Effects of Ammonium to Nitrate Ratios On Substrate pH Shifts During Growth of Calibrachoa with Alkaline Water

Calibrachoa is considered an iron inefficient plant that commonly has problems with iron deficiency.  This problem is often the result of high substrate pH, rather than an undersupply of the nutrient.   Fertilizer composition can cause substrate pH to decrease or increase and the direction of the shift is controlled primarily by the form of nitrogen.  Fertilizers high in NH₄⁺ have an acidifying effect and the opposite is true for fertilizers high in NO₃^-.  The purpose of this experiment was to determine the ammonium to nitrate ratio that would hold substrate pH constant for Calibrachoa grown with either deionized (DI) water or tap water with high alkalinity.  Calibrachoa hybrid 'Superbells Royal Blue’ plants were greenhouse grown and received irrigation with two concentrations of water alkalinity (0 and 300 ppm calcium carbonate equivalent) and 5 different fertilizer treatments with ammonium to nitrate ratios of 90:10, 70:30, 50:50, 30:70, and 10:90.  Substrate pH was determined every two weeks by calcium chloride substrate extraction.  When plants were grown with DI water, substrate pH decreased over time regardless of fertilizer treatment.   Plants that received fertilizer treatments with the two highest amounts of ammonium had the lowest end-of-crop substrate pH of 4.4 and as the amount of ammonium in the fertilizer decreased, end-of-crop substrate pH increased significantly to 5.2.   When plants were grown with highly alkaline tap water, substrate pH increased over time regardless of fertilizer treatment.   When plants were fertilized with 90% nitrate end-of-crop substrate pH was 6.2 and was significantly higher than from plants that received all other fertilizer treatments, which ranged from 5.8 to 5.9.  These results indicate that when water is devoid of alkalinity, substrate pH may decline regardless of the ammonium to nitrate ratio.  Indeed, the rate at which pH declines can be controlled by the ammonium to nitrate ratio.  When water alkalinity is excessive, substrate pH may increase despite the ammonium to nitrate ratio.  Water with this level of alkalinity is not recommended for crop production and should be blended with other sources to reduce the strong effect on substrate pH.  High ammonium fertilizers alone may not be able to counter act the increase in pH and may also reduce growth and increase the incidence of ammonium toxicity.