EXETER, England — The world's longest continually running experiment has revealed traces of plutonium in British soil from nuclear tests in the Nevada desert 50 years ago, scientists said on Monday.

The amounts are "very, very small" and pose no danger Professor Keith Goulding of Rothamsted Research in Hertfordshire, England, told a science conference.

But they show the value of the experiment that began 160 years ago when John Bennet Lawes, the owner of the Rothamstead Estate, appointed a researcher to take soil samples to measure the effect of fertilizers on crop yields.

The experiment has been going ever since, and the soil samples, which have been stored since the project began, have enabled scientists to study the impact of environmental changes, the industrial revolution, nuclear weapons tests, and accidents like Chernobyl on the soil.

Scientists at the Geosciences Advisory Unit at Southampton University in southern England are using the archive to produce a record of nuclear fallout in the U.K.

"They have provided us with the first evidence that plutonium from the Nevada Desert tests in 1952 and 1953 contaminated northwest Europe," said Dr. Ian Croudace of the Southampton Oceanographic Center.

Goulding told the meeting of the British Association for the Advancement of Science that such experiments are very valuable.

"Events such as the Industrial Revolution, the introduction of unleaded petrol and acid rain can all be seen in the changing chemistry of the sample archive," he said.