Monitoring changes in soil organic matter

There are several factors to consider when determining the concentration of organic matter in your soil.

The amount of organic matter present in soil depends on:

• The input of organic materials and their decomposition rate
• The rate of existing soil organic matter decomposition
• Soil texture (i.e. sand, silt and clay content)
• Environmental factors such as soil moisture, temperature and aeration

The natural levels of organic matter that can be held in a sandy soil are much lower than in a heavier soil because of how clay can stick to and stabilise organic matter in soil.

It is important to take into account both soil texture and climate when assessing organic matter levels in soil.

Grassland soils usually contain more soil organic matter relative to a continuous arable rotation. It is generally good to increase soil organic matter content. However, vegetable growers on a sandy soil should not expect to achieve and maintain the same soil organic matter content as pasture-fed livestock-producing neighbours on a heavier soil.

Ensuring regular additions of organic matter to 'feed' the soil is more important than achieving a specific, measured value of soil organic matter.

There is little consistent scientific evidence of a critical threshold of soil organic matter, above or below which soil properties change significantly.

Instead, it is important to measure soil organic matter periodically over time to determine the direction of change and understand the impact of management practices.

The amount of organic matter in soil is relatively large compared with the changes that result from management over a short period of time.

Sampling to monitor changes in soil organic matter should take place regularly and, ideally, always at the same point in the rotation. It is usually not useful to measure soil organic matter every year.

Trends can usually be detected for a field using samples collected in the same way at 3–5-year intervals over a decade or more. Detecting changes in the total soil organic matter in the field is often a slow process.

Organic matter is naturally concentrated at the soil surface and decreases with increasing depth unless inversion tillage takes place.

It is difficult to pick up small changes in soil organic matter using the ‘loss on ignition method’ commonly offered by soils laboratories. However, there is increasing interest in measuring a particular fraction of the total soil organic matter known as the ‘light fraction organic matter’.

This is the active fraction of organic matter that can be much more responsive to changes in soil management practices over short periods.

Evidence suggests that light fraction organic matter is a very good indicator of whether management practices implemented on the farm are likely to result in long-term benefits to soil organic matter content if continued over time.

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