The nitrate-based Peters 15-0-15 Dark Weather Feed fertilizer recommended by Mark looks like an excellent solution for winter growers. Ammonium and urea-based fertilizers (ammoniacal nitrogen) should be avoided during the winter months. Soil bacteria convert ammonium and urea (urea via ammonium) to nitrate, the form of nitrogen most commonly used by plants. The rate of soil nitrification (conversion of ammonium and urea to nitrate) significantly decreases as soil temperatures drop below 50F (10C) but a significant decrease also occurs even below 60F (16C). This means soil ammonium concentrations can significantly increase in cold winter soils when using the wrong fertilizer. High soil ammonium levels in cold soil frequently result in ammonium toxicity in plants. Oxygen is also required for nitrification to take place so wet, poor draining soils can further exacerbate the problem. The symptoms of ammonium toxicity are well documented in the greenhouse industry for many species but usually include some combination of chlorosis, curling and burning of leaf margins, necrotic leaf spots, and overall stunting. Many plant species of interest here, including West Coast natives, grow during the winter months and benefit greatly from supplemental nutrition. Growers typically use nitrate instead of ammoniacal-based fertilizers during the winter to avoid ammonium toxicity. > On 03/05/2023 6:47 PM Mark Akimoff via pbs <pbs@lists.pacificbulbsociety.net> wrote: > > I have had this dilemma more than a few times over the past few years. The ammoniacal nitrogen form often messes with the pH in cold weather sometimes dropping it significantly, and at low temps the ammonia won’t nitrify and can cause toxicity. Using a nitrate form is better in the cold weather, in the greenhouse many of the bulbs are starting into very active growth now and do need some feeding. I recently discovered a product called Peters Dark Weather feed, it’s a 15-0-15 with cal mag and micros that is mostly nitrate nitrogen. I have been doing 50 ppm which is a pretty low dose but the plants have responded very well and I’ve even seen a few nutrient deficiencies corrected despite the temps in the 40’s. > > Mark > > Illahe Rare Plants > Salem, Oregon > _______________________________________________ pbs mailing list pbs@lists.pacificbulbsociety.net http://lists.pacificbulbsociety.net/cgi-bin/… Unsubscribe: <mailto:pbs-unsubscribe@lists.pacificbulbsociety.net> PBS Forum https://…