Loss of Ignition compared to the Dumas test when testing for soil organic matter levels, and the implications for soil function and crucially the Carbon to Nitrogen ratios – C:N ratio.

Soil organic matter is predominately negatively charged, so as with silt and clay it also plays a significant part in retaining nutrients.

There are two methods of analysing the soil organic matter content, Loss of Ignition (LOI) and Dumas.

  • Loss of Ignition (LoI) tends to be more unreliable as if the soil has a high calcium content that will be burnt along with the carbon potentially skewing the results.
  • The Dumas test is an international laboratory standard methodology for assessing soil organic matter content and therefore results can be directly compared and contrasted. Currently the availability of the helium gas required for the test is significantly delaying the analysis process.
Carbon to nitrogen (C:N) ratio

If we consider the ideal carbon to nitrogen (C:N) ratio in the soil for the microbes to thrive and survive is 24 to one (24:1). Consider also that 16 parts of that are constantly fluctuating during their respiration cycle to create energy for the microbe’s survival, while eight parts are used for maintaining the microbial carbon structure within the soil.

Farm-yard manure has an average carbon to nitrogen (C:N) ratio of 20 to one (20:1), and that wheat straw has an average carbon to nitrogen (C:N) ratio of 80 to one (80:1). While woodchip has a high carbon to nitrogen (C:N) ratio of over 100 to one (100:1), and in its raw form takes well over two years to degrade to become useful to soil productivity. What is the impact they are having on soil conditions? Acknowledge their relationship when they interact with the soil classification, the soil microbiome, and their speed of degradation for crop utilisation.

As the decomposition process continues both carbon and nitrogen are now subjected to loss. The carbon as carbon-dioxide (CO2) and nitrogen as nitrate (NO3) which are both leached or adsorbed by plants.

The UK agricultural industry guideline for the soil carbon to nitrogen (C:N) ratio for a crop to function is 10:1. When the base carbon microbial saturation of eight parts of carbon to one of nitrogen is considered, then this is a good guideline of carbon function in the soil.

Higher than that and the carbon breaks down more slowly, lower that that and the speed of breakdown increases.

The Sample site is ‘unimproved grassland’ within a certified organic regimen. The sampled grassland has only had sporadic (due to the topography and elevation) mechanical intervention to improve sward function.

In essence the AS and GG sites have been allowed to go through the natural cycle of growth, grazing (sheep & cattle) and decay for over 30 years. In contrast BB1 was ploughed seven years ago to produce a spring barley crop.

SAMPLE TABLEWhat can we discern from the results?

1. The results above clearly confirm the low productivity potential of the soil types.
2. Productivity potential is low because of a combination of factors not the least of which is the sites topography and the elevation. Also, the relatively low silt and extremely low clay content and their corresponding lack of negative charge to hold on to the cations (calcium, magnesium, copper etc.) required for a nutritious sward will significantly limit productivity potential.
2.1. The relatively low stocking density of both the Swale Dale sheep and Luing cattle on the impoverished soil means that the stock has access to enough grass to thrive. Even in this challenging environment.
3. The sward was populated with native grass species, particularly:
3.1. Cocksfoot Dactylis glomerata – deep rooted to 1.0m, and impervious to the Great British weather.
3.1.1. The deeper the root mass, the greater potential to stabilise the environment and access nutrient access from the soil
3.1.2. Has the same feed value as Ryegrass but without exhausting its productivity potential after just a few years.
3.1.3. Cocksfoot extracts more copper from the soil than other grass species, considerably helping with fecundity in this impoverished environment.
3.1.4. As an example, I manage a pure cocksfoot sward that was established in 1953 and it is still highly productive today fattening lambs 10-14 days earlier than the ryegrass leys in the area.
3.2. Meadow Fescue Festuca pratensis – deep rooted to 0.75m and thrives in a soil with a low pH.
3.3. Sheeps Fescue Festuca ovina – relatively shallow rooted at 25-35mm but has the unique ability to generate its own mycorrhizal activity in the soil to interact efficiently with the soilborne AMF (Arbuscular Mycorrhizal Fungi) and consequently release soil nutrition to help the sward flourish in a challenging environment.
3.3.1. A hardy, and drought resistant species.
4. The pH in GG will further reduce the availability of the magnesium and phosphate. Magnesium levels were average (low) for the region, but the phosphate levels were within acceptable ranges.
5. Crucially the Loss of Ignition results were significantly higher than those from the Dumas test for organic matter results.
5.1. Be aware of this critical factor when utilising soil organic matter analysis for carbon capture assessments.
5.2. What is considered when extrapolating data to assess the level of carbon dioxide that a particular soil can retain, or ‘capture’? This is calculated from relating the Bulk Density of the soil to the organic matter content, and assuming that a hectare of soil to 15cm of soil weighs 2200 tonnes. Also, that there is 58% carbon in the organic matter and that contains 44g of carbon dioxide in each 12g of carbon.
5.3. It can clearly be seen how skewed the results could be if LoI was used for this calculation instead of the Dumas analysis.

The soil texture triangle for reference:

soil texture triangle