The government says the creation of new nature reserves will allow for the construction of 3,700 new homes.
Dorset Wildlife Trust (DWT) previously announced that it had partnered with Natural England to purchase 833 acres (335 hectares) of former farmland and Down at Liscombe, between Dorchester and Blandford Forum. .
He said the agreement would reduce pollution at Poole Port.
The government said the cuts would offset the environmental impact of housing and allow more homes to be built.
Rob Farrington, the trust's head of natural landscapes, said the port was “on the brink of ecological collapse” due to agricultural runoff and sewage.
He said former farmland would be grazed less intensively in the future to reduce excess nutrients leaching from the land into the ocean.
Other plans for the trust include restoring wetlands and improving public access.
The government says developers will be able to purchase Nutrient Relief Scheme credits to offset their housing projects.
Marian Spain, CEO of Natural England, said: “The acquisition of Liscom is an important step forward in reducing nutrition sources and promoting sustainable development.”
“Providing nutrition credits through on-site work like this allows us to continue building homes into the future without harming our waterways.”
DWT said Liscom, purchased from private landowners and farmers, was its most expensive land purchase.
The site, approximately 123 acres (50 hectares) of chalk slope reserve, was purchased by Natural England and leased back to the trust for management.
Brian Brace, chief executive of the trust, said: “Projects like Liscombe ensure that development also contributes to improving the environment.”
In September 2023, the House of Lords blocked the government's plan to ease water pollution regulations in England to encourage housebuilding.
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