Soil testing is the best tool available to estimate soil nutrient
levels.
But Adrian Johnston of the Potash and Phosphate Institute of Canada
says research is needed into new and better ways of testing.
Johnston said new management practices, such as direct seeding, are
affecting soil test results.
“We need to start looking at how changing systems is influencing the
results we’re getting,” he told the Saskatchewan Soil Conservation
Association direct seeding seminar.
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He said much of the information used to estimate crop requirements was
established 20 to 40 years ago. While databases are updated as more
information becomes available, different farming methods have to be
considered.
For example, research has found that phosphorus accumulates at depths
of 10 centimetres because that’s where direct seeders are placing
fertilizer bands.
That is a good situation as far as phosphorus is concerned, Johnston
said, but nutrient stratification can also give false readings on soil
tests.
“It’s sort of a balancing act of looking at where we’re doing the
measurement and from what database we’re using to make the assessment
of a recommendation,” Johnston said in an interview.
He said testers are having a more difficult time estimating nitrogen
concentration. Nitrogen is a more mobile nutrient.
“The ability to estimate soil N supply with current soil testing
methods appears to be limited in some situations and innovative changes
to soil testing will be required to improve the ability to predict soil
nutrient supply rates,” he wrote in a paper submitted to the seminar.
Johnston added soil testing alone should not be used to determine how
to optimize crop responses.
“(Farmers) need to consider looking at the removal of nutrients in the
grain so that they can use removal values as part of their estimate as
to what they should be putting on,” he said.
“That is, look at what you grew and potentially what was removed and
what your future target yields are and then start looking at replacing
those nutrients.”
Farmers also need to look at field history – last year’s yield,
previous fertilizer management, manure application, which forage or
legume crops were grown – and next year’s cropping plan to make their
assessments.
