I am Scott Sturgul with the Nutrient and Pest Management
program and i’m back again to discuss with you soil potassium. And this discussion is
again part of the larger CCA pretest training videos we’re working on for this
year. Potassium as you know is the third of
the big three nutrients that we’re concerned about, with nitrogen and
phosphorus p and potassium k. Just like phosphorus, soils contain large amounts
of potassium. However most of that potassium is fixed
or held by soil particles particularly clay particles and unavailable to plants.
Thus we need to supplement soils for crop production with the potassium more so than we do with phosphorus. Because large amounts of potassium are removed with
the crops commonly grown in Wisconsin. Alfalfa, corn silage, other forage legumes,
all remove a great deal of potassium from the soil. Sands and organic soils
tend to lose soil, excuse me, lose potassium from the
soil profile and typically test low for plant-available k. We’ll talk about some
unique management considerations for sands and organic soils they just don’t
hold onto potassium very well and intuitively obvious if we don’t apply
potassium we will draw down soil test k levels and these cells k levels can draw
John much more quickly than the drawdown we talked about with soil phosphorus
levels and we’ll have a graphic or two that look at that in a moment this is the potassium cycle and we will
look at inputs of potassium to the soil and exports of potassium from the soil
by far the most important input of potassium to the soil is the parent
material of the soil minerals bedrocks as they whether they release potassium
and and other nutrients fertilizer is a for an input manure biosolids all
important inputs to potassium in the soil profile losses of
potassium from the soil cycle are of the foot potassium cycle crop removal
potential loss of potassium if we’re losing any of the fertile topsoil
through erosion or runoff I just like phosphorus potassium is taken up by the
plant in the dissolved or soluble form this soluble levels of soil test
potassium are relatively low relative to overall concentrations of potassium the
soil profile and we’ll talk about a minute about how potassium is released
from the soil available for crops again the parent materials bedrock minerals
such as Micah bells bars over time and very slowly release potassium some of
this potassium or is fixed or held by clay particles this is what we call the
non available potassium it gets trapped as it as the parent material whether it
gets trapped in the clay layers you remember our clays are made up of layers
of molecules we have one-to-one clays into one expanding plays well potassium is strongly held by these
clays if it gets in the clay layers between layers of clay it’s strongly
held in hard to be released for plant uptake over time some of this potassium
is released we have what we like to call the non-exchangeable potassium and the
exchangeable potassium potassium that is in equilibrium with the soil solution is
the exchangeable potassium here soluble potassium is either taken up by the
plant or can be immobilized by the soil itself specifically when I say mobilized
it can be bound to clay particles this potassium is exchangeable are available
to come into solution and be available plants because this potassium is held on
the outside of clay particles rather than inside clay particles think about
cation exchange capacity state can exchange capacity is the
ability of soil particles organic matter particles to hold on two cations and
this is what’s going on here with the exchangeable potassium so there’s an
equilibrium here between plant-available or dissolved potassium the soil solution
with the exchangeable potassium which is held on the exchange sites of certain
clay minerals and other soil particles and there’s equilibrium is also going
this way in terms of the fixed or unavailable are very slowly available
potassium from the soil itself again the release of potassium from parent
material to fix clays to available clays is very slow and we can’t rely on that
to supply potassium at levels to meet the agronomic needs or the nutritional
needs of the crops we tend to grow one other lost that i didn’t mention of
potassium from the soil can be through leaching fortunately we don’t worry
about leaching are losing potassium or when we do lose it we don’t worry about
it from any environmental standpoints however on our sands which have very low
can exchange capacity we tend to lose potassium as as water moves potassium
ions through the soil profile also we tend to leach potassium or lose
potassium on our organic soils now we know organic soil particles have a very
high cation exchange capacity but even though organic matter has a hike at
exchange capacity does not hang on to potassium cations very strongly so we
can lose potassium through through leeching again are sandy soils and our
organic soils explain a little better what I talked about that diagram the
pools of potassium in the soil profile the unavailable potassium is contained in the soil and
some clay minerals that’s this here the slowly available potassium is held by
other clay minerals and this is the storehouse of potassium over time this is where the potassium is
released over time going the wrong way and this is the available or
exchangeable potassium associated with other species or other types of clay
that are are more available again low amounts of clay on our sandy soils
that’s low amounts of of potassium on our sandy soils are movement out of the
soil profile in sandy soils and for organic soils they don’t hold onto
potassium very well the readily available potassium that is the
potassium that’s in the soil solution is available for uptake by the crops this
