Merino wool is an excellent fabric for keeping your feet dry, warm and comfortable. But you've probably noticed that many of the merino wool socks available contain a blend of merino wool and other synthetic fabrics. The reason for this is very simple. 100% Merino wool socks do not last as well as blended socks. While popular sock companies like Smart Wool are working on new knitting designs to improve the durability of merino wool socks, nothing lasts today as well as a blend of merino wool and some other popular synthetic fabrics.
You will typically find that merino wool hiking socks contain nylon or polyester as well as a small percentage of lycra/spandex. The lycra/spandex is there to help the sock stay up and keep its shape. Nylon and polyester are very durable fibers. When added to merino wool socks, they extend the life of the sock dramatically. A few years ago, I bought 2 pair of 100% merino wool socks. After just one summer of wearing them hiking, the heels and toes wore completely out of them. I bought the same model sock from the same company after they had changed up the sock a little and had added in some nylon. So far they've lasted me a whole summer of hiking and will likely last a couple more.
So, when you're shopping for some nice merino wool socks that you expect to last more than just a summer, you'll want to get a pair that have some nylon or polyester blended into the fabric.