Carbon Sequestration

The question has been asked; how much carbon should we have in our soils?

How long is a piece of string? A lot depends on the type of soil, the weather conditions and the type of farming. First of all we need to accurately know how much carbon aka soil organic carbon [SOC] you have actually got.

The first part in testing for SOC is to have the soil bulk density measured. It is a test to measure the weight of the soil. The higher the density the higher the weight.  This is done by filling container with a specific volume area with soil from the sample and weigh it.

The weight measurements are based on the formula of one litre of water equals one kilogram. So when the soils are dried light volcanic soils BD is around.65 and heavy clay soils are around BD 1.1.

The next test is the extractable carbon which is reported as a percentage of the weight of material tested.  These levels vary dramatically in the 0mm to 150mm and 150mm to 300mm profiles that we test.  The top 150mm invariably has more SOC than the 150mm to 300mm profile.  By multiplying the BD x % Carbon x soil depth measured x hectare of the block = the amount of SOC held in the soil per hectare.

Another slightly confusing part is that the NZ Carbon Units that are being traded are Carbon Dioxide Units.  To calculate the $ value of the SOC we convert SOC to CO2 by multiplying the SOC per hectare reported X 3.67 = Carbon Tradable Units. This is how much carbon dioxide could be made with all the SOC in your soil.

Returning to how much SOC we should have in our soil: In healthy biologically active pastured soils the levels of carbon can range from 5% to 8%.  This does not include peat soils.

The SOC percentage tends to be higher on lower Bulk Density soils. We see levels of 7.5% on volcanic ash soils and 5% on heavier clay soils, which when calculated up with the bulk density can end up with a figure of total SOC that can be quite similar in most healthy soils.

Soils don’t tend to hold any more SOC than the 5% to 8% and as the soil becomes more active and healthier the carbon goes deeper and this is where we see the major increases in carbon sequestration.  This also tends to express the absolute health and productive level of the soil.

The objective is to have SOC levels at 5% to 8% from the surface down to 300mm and beyond and we can see this taking place as we overcome limitations in mineralogy, biology.  And environment management.

The SOC in the soil is held in Organic Matter in healthy soil conditions. The organic matter is approximately 50% to 60% carbon, 30 to 40% oxygen, 3 to 4% hydrogen, 4 to 5% nitrogen, 0.4 to 0.6% Phosphorus and 0.4 to 0.6% sulphur.  As you can see from these figures Organic Matter holds a huge amount of available nutrient and you can start to see why production increases start to get under way when SOC levels rise in the soil.

What most people do not understand is if the carbon levels are falling it is not only carbon dioxide being released into the atmosphere but also large quantities of Nitrogen are being released which is one of the largest if not the largest supply of nitrogen into waterways.

On a healthy farm we have approximately 100 tons SOC per hectare x 2 = approximately 200 tons organic matter per hectare. The nitrogen content of organic matter is 4 to 5%.  This equates to approximately10 tons of nitrogen per hectare.

Dairy New Zealand Research states Waikato farms are losing 1% SOC a year which also means they are losing 100 kg nitrogen per hectare plus whatever nitrogen has been applied.

The results exemplify that we need to be keeping the SOC in the soil and growing it if we wish to have a clean environment.

Water samples from farm runoff water leaving the farm in Waihi showed nitrogen levels at 1.3% of the WHO highest acceptable levels for drinking water and when calculated up utilising the maximum amount of water leaving the farm based on a two meter annual rainfall, this came to a nitrogen leaching figure of 1.4kg per hectare in a calendar year.  This from an intensively stocked farm!

What about the claim that cows belch methane into the atmosphere and are adding to greenhouse gases?  But wait!  It has been worked out that methane converts to carbon dioxide very quickly and is then the source of carbon for carbon sequestration in your soils and for the pasture your animals consume.  So the cows can only belch what they have eaten and that has already been taken from the atmosphere. There is no excess in this situation.

This knowledge allows the farmer to accurately measure the SOC content of their soil each year and can monitor the SOC increases / decreases and possibly sell the carbon credits. This is great as you get to keep and utilise the carbon!

You can also test the water leaving the farm with a water runoff test if that water rises on your property.  This information can be added to the evidence that you create for your own environmental plan based on testing and not on assumption. This will enable you to keep two steps ahead of any compliance issues and save a bucket load of money.

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EcoFarm Aotearoa operate Monday – Friday 9am – 5pm NZ time . To contact Ewan or his team please feel free to email:

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