grown with deionized water and tap water containing excessive alkalinity.