Nitrogen use efficiency and organic matter

By Simon Cowell, Essex farmer and founding member of the Green Farm Collective

Having conducted 20 years testing for organic matter in my soils, I thought it would be a good idea to put all the results into a spreadsheet and see what has happened over that time.

Getting to the root of it

I have a total of 63 tests, some fields have only been tested three or four times, others up to eight. The first thing that became obvious was the randomness of individual results in each field as the organic matter percentages appear to fluctuate up and down over quite a wide range. This just shows the inaccuracies involved when sampling and testing in the lab. It is highly unlikely that a few grams of soil will ever be representative of the thousands of tonnes of soil in a whole field with variable soil type, however many sub samples are taken.

By putting all the results onto one spreadsheet and then allowing the wonders of Excel Graphs to do its thing, the trend line is showing a definite increase over time. It would appear that, on average over the last twenty years, my organic matter percentages have gone from 4.8% up to 6%. You can see on the graph that each dot represents an average of all the tests done in each respective year.

Digging deeper

I have also taken quite a few samples in the last five years from different depths down to 45 cm. There is a gradual reduction of organic matter percentages as you go down, but even at the lowest depth, they are still around 4. I didn’t do these deeper tests twenty years ago for comparison, but I believe it is fair to assume that organic matter is forming throughout the soil profile, albeit at a slower rate than near the surface.

Having taken into account the weight of soil per hectare, soil bulk density, and the fact that organic matter is fifty percent carbon, it appears that I am sequestering just over one tonne of carbon per hectare. The carbon to nitrogen ratio of organic matter is 10:1, which means that I am sequestering 100 kgs of nitrogen per hectare. This is nitrogen that would otherwise be available to our crops. Most nitrogen requirement calculations assume ten to twenty kilograms is available from soil mineralisation, but regenerative farmers who are increasing their organic matter are immobilising 100 kgs per hectare. The nitrogen entering the system from fertiliser and fixation from the air is effectively being used to grow two crops, the above ground cash crop and the below ground organic matter accumulation. If both these are taken into consideration, I wouldn’t be surprised if my Nitrogen Use Efficiency is above 100%.

In other words, fertiliser applications are not being lost to the atmosphere or leached into the drainage water but sequestered into soil organic matter along with the carbon.

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