What are the white crystal-like substances that appear on my retaining wall after it rains?

After it rains these white crystals appear on my retaining wall but only below the part where there is dirt on the other side of the wall. I assume they are caused by water seeping through the concrete as it takes a few days after it has stopped raining for them to appear. Part of the wall is covered (and so it does not get rained on) so i know it is not a surface problem caused by the rain itself. Is this a symptom of a problem I should be worried about?

Here is a picture: <a href=“http://imgur.com/hup28.jpg” title=“Hosted by imgur.com”>http://imgur.com/hup28.jpg</a>

[URL=http://imgur.com/hup28.jpg][IMG]http://imgur.com/hup28.jpg[/IMG][/URL]