Many grazing paddocks have a long history of the same blanket rate of single super each year.
But a growing number of producers are turning to variable rate fertiliser application.
Vic company Precision Agriculture, based at Ballarat, has seen a spike in interest in its grid mapping to identify different zones within a paddock for soil pH and nutrient levels such as phosphorus, sulphur and potassium.
In the past six months it has precision mapped more than 3000 hectares of grazing land on Kangaroo Island and significant areas in western Vic.
Using an aerial photo, each paddock is divided into two-hectare grids.
Eight soil samples are taken along a 120 metre arc in each 2ha, using a soil corer specially fitted onto an ATV.
The samples are analysed at Australian Precision Ag Laboratory, before PA’s GIS team generates prescription maps.
The mapping costs range from about $15/ha for pH to slightly more than $30/ha for electrical conductivity, phosphorus, sulphur and potassium and pH.
PA field technician Daniel Bell says in most cases producers are looking at “trouble paddocks” they want to boost pasture production.
“The cost savings are great but it is really the reallocation of either nutrients or soil ameliorates such as lime and gypsum to places that actually need it rather than overapplying to places that don’t need it that you get the best bang for your buck.”
Mr Bell says most spreading contractors had variable rate capabilities but if not there were still ways around it, such as being able to overlay prescription maps with background imagery to enable for selective manual spreading.
Victor Harbor-based livestock consultant Tim Prance sees the greatest value in paddocks run at high stocking rates with a history of high fertiliser application.
“On properties always putting out around 14kg of P/ha/year, if they can bring it back to 5-6kg of P/ha/year over, say, 30 per cent of their property, it can pay for the cost of the sampling,” he said.
“There is also opportunity for farmers using high rates of single super to be able to use the cheaper alternative such as MAP or DAP on parts of paddocks where sulphur levels are already sufficient .”
Mr Prance says one KI farmer recouped the $18,000 cost to map his 580ha property in the first year with $21,000 in fertiliser savings.
“He had been putting out a flat rate of 1kg/P/DSE over the whole farm for quite a few years and he is running 16DSE/ha, so 180kg/ha of super every year,” he said.
“After he did the precision agriculture mapping over his whole farm, half his farm was well above the critical level for soil phosphorous, but only 25pc was less than 35 parts per million P, so basically he was putting out a lot more super than he needed to be.”
On-farm trials test economics
Producer demonstration sites in the South East have shown some variation in soil phosphorus but considerable variation in sulphur and potassium levels.
The three paddocks at Taratap and Keith were sampled by Precision Agriculture as part of a project funded by the South East Natural Resources Management Board, with money from the national Landcare program.
Livestock consultant Tim Prance says the aim of the work undertaken through the Limestone Coast branch of the Grasslands Society of Southern Australia was to see if generating variable rate soil fertility maps could be economically viable on broadacre grazing properties.
The paddocks at Taratap owned by Lachie Stewart were a heavy clay over limestone paddock of ryegrass and balansa clover prone to waterlogging and an undulating paddock on the western side of the range.
The Keith paddock owned by Chad Makin was a sandy soil which had been clayed and delved and had an excellent lucerne stand.
Mr Prance said the variations in both sulphur and potassium ranged from very deficient to more than adequate, especially in the two sandy paddocks.
“This opens up the possibility of applying cheaper high analysis low S fertiliser to parts of paddocks previously considered to be sulphur deficient,” he said.
“Likewise potash fertiliser may now be economical if only required over a small part of the paddock.”
The full results, including an economic analysis, will be published in the June Grassland Society newsletter and also on the NRM SE website.