Soil testing needed to inform fertiliser programs in 2017
Author: Toni Somes | Date: 02 Mar 2017
Nutrient off-take in 2016 is likely to have been significant in many grain-growing regions, so replenishing soil nutrients to sufficient levels to support this year’s crops will be a priority.
This key agronomic message for 2017 has been delivered to growers and advisers at Grains Research and Development Corporation (GRDC) Grains Research Updates in recent weeks.
Dr Rob Norton, Australia and New Zealand Regional Director of the International Plant Nutrition Institute (IPNI), has told Update attendees that in terms of nitrogen (N), it “may very well be that the cupboard is bare for the coming season”.
“But the only way to find out is to take a deep soil N test,” says Dr Norton, who is also chair of the GRDC-funded Making Better Fertiliser Decisions for Cropping Systems in Australia (BFDC) Program Advisory Committee.
“Assessing subsoil nutrient supply, particularly N and sulphur (S), will be important to get a picture of the potential subsoil nutrient availability in a wet profile and to ensure nutrients are supplied at the right time.”
Dr Norton says that following a wet year, mobile nutrients such as N and S, and on some soils potassium (K), are likely to have moved down the profile, necessitating deep soil tests to gauge to extent and location of available nutrients.
In the north in particular, assessing deeper (10-30 centimetres) phosphorus (P), K and S supplies has shown benefits as this is where the roots will draw water and nutrients in those environments, especially as the topsoil is often dry during the growing season.
“Fertiliser rates applied at sowing may need to be increased for these nutrients to ensure they remain at adequate levels in the developing root zone,” he says. “Care of placement of fertiliser relative to seed will be needed to avoid toxicity.”
Dr Norton says that N uptake by crops was large in 2016, given big yields that were often coupled with reasonable protein levels in grain. Supplies of N from the mineralisation of organic matter were larger than normal because of wet spring conditions.
The high-yielding 2016 cropping season is also expected to have impacted on the N fixed by legume crops. “Even though legumes flourished and fixed large amounts of N in 2016, their high yields, high harvest indices and N removal may mean that little fixed N is left for 2017’s crops,” Dr Norton says.
N tied up in heavy cereal stubbles should also be factored into nutrient budgeting considerations.
“For each tonne of stubble, an extra 6 kilograms of N/tonne will be needed to break down the residue,” Dr Norton says.
“Therefore, for a 6 t/ha wheat crop, there could be 8 t/ha of stubble demanding up to 50 kg N/ha for residue breakdown, which might be twice as much as normal.”
Dr Norton says while burning heavy stubbles can have logistical benefits, most of the N in the stubble will be lost: “The GRDC Managing Stubbles publication suggests that 80 percent of N, along with 40 pc of both P and K, are lost in a hot burn. With many lodged crops in 2016, the stubble is not likely to be tall, so achieving a hot burn might be difficult anyway.”
Due to the combined effect of heavy stubbles, high N removals and losses through denitrification (where flooding occurred), extra fertiliser N should be considered at seeding, according to Dr Norton. “Where there are fewer opportunities for effective N topdressing after crop establishment such as in drier regions or in the north, more N at seeding is likely to be beneficial for 2017.”
Dr Norton advises that fertiliser P rates for 2017 could be targeted to replace the P removed in the 2016 yields, but a better strategy is to use a soil test at 0-10 cm depth. If the soil test is above the critical P value, plants will be able to draw down some of the soil P reserves and fertiliser applications below the removal rates will not limit yields. If the soil test is below the critical value, then fertiliser P rates greater than removal rates will be needed to meet soil and crop demands.
He recommends using an N-rich strip to help monitor what is happening during the season with the balance of losses and gains in soil N. These strips will help highlight if the crop is limited by N.
A “4Rs” approach is advocated by Dr Norton and other crop nutrition experts: “Getting the RIGHT nutrient source at the RIGHT rate, RIGHT time and RIGHT place is the basis of good nutrient management.”
Dr Norton says lime to address soil acidity, gypsum to improve soil structure on sodic soils and weed control are the “big ticket items” between now and sowing. “In many areas there is some moisture carryover from summer storms and it is vital to ensure that this is conserved for the crop this year. Also early sowing, good disease control and all the other basics of agronomy set up the crop to make full use of any nutrient inputs.”
For analysis of soil tests, Dr Norton recommends laboratories with comprehensive certification by the Australian Soil and Plant Analysis Council (ASPAC), and these are listed at the following link.
The BDFC website provides information about soil test critical levels for the four key nutrients – nitrogen, phosphorus, potassium and sulphur.
More information on soil testing is available in the GRDC Southern and Northern Fact Sheets, as well as the GRDC crop nutrition extension hub at and the IPNI website.
Rob Norton, IPNI/BFDC
Phone 03 5381 2673
Toni Somes, Cox Inall Communications
Phone 0427 878 387
GRDC Project Code DAN00166, UQ00063, DAN00168, DAN00165, BWD00021, DAQ00183