A Brief Note on Carbon

Where soil Organic Matter is 10% in the top 7.5 cm, that soil is about 5.8% carbon. The bulk density of soil in the field is generally greater than one, if we use a bulk density of 1, this is 43,500 kg C in the top 7.5 cm, which is quite a large amount. There will be some C further down the profile so it is quite possible in summer moist districts that the C in the top 30cm of soil could easily be 100,000 kg/ha (100 t/ha).

Cultivation to grow one crop or to re-grass is likely to result in the loss of about 3% of the C during that growing season mineralised and released as atmospheric CO2. Repeat cultivation or cropping during successive years will result in a gradually reducing rate of C loss, but loss of C will continue and can be monitored with the soil OM test. Heavy or excessive applications of N fertiliser can eventually have a similar effect.

The good news is that management options can be taken aimed at accumulating soil C. Adoption of crop rotations where soil C (and N) is alternately accumulated under grass pasture and reduced with cropping was the basis of ‘old fashioned’ crop rotations practised before N fertiliser use increased dramatically. Irrigation of pastures has a beneficial effect, increasing plant growth (utilisation of atmospheric CO2) and decreasing the negative effect of summer dry conditions. Soil husbandry aimed at stimulating deep root development, application of lime to enhance soil structure and biological activity all have an effect on the soil C reserve.

If NZ is serious about managing C emissions and involving horticulture and agriculture, then monitoring of soil C is essential. There should be some incentive for land-based industries to be good guardians of the soil C ‘pool’. It could be worked into a carbon credit system. However this would be a ‘two edged sword’, if loss of C occurred, a debit would be incurred…

Food for thought

The potential is BIG
If Carbon leaves the soil it has to go somewhere.
Yes…into the air.
If Carbon builds up in the soil it comes from somewhere…
Yes…the air