Bristol farmhand Lee Mason was "virtually strangled" to death after being caught in an unguarded soil-sifting machine, a court heard.

Bristol Crown Court was told that, just three days short of his 18th birthday, Lee got his left arm caught while operating the device alone at Bridgwater Farm, Dundry, causing him to be pulled into the machine's framework, fatally crushing his neck.

Farm owner Roy Hill, aged 66, and his son Michael, 32, both deny manslaughter by gross negligence in April 2007.

Both father and son also each deny breaching the Health and Safety at Work Act 1974, but Roy Hill has admitted a separate health and safety charge in employing Lee without taking a safety risk assessment.

Philip Mott, QC, prosecuting, told the court on Tuesday that Lee, from Knowle, was not only young but of "very limited intelligence", having left school aged 16 with four F-grade GCSEs in design technology, English, science and religious studies.

Mr Mott said: "The general effect is that Lee needed things to be explained simply and he needed things to be explained over and over again before he understood.

"When he started at the farm, he really didn't even understand how to use a spade."

Mr Mott said that after Lee had worked at the farm for 18 months, manager Michael Hill had considered sacking him but had a word with Lee's stepfather, which led to Lee improving in motivation.

Mr Mott told the court: "He lacked any natural common sense or instinct about danger, in particular working with machinery.

"At the same time, he loved machinery and driving diggers, and that was a toxic combination which meant it was probably the last place for him to be asked to work."

Mr Mott told the jury the farm had no health and safety policy, training courses or risk assessment.

He said it was up to employees to use their common sense when working to keep themselves safe - and, due to Lee's youth and inexperience, he was not able to look after himself.

Mr Mott said the soil-sifting machine was used to sift ungraded soil down into topsoil, operated with a conveyor belt on rollers.

The court heard the belt had been running too fast, but this was rectified by the suppliers, and a tracking problem meant the belt sometimes did not run true.

It is the Crown's case that, when the accident occurred, a safety guard was either not in place or not mounted properly.

But Mr Mott said that Lee, who weighed only seven-and-a-half stone, would not have been capable of lifting off the guard on his own, which weighed five-and-a-half stone.

The jury also heard that, with the safety guard not in place, the machine was still able to run.

Mr Mott told the court that for some reason Lee must have put his arm into the machine, he was trapped and pulled in, and he lost his life.

Bristol Crown Court heard the health and safety charged denied by the father and son related to Roy Hill as employer and Michael Hill as manager.

When interviewed by police in July 2007, Roy Hill said: "Sometimes you had to explain to him (Lee) what common sense was."

The case continues at Bristol Crown Court.