Often root vegetables are put in the tough and gnarly category, underground knobs that are considered best when cooked. But it may surprise you to know that cooking isn’t always necessary. Just think of carrots, radishes, and even grated beets, which are wonderful raw. In addition, fresh turnips (the ones with their greens attached) are sweet, crunchy, and moist when raw, and can be served as a snack or appetizer when thinly sliced or quartered and sprinkled with a pinch of sea salt. (I’ve even snacked on slices of big old storage turnips and don’t find them too strong, although they lack some of the better qualities possessed by the summer turnip.) Tender kohlrabi can be treated the same way.
However, there are no guarantees, because tenderness, moistness, and delicacy depend in part on how and where a vegetable is grown. Turnips and kohlrabi grown in a hot, dry climate without sufficient irrigation might turn out to be pretty hot and spicy, just the way radishes get as the weather warms up, so taste to make sure you know what you’re getting. If you’re using such strong, aggressive vegetables for soup, you can reduce their spiciness with a quick blanching of the cut pieces in salted, boiling water; then again, it’s the nature of soup to temper all flavors, so even that isn’t strictly necessary. (Besides, stronger flavors are what give soup character.)
As for rutabagas, I have never seen one at the store or farmers’ market with greens attached. My guess is that by the time they mature those greens are pretty ragged, so they just get lopped off at the farm. Nonetheless, pretty, pale yellow rutabaga flesh is mild and often sweet, but drier than turnips or kohlrabi. Thinly sliced, rutabagas can be nibbled perfectly raw.