There is no need to put a cutting very deep into the soil. Most commercial growers barely put them into the soil and support them with a stake or post. The flesh of the segment will just rot by having it planted in the soil. This has been written about quite a bit by growers in the U.S., Asia, etc. in the pitaya discussion group at
PitayaFruit : Pitaya And Other Dessert-Fruiting Cacti