To the untrained eye, a smear of mud might seem to be of less consequence at a crime scene than a speck of blood, a strand of hair or fingerprints.

But in recent years, soil forensics has become a remarkable weapon in helping detectives unpick important information.

Such as the case of Emma Caldwell, whose killer, Iain Packer, was convicted after soil found on his van was found to match the wood in which she was buried - the final piece in the jigsaw to seal a conviction.

Pioneered in the UK by Prof Lorna Dawson at the James Hutton Institute, soil forensics is now moving into a new era.

Not just able to link a suspect to a crime location, eventually soil will may even pinpoint when they were there.

Iain Packer is serving at least 36 years in jail for the murder of Emma Caldwell.Iain Packer is serving at least 36 years in jail for the murder of Emma Caldwell. (Image: free)

DNA technology and big data crunching is being worked on which has potential to trace to within a brief window of time when someone was in a particular location.

Once added to what forensic soil scientists can already unravel from the tiniest spot of soil, such precious extra detail could be the ‘smoking gun’ to seal convictions in future cases and, crucially, cold cases which have lingered for years without a conclusion.

Like a fingerprint, each teaspoon of soil has its own unique set of components.

There are the inorganic minerals derived from the geology around it: the old gneiss rocks of the northwest will leave a different mark to the ‘newer’ sandstone of the central belt. Drift material left behind from the last Ice Age will also affect its makeup.

In addition is water, air and organic material such as the plants, and the chemicals left behind when they and other living things decompose.

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In the forensic soil laboratory, even small soil samples removed from a suspects clothes, footwear, carpets and vehicles – indeed, anywhere it might be found – are analysed using x-ray diffraction for the minerals, gas chromatography for the organics and inductive coupled plasma mass spectrometry to uncover elemental concentrations, such as magnesium, potassium and calcium.

The scientists can also look for traces of pollen, fabric fibres, human and animal hair, fly ash from industry and diatoms which can reveal if someone has walked through a water course.

Because Scotland has an enormous soil archive dating back 90 years and with samples collected across 10 kilometres ‘grids’, they can cross check their sample with thousands of others to narrow their search to specific locations.

It can tie soil from a victim or suspect to a particular location with up to 97% similarity.

“Because soils are continuous horizontally and vertically, we can’t exactly say ‘X marks the spot’,” explains Prof Dawson, “but we can say that it’s within 97% similarity, and based on results it’s much more likely it’s come from one place than another.”

“What is really exciting is the potential to use soil DNA – the third component of soil.”

Alongside organic and inorganic material are the living things which exist within soil such as bacteria, fungi, algae and nematodes.

All leave behind their DNA which once unpicked, has potential to help identify not just seasons of the year, but potentially down to a matter of weeks.

“We extract all that DNA, and we have a huge amount of data,” says Prof Dawson.

“If we understand it fully, the microbial data tells us whether someone has been there in spring, summer, autumn or winter.

“That changes possibly on a week’s basis, because the bacteria respond to very subtle shifts in temperature moisture and predation.”

“But,” she adds, “there are more organisms in a little teaspoon full of soil than there are people on the planet.

“We need to work with bioinformatics people who understand big data, and also with microbial scientists so we can understand why certain organisms change and help answer ‘when did that person visit that site?’

“We’re not using it yet. But ten years from now, that will be a very powerful tool.”