Please advise on mode of travel apart from by air, the travel from Hong Kong to Nanning.

If you do not want to travel by air, you can choose the bus and the trains, but you may have to transfer.
First you need to take trains or buses to reach Guangzhou. Then you can go to Guangzhou Railway Station where you can find the trains to reach your destination city.
The train from Guangzhou takes about 12 to 14 hours and the ticket price is around 184 to 196RMB for the hard sleep.
The bus from HK to Guangzhou takes about two hours and a half and about 100 to 200HK$ for the bus fare.