Interactive effect on pH of ammonium:nitrate fertilization ratio and the pH buffer of growing media

J.B.G.M. Verhagen
For years peat was the main component for growing media. Fertilization in pot plant cultures as such is completely adapted to it. Due to its decomposition peat has a high content of humic and fulvic acids which buffers cations. This cation exchange capacity (CEC) does not only buffer nutritional elements such as ammonium, potassium, calcium and magnesium. It also buffers the pH of the growing medium. Whenever the pH is forced to rise or decline due to uptake of nutrients by the plant, this will be inhibited by this pH-buffer. In fertilization schemes pH is mainly conducted by the ratio of ammonium:nitrate. Schemes which dominantly contains ammonium will lower the pH, whereas schemes dominantly contains nitrate will let the pH rise. In most cultures a large change of pH is unwanted. In peat based growing media this pH change is inhibited to a large extend by the pH-buffer of the medium. Nowadays growing media do consist of other components than peat. Among others coir, woodfiber, perlite and bark are often used in mixtures. In practice these mixtures showed a more labile pH behavior in comparison with 100% peat based growing media. Based on a two way titration it was concluded that coir, woodfiber and other constituents do have a much smaller pH buffer when compared to average peat materials. In a pot plant culture with Pelargonium, growing media differing in pH buffer were tested under fertilization schemes which differed in ammonium:nitrate ratio. This experiment showed clearly the interaction between the pH-buffer of a growing medium and the behavior of the pH under various ammonium:nitrate ratios in a pot plant culture. Based on this research it is advised that the pH buffer of a growing medium should be taken in to account when composing a fertilization scheme for a culture.
Verhagen, J.B.G.M. (2019). Interactive effect on pH of ammonium:nitrate fertilization ratio and the pH buffer of growing media. Acta Hortic. 1266, 249-254
DOI: 10.17660/ActaHortic.2019.1266.35
growing medium, peat, coir, woodfiber, pH, pH-buffer, humic acid, fulvic acid, cation exchange capacity, ammonium:nitrate fertilization ratio

Acta Horticulturae