Managing Fertilization of Bedding Plants: A Comparison of Constant Fertilizer Concentrations versus Constant Leachate Electrical Conductivity

Abstract

In the last decade, there has been an increasing focus on maintaining the electrical conductivity (EC) of leachate of bedding plants within an optimal range. However, there has been no research determining whether an optimal leachate EC results in better growth than using constant fertilizer concentrations throughout the production period. To evaluate the effects of constant fertilizer concentrations and constant leachate EC on the growth of wax begonia (Begonia × semperflorens-cultorum Hort.) ‘Cocktail mix’ and petunia (Petunia × hybrida Hort. Vilm-Andr.) ‘Gnome white’, we grew plants either with one of six different fertilizer concentrations (fertilizer EC of 0.5, 1.5, 2.5, 3.5, 4.5, or 5.5 dS·m−1) or by maintaining a leachate EC close to 0.5, 1.5, 2.5, 3.5, 4.5, or 5.5 dS·m−1. The leachate EC of plants fertilized with constant fertilizer concentrations increased throughout the experiment if the fertilizer EC was 2.5 dS·m−1 or higher, was stable in the 1.5 dS·m−1 treatment, and decreased in the 0.5 dS·m−1 treatment. In treatments in which we tried to maintain the leachate EC constant, the leachate EC on average was within 0.2 dS·m−1 of the target EC. As a result of the acidic nature of the fertilizer, the pH of the growing medium decreased throughout the experiment with increasing leachate or fertilizer EC. When plants were fertilized with constant fertilizer concentrations, fertilizer solution EC of 0.52 and 1.24 dS·m−1 were estimated to be optimal for begonia and petunia, respectively. When the growing medium was maintained at a constant EC, 1.0 and 1.7 dS·m−1 were estimated to be optimal for begonia and petunia, respectively. Growth of both begonia and petunia was greatly inhibited when high fertilizer concentrations caused accumulation of soluble salts in the growing medium. Growth was reduced more by high fertilizer EC than by high leachate EC treatments. This difference probably occurred because superoptimal fertilizer concentrations resulted in very high leachate EC (up to 10.5 dS·m−1 for petunia and 12.5 dS·m−1 for begonia), which in turn inhibited growth. Periodic measurements of leachate EC can be a valuable tool in fertilizer management to prevent such excess buildup of salts in the growing medium.