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nitrate toxicity levels for plants

     I was reading an old TAG issue last night on Cryptocorynes (4:1 I 
     think) and the author was discussing substrates for Crypts. He was 
     using sphagnum peat moss in pots. I was wondering about mixing it with 
     anything else or covering with gravel. Anybody care to describe the 
     use of peat in substrates especially those designed for a variety of 
     plants? I suspect peat might have a tendency to decompose a little 
     faster if mixed with soil. I also suspect that large ratios of peat 
     might not be suitable for certain plant types.
     My question is about nitrate toxicity especially with Crypts. The 
     author of the TAG article mentioned that certain types of Crypts 
     didn't grow well in high nitrate environments (i.e. ~100ppm) He 
     suspected this contributed to Crypt meltdown. I've grown a variety of 
     Crypts (not as many as the author) in a tank with high nitrates but 
     found the plants to grow rapidly and they were quite robust. I've not 
     experienced meltdowns but I doubt that I'm growing anything sensitive. 
     I also use strong lighting so I suspect this enhances the Crypts' 
     vitality and health. I suspect that meltdown actually caused by 
     nutrient deficiencies and poor conditions in general. Any comments?
     I was able to previously grow Rotala macranda using a less fertile 
     substrate with good success however in the highly organic substrate 
     tank with high nitrate concentrations, it did not grow well and died 
     out. I wonder if this was due to nitrate toxicity or to an intolerance 
     for a very low redox potential in the substrate or to a lack of other 
     nutrients since some other nutrient must have become the limiting 
     growth factor. It could be that the R mac couldn't get enough iron 
     from the substrate since I was maintaining relatively low levels of 
     chelated Fe in this tank and there could have been strong competition 
     for it. I can't be sure if this was a case of nitrate toxicity or not. 
     Any comments?
     Steve in Vancouver where the heat wave may be ending