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Soil Test
“Interpretation Guideline Tool”
Introduction:
“When we look at an A&L soil test there are many different analysis and
ratios applied to that particular soil. The key to achieve the best
utilization of the soil test is to understand the components and what they
mean for that soils ability to grow a crop and your ability to translate that
to your growers.”
We will break a soil test down into different components and identify key
things to look for when building or interpreting a recommendation.
1. Sampling Considerations: - Most important step to develop
a good soil test
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Who takes the sample
What time of year (spring, fall, freeze-up)
Locations on a field (with grower picking them out)
Depth of sample (two depths preferably)
GPS (repeatable site year after year)
Cropping rotations (Understanding the history)
Row spacing and fertilizer placement (zero till or conventional)
(don’t want to sample on an old band)
Soil Compaction problems (how deep can the roots grow) use
of a soil compaction probe
2. Physical and Chemical Properties:
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At a quick glance this gives us an indication of the native
makeup of he soil we are working with
Components to look for on the test are Organic Matter, ph,
CEC, ENR and sodium levels
CEC is basically the measurement of the soils ability to hold
and release nutrients.
The higher the CEC usually the better the organic matter of
the soil and the greater the clay content
OM in the 3-5% would indicate a soil with greater opportunity
for in season N production. The higher the OM the higher the
ENR (estimated nitrogen release) will be.
pH is an indication on whether our soil is acidic (less then 7)
or basic (greater then 7). 6 to 7 would be considered an ideal
range. pH is important to understand when it comes to the
breakdown of certain herbicides as well as low OM can cause
problems with carryover as well.
Most soils in our trading area have a greater then 7 pH.
The lower the CEC number (<12 more sand) means lighter
textured and the higher the number >30 usually will indicate a
clay or heavy soil. We can use this number as an indication of
the soils ability to hold water as well.
Soils with excessive calcium or free carbonate can cause and
artificially inflated CEC.
With this field you can see a real change in properties as you
enter the subsoil. Sharp increase in pH, CEC and sodium
content with a drop in OM.
3. Nitrogen Considerations:
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Nitrogen is a key element to growing any crop and one of the
hardest elements to determine the availability to a crop in a
certain year due to its constant change in chemical properties
and its volatility in the soil.
Most labs tend to just focus on nitrates where as A&L likes to
give you as much info as possible to make the most informed
decisions.
When looking at making a nitrogen recommendation I look at
the nitrate numbers, organic matter, CEC, previous crop,
intended crop, time of sampling, whether or not manure has
been applied and moisture conditions.
A&L offers two different methods for calculating nitrogen
when using the online program. You can calculate by using
nitrates or by using the ENR calculation. By clicking on the
“Config Calc Params” tab in the recommendation area you can
toggle between ENR and STN (standard nitrate).
At A&L we convert our Nitrate ppm’s to Ibs by taking the ppm
(7) x by the depth of sample taken (6 inches) x .3 (which is a
conversion factor used by most labs) this equals 12.6 Ibs of
nitrate in the top six inches. The same applies to the sub soil.
2ppm x 18 inches (6 – 24 inches) x .3 = 10.8 Ibs nitrate in the 6
– 24 inch depth. This works well if you only achieved 19
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inches on your sub-soil depth so it would be 2 ppm x 13 inches
(6 – 19 inches) soil x .3 = 7.8 Ibs nitrate. Total amount of
nitrate in this soil is 23.2 Ibs in the 0-24”. Based on variability
I would never account for more the 50 to 70% of the total
nitrate in the soil when building a recommendation when using
the ENR values as well.
Organic matter is the life blood for in season nitrogen
production as well as water holding capacity of our soils. A&L
uses an ENR based on OM, soil type and length of season of
crop being grown (long –corn or short season - barley crop).
