Question About Fertilizing

[Rick]

Antec has a great list on which paphs need calcium supplement. http://ladyslipper.com/calsub.htm

Just to add a bit onto the fertilizing question. What's the difference with fertilizers with and without urea?

Thats not really a list of species that require calcium supplements, but a list of species that are found in association with calcareous environments. Refer back to my post on pH.

Urea is a nitrogen yeilding molecule that is somewhat more complex and less bioavailable than ammonia. Some orchids apparently find it difficult to consume and sometimes toxic. Ammonia and Nitrate are the easiest nitrogen forms for orchids to utilize. Ammonia can be toxic at pH levels from 7.5 and up. So if you wanted to buffer up the fertilizer mix for a roth to duplicate the ultra basic conditions of serpentine (the geology that roths are found over) the pH could be greater than 8, and you should use a nitrate based fert rather than ammonia.

 

[gonewild]

Urea is a nitrogen yeilding molecule that is somewhat more complex and less bioavailable than ammonia. Some orchids apparently find it difficult to consume and sometimes toxic. Ammonia and Nitrate are the easiest nitrogen forms for orchids to utilize. Ammonia can be toxic at pH levels from 7.5 and up. So if you wanted to buffer up the fertilizer mix for a roth to duplicate the ultra basic conditions of serpentine (the geology that roths are found over) the pH could be greater than 8, and you should use a nitrate based fert rather than ammonia.

Urea is used as a cheap source of Nitrogen for fertilizer mixes. Urea is not well suited to fertilizer mixes for container grown plants.

The Nitrogen in Urea quickly converts to ammonia gas and is released to the atmosphere unless contained by the soil. In an orchid mix which is normally very course, loose and has an atmospheric air flow, nitrogen from Urea may not be available to the plant. The Ammonia gas quickly leaves the potting media to escape upward to the atmosphere. So with fertilizers containing Urea you may not actually be getting much nitrogen available to your plant. Or too much).

I suppose the toxicity you refer to with orchids may actually be a reaction to the the ammonia gas by the orchid root?

 

[Rick]

Urea is used as a cheap source of Nitrogen for fertilizer mixes. Urea is not well suited to fertilizer mixes for container grown plants.

The Nitrogen in Urea quickly converts to ammonia gas and is released to the atmosphere unless contained by the soil. In an orchid mix which is normally very course, loose and has an atmospheric air flow, nitrogen from Urea may not be available to the plant. The Ammonia gas quickly leaves the potting media to escape upward to the atmosphere. So with fertilizers containing Urea you may not actually be getting much nitrogen available to your plant. Or too much).

I suppose the toxicity you refer to with orchids may actually be a reaction to the the ammonia gas by the orchid root?

Urea doesn't breakdown spontaneously and needs a fair amount of help from bacteria and bluegreen algae to be utilized by most plants. Thats why its fairly useful for crop plants like soya beans and rice, but not real helpfull for most orchids. If your potting mix is dense with lots of bacterial action to it, then you might end up with an excess of ammonia, which could ulitmately be toxic to the orchid.