Decades of easy access to cheap nitrogen appear to be coming to an end as urea prices surge toward $1000 a tonne. The hunt is on for alternatives.
As CSIRO scientist Dr John Angus observes, oil-derived fertilisers "are the only flexible nitrogen option we have".
But there are less flexible options, and some are proving to have advantages well beyond their role in replacing synthetic nitrogen.
After 12 years of research into legume rotations with cotton, CSIRO/Cotton Catchment Communities CRC scientist Dr Ian Rochester is confident that farming systems can be developed that won’t always need annual nitrogen fertiliser applications to be profitable.
In his own work around Wee Waa, NSW, Dr Rochester has seen vetch rotations regularly fix up to 200 kilograms of nitrogen per hectare under dryland conditions—and deliver a wealth of other benefits that combined, increased gross margins by up to $540 per ha.
"This last year we got over 5.5 bales of cotton an acre, with no nitrogen, using legume systems," Dr Rochester said.
Cotton grower John Phelps, who has been trialling vetch rotations on his Wee Waa property "Havana" for about six years, said he has reduced his formulated nitrogen applications by 40 per cent, and hopes to reduce it further.
Vetch, which is grown out and then slashed and mulched into the soil as a "green manure" before it begins to hay off, has proved a stand-out soil ameliorant, but faba bean runs a handy second and provides the additional attraction of a cash crop.
"Within a continuous cotton system where cotton was planted year after year, growing vetch reduced the amount of nitrogen fertiliser required for cotton by 140 kg per hectare to achieve maximum yields," Dr Rochester said.
"Coupled with increased yield, the gross margin per hectare for this system [compared to not growing vetch] was increased by $390."
Average nitrogen fertiliser applications in the cotton industry now stand at around 200 kilograms per hectare, and are increasing.
Under a wheat-vetch-cotton rotation, gross margins improved by $270/ha compared to not growing vetch, with the biggest results coming in a vetch-fallow-cotton rotation that saw gross margins leap by $540/ha.
"Although vetch is not an income producing crop itself, the $100 per hectare cost of growing it is substantially outweighed by the financial benefits accrued for the following cotton crop."
Those benefits cover a range of soil improvements, including and increase in soil organic matter of 14 per cent—and the annual accumulation of a tonne per hectare of total soil carbon, which itself may deliver a cash return under an emissions trading system.
Dr Rochester attributes the jump in organic matter and carbon not just to the vetch, but a permanent-bed tillage system that minimises soil disturbance, and thus the soil nitrogen that is normally consumed when soil is turned over and microbial activity accelerated.
The combined effect has been the improvement in a range of soil properties, including water infiltration and water-holding capacity, that collectively deliver a beneficial boost to crops.
Following vetch, cotton crops are able to absorb greater amounts of nutrient, including nitrogen, phosphorus, potassium, zinc and copper; while detrimental sodium uptake is reduced.
Management also becomes easier. The interaction between plants and healthy soils tends to self-regulate the delivery of nitrogen, so that the crop isn’t hit with a nitrogen overdose if conditions start to dry out.
"The nitrogen is in an organic form, so it’s slow release, and it cycles between organic and inorganic forms, so that its cycling more rapidly and the crop has access to the mineral nitrogen," Dr Rochester said.
"It all becomes less risky—you can rely on the nitrogen being there when you need it."
It doesn’t have the flexibility of nitrogen fertiliser, he observed, but because organic nitrogen is more stable, the last-minute rush to add N to a potentially high-yield crop may not be so necessary. However, it remains an option to those who want to capitalise on yield in a big year.