When the plants die these roots also add organic material to the soil. Animal droppings and carcasses are additional sources of organic materials. This humus helps to retain the moisture in the soil. Different types of plants have adapted to receiving varying amounts of sunlight. The intensity and amount of sunlight varies.
Many plants require bright sunlight, but many others have adapted to growing in the shade.
Plants growing in a meadow have adapted to much brighter light than the plants growing on the floor of a forest. Forest floor plants receive far less intense light for fewer hours of the day. Another factor that influences which plants grow is the Ph level, a measure of the amount of acid in the soil. Again most plants are tolerant of a range of Ph level. If the Ph level rises too high, many trees die.