YHS holds contracts to process green waste on behalf of Barnsley, Doncaster and Rotherham local authorities.
William Sinclair said wit was now looking to invest in the acquired business and expand the tonnage processed at the site.
The terms of the acquisition were not disclosed.
The YHS site processes 40,000 tonnes of park and garden waste per annum, most of which is turned into green compost.
Bernard Burns, chief executive of William Sinclair Holdings, said:
"The government is determined that peat use in horticulture will reduce and eventually end. This move into composting is a natural progression for our Freeland business. We plan to expand Freeland's composting capacity as demand for peat free products increases."