Previous owners salvaged broken sidewalk pieces to use as stepping stones for paths and flat areas. I want to use these areas for garden plants. Should I dig up the concrete pieces or will it be OK to cover them with soil? Will the chemicals in the concrete adversely affect the soil ph?
In general, concrete will have a high PH. This is due to the alkaline base most concrete mixes employ. If I was to guess, I would say leaving it in the soil could actually help keep your soil's PH stable. As our SOIL PH CONTROL ARTICLE explains, most yards tend to drop their soil PH over time. If this exists in your yard, the concrete will most likely work to stabilize the PH so in the end, I'm not sure you'll need to remove it nor worry about any other "chemical" releases that could impact plants in a negative way.
At this point the only thing I suggest is to get a SOIL PH METER so you can get a "ground" or control measurement of where your PH is now. Use this to compare what the PH becomes, should it change, by checking it every 3-6 months. I think you might actually find it remaining quite stable and wouldn't be surprised if you didn't have to do nearly as much to keep your PH in check compared to most homeowners. In the end, this might turn out to be something positive
Here are direct links to the information and products listed above:
Soil PH Article: http://www.soil-ph.com/soil-ph
Soil PH Meter: http://www.bugspray.com/catalog/products/page1736.html