Hi folks I'm Kolby Grint and I'm the northwest regional specialist with the Nutrient and Pest
Management program. Today I'm going to be talking about crop responses based on soil test levels.
So, how do crop response levels work? We'll typically see crop response levels in the
soil test interpretation section of our soil test report and here we'll see different crops
referenced in terms of test categories for our immobile nutrients, phosphorus and potassium,
with these test categories ranging from very low to excessively high. Each of these categories
represents significant changes in the probability of seeing a yield response that will be profitable
from applying nutrients on an annual basis. When we're in the very low to low test categories
you know there's a very high probability that we'll see a positive crop response
which will be profitable for applying nutrients. And then when you get to the very high
to excessively high categories the probability of seeing a crop response is low, so it likely won't
be profitable to apply nutrients. Another good way to think about these is in the very low to low
test categories, the recommended rates will be based on the crop removal rate
plus a little more for those nutrients so that we can build that soil test category level over time,
ideally to the optimum test range. Once you're at the optimum test range this is thought of as
the most economically and environmentally desirable level and here the recommended rate
is going to be at the crop removal rate so that we can maintain that test level.
Once you get to the high rate, the probability that we'll see a positive crop response
starts to really diminish and you'll usually see recommended rates at around
half of the crop removal and that's so that we can bring some new available nutrients into the system
without significantly drawing down the test category level if we have some you know really
productive years. And then once we're in that very high to accessibly high range
there's usually a low to no recommendations for making a fertility application
We commonly see questions similar to, "Should I try to maintain my soil test
phosphorus and potassium values above the optimal level so that i can maximize yield?"
The answer to this question is going to be NO! so when we're in those
soil test levels above optimal we're in the non-responsive range so, there's a
less likelihood that we'll see a positive cross response to making that nutrient application.
When we're trying to maintain our test level in these areas, there's usually a higher
risk that there's going to be environmental damage from runoff occurring because there's
going to be a higher load, especially in the case of phosphorus, of that nutrient
in the soil. It can also be a good idea to think about the rates that we're going to be applying
depending on our soil test category differently depending on the fertilizer price. So in some
years availability or other issues might be causing the price of phosphorus and potassium
sources to be higher, and if you remember from before when we're in our very low to low testing
categories the rate is going to be allowing for crop removal plus more to build but if our
fertilizer prices are higher we're going to be paying more in those years to build fertilizer.
So one potential thing you can do to increase profitability in that situation is to only apply
the optimal nutrient rate for our very low to optimum test categories. That way we can
maintain productivity and then when you're in those higher testing categories you can
probably get away without applying any nutrients as the most profitable decision
Another way to think about our soil test categories is when planning for phosphorus
or potassium applications. So some of us in Wisconsin live in areas that have limited access
to fertilizers or we all might experience years where availability is limited. So it's always
a good idea to prioritize our applications for our lowest testing fields in these situations.
If you're just working with soil test reports you can just compare the reports for individual fields
and make those applications to the lowest testing fields that are in the lowest testing category.
That's going to show you the greatest probability of seeing a response. Or if you're using SnapPlus,
a trick to do this in the cropping screen is to select the plant icon in the top left corner,
and then that's going to show us you know our nutrient recommendations for all of our fields.
Then from there you can click on the column headings for our recommendations for phosphorus
and potassium, and that will allow us to filter our fields based on our lowest and highest
rates recommended rates for all of our fields and we can prioritize fields there
Thanks for watching my video on crop responses based on soil test levels! If you have any
questions feel free to reach out to my email or my phone and I'd be happy to answer them!