Lecture 13a
Soil Fertility – N-P-K
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The study of supplying plant
nutrients from the soil.
Understanding CEC is the
single most important concept
in understanding soil fertility.
(However soil pH is the most
important chemical property)
Nutrient analysis of the soil
determines the potential of
the soil for supplying N,P,K,
Ca, Mg, S, plus micronutrients
to plants during the growing
season.
Nutrients become available
through organic matter
decomposition, chemical
weathering of minerals,
airborne additions, and
fertilizers.
Fertilizers
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Manufactured fertilizers have been
used for over a 100 years.
The numbers on a bag of fertilizer-"10-5-5", = guaranteed chemical
analysis.
These numbers indicate the bag of
fertilizer contains:
10% N,
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5% P2O5,
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5% K2O.
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These numbers--"10-5-5“ = the fertilizer
grade.
Fertilizer ratio - proportion of primary
nutrients (N-P2O5-K2O) in a fertilizer grade,
divided by the highest common divisor; or
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10 ÷ 5=2, so the ratio of this fertilizer is 2-1-1.
Fertilizer Plant
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For phosphorus and potassium,
the chemical analysis is given in
the oxide form.
This is the way the nutrients
were first thought to be
absorbed by the plant and is still
used today to express the
analysis of fertilizer.
For a grade of 18-24-12
elemental analysis = 18- 10.5 - 10
The elemental analysis can usually
be found on the bag near the
fertilizer grade.
To convert from the elemental
analysis to the chemical analysis
for phosphorous and potassium
fertilizers, use this formula:
% P2O5 x .44 = %P
% K2O x .83 = %K
Converting P2O5 & K2O (oxide from) into P and K (elemental form)
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% P2O5 x .44 = %P (or there is 44% P in P2O5)
&
%P x 2.29=%P2O5
%K2Ox.83=%K ( or there is 83% K in K2O )
%K x 1.2=%K2O
For Example: A 30 pound sack of fertilizer (25 - 5 - 5) , is
applied to your garden.
The amount of N applied=.25 x 30=7.5 lbs N
amount of P205 applied=.05 x 30=1.5 lbs & P=.44 x 1.5=0.66 lbs P
amount of K20 applied=.05 x 30=1.5 lbs & K=.83 x 1.5=1.245 lbs.K
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Dyad = If you apply a 14 lb
sack of 22-3-14 how many
pounds of N, P2O5, and K2O did
you apply?
N – 14 x .22 = 3.08
P2O5 – 14x .03 = 0.42
K2O – 14 x .14 = 1.96
Soil Fertility
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Nitrogen Fertilizer -
management of Nitrogen
requires an understanding
of the Nitrogen Cycle.
Nitrogen is dynamic in the
soil and forms of N will
change depending on the
soil conditions.
Reed Palm – N Def on left
Nitrogen
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NH4+ and NO3forms taken up
by plants
Loss of N can
occur: 1) leaching
of NO3- , 2)
volatilization of
NH4+ to NH3
(high pH soils),
3) immobilization
by plant or
microbe uptake,
4) Denitrification
Nitrogen Cycle
Fertilizer Nitrogen
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Nitrogen fertilizer should
be added to the soil when
the crop will use it, adding
excess N will cause losses
that may harm the
environment.
Keep N & P out of water
bodies by only applying it to
soils that need the
nutrients.
Nitrogen is expensive and
using only what the crop
needs for adequate growth
is important
Wheat with N response
N Fertilizer Rate and Application
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Rate of N application is
the management
practice that most
influences nitrate
concentrations in the
tile drainage water.
Reducing the N rate
by 33% ( to 80 lbs N)
from the recommended
120-lb N rate for corn
after soybeans
decreased the nitrateN concentration in the
subsurface, drainage
water by
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2.6 mg/L (14%)
and decreased corn
yield by 25 bu/A
(16%).
http://www.ewg.org/reports/deadzone/top10.php
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On the other hand,
increasing the N rate
by 33% above the fallapplied 120-lb rate,
increased nitrate-N
concentration by 4.3
mg/L (23%) but
increased yield only 9
bu/A (6%).
