Re: Why do venus fly traps need acidic soil to grow properly?

That is a good question. Venus fly trap (Dionaea muscipula) naturally grows 
in soil that consists of sand and organic matter with a pH of 3 to 5, which 
is very acid (Pietropaolo, 1986). In a very acid soil, some mineral 
nutrients, such as iron, manganese, copper and zinc are much more available 
to the plant than in a less acid soil (higher pH)(Brady, 1984). Thus, a 
plant adapted to very acid soil may become deficient in those mineral 
nutrients when the soil is less acid. Other mineral nutrients, such as 
calcium, magnesium, and phosphorus are usually less available in a very 
acid soil than in a less acid soil. Thus, a plant adapted to a very acid 
soil may absorb toxic amounts of those mineral nutrients when grown in a 
less acid soil.

Venus fly trap is unusual for a plant because it does not like normal 
levels of mineral nutrients. It is adapted to soils that are poor in 
mineral nutrients and obtains a significant amount of its mineral nutrients 
by trapping insects or tiny animals. Some people who cultivate Venus fly 
trap recommend not giving it any fertilizer and even watering it with 
rainwater or distilled water (Pietropaolo, 1986).   

References

Brady, N.C. 1984. The Nature and Properties of Soils. New York: Macmillan.

D'Amato, Peter. 1998. The Savage Garden: Cultivating Carnivorous Plants. 
Berkeley, CA: Ten Speed Press

Pietropaolo, J. and Pietropaolo, P. 1986. Carnivorous Plants of the World. 
Portland, Oregon: Timber Press.
 http://www.californiacarnivores.com