It sounds like fertilizer/salt build-up, indeed.
Could also be precipitate from your water source, if your water isn't filtered.

Josh

What Josh said.

Solids don't evaporate, so when water at the surface of the soil evaporates, everything that was soluble gets left behind. Those of us using fast soil (soils that drain quickly and aren't water retentive) are able to guard against this build-up because we water copiously when we water. The salts go back into solution and are flushed from the soil and go out the drain hole. When using water retentive soils that require you to water in sips to prevent root rot, ALL the dissolved solids from fertilizers AND your tapwater remain in the soil where they can build up to critical levels. This is an issue that needs to be managed properly with all plantings, but it is a MUCH larger issue with houseplants than most growers ever realize and usually goes completely undiagnosed or misdiagnosed.

Al

Any ideas then for dealing with this, preventing it from happening? If I'm getting a fertilizer/salt build up after 2 weeks, it can only get worse through the summer. Am I better off flood-watering rather than 'sip watering'?

I'm in a condo and have only patio space for my vegetable/fruit/herb garden, so the giant 'earthbox' type of planters don't work here. I've used what I thought was a good choice for soil - the balcony faces west and gets 10 hours of intense sun daily through the summer, so the moisture retaining bits in the soil were essential, complete dry-outs of the soils seem to be my biggest obstacle in the summertime.

I keep reading up on hints/tricks/experiencial mistakes to learn the best way to do patio-container gardening, but it's hard to get good clear info sometimes. :(

The only way to prevent it is to flush the soil on a regular basis. Ideally, you would be using a soil that allows you to water copiously enough so salts are being flushed before they can accumulate. If you think your soil is too water retentive to allow you to water in such a manner w/o risking root rot, there are some things you can do. After flushing the soil, tilting the container at a 45* angle will drain a lot of excess water that would not have drained if you kept it level. Also, employing a wick through the drain hole and allowing it to dangle below the container will drain even more water when used in combination with tilting after flushing the soil. Partially burying a pot is a great way to eliminate excess water from containers, too.

The negative effects of saturated soils do diminish somewhat as the planting matures and roots colonize the entire container, thus allowing a more favorable volume of air to return to the soil faster.

FWIW - I'm pretty sure you'll get good information here. If someone offers the wrong information or offers something that can be looked at from more than one perspective, someone usually speaks up to clarify or present another view for your consideration. Hopefully there will be some sort of majority consensus, or someone will stand out as the voice of reason and instill enough confidence for you to decide what's right for you.

Take good care, and good luck, CL.

Al