ANECDOTAL evidence of soil nitrogen consistently being above expected levels in no-till, cereal-on-cereal farming systems raises questions over the established wisdom of how nitrogen replenishment works, according to Western Australian consultant and no-till advocate Bill Crabtree.
Mr Crabtree said no-till farmers who, for a range of reasons, had been unable to include legumes in their rotations were finding that soil nitrogen levels were not depleting as fast as expected.
He said he’d observed the phenomenon in no-till farming systems internationally, including Western Australia where many producers had been forced to drop lupins from their rotations because of the crop’s poor yields and returns and the difficulty of managing weeds like radish and ryegrass.
“I have found that people who have kept all their stubble and not grown a legume in the system have more nitrogen in their soil than what we would expect them to have,” he said.
“A lot of people are now growing wheat/wheat/wheat/canola then wheat/wheat/wheat and we are not finding it crashing. In fact we are measuring good units of nitrogen in the soil.
“Some people will say they are getting the nitrogen out of the straw, but if the organic carbon is not going down across 10 years and you are harvesting 75 units of nitrogen (in the grain) every year and you are only putting on 25 units (in fertiliser) every year, then it has to be coming from somewhere.”
In such situations, farmers have found they have been able to dramatically cut back on the usual fertiliser top-up rates.
Mr Crabtree challenged scientists and researchers at the Fifth World Congress on Conservation Agriculture in Brisbane to investigate why nitrogen levels were holding up under legume-free, cereal cropping regimes.
“Some scientists will think it is not possible to have a non-legume rotation and be fixing nitrogen,” he said.
“But the science community needs to work out why farmers are seeing what they are seeing.
“If we don’t there will be very good no-till farmers who get frustrated with the establishment who are disagreeing with them and they will go to ‘muck and mystery’ fertiliser companies and buy products that rarely add value to a farmer’s bottom line.”
Mr Crabtree suggested there might be other factors at play in fixing nitrogen in addition to the known sources of lightning and rhizobial bacteria.
“We know lightning can give you one or two kilograms of nitrogen. In the air we breathe there is 78 per cent nitrogen with bonds that are unbreakable except by lightning. The lightning will crack that open and that is why you get a little bit of nitrogen,” he said.
“Or it can be broken open by rhizobia in legume crops like peas, chickpeas and lupins that fix nitrogen from the air.
“The group of rhizobia bacteria aren’t the only ones that can do it. There are others that live in the soil that can do it.”
Mr Crabtree said free-living bacteria and algae – and even stubble-eating termites – might be part of the nitrogen story.
“Termites have bacteria in their stomachs that fix nitrogen,” he said.
“A CSIRO study found that having termites in a crop gave yield benefits by keeping open pores in the soil, reducing compaction and nutrient cycling.
“In the Geraldton area a guy who has been no-tilling for about 20 years sprayed out the termites in the field.
“By taking the termites out he found quite significant yield reductions happened a couple of years later.”