is one last graphical representation of how soil potassium interacts with the
soil clay particles again remember clay particles are layers of minerals that
are forming a clay particle those layers that are on the outside on the cation
exchange sites are readily available to come into solution and can be utilized
by plants the non-exchangeable are slowly released buffer if you will
potassium is that that’s contained within the clay particles soil tests
measure the amount of exchangeable potassium are we estimate through our
extractants the amount of exchangeable potassium as well as the potassium
that’s dissolved and readily available in the soil solution factors that
influence this exchangeable or readily available potassium in our soils are
wetting and drying cycles freezing and throwing some buying cycles and
oxidation state are the the content and state of iron in our soil profile in
ours in our soils and we’ll talk about that in a moment wetting and drying exchangeable
potassium can content can increase or drink decrease when the soil is dry and
that depends on the type of clay minerals that are plugged present some
minerals when they dry the layers will collapse around any anions in this case
cations excuse me cations that are contained within the layers of the so
profile and it will hang onto them other types of clays when they are dry the
layers will open up and the potassium can be released from
these types of clays and out into the soil solution so time of sampling if we
do soil sampling relative to soil wetness can influence cell test k levels
and you’re gonna hear me say this a couple of other times if when we’re
sampling fields for routine soil testing it’s important that we try to sample the
same time of the year more or less whenever we’re sampling whether it’s
once every four years once in the rotation or or more frequently than that
because soil test potassium levels can change as a function of temperature as a
function of soil wetness so we’d like to make as close as we can get to apples to
apples comparison so if your soil sampling in the fall in one year try to
sew sample in the fall the next time your soil sampling a given field likewise with soil moisture freezing and
thawing or so temperatures can impact the amount of potassium that’s released
by soils this all depends on clay mineralogy and and other factors in the
soil soil that contain consider bottom I cos they can release potassium is a
function of freeze-thaw cycles again this influences the the layering the
strength of the layers that are holding the soil potassium in the soil other
soil types that have smaller amounts of Mike and greater amounts of exchangeable
potassium they’re not so affected by freeze-thaw cycles so as I mentioned in
the previous slide you know the time of soil sampling
influences nutrient levels or potassium so we can see differences due to
moisture content due to to the temperatures in spring vs fall sampling
depending on the clay minerals that are present so again with the repeated soil
sampling of a given field it’s important that you sample the field at the same
time of the year from sampling cycle to sampling cycle the mineralogy are the content of the
clay specifically the iron content of the clay also has some influence on the
potassium content in the potassium release from soils this is a rough
diagram the soil scientist at uw-madison prepared looking at general
generalizations of clay mineralogy across the state of Wisconsin and you
can see our clay soils different different have different properties
across the state depending on where they’re found in one of these properties
iron iron content and iron oxidation state and these all influence potassium
availability as well ok the soil test when we stick a soil
probe in the ground we collect a soil sample we sent it off to the lab what is
the lab measuring the limit but the lab is measuring two parameters regarding
self-test potassium they’re measuring the potassium that’s in the soil
solution this is the dissolve readily available solution and it’s all so as to
also estimating the exchangeable potassium how much of that potassium
that’s on those clay particles the soil particles into some extent the organic
matter particles is available to come into solution and be available to plants
last time I’m going to say this and I use you see it here again when you’re
sampling for potassium which is part of routine soil sampling it’s important
that you sample the same time from from year to year because there can be
seasonal very variation in the soil test potassium levels were going to find what
are the optimum self-test potassium levels for the major crops grown in
Wisconsin their illustrated here on Arlo me which are medium and fine textured
soils you’ll see the optimum ranges for alfalfa corn and soybean a range from a
hundred 240 parts-per-million soil test potassium are not responsible levels for
Al far greater than 240 parts-per-million potassium 194 corn and
for soybeans here’s an example of what the potassium recommendations are for
alpha alpha i showed you a similar table 4 phosphorus recommendations for corn you
can see that the amount of potassium we need to apply is a function of the soil
test level the higher the soil test level for potassium the less fertilizer
we need to apply the higher the yield goal for the given crappie alphabeat
corn the more potassium we’re going to have to apply again these
recommendations can be found in detail in UW extension publication a 2809 are
so test recommendation publication environmental factors that affect
potassium availability to plants soil moisture influences that when we are
going through dry soil conditions we have little or low soil moisture there
is less diffusion of soluble potassium to the roots diffusion is the major form
