Soil - a weighty issue

Do you truly know how much carbon is in your soils? Isn’t this supplied on your soil test result? While a standard soil test usually does supply a value for carbon (either Organic C or Total C) this is not enough to calculate the amount of carbon per hectare (t C/ha). As the soil test value is expressed as a percentage, it actually is telling you how much carbon there is for a known weight of soil. Therefore, the weight of soil per hectare is also needed to fully understand how much carbon there is.
So how do you know how much soil there is per hectare?
To do this you just need to know the weight of soil for the volume sampled (otherwise known as bulk density). Sound simple? Well, while it is simple in theory, in practice measuring the tonnes of soil per hectare can be a bit fraught for the unwary...
The most common method is to carefully collect a sample of soil using a metal cylinder of known volume. This sample is then dried in an oven (to remove the water) before being weighed. The end result is a weight of soil per volume which then can be converted to tonnes of soil per hectare.
So if bulk density is needed to get a true value of t C/ha, why isn’t this done with the regular soil test sampling?

The simple answer is time and cost. To get an accurate value for bulk density careful collection is needed. Small changes in soil weight can have large changes when converted to tonnes per hectare. Multiple samples should also be taken to account for any variability across a paddock. Taking meaningful samples for bulk density is therefore a time consuming process and takes a lot longer than normal soil sampling.