FARMERS have been flocking to Healthy Soils Sustainable Farms field days in Gippsland to learn the secrets of life below their feet. JOHN PARRY reports
One of Australia's top specialists on the complex world of soil carbon, Clive Kirkby, has a succinct warning for farmers.
"There is a lot of hoodoo around about soil carbon," Clive said.
"Get the idea of carbon out of your head. We are not talking about carbon, but organic matter," the CSIRO Plant Industries scientist told farmers at soil health field days in Gippsland recently.
"You don't put carbon in soil, you put organic matter in the soil.
"Yet we have people running around saying we can sequest carbon. I wish they would be more accurate and say we might be able to sequest organic matter, but that involves much more than just carbon."
Clive said soil carbon levels were measured as a means of indicating how much organic matter was in the soil.
"I think we can build soil carbon, but the cost is going to be all-important and this hasn't be raised sufficiently," he said.
Clive urged farmers thinking of getting into carbon trading to understand the two distinct pools of carbon in the soil - the soil organic matter pool and the particulate organic matter pool.
SOM is the stable organic fraction of the soil or humus and POM is the undecayed plant and animal residues.
"The two pools behave very differently, but unfortunately most of the soil-test laboratories lump them into one pool and analyse it all as soil carbon," Clive said.
"I think it would be very wise to know where your carbon is in those two pools.
"If you have a lot of carbon in the POM pool and something happens to it, you might have to pay the (carbon trading) money back."
In untouched and minimum-till soils there was generally much more POM than in cultivated cropping soils, Clive said.
"As soon as you cultivate, you encourage the decomposition of this plant material and the total carbon levels drop, but not the humus or SOM carbon levels," he said.
"The main increase in total soil C under any minimum till is POM, not SOM, and it disappears quickly under cultivation.
"So you have to question whether the POM is good for carbon trading."
Clive said building SOM was like building a brick house.
There was a relatively constant ratio between carbon (C), nitrogen (N), phosphorus (P) and sulphur (S) in the soil's microbial biomass or humus.
"If the C is equivalent to bricks and the N, P and S are the cement in the mortar, you need them all and in the right proportions if you are going to build a house."
A shortage of any one of these four minerals would limit the production of humus or organic matter.
"Without those other elements (N, P and S), you will not hold on to the C - that is a key message," he said.
"If you want to build up SOM levels, you must have all the elements and the right proportions."
Clive said that sequesting, or building up, one tonne of C as humus, required the sequestration of 83kg of N, 20kg of P and 14kg of S.
"Humus decomposes by 2-3 per cent a year and, during this process, the N, P and S in the humus is mineralised, essentially becoming fertiliser N, P or S," he said.
"In the mineral state, this N, P or S can be either leached, gobbled up by the plants or re-used by new microbes.
"If it is re-used by new microbes, the N, P and S can eventually become new humus and the net result is no overall loss of humus.
"However, if the mineralised N, P or S are either leached or used by the plants and subsequently removed from the system (in, for example, grain) then extra N, P or S will have to be added just to maintain the status quo."
So a soil lacking N, P or S could hold back the build-up of carbon, and soils low in P and S, in particular, were common in Australia, Clive said.
Humus should be considered as a crop, like any other crop.
"Humus is a crop below the ground and it needs its own set of nutrients, not the bits and pieces left over from cropping," he said.
"The question is: can we do anything in the real world to build it up?"
Laboratory incubation trials, using soil from Leeton in NSW showed that adding the equivalent of 10 tonnes/ha of stubble every three months, for 18 months, lifted soil C from 1.2 to 1.4 per cent and to 2.1 per cent if balanced nutrients were added as well.
"It is no different to feeding a cow - microbes are exactly the same," Clive said. "Essentially all you are doing is adding a nutrient supplement to their diet, the stubble."
But what about the cost of sequesting soil organic matter?
"The cost of the fertiliser used in the trials was about $248/ha. But even at the top price for carbon around the world ($40/tonne), you would be about $100/ha behind and no farm can operate like that," Clive said.
He said SOM could do two things - act as a soil conditioner, mainly by improving soil structure, or act as a fertiliser, but not both at the same time.
"If you suck out the nutrients from SOM to build a crop, it can't act as soil conditioner, as you will lose the carbon," he said.
"You have to decide which strategy suits your conditions."
But adding microbes, rather than nutrients, would not work.
"The microbes might decompose fresh organic matter, but if nutrients aren't there they still can't make humus," he said.
"You cannot ignore those ratios. I'm not saying the N, P and S have to come out of a bag, but they have to come from somewhere."