of uptake of potassium in the plants so dry soil conditions lead to reduce
potassium uptake if we increase soil moisture from 2010 percent to
twenty-eight percent for example you can see an increase in potassium transport
in the soil solution to the roots by up two hundred and seventy-five percent now
too much moisture on the other hand can result in restricted root growth low
oxygen and slow potassium absorption by the roots so you know saturated soils
cold soils are something that’s going to inhibit potassium uptake by plants i
already mentioned the impact of soil temperature here is a slide that just
reiterates what I said you know low soil temperatures will restrict plant growth
but growth in the uptake of potassium if we have high potassium levels in the
soil it will increase potassium uptake at these low soil temperatures so this
is one of the reasons we see positive responses to starter fertilizer banded
fertilizer you know the the plant roots are slowly expanding if they can get to
that band of concentrated phosphorus and potassium the plant will respond
positively soil pH we want to keep our ph and optimum levels for crop
production at low PH or acidic soils potassium has more competition for
getting change sites that’s less of its
available and held by the soil when soils are limed greater amount of
potassium could be held mechanics changes sites and applause excuse me
potassium leaching will be reduced as i mentioned earlier we’re talking about
the potassium cycle potassium leaching can occur on our course textured sandy
soils and our monkey soils particularly organic soils if they’re irrigated large
fall applications of potassium to sandy soils organic soils are discouraged it’s not a good idea to apply these in
the fall just like nitrogen because chances are we’re going to lead to
remove that potassium through the solar poor soil profile and out of the root
zone and it won’t be there in the spring when the crops needed particularly on
our sandy soils and organic soils annual maintenance of our manual applications
of potassium is important we need to monitor our soil test phosphorus excuse
me self-test potassium levels a little more closely and perhaps a little more
often on our sandy organic soils regarding potassium on these soils like
to talk a little bit about some potassium management concerns
considerations that we have to worry about Wisconsin many of our soils are
potassium deficient and this trend is getting worse in the state of Wisconsin
you’ll see some data in just a few minutes looking at the decrease since
average soil test potassium levels over time in the state of Wisconsin this is
due to a couple concerns the most important is probably economics and I’ll
elaborate later but also any fields where we have alfalfa or corn silage in
the rotation are prone to utilizing a lot of soil potassium and these fields
should be monitored closely to make sure we don’t drop our soil potassium levels
below optimum ranges for crop production potassium should be monitored in reduced
their conservation tillage systems switching these systems can cause some
called soil compaction soil compaction and other factors can lead to lower availability of potassium in the
soil so we need to keep a handle on it their top dressing is important to our
Forge needs our elf and other forms legumes applying supplemental potassium
during the growing season typically after the first and third cutting to
optimize our alpha yields and to maintain forage quality or important one
other concern to we do have to be a little bit aware of excessive levels of
potassium in our soils translating x excessive levels of potassium in our
forages if we have such a situation folks that work in the dairy nutrition
side are keenly aware of the problems that excess potassium in the diet can
cause things like milk fever and other concerns with cattle so if we’re dealing
with excessively high soils soil test potassium we likely are going to have
high levels of potassium and the forages were harvesting and we need to be aware
of that when we’re looking at the diet were feeding to our dairy cattle in
Wisconsin this is a chart similar to the
phosphorus chart you looked at which shows the average soil test potassium
levels in Wisconsin over a 50-year period from the mid-sixties 2 2009 the
the bar you can see a steady increase but this is what I’m worried about here
which I talked about in the previous slide the decrease where we’re seeing in
soil test potassium levels in the last couple of sampling periods when I showed
this diagram for phosphorus we had this bar here going across which was the
excessively high range for most of our crops are commonly grown crops in
Wisconsin well with potassium this isn’t are excessively high ranges are optimum
range you can see what a serum levels are you know in that optimum range but
certainly aren’t exact excessively high as we see in the celtics phosphorus
category and again we’re worried about this decline in soil test potassium
values the rise in the build-up of soil test potassium levels of in terms of
County averages across the state county average so test pic I potassium levels
are illustrated here and this graphic showing data from 1974 2009
you’ll see the darker the red is where we want to be with soil test potassium
levels in the range about optimum for for crop production in the state of
Wisconsin you can see a general a buildup in soil test p across the
seventies through the eighties into the nineties once we get into this . the
latter part of the nineties in the early two thousands you’ll see up a paling or
pinking if you will of this map showing general decline and soil test potassium
levels and this is not a good sign we don’t like to see it but there are some
valid reasons for it those of you that were out there in the
