Dorset Council is working with Dorset Police, the National Trust and Forestry England to track down those responsible, and appealed to the public for information, “no matter how small it may seem”.
A spokesperson said: “Although recent fly-tipping incidents occurred on private land, we are committed to supporting partner agencies in tackling waste crime in rural areas.”
Dorset Police and Crime Commissioner David Sidwick said: “The dumping of waste earth at this precious SSSI nature reserve, which has already been so badly damaged this year, is nothing short of despicable.
“This type of irresponsible fly-tipping blights our countryside, damages protected habitats and undermines the community’s trust in keeping our environment safe.”
Holt Heath is a Site of Special Scientific Interest and a National Nature Reserve.
About 69 hectares (170 acres) of nature habitat was destroyed there in the summer that was started deliberately and took six days to put out.
Dorset Council said the area between Holt Heath and Uddens Drive was a current hotspot for fly-tipping.
It estimates the issue costs the council £100,000 a year to clear up.