The typical calculation would be for this soil, 0-6 inches of soil
weighs roughly 2,000 000 Ibs. This soil has 4.1 % OM so 2,000
000 x .041 = 82,000 Ibs of OM in the top 6 inches. Research
indicates that OM is roughly 5% nitrogen, so 82,000 Ibs OM x
.05 = 4,100 Ibs of total nitrogen/acre in the top 6 inches. Now
in any given year we could expect 2 – 4 % release from the
total nitrogen in that acre (2% being a poor year in a cool
climate and 4% in a good year in warm environment). 4,100
Ibs total nitrogen x .02 (colder climates like western Canada)
= 82 Ibs of nitrogen released in that given year. A&L then
takes that number and tweaks it based on cropping and soil.
The number is usually a little greater then half of the lowest
value produced from the calculation. A&L produced a number
of 53 Ibs of N potentially available for the next growing season
from this soil test. A&L Canada’s ENR numbers for Western
Canada are very conservative compared to the historical
research done across the Midwest.
Another way to calculate available nitrogen from the Organic
Matter is to assume 6-7 Ibs of N released for every 1 % organic
matter. So for this soil we could assume there would be 25 to
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30 Ibs of N released. This would be very very conservative
based on documented research but is still better then not
accounting for it at all.
I look at the CEC to help me determine under wet conditions
which way my nitrogen may go. Under low CEC we will lose
our N to leaching and under high CEC will can loose N to
denitrification (gases off into the atmosphere). Don Flaten
from U of M states that we can loose upwards of 50% of our N
after 10 days of standing water on over saturated soils. You
may want to alter your N applications based on these
circumstances.
I look at the previous crop to see if I will need extra N or have
an N credit. If the Carbon to Nitrogen ratio is greater then
30:1 it will take extra nitrogen to break down the straw, less
then 30:1 you will probably get a nitrogen credit. Cereals are
in the area of 60:1, canola is around 30:1 and pulses can fall
into the range of 15:1. For cereals, depending on the mass of
the previous crop it can take upwards of 20 Ibs N/acre to
break down straw residue. This is why we sometimes see
striping from last years swaths if the straw was not spread
properly. Canola is a wash and we may get 10 to 15 Ibs N/acre
credit from pulse crops. Alfalfa breaking can be as high as 50
Ibs N/acre credit.
CFI has excellent charts which will become the Manitoba
Agriculture standard for amounts of nutrient it takes to grow
certain crops. We use this to determine the amount of N it
takes to grow a targeted yield then subtracting all the factors
we have looked at from our soil test and cropping systems to
come up with our N recommendations.
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If manure has been applied it may not show up initially on the
test and can take up to three years to get total utilization. A
manure analysis and rate applied would be beneficial here.
For this field I want to grow 40 bushels of hybrid canola.
According to the CFI chart it takes 3.5 Ibs of N per bushel on
the high end (hybrid) so 40 bushels required 140 Ibs of N to
grow the crop.
Looking at a couple different calculations from this soil test
we could say we have 23 Ibs of nitrate x by .6 (60 % available)
= 14 Ibs of N from nitrates, we have 4.1 % OM x 7 (Ibs N per %
OM) = 29 Ibs released from OM. Total nitrate availability +
release = 43 Ibs N from the soil. Subtract that from what it
takes to grow the crop and our N recommendation would be
97 Ibs nitrogen/acre. Another option would be to substitute
the 7 Ibs of N per 1 % organic matter with the A&L ENR
number of 53 Ibs. + the 14 Ibs from the nitrate and our total
contribution from the soil would be 67 Ibs. Subtract that from
what it takes to grow 40 bushels of canola and our N
recommendation would be 73 Ibs nitrogen/acre. You would
also have to factor a plus or minus credit from previous crop
for straw breakdown (+ 20 Ibs/acre for wheat).
A good exercise would be to take a field you know of and have
the soil test from. Ask the grower what kind of crop he grew
there last year, yield wise, and how much nitrogen he applied.
