Because soils are dynamic systems, they are constantly changing, making accurate measurement of carbon and other components challenging.

There can be as much variation in soil carbon within a metre, as within a whole field. A sample taken one day will be different from one taken the next day.

The time of year, weather conditions, geographic orientation, altitude, depth of soil and farming techniques all contribute to the variation that occurs across every part of a field, farm or region.

At each geographic scale, different processes are at work from climate extremes and daily weather patterns to seasonal patterns of growth and microbial turnover.

In the growing season plants absorb carbon from the atmosphere through photosynthesis and use it to build their roots, stems or leaves. Carbon is transferred to soil through the release of organic compounds by plant roots or through the decay of plant material or soil organisms when they die.

Microbial breakdown of the organic matter finally releases the nutrients which plants use to grow. During this decomposition, some carbon is released as carbon dioxide through soil respiration, whilst other carbon is converted into stable organic compounds that are locked into the ground.

How fast this happens depends on the combination of temperature and rainfall, the soil-water balance, and the composition of the organic material, making it hard to predict.