industry in the latter part of the last decade remember what happened to energy
prices gas prices in fertilizers in 2007 2008 2009 all those commodities went up
energy-related commodities and so did the cost of fertilizer potassium
fertilizers really spiked in some cases Dumbledore beyond what their previous
prices were as a result k applications were were limited on
farms because of the the lack of affordability of potassium we’re growing
more corn silage today than in the past with the expansion of the dairy industry
in the state of Wisconsin more corn is being harwich harvested for silage then
been in the past and when we harvest corn silage we’re removing significantly
more potassium than we would if we were just harvesting corn grain there’s also
a concept i’ve heard a few people talk about out there is that folks feel that
since potassium is derived from soil bedrock soil mineral soil clay that the
soil ruhr replenish itself in time that the subsoil can supply potassium and it
can but it can’t supply enough to a crop to give us an agronomic or a profitable
yield so in terms of growing agronomic crops the soil potassium will not
replenish itself or will not replenish itself enough to allow us to read achieve our
optimum yields for for crop production we if you saw the soil phosphorus
discussion that I talked about how long it takes to build up so test phosphorus
levels and how long it takes to draw them down the at the story is just the
opposite with potassium we talked about a buffering capacity buffering capacity
is the soil property and it’s the amount of fertilizer that’s required to change
a soil test value by one parts-per-million ah for phosphorus we
need to apply 18 pounds of p2o5 to result in a 1 part per million change in
in soil test phosphorus for potassium were the buffering capacity is
significantly lower only six to seven pounds of k2 all will result in one part
per million change in soil test p therefore significantly less time is
required to lower or raise soil test k levels compared to phosphorus also the
demand of our crops particularly our forage legumes and corn silage or
greater for potassium than they are for phosphorus so we’re going to draw down
potassium levels quicker in the rotation then we are in phosphorus this is that
same example we did for phosphorus only this time we’re substituting potassium
let’s say we start out at the beginning of this two years corn one-year-old
three-year a rotation with an excessively high level of soil test
potassium 200 parts per million if we don’t any apply any supplemental
potassium we look at crop removal of the rotation we look at the change in soil
test p by subtracting that removal of ptk 20 subtracted by the buffering
capacity results in a hundred forty-five partner millions part per million
potassium change so 200 minus that 145 part million gives us a 55
parts-per-million soil test potassium value after the six year rotation so
unlike attack phosphorus excuse me we went from excessively high and stated
excessively high after the six year rotation with potassium we started at
excessively high and after six years of this rotation no supplemental
potassium we drew our soil test levels down to the very low category so soil
test potassium levels change very quickly soil test phosphorus levels
change very slowly and again here we’re looking at crop removal of various crops
grown in the state of Wisconsin and you can see relative to potassium the large
removal rates are associated with alfalfa and corn silage you can see the
degrees here of potassium removal relative to corn grain soybean and wheat
potassium deficiency will wrap up here with our discussion of potassium
deficiency on how to use your your eyes as a diagnostic tool for identifying
potassium deficiency protest potassium deficiency is a pretty easy deficiency
to identify out in the field and if you’re going to be a CCA worth worth
your salt you know you’re going to have to be able to identify this right away
the classic potassium deficiency symptoms in corn are a yellowing or
chlorosis of the leaf margins starting at the tip and going down the leaf
margins and beginning at the older plants the the bottom plants of of a
corn plant potassium is a mobile nutrient in the plant meaning that when
the plant is stressed for potassium it will feed off the older leaves and shoot
the potassium to the new growth so you’re if your crops are proficient
potassium you’re going to see it on the older the lower leaves again here’s a
transgression of a healthy corn plant one that’s starting to get a little low
in terms of soil test potassium you’ll see the yellowing along the margins
starting at the tip and worsening conditions as we lower our soil test
potassium levels with alfalfa we have to be keenly aware of potassium
deficiencies and alfalfa and the symptoms they’re pretty telltale there
you’ll see the white or yellow some cases brown spots on alfalfa leave
starting at the tip and working down the margin you don’t see
bands or streaks like we see in corn you see these spots don’t confuse this with hopper burn
Auburn is broad bleached areas along the margin of the plants the spots are
telltale signs of potassium deficiency in alfalfa and here’s another shot of
what it looks like in elfelfa potassium same bit chlorosis yellowing browning of
the leaves the older leaves first starting at the tip and progressing down
the margin of the soybean plant and here is another photograph of potassium
deficiency beans in the field you’ll notice the closest starting at the tip
going down the leaf margins and the older or lower leaves are being affected
by the potassium deficiency prior to the new leaves on the plant so with that that wraps up my discussion of potassium
in soils I thank you very much for your time and attention