THERE are signals from last season that graingrowers can pick up on when making their fertiliser decisions this year.
This was one of the key messages from International Plant Nutrition Institute regional director Rob Norton, who was one of the guest speakers at a recent GRDC update held at Spalding.
”Heavier soils had denitrification and on lighter soils there was leaching of nitrogen, sulphur and boron,” he said.
Dr Norton said with generally good yields, there would have been a lot of removal of N and other nutrients, and even high-yielding pulse crops would not have fixed much N, because of the amount coming out with the grain.
“With high stubble loads, there would have been immobilisation of N and with burnt stubbles, there would have been the removal of N in the straw,” he said.
These factors added to there likely being little N reserves in many SA paddocks at seeding, but summer rains would have helped kick pre-crop mineralisation along.
Dr Norton said each season offered growers a new challenge, but good nutrient stewardship could help them make the most of their crops.
“There’s four Rs related to nutrient stewardship – having the right source, applied at the right rate at the right time and in the right place,” he said.
Dr Norton said a fertiliser rate should set by available water plus the known removal of N.
He said a quote from former United States Secretary of Defense Donald Rumsfeld, that there were known knowns and known unknowns, also related to crop nutrition.
“The known knowns is what N there is and whether it’s accessible,” he said.
“The known unknowns are the soil mineralisation in the crop, the losses of applied N and how much N has been tied up in grain.
“The unknowns are things like frost and heat.
“Part of crop management is to shift as many known unknowns into known knowns.”
Dr Norton said this shift could be made through soil testing, the use of yield estimators like the Yield Prophet tool and looking at the paddock history of water use efficiency.
“N-rich strips are another really good tool, as it gives a good estimate of what nutrition you need,” he said.
Dr Norton recommended growers set their N supply to meet yield potential.
“Water should be the limiting factor, not nutrition,” he said.
“Rate is more important than source or timing and it’s not all about N, also keep an eye on S and copper.”