Soil Biological Properties

The Importance of Soil Organic Carbon

Soil Carbon whilst not a plant nutrient in itself, (plants obtain carbon from the CO2 they use in photosynthesis) is the principal reservoir from which the elements Nitrogen, Phosphorus and Sulphur are liberated into the soil solution. Thus whilst not a nutrient, soil organic carbon is an important factor in maintaining the soils reserves of Nitrogen, Phosphorus and Sulphur.

The rate at which these nutrients are then liberated and made available to plants is dependent as much, if not more, on the activity of the soil microbiology and their requirement for nutrition as it is on the availability of organic carbon.  It is the requirements of this microbiology alone that determines the availability and rate of mineralisation of Nitrogen, Phosphorus and Sulphur within the soil and therefore it’s subsequent availability to the plant.

Not withstanding that the mineralisation mechanisms of Phosphorus and Sulphur are not the same as that of Nitrogen, it is fair to assume that if attention is paid to only maintaining the Nitrogen dynamics and mineralisation rates that the conditions for both Sulphur and Phosphorus mineralisation will be sympathetically catered for as a consequence. The correct management for one should suit the other two and in the event that this is not the case it is still logical to begin with Nitrogen management.

Carbon: Nitrogen ratio

As a rule of thumb a fertile soil will have a C:N ratio of between 10:1 and 25:1 with a ratio below leading to excessive liberation and losses, and above, as the microbial community scavenge all the available Nitrogen, to shortages. The optimum for a mineral soil is considered to be between 15:1 and 20:1.

Important though it is, the role of soil organic carbon in supplying N, P and S is somewhat eclipsed by the role it plays in maintaining life and life processes on this Earth.

Soils through our use in agriculture have become severely depleted of Organic Carbon . Some (Lal et al 2000) have estimated that as much as 1/3rd of the elevated CO2 levels in our atmosphere have resulted directly from the soil as a consequence of agriculture. Re-invigorating the soil will therefore not only improve crop nutrition but can also play a significant, and some would argue potentially the most significant mechanism in sequestering this carbon out of the atmosphere.

Organic Matter Content


Potential to chemically measure ?

Given the difficulty of maintaining the appropriate high temperatures that this method requires and the fact that Hydrogen Peroxide was used to oxidise the organic carbon in the settlement exercise earlier; it would seem reasonable to assume that chemical oxidisation is a viable alternative and that simply evaporating the hydrogen peroxide off and to air and oven dry would reveal a sufficiently accurate figure for carbon content without affecting structural water content. Unfortunately I currently have no data to support this hypothesis.

The measure of nitrogen

Nitrogen, the most significant nutrient in terms of discussion in agriculture exists in two pools in the soil: the tiny inorganic fraction that is the pool most talked about, and the relatively much larger and more significant fraction that is the reserve and source of the smaller inorganic fraction but which is not talked about.

The measure of the inorganic soluble fraction, the nitrate and the nitrate states of nitrogen within the soil matrix is possible. However the concentration of Nitrate and Nitrite  is highly variable from both a spatial and temporal perspective within the field and no amount of sampling can compensate for this variability. As a measure it has no use whatsoever and tells the enquirer nothing about the soil or it’s nitrogen dynamics: but it can be measured.

The approximate mineralisation rate and the conditions under which the greatest rates occur is by far, more valuable information for the Agronomist to know than the a random value reflecting a unique nitrogen state of the soil solution.

Whilst mineralisation rates cannot be measured directly they can be approximated from measuring the total organic carbon, the carbon: nitrogen ratio, and the microbial respiration rates of the soil.

That said the measure of the total nitrogen content is not a simple procedure outside of a laboratory and whilst I considered outlining it here I felt it would only add unnecessary complication to an already complex document. The full method though is, as with all the methods contained here to be found in ‘Soil Science, Methods and Applications, D Rowell (1994).

Obtaining a total nitrogen content analysis from a specialist laboratory is a perfectly valid option. In many cases a specialist laboratory will be able to measure the fractions as well as the total Nitrogen content. Furthermore there are many laboratory tests for nitrogen which are not agricultural related but many are suitable  and can provide this data at a reasonable cost.

Measuring Soil Organic Carbon

Measuring Microbial Respiration

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