Scotts admitted failing to ensure the safety of employees at the Stainforth Moor Peat Works in Thorne, Doncaster, during a hearing before Sheffield Crown Court.
The court heard that employee Geoffrey Burnitt, 54, was killed after he was trapped by the hydraulic arm of a baling machine.
Evidence was given that a gate that led to the machinery was incorrectly secured.
Alex Offer, prosecuting on behalf of the HSE, said that the electronic security system on one of the three gates that guarded the machinery was not working properly and that it had been secured on a temporary basis with only a handwritten warning sign added.
Offer continued: "The situation remained like that for four days. The gate remained inadequately secured and no one made any attempt to finish the job."
Offer added it was not possible to say why Burnitt had entered the area in March 2006.
In mitigation it was said that safety procedures had now been upgraded so there could be no repetition of the accident.
Fining the company, Judge Graham Robinson said that Burnitt, whom he described as an experienced and valued employee, had deserved better protection.
Judge Robinson added: "As a minimum, the gate should have been secured by a chain and padlock and, following that, should have been welded together - a process that took just 30 minutes from start to finish.
"The problem with a handwritten sign is that it is unprofessional, does not demand attention and loses impact very quickly. This was a warning that remained in place not for four minutes, or even four hours, but for four days."
The company was also ordered to pay £20,000 in costs.
Scotts would not comment.