Waihi Soil Carbon Study

September 2009

Students at Waihi College compare carbon levels of different soils

As part of a scheme to get College pupils interested and involved in practical science, Clyde Smith has worked with teachers, Dr Peter Brumby and others to put together a quick and uncomplicated protocol for estimating total carbon in soil samples by heating them in a barbecue and measuring weight loss. On a dairy farm they compared three paddocks that had had different management over five years one that had standard fertiliser, one irrigated with dairy effluent, and a third that had been cropped. The irrigated paddock had double the carbon of the standard paddock, while the cropped paddock had lost carbon. The increase in soil carbon is far more than the small amount in the effluent, indicating that increased pasture growth and soil organism activity are largely responsible.

Getting High School students interested in science can be difficult when school experiments are seen as not having any practical value. As a tool to attract their attention Waihi College staff have worked with local people to find a suitable topic and method that would not be too complicated, not take too long, and would have some practical use. Clyde Smith, the Waihi College farm manager, sought advice and some funding from a local trust set up to benefit local people. They came up with the idea of estimating soil carbon and comparing the fate of adjacent soils under different management regimes.

The issue of soil carbon is topical and controversial. Soil is the largest terrestrial carbon sink, and some people believe that if pastures and orchards were managed differently then topsoil depth would grow, soils would become more resilient and productive, and a considerable amount of atmospheric carbon would be sequestered and this would offset greenhouse gas output from the farm.

Conventional soil science holds that NZ soils are saturated with carbon and in a steady state, that soil carbon is too difficult and expensive to measure, and that if soil growth is possible it is too slow to get meaningful results. However, such studies that have been done have not looked for any increase in soil depth, only the amount of carbon per unit of soil in the top few cm.

With grants from the local trust and from AgMardt (this was seen as a pilot for a national scheme for high school students) and guidance from retired agricultural scientist Dr Peter Brumby, Clyde decided on the topic. He asked the physics teacher to show him and the students how to dry soil and then heat it to 300C to burn off the carbon and check the difference in weight, and there would be a very strong correlation between that and soil carbon.

Using a crucible and Bunsen burner took too long, so Clyde modified his barbecue so that it could be heated to 300C. He then talked the school engineering shop into making some crucibles out of stainless steel because when you heat stainless steel there is a very strong correlation between temperature and the colour it goes, so the students could just look at the colour and know whether it had reached the right temperature.

In the barbecue, students were able to cook soil in large volumes and very quickly. Samples were tested using same method several times, and the results were consistent and reliable. Topsoil depth was also measured and the results expressed as depth of topsoil and percentage carbon rather than the conventional way of expressing this as tonnes of carbon per Ha. The number of soil samples tested (about 150) was not enough to make any definitive statement, but the results show trends.

Students sampled paddocks on dairy farms nearby where the paddock histories were known, including Peter Brumby's, and they measured the soil that had effluent is sprayed on it and compared it with paddocks that hadn't. The result was that there was double the carbon, double the soil depth on the effluent treated paddocks.

Peter Brumby says that the soil growth is far and away above the nutrients that have been applied, and that the results are eminently repeatable. Clearly some practices dramatically increase soil growth and farmers benefit from the improved soil function that comes with it. Some practices will reduce soil carbon, and clearly the soils are neither saturated with carbon nor in a steady state.

Students have presented their results to several local farming groups, and to the Parliamentary Select Committee on the Emissions Trading Scheme to which they submitted a number of additional points:

Carbon levels in the soil can be changed quite cheaply and easily just by altering pasture management practices.

The increase in carbon in the soil is followed by a increase in production.

By the time the production, be it milk, calf body or Kiwifruit, goes out the gate as fixed or sequestered carbon, is put in the calculation against the emissions, the net outcome we see as Carbon Credits to sell.

The need for research scientists to research these findings is of great urgency. Clearly if our results do hold true, then NZ Dairy Cows and Kiwifruit will be Carbon sequesterers.

It is to be hoped that decision makers in Wellington have taken on board their final point that research is needed urgently to check out their findings in a scientific manner and determine to what extent altered pasture and orchard management can sequester atmospheric carbon and offset other greenhouse gas emissions.