Kale and spinach are both nutrient-packed greens. Kale is a member of the Brassica family, which includes cabbage and Brussels sprouts, while spinach is softer, and more lettuce-like. In some dishes, the two work well as substitutes for one another, with slight adjustments required for cooking. In other dishes, however, the differences between kale and spinach pose greater problems for direct substitution.
Kale requires longer cooking time than spinach, due to its thicker, coarser structure. Similar to cabbage in terms of its toughness, kale takes longer to wilt than spinach and needs more time to soften in recipes that call for the green to be cooked down. Similar to spinach, though, kale will greatly reduce in volume once wilted. Additionally, kale takes on the flavor of what it is cooked with, even more so than spinach. From a textural standpoint, even once cooked, kale is leafier than spinach.
Kale lends itself well as a substitute for spinach in casseroles, as the baking time allows the green to soften and the texture is somewhat hidden by its incorporation into the overall dish. For the best results, cut the kale into thin strips or smaller pieces than you ordinarily would the spinach, so that it cooks evenly and completely. Use approximately the same amount of kale, by volume, as you would spinach for the recipe.
When substituting kale for spinach in a soup, it's important to remember that the kale will take significantly more time to soften than the spinach. Fortunately, soups often simmer for long periods, making kale a viable ingredient. Use the same volume of kale as you would spinach, cut in the same manner, but add it to the soup with the liquid at the beginning of the cooking time, rather than toward the end of the cooking, as you would with spinach. If the soup does not simmer for a long time, wilt the kale in a pan before adding to the soup to ensure that it gets fully cooked down.
Substituting kale for spinach will not work as well for salads as for cooked dishes, as the texture differences between the two are so significant. However, kale can be eaten raw and used in a salad. Instead of using the kale in whole leaves, as you would with spinach, thinly slice or roughly chop the kale into smaller pieces. Toss the kale with a lemon vinaigrette and almonds, or drizzle with olive oil and salt and add the ingredients you would have for a spinach salad. Alternatively, add kale to a spinach salad as a complement. The texture will be different, but still delicious.