I can't say specifically for venus fly traps, but carnivorous plants exhibit a range of needs. The benefit of carnivory is that the plant can get nitrogen and phosphorus (nitrates and phosphates) from a source other than the soil. This means that it can survive in soils that would not support most plants e.g. acid bogs. Some of these plants have become so specialised that they are unable to absorb and process nitrates from the soil, in which case they are compelled to get nitrogen elsewhere or die. Others retain some ability to use inorganic nitrates, but grow better with a mixture of nitrogen sources.
