Can I put other vegetables in a manual wheat grass juicer? I'm thinking about buying a wheat grass juicer, but the fact that it only juices one thing gives me pause.  Are they adequate at juicing vegetables such as spinach, kale and carrots?
Some juicers are better than other kinds for different things.  A dedicated wheat grass juicers probably isn't going to juice spinach, carrots, and kale well.

A centrifugal juice usually doesn't handle wheatgrass well and in general isn't supposed to be as efficient as some other kinds for leafy greens.

A single-gear or double-gear masticating juicer works great for leafy greens, but isn't supposed to be as efficient for softer fruit.

If you're looking for something that will do wheatgrass, leafy greens, and a variety of other things - consider something like the Omega 8004.  Long warranty, easy to clean, and gives good quality juice.  If you need to make juice for a lot of people though, it is kind of slow for large quantities.