You can use the CFI chart to determine how much N per
bushel it takes to grow the crop and figure out if any how
much N the grower received from the soil to arrive at the yield
of the crop. Ex Grower grew 60 Bushels of red spring wheat.
Applied 90 Ibs of Nitrogen/acre. Each bushel of red spring
wheat takes 2.3 Ibs of N to grow = 138 Ibs N on a 60
bushel/acre crop. He would have received 48 Ibs of N from
nitrates and OM to achieve his goal. If you knew of the nitrate
level before planting x 60% expected availability you could
figure out how much N the organic matter released or the crop
accessed during the season.
4. Sulfur Considerations:
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Sulfur is probably one of the toughest nutrients to determine
from a soil test.
The extraction used takes all types of sulfur off the soil and
adds them up whether they are calcium sulfates, sodium
sulfates, magnesium sulfates etc. Not all of these compounds
are plant available and the bonds have to be broken in order to
achieve plant uptake.
Rule of thumb if your sulfur level is low then there is a high
probability that the whole field requires sulfur. If the test is
moderate then still good chance a higher portion of the field
requires sulfur. If the test is high there may still be parts of
the field sulfur deficient and you should apply a maintenance
application on crops that are high sulfur users regardless the
number.
A&L likes to see at least 25 Ibs/acre for normal crop
production.
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We want to maintain an N: S ratio of 5 to 7 to 1 on canola and
a 10 to 12 to 1 on cereals.
Again looking at our CEC can indicate on the potential
problem of leaching on our lighter textured soils where you
may want to use combination products which are not as prone
to leaching all at once (elemental/sulfate combos).
The higher the percent sodium or the higher the EC or soluble
salts the higher our sulfur test usually reads.
Again refer to a CFI crop growth and removal chart to
determine how much S it takes to grow a bushel of your
desired crop. .55 Ibs sulfur/bushel on canola, .25 Ibs
sulfur/bushel on red spring wheat.
5. Phosphate and Phosphate Interactions:
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At A&L they offer two different phosphate tests on their
Western Canada reports but can run five different extractions
depending on what region of the country you are from.
The most commonly used extraction is the Bicarb test. This
test works best in soils with pH over 7 and calcium levels over
2000 ppm.
The Bray P1 test works well in soils under 7 pH and with less
then 2000 ppm of calcium.
Ideally we would like to see over 20 ppm on the bicarb phos
test, but in all reality most of our soils will be less then that.
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Exceptions will be manured land or land that has had
continuous phos applications exceeding removal.
Good crops can be grown on lower phos soils but are more
susceptible in adverse years.
A&L will always try and recommend replacement values for
the crop to be grown and if the phos level is low they will have
a small build (over 10 years) to work on bringing the
phosphate number back up.
It can take 4 to 20 actual Ibs of Phosphate to build your soil
one ppm. The heavier the soil the more phosphate it takes to
build the soil, this task is easier in light coarse textured land.
Look at removal chart to see what it takes to grow the crop
intended.
We also look to see what kind of pH environment we are
working in. 6-7 pH is best for phos availability. Either side of
that we will see tie up by different cations in the soil.
%P Saturation is an indication of the availability of the
phosphate that has been found in the soil. This number is
mainly controlled by aluminum and soils with high aluminum
will have a low % P saturation. In this particular soil it is
stating that of the 14 ppm we have we are only achieving
roughly 9% because of the aluminum tie up.
You need roughly .88 Ibs phosphate/bushel to grow red spring
wheat (.60 Ibs phosphate/bushel removal) and 1.5 Ibs
phosphate/bushel to grow canola (1 to 1.14 Ibs/bushel
removal).
If we were to target 60 bushels of red spring wheat we would
want to apply 35 to 40 Ibs of actual phosphate/acre just to
maintain the 14 ppm we have in the soil providing we put the
straw back. If we were to apply only 20 Ibs/acre actual
phosphate we may see our phos number drop 2 ppm next
season if we achieved our target yield.
