LOWER crop yields and less nutrients removed in the 2019 growing season does not mean inputs can be reduced in 2020 to save money and utilise what wasn't used last year.
The biological process of the soil and nutrient fixation and release is a little more complicated than simple addition and subtraction, so when it comes to nutrients, levels of potassium (K), nitrogen (N) and phosphorus (P) need to be measured and tested.
CSBP senior agronomist Luke Dawson said potassium was a very residual nutrient and any unutilised potassium from 2019 would be able to be accessed by this year's crop.
"The K requirements of this year's crop will be determined by the crop yield potential and supply of K needs to be determined by soil sampling to 30 centimetres," Mr Dawson said.
"A long-term K trial near Mingenew showed that the best yields in 2019 were on plots that had received a capital application of K in prior years.
"Those plots also received as a maintenance application of K each year supplied by an NPK compound at seeding time."
For phosphorus, if a crop yielded two tonnes per hectare of wheat last year, then the rules of removal suggest that only six kilograms per hectare of phosphorus was removed.
Mr Dawson said since last year was tough and if enough P was applied for 3 tonnes a hectare (about 9 kilograms a hectare), then theoretically there should be 3kg/ha of P left in the soil.
"By following the removal rationale if we are aiming for a 3t/ha yield this year then we should only need to apply the 6kg/ha that we removed last year to make up the deficit, however the P cycle is a little more complicated than this," he said.
"In any given year, depending on soil P levels and Phosphorus Buffering Index (PBI), we may only ever have access to 30 per cent of applied P and the rest of the plant's requirements will need to come from P in the soil, largely from the P bank."
Mr Dawson said although last year's P may have contributed slightly, it is more dependent on the past 10 to 20 years of P application, than last year's in isolation.
CSBP has recently conducted 10 different phosphorus trials around WA and have proven that P requirements are largely driven by two factors, nutrient distribution in the soil profile and the Colwell P and PBI levels in the soil profile.
Mr Dawson said one of the best examples came from a P trial hosted in Mt Madden in 2019, where the paddock in the trial averaged 1.6t/ ha in 2018.
"If we had taken the removal approach, then technically this paddock should have only needed 5kg/ha of P, however, 5kg/ha of P potentially would have returned no yield benefit and we may have lost money on the P," he said.
"By using only the soil results in the topsoil we had a recommendation of 0kg/ha P, in this situation we make the assumption that the paddock is not going to be overly responsive to P, and only removal or maintenance rates of P are required to keep soil P levels stable.
"By using the complete soil profile results we had a recommendation of 15kg/ha of P which was a lot closer to the rate of 18kg/ha giving us the highest yield and profit from P."
Overall, the P response was driven by the P and PBI levels of the whole soil profile, while P application from 2018 played a minor part in contributing to the overall P status of the soil.
Mr Dawson said while it may be tempting to cut P rates to reduce costs, it's important not to punish this year's crop and reduce yield potential.
"By soil testing to depth and using a reasonable long-term yield target we will be able to maximise yields and returns," he said.
"This will have a greater impact on your bottom line than saving a few dollars per hectare at this end of the season and more than likely reducing overall yield potential and returns."
Nitrogen is slightly simpler than P, theoretically unutilised N should still be available to this year's crop however, the difficult part is calculating when it will become available.
The availability of this residual N will be determined by a few factors including summer rainfall events above 15 millimetres which will have started to mineralise any N in the soil to nitrate N and summer weeds that have germinated off the back of recent summer rainfall events which will be using up any available soil N before they are controlled.
The stubble load remaining from 2018-2019 will also be a factor as high yields saw stubble loads at an all-time high, this residue has still not fully broken down due to the low rainfall from last year.
Mr Dawson said in order to determine the value of any residual N, the only option was to soil test to 30cm to identify exactly what the current and potential N supply is going to be.
"Nitrate and ammonium N levels will give you an indication of the current N status of the soil while your organic carbon levels will indicate potential N that will be mineralised over the year," he said.
"Banding a robust rate of N is the best way to maximise Nitrogen Use Efficiency (NUE), by banding a high rate of N at seeding time you allow the crop to set up a decent yield potential and give yourself more time to make any N decisions in season.
"In years like 2019, banding high rates of N allowed us to shut the gate and gain the biggest benefit from the N applied at seeding time."
Mr Dawson said overall, before taking the red pen to fertiliser expenditure this season, it was important to consider what was this year's yield potential, what nutrients were needed to achieve this potential and what nutrients were available?