Time of application
studies showed
nitrate-N losses from
a corn-soybean
rotation to be ranked
fall N > split(fall and
spring) N > spring N =
fall N with a
nitrification inhibitor.
From G. Randall,2004
U.S. Geological Survey, 1993; 26-38.
Alfalfa and
grass CRP
effectively filter
tile drain water
Randall et al., 1997
Nitrogen Pollution from Manure
Nitrogen soil testing
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NO3- Mobile nutrient
In drier areas use a fall or
spring nitrate-N soil test.
In humid areas, use spring
nitrate-N test or table
value based on previous crop
and organic matter.
The amount of rain after
the soil test may determine
if the NO3- tested for has
been leached below the root
zone.
Nitrogen Credits
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Once the amount of N
needed for the plant is
determined N credits need to be
taken for:
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Previous crop
Previous manure
applications or sludge
2nd year after alfalfa
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THUS it becomes important to
give N - CREDITS for previous
management activities.
legumes, manure or other organic
additions with low C:N ratios are
adding N to the soil
Thus this organic N needs to be
counted into the total N available
for crop growth
And thus reduce the need for
fertilizer N.
Green Manure
Cow Manure
Price of N P K
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2005
NH3 - .26 $/lb N
Urea -.37 $/lb N
P2O5 -.33 $/lb P
K2O - .20 $/lb K
2006
.32
.40
.36
.23
2007
.32
.50
.46
.27
2008
.47
.60
.88
.47
2009
.42
.52
.71
.71
Source- http://www.ers.usda.gov/data/fertilizeruse/
2010
.31
.49
.56
.42
P - Fertility
Phosphorus is low in total
amount in the soil and low
in solubility and is readily
fixed by Fe and Al at low
pH and Ca at high pH.
H2PO4- and HPO4-- forms
taken up by plants
P Fertilizer is made from
rock phosphate
Rock phosphate mines are in
Florida & South Carolina
Soybean Yields with P fertilizer
_______________________Yield bu/acre___
Lbs P fertilizer added
0
23
46
69
92
Ridge Till
Soybeans
Waseca
31
33
35
36
37
Lamber.
27
29
30
30
32
P
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Soil test for P (Bray pH<7.4 of soil)
0-5 ppm = very low
6-10 ppm = LOW
11-15 ppm = med
16-20 ppm = high
> =21 ppm = very high
No reason to have
soil test
> 21
environmental problems
when P >16
ppm x 2 = lbs/acre
P deficient tomato
Soil P
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Crops need more P than is dissolved
in the soil solution at any one time,
therefore, this P in the solution
phase must be replenished many
times during the growing season.
The ability of a soil to maintain
adequate levels of phosphorus in the
solution phase is the key to the
plant available P status of the soil.
The solid phase P is both organic and
inorganic
Solid P Phase
Solution Phase
P deficiency reduces root growth
Root Hair
Long term phosphorus soil test trends for NW Ohio
120
lbs/ac Bray P1
100
80
60
40
20
0
1960 1965
1970 1975 1980
1985 1990 1995 2000
2005
160
140
120
100
80
60
40
20
0
NW OH
SE MI
NE IN
19
96
19
97
19
98
19
99
20
00
20
01
20
02
20
03
20
04
20
05
20
06
lbs/ac Bray P1
Phosphorus Soil Test Trends
(A&L Great Lakes Laboratory, Inc.)
John P. Crumrine
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Potassium Fertility (Potash)
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Potassium (K+) is a problem
on
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acid soils,
soils with low CEC
soils with irrigation or high
rainfall where leaching can
readily occur.
Potassium can be stored in
the soil from one year to
the next
K is not a pollutant - even if
leached from soil, K does
not cause environmental
problems.