6.
Looking at the Cations (Positive charged ions):
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These are the positively charged ions in the soil. They occupy
most of the space on the soil shelves, in which the soil carries
a negative charge. The positive sticks to the negative.
Most of the cations are at levels of hundreds of pounds/acre
and in the case of calcium can be in the thousands of
pounds/acre.
A soil with high amounts of cations (potassium, magnesium,
and calcium) usually indicates a silt or clay type of soil
because of its ability to hold all the positive charges.
We use the ppm of the cations (potassium, magnesium,
calcium and hydrogen) to calculate our CEC. This is done by
taking the ppm of each nutrient and dividing it by
milliequivalents which represent each cation then adding
them up.
1 meq K = 390 ppm
1 meq Mg = 120 ppm
1 meq Ca = 200 ppm
1 meq H = 10 ppm
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Let’s look at the top 6 inches of this soil and see if we can
calculate the CEC. We calculate hydrogen if present by taking
the pH of 7- buffer pH x 12 = meq H.
219 ppm K/ 390 = .56 meq K
360 ppm Mg/120 = 3 meq Mg
1950 ppm Ca/200 = 9.75 meq Ca
(7pH – 6.9 buffer pH) x 12 = 1.2 meq H
By adding up the meq of the cations, the CEC for the top soil
on this field is: 14.51 CEC
If the cations are more then the soil can hold they end up in
solution and can start causing problems like saline or sodic
areas of the field.
A&L will rate the cations based on the soil type from the field.
We may achieve a high reading on a soil with 130 ppm
potassium that is coarse textured and receive a low reading
on a soil with 130 ppm potassium that is heavy clay based.
When we look at a soil and see the sodium number creeping
over 100 ppm at any depth then it starts to create problems.
High sodium levels disable the uptake of other nutrients and
water and the plants will either not establish or die back in
the growing season.
We will establish a better view of appropriate levels of the
cations when looking at the % Base Saturation of the cations.
Looking at this field from your trading area there is a drastic
change in cations from the surface soil to the subsurface. The
ppm of potassium drops off but calcium shoots through the
roof. The pH climbs dramatically as well. I would estimate the
subsoil potential would be limiting in this field (notice the ppm
of sodium)
7.
Base Saturation Snapshot:
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I look at the base saturation as a simplified view of the
cations and gives us an idea of the balance of the cations in
the soil and more importantly the availability of potassium
which is almost as important as nitrogen in growing a crop.
Base Saturation is calculated by taking the meq of each
nutrient divided by the total CEC number. We did this exercise
in the previous point.
Looking at this soil, lets try and calculate the base saturation:
.56 meq K/14.51 CEC = .038% = 3.8%
3 meq Mg/14.51 CEC = .206 % = 20.6%
9.75 meq Ca/14.51 CEC = .67% = 67%
1.2 meq H/ 14.51 CEC = .082% = 8.2%
We have now determined our base saturation by starting our
calculation with the ppm of the cations.
We would do the same calculation to find the %Base
Saturation for Sodium as well. 1 Meq of Sodium = 230 ppm.
For this field we have 23 ppm of sodium in the top soil. 23/230
= .1 meq Na. Using the same formula as the other cations we
divide .1/14.51 CEC = .0068 or .07%.
Through years of research based on the work done by Fisher
optimum ranges of % Base Saturation have been developed for
best availability of the cations:
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3 -5 % Potassium (the lighter the soil the higher the
percentage you want with K)
10 – 20 % Magnesium
65 – 80 % Calcium
10 – 15 % Hydrogen
Less then 2% Sodium
If your soil is out side of these ranges it disrupts the balance
for optimum root uptake even though their may be sufficient
amount of pounds of that individual element in the soil.
Looking at this soil test the surface looks to have a real nice
balance and has good potential for the roots to find enough
potassium. The barrier here would be the subsoil where our
%K drops off and we have an increased % Na over the 2 range.
