Low mash efficiency w/ soft water?

I brewed my first all-grain batch and hit 82% efficiency, using hard water from a friends well. The last two batches I've done, I used the soft city water from my home. On both of those runs I've seen about 65% efficiency. The crush looked to be the same on all batches and all brewes were similar, all wheat beers. Is there an issue using soft water causing my low numbers? I felt that my brewing technique was improved the last two runs, but my numbers just don't show it. Only difference was the water.

The soft water has less minerals. Minerals play a part pH, which impacts the efficiency of the extraction of sugars from the grains. This is the very simple version of water chemistry. I would recommend going to the How To Brew website and reading the basic water chemistry info. You can also go to the Brew Science Forum on this site and read the stickies on water chemistry. There's a little more to AG brewing than just mixing water and grains.

I agree with JJL - it probably has to do with mash pH. When I brew light beers with soft water, I often add a bit of saur malt to bring the pH down.

By soft water, do you mean softened water? Or is the water at your house naturally soft from the source?

If the source water is hard and the house has a softener, then its probable that the softened water still has high alkalinity. That is the real drain on mash performance. Hard and alkaline water can still produce a good beer when brewing some styles. Soft and alkaline water cannot typically make a good beer since the mash and kettle pH is likely to be higher than desirable. Bypass the house softener if there is a softener. If the bypassed water tastes good, brew with it.

Cool, thanks for the input! I do have a softener and have seen my water report and it's not pretty. I really was just doing some tests since I'm new to AG brewing.