K deficient corn
K
Different crops have different K
requirements
 Soil test :
 0-40 ppm = very low
40 – 80 ppm = low
120 ppm = medium
120 – 160 ppm = high
> 160 ppm = very high
 ( > 160 = no K needed)
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80
K deficient soybeans
Potassium
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Feldspar Mineral
Potassium is found in minerals like
feldspars and micas (90% of Soil K)
K is fixed inside of clay minerals ( 9% of soil K)
K is on the soil exchange sites ( 1%)
K is in the soil solution (0.1%)
Annual K Recommended for 160 bu corn yield goal
Soil Test
CEC
10
20
30
50
130
150
170
150
90
110
130
250
50
70
90
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Ca, Mg
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Calcium and Mg - when
soils are low in Ca, Mg,
they have a pH problem
and by adding lime or
dolomite the pH and Ca,
Mg problem is corrected.
On some acid, sandy soils
Mg deficiency on corn
has been noted. 15 lbs
as a starter or 75 lbs
broadcast corrected the
problem.
Magnesium deficiency on corn
S
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Sulfur - Most soils in Mn
would not respond to
additions of Sulfur
because of adequate levels
and atmospheric
deposition.
However, on sandy soils in
NE Minnesota yields have
increased with 25
lbs/Acre to Alfalfa, Corn
and Small grains.
Sample Fertilizer ProblemFor your notes
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Nutrient Analysis - N - P - K = N -P2O5 - K2O
% P = .44 x %P205 & %K = %K20 x .83
If you apply 25 lbs of P205 how much elemental P did you
apply, 25 x .44 = 11 lbs P
Fertilizer Bag = 20 - 5 - 10 =
20% N, 5% P20%, 10% K20
If you apply 50 lbs of fertilizer you put on ?
.20x50 = 10 lbs N,
& .05 x 50 = 2.5 lbs P205 and 1.1 lbs of P,
& .1 x 50 = 5 lbs K20 and 4.15 lbs K.
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If you need 85 lbs of N for Corn how many lbs of
this fertilizer(20-5-10) do you need?
85/.20 = 425 lbs fertilizer
How many lbs of elemental K will you apply with
the 85 lbs of N.
425 x .1 = 42.5 lbs K20 and 42.5 x .83=35.275
lb.K
Micronutrients
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Iron - Fe
Boron - B
Zinc - Zn
Copper - Cu
Molybdenum - Mo
Chlorine – Cl
Manganese - Mn
*Cobalt Co
*Nickel-
grape leaf symptoms of boron deficiency;
right: boron toxicity)
Reasons why use of micronutrients has increased in recent years:
Increased knowledge of their role in plant nutrition, deficiency symptoms in plants, and
behavior in soil.
Higher crop yields which have increased micronutrient needs.
Improved analytical methods, resulting in more soil tests and improved fertilizer
recommendations.
Improved sources of micronutrient fertilizers and methods of incorporation into other
fertilizers.
Land-shaping practices, such as terracing or land leveling for irrigation, which remove topsoil
containing organic matter, often associated with most of the available micronutrient supply.
(Reference: The Fertilizer Handbook / The Fertilizer Institute - pp 91-92)
Role of Micronutrients
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Boron is believed to be involved in carbohydrate
transport in plants; it also assists in metabolic
regulation .
Chlorine is necessary for osmosis and ionic balance; it also
plays a role in photosynthesis.
Zinc participates in chlorophyll formation, and also activates
many enzymes
Copper is a component of some enzymes and of vitamin A.
Iron is essential for chlorophyll synthesis, which is why an iron
deficiency results in chlorosis.
Manganese activates some important enzymes involved in
chlorophyll formation
Molybdenum is used by plants to reduce nitrates into usable
forms
*Cobalt is thought to be an important catalyst in nitrogen
fixation. It may need to be added to some soils before seeding
legumes
*Nickel has only recently been shown to