Leachate Concentrations of Ammonium, Nitrate, and Phosphorus as Affected by Nutrient Release From Four Different Types of Controlled-Release Fertilizers and Crop Development of Containerized Azaleas

Concentrations of ammonium, nitrate, and phosphorus in irrigation leachate were measured weekly over a 47-week period from a low-fertility, acid-based substrate into which four types of 12-month controlled-release fertilizers (Osmocote, Nutricote, Polyon, or Multicote) were incorporated. Containers of substrate were placed in an unheated greenhouse, with one-half of the 2.4-L containers planted with liners of '' azalea, while one-half of the containers remained unplanted. In substrate without plants, concentrations of ammonium remained low during the first 4 weeks, rose to a peak around week 8, declined until around week 30, and remained low thereafter. Nutrient uptake by developing root systems significantly reduced leachate concentrations of ammonium from week 8 and throughout most of the remaining weeks of the study, with the most notable uptake indicated by reductions in ammonium in leachates during the period of maximum nutrient release. Concentrations of nitrate also generally remained low during the first 4 weeks, rose to peak around week 8, then decreased and stabilized for the remainder of the study in all substrates except the substrate containing Osmocote. Nitrate concentrations in leachate from substrate containing Osmocote tended to be more stable compared with other fertilizer treatments, and were otherwise similar to the other fertilizer treatments in most weeks from around week 17 to the end of the study. Nutrient update by the growing plants was evident by consistent reductions in leachate nitrate concentrations from week 18 to the end of the study. Concentrations of phosphorus in leachates from substrate without plants generally tended to be at their highest (with some fluctuation) during the first 10 weeks of the study. From week 9 to the end of the study, presence of growing plants resulted in a mostly consistent reduction in phosphorus concentrations in leachate.