I recently bought some clay pots and put a one year old date palm in it using a potting mix of 1 year old compost, leaf mold and very few of bought horse-manure-mix. For drainage I used some stones.
Today I noticed that there is some white foam visible pn the outside of the pot. It looks like a fungi. I read somewhere that it could also be salpeter.
So what it is and is it dangerous for my plants?
EDIT: as requested I upload a photo: (click for a larger version)
It is efflorescence or mineral salts. They are not harmful to the plant. I assume if the whole outside of the pot was covered in a solid crust it could slow down the transfer of water from the soil to the air through the clay walls. At this is quite small you could just use a wire brush to remove it or vinegar as Niall mentions.
It was likely to be the choice of horse manure that caused this as some manures, even after being composted, still have a high level of dissolved salts. Don't let this stop you from using it. Composted manure is a good source of organic matter for soil mixes.
