Originally written for CCA Soy Envoy Blog
Having spent
my time as a CCA Soy Envoy writing about micronutrients, sulfur, nitrogen and
phosphorus, it seems appropriate to close out the year focusing on potassium. Of
the plant nutrients, potassium is part of the big 3—nitrogen, phosphorous and
potassium (NPK). When I run soil tests in my area potassium is the most likely
to be low, although some soils I work with can be high and stay high.
Potassium helps
plants withstand drought damage. It is also credited with general plant health
and preventing lodging. Potassium helps the plant produce starch and transform
sugars. This nutrient is also essential for photosynthesis. Potassium
deficiency is characterized by yellowing and then tissue death starting at the
edges of leaves in both corn and soybeans.
This Iowa State University Bulletin has more information on potassium
deficiencies. They go into some explanation of causes of potassium
deficiencies other than low soil test.
In past years we have seen some drought-induced
deficiencies in fields. Fields with good potassium levels to begin with seem
to resist that deficiency better. We also thought we saw some compaction-induced
deficiencies because of working and planting in wet soil conditions this spring.
Several years ago, University of
Illinois (UIUC) researchers hypothesized that we do not need potassium
fertilizer. Research from other credible sources including the International Plant
Nutrition Institute has called the
researchers’ premise into question and disproved some of their findings. Emerson Nafziger, Ph.D., also argues for potassium fertilizer in this article.
Potassium is common in our Illinois
soils because it is present in the crystal lattice of a prevalent clay mineral,
illite. The nutrient seems to move in and out of the lattice depending on
moisture. This makes potassium soil testing imperfect, but it is all we have to
establish a scientific basis for determining fertility needs.
One reason that we test soils more
often than the university recommendation is that it helps us keep better track
of potassium. It has been my experience that crops do respond to potassium when
soil test levels are below 230 pounds per acres. Ideal levels are
considered to be 300 to 400 pounds per acre in Illinois. Potassium is critical
to help weather a drought. Levels need to be high because availability goes down
when soil is dry.
We do have clients with soils that
have not needed potassium fertilizer in twenty years or more. These
growers haven’t had any apparent yield loss and the test levels remain
high. We also have clients who have low potassium test levels who have
benefited from potassium fertilizer.
One of my clients raised 80 bu/A
wheat a few years ago after increasing his potassium fertilizer on that field. We
do find that potassium levels sometimes remain low for several years as we
attempt to build the nutrient up. We will then have a sudden and unexpected
rise in soil test potassium. I will sometimes suggest 100 pounds per acre of
potassium fertilizer to prevent wild swings in soil test levels even though
soil tests would seem to indicate we can skip a year.
Iowa State
University has endorsed a moist soil test where they do not let the soil dry
down. I have no problem with using the moist test, but we continue to use the Mehlich 3 extraction. I am not ready to give up on potassium testing
even knowing the limitations of the test. We are going to continue to test
frequently and recommend potassium fertilizer on soils that have low soil test
levels, making adjustments as needed.
