Cropper's urged to start testing paddocks for N, P and K now

By: "Farm Tender" News
Cattle News
Dec 21, 2017
611 views
Share

Growers are being encouraged to get into the paddock now to understand what their crop nutrition needs are likely to be in 2018.

International Plant Nutrition Institute (IPNI) regional director Dr Rob Norton said it was important that growers test for nitrogen (N), phosphorus (P) and in some cases also for potassium (K) following what was a predominately low rainfall season.

IPNI regional director Rob Norton is encouraging growers to get into the paddock now to help understand what their crop nutrition needs are likely to be in 2018.

“Growers need to be approaching their nitrogen management in a way which is both strategic, that is developing management approaches for nutrients where there is little or no other options for intervention, and tactical, where nutrient interventions can be implemented effectively,” he said.”

“When I say tactical, I mean mapping nutrients, but also understanding what can change. I often use Donald Rumsfeld’s expression of the ‘known knowns’ which are what has been removed from your paddocks (yield), what are your nutrient concentrations and what is your likely nutrient supply, which is where soil testing comes in.

“This then helps you to make decisions and offset the ‘known unknowns’ which are what will be the demand of your next crop and what are the likely losses of supplied nutrients due to seasonal conditions, and then there is the ‘unknowns’ which includes things like frost, bugs and late heat.”

Dr Norton said with nutrients the first and most important thing to consider is removal and soil testing.

“If you consider, with removal, what’s going out the gate. A 5 tonne/ha 12 per cent protein yield of wheat is likely to remove 100 kg/ha of nitrogen, 15 kg/ha of phosphorus, 18 kg/ha of potassium and 5 kg/ha of sulphur (S).

“While canola figures for a 3 tonne/ha yield will be 90 Kg/ha nitrogen, 15 kg/ha phosphorus, 20 kg/ha potassium and 15 kg/ha sulphur.

“These numbers changed in the case of a frosted wheat crop being cut for hay, which may be a 9 tonne/ha hay yield, then nitrogen removal goes up to 160 kg/ha and potassium to 180 kg/ha. Stubble burning will also change those numbers.

“These figures are the mean numbers from long-term data on the Grains Research and Development Corporation (GRDC) National Variety Trials (NVT), so it should be noted the mineral concentration variation can be significant from site-to-site and year-to-year, and if you want to be sure then nutrient sample testing is a good strategy.”

In terms of replacing these nutrients, in the example of phosphorus, Dr Norton said it needs to come from either soil supplies and or fertiliser inputs.

“So, for that 5 tonne/ha wheat crop, you will need 75 kg/ha monoammonium phosphate (MAP), 200 kg/ha urea and 45 kg/ha Muriate of Potash (MOP).

“But to be certain on inputs, soil testing, deep soil testing is absolutely critical to check if a crop will respond to nitrogen and potassium in particular.

“Nitrogen and sulphur, as anions (negatively charged), tend to move deeper into the profile with rainfall, but on sandy acid soils, potassium (a positively charged cation) can also be mobile and leach into the lower root zone, although it is not as mobile as nitrogen or sulphur.”

Dr Norton said in the north, assessing deeper (10-30 centimetres) phosphorus, potassium and sulphur supplies has shown benefits as this is where the roots will draw water and nutrients from, especially as the topsoil is often dry during the growing season. These deeper zones can be nutrient depleted and so deep-placed nutrients can provide a strategic nutrient backup in those regions.

“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 said.

“By combining these ‘known knowns’ and doing your best to understand the ‘known unknowns’ growers can maintain a nutrient strategy which is both tactical and strategic.”

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.

https://grdc.com.au/