But carnivorous plants often live in "awful", nutrient-poor places. In other words, very nutrient scarce environments where they're very boggy, so most of the nutrients have probably been leached away by water. As a result, the soil is so poor that many of those trace elements that keep plants growing normally, just aren't available in appreciable amounts. So the plants need to look to the air to obtain that nutrition and they do it by catching insects because if they catch an insect, insects have got lots of iron, they've got lots of proteins, they've got lots of other micronutrients in them that the plants have adapted and evolved to make use of, and to supplement the poor source of things that are coming in through the soil.
