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A typical teaspoon of agricultural soil contains more than a billion bacteria of several thousand different species, a million other single-celled organisms, a million individual fungi, and hundreds of larger creatures. This biodiversity plays a key role in the ability of soil to support plant life, including agricultural crops.
Earthworms are the most important of the easily visible creatures, acting as the chief engineers of soil structure and providing material for finer decomposition by their microbial colleagues. Worms are also an indicator of soil health, being affected by matters such as acidity, water logging, and compaction. They're easy to identify and count. But with soil microbes, that task has been far harder.
However, new molecular fingerprinting technology, known as metagenomics, is coming to the rescue. Scientists can take a soil sample and sequence all the DNA in it. Then powerful computer programmes try to sort out the various contributing organisms. The first attempt to build a global atlas of soil microbes mapped around 25,000 types of bacteria.
At a biological level below bacteria are the viruses that infect them, known as phages. But their impact on microbial activity and soil is still almost entirely unknown. Another challenge is determining the best way for farmers to use this new soil composition information. For example, one of the biggest changes in European arable farming recently has been to drill seeds directly into the soil without tilling or ploughing.
Minimum tillage increases biodiversity in the upper layer, as organic material builds up. But it's still not certain if this improves crop productivity or its ability to store carbon. The knowledge may still be a bit muddy, but as soil quality rises up the political and environmental agenda, so will the sophistication of methods to assess the impact of its myriad crucial inhabitants.
