The soil’s structure is a vital component and vital indicator of its health. It is created mostly by microbes, which turn much of the soil’s organic carbon into polymers – or glues – with which they stick particles together to create “microaggregates”. This is why soil carbon is so important: lose the carbon and the structure collapses.

Microbes do this to build habitats for themselves. The tiny pores between their microaggregates are home to a remarkable diversity and abundance of life: on some estimates, they harbour more than half of all the Earth’s species. Thanks to their remarkably dense network of pores, many soils harbour a greater weight of living creatures than exists aboveground.

Over time, soil building by microbes creates an ever more complex architecture. Long strands of fungal hyphae bind microaggregates into larger clusters. The pores and passages enable water, oxygen and nutrients to pass. The pore space created by microbes, fungi and small animals (such as microarthropods and worms) accounts for around 50% of the total volume of an average, healthy soil.

The self-organised, adaptive structure that living creatures build to suit themselves helps to explain soil’s astonishing resilience in the face of droughts and floods: it survives crises that would otherwise reduce it to amorphous powder. But it also helps to explain why soil can start to break down when it’s farmed. When farmers or gardeners apply nitrogen fertiliser, under certain conditions the microbes respond by burning through the carbon in the soil. Without glue, the pores cave in. The passages collapse. Oxygen and water can no longer permeate. The soil becomes sodden, compacted and airless. This loss of structure accelerates erosion and reduces crop yields.

More than 75% of the world’s soils are classed as “substantially degraded”. As a result of degradation, crop yields are projected to decrease by an average of 10% globally by 2050, and by as much as 50% in some areas. Soil damage contributes to one of the existential threats facing humanity: the possible chronic decline or acute collapse of our food supply as a result of environmental shocks and system-scale crisis.

For this and other reasons, it’s essential that we develop cheap, easy and scalable ways to measure and monitor soil health. Many potential indicators of soil health have been proposed, including earthworm abundance, microarthropod diversity and microbial biomass, available nutrients, soil organic carbon concentrations, bulk density, penetration resistance and visual evaluation of soil structure. But all these indicators have limitations, not least because they rely on point samples, that fail to capture the high levels of variation in soil properties over small distances. They tend to be expensive and time-consuming. None are easily scalable.

One of the many potential breakthroughs enabled by the Earth Rover Program is the development of a spatially explicit and immediate means of measuring and monitoring soil health. Our aim is to make these technolgies cheap and accessible enough for even the smallest and poorest farmers to use.