Shallow rooted crops may do better here as opposed to deep
rooted crops.
A&L Analysis Ratios:
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A&L Canada offers a couple extra ratios that help us
understand availability of two of the major crop growing
nutrients.
The K/Mg was developed from years of yield maps overlaid on
one acre soil grid maps. Anytime they were achieving high
yields the K/Mg ratio was always between .2 and .35. Outside
of that range the yields drop off.
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K/Mg is calculated by dividing the meq of K by the meq of Mg.
For this field it is .56 meq K/ 3 meq Mg = .19.
This allows us a third dimension to viewing the availability of
K in our soil.
Saturation of Phosphate (P) is a relatively new ratio and gives
us an indication of the percentage of phosphate that would be
available that is showing in the ppm (14 ppm). This number is
strongly guided by the amount of aluminum in the soil.
Aluminum has a strong affinity to phos and when the
aluminum number raises in the soil our phos availability
decreases.
With this soil we have close to 600 Ibs of aluminum/acre in the
top 6 inches.
This is not indicative to all the soils but we do see it on
occasion.
According to Greg Paterson we would need at least 35% free
lime of excess carbonates in the soil before calcium starts to
affect phos availability.
Micronutrient Considerations:
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We do not always request the micronutrient portion on our
tests but it is important to understand where the levels are in
our soil when we are targeting higher end production.
If we have developed a good handle on our macro fertility then
the micronutrients will come into play.
Most of our micros are immobile except for boron and will be
found in the upper 6 inches of our soil. With the weather
conditions hot, cold, dry and wet can have an impact on
availability in a given year. Take last year we seen a higher
amount of boron deficiencies early on as boron had probably
moved out of the root zone and with cool temperatures our
roots were slower in development.
A&L Canada uses the Melic 3 or HCL method for extracting
their micronutrients. With the different more aggressive
extraction we have higher threshold numbers compared to the
familiar DTPA extractions used.
Boron is extracted using the hot water extraction which is
common place in the industry.
The threshold soil levels are:
Copper – 2.3 ppm
Zinc – 5 ppm
Manganese – 33 ppm
Iron – 24 ppm
Boron – 1 ppm
If we see levels below the numbers indicated I would flag the
field and make sure it was followed up with a tissue to see
where the levels are in the plant. Most micronutrient
deficiencies can be corrected in crop and allows us more time
to monitor the conditions.
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If we see critically low levels the problem may have to be
addressed through a soil build.
pH also plays a role in availability and as it rises we see tie-up
of zinc and manganese especially. This field has an ideal pH
so it should not be a problem.
High organic matter soils or heavily manured soils can cause
tie-up of copper and manganese so again something to watch
for on a tissue test which will not show on the soil test.
Certain crops have a higher demand then others for certain
micronutrients:
Copper – Cereals, flax
Zinc – Corn, flax, beans, potatoes
Manganese – Oats, barley, peas
Boron – Canola, peas, potatoes, sunflowers
With this field I would flag boron if canola or peas were
targeted, copper if I was growing wheat and zinc if corn, flax
or beans was the intended crop.
New for 2005 Chloride and Soluble Salts:
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Again by adding more information to the soil test it allows us
to make better decisions on cropping, target yields, rotations,
and overall fertilizer requirements.
Soluble Salts is an indication of the salinity in our soil.
Salinity can be caused be excessive amounts of magnesium,
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sodium, potassium, and chloride salts which can cause white
crusting of the soil. If the soil is jet black and nothing will
grow there it is probably just a sodium related issue.
We want to try and avoid the white crusting (Saline) and black
soil areas that did not grow a crop (Sodic) when soil testing
unless we want to sample them separate to understand what
the levels are there and if there is anything that can be done
to correct the problem.
Leaching the soluble salts out of the root zone is one of the
only ways to re-establish crop growth. High water table areas
can be a problem.
If your problem is just sodium we can use elemental sulfur or
gypsum to reclaim the soil but will still need a leaching event
to move Na downwards.
The general guideline is anything over 1 mmhos/cm will start
to limit crop production on salt sensitive crops. Crops like
barley, sugarbeets, and Salt grass are more tolerant and can
take levels a fair bit higher then that.
Chloride testing again is not new and there has been a fair bit
of work done on chloride and its ability to help plants fight
disease.
Unfortunately we do not have a specific chloride product to
address deficiencies and our only form of application is
through the use of KCL or Potassium Chloride. Under proper K
fertilization we should always be supplying adequate chloride
to our crops.
Generally if we have 15 to 20 ppm in the soil it is adequate for
normal crop production.
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Chloride is highly leachable and can become a problem in wet
conditions and typically we will find more at depth in coarse
textured soils because of this.
We want to make sure we have enough chloride when entering
into cereal production.
Building A Recommendation
If we were to build a quick recommendation for this soil based on the
notes above this is what we may come up with:
Crop: Canola
Target Yield: 40 Bushels
N Requirement:
4.1 % OM x 7 (7 Ibs N per 1% OM) = 28.7 Ibs
23.2 Ibs total Nitrate 0-24” x 60% (May have trouble accessing lower
nitrate because of higher sodium levels) = 13.92 Ibs.
40 Bushels x 3.5 Ibs N/bushel = 140 Ibs N required
140 – 28.7 – 13.92 = 97 Ibs N
P Requirement:
14 ppm of Phosphate in the soil (Decent range)
40 Bushels of canola requires 42 Ibs phosphate/acre in grain removal
alone.
In-order to maintain our P level we would want to apply replacement.
If there is cash to due so, an additional 10 Ibs Phos actual over removal
over the next 5 – 10 years would bring his phos number into an optimal
range.
Rec here would be 42 Ibs P205
K Requirement:
219 ppm K in top 6 inches
3.8% Base Saturation
.19 K/Mg
This is a good soil with adequate K at all views of this nutrient
If the grower is looking for maintenance, canola requires .5 Ibs/bushel
(grain removal) so 40 Bushels would require 20 Ibs/acre KCL
My rec would be to skip the K and apply those dollars to building P levels.
(May be a benefit from added chloride if chloride levels were low)
S Requirement:
Looking at the test we have 35 ppm Sulfur 0-24” depth most of which is in
the subsoil. There is roughly 16 Ibs/acre in the surface
In the case of canola and understanding the variability of sulfur in a field
as well as the higher sodium levels at depth I would apply crop growth
values of sulfur to this canola crop
With canola I always like to apply enough to grow the crop
Canola requires .55 Ibs/bushel x 40 bushels = 22 Ibs/acre sulfur/sulfate
combo
This also keeps my N : S ratio in that desired 5 – 7 : 1
Micronutrient Requirement:
When we look at growing canola a key micronutrient is boron. This aids
in adequate flower, seed set and pod fill
Most other micros look in check for canola production
With the boron levels borderline I would recommend a tissue test to see
where the levels are before flowering to make sure we fulfill that
requirement. Weather conditions can greatly affect boron uptake and
availability at time of flowering. Foliar application insures we get it into
the plant.
.4 litres/acre if required from tissue
Summary
Hopefully these pointers will help you better understand what to look for
on your soil tests and how to interpret the information. The time and
information required to build a recommendation is lengthy at first, but
does become more user friendly the more you do it. The more you work
on building your own recommendations with your growers, the more you
will enjoy reading and translating soil information and the more value you
will have to offer your customers.
As always budgets will play a factor on how many dollars an acre the
grower has to work with so it is your job to insure they receive a
balanced blend for best efficiency of dollars spent.
For more in-depth information I would recommend reading through the
A&L Agronomy handbook which takes on a broader scope of the
information provided here.
Feel free to contact us at any time as we enjoy discussing your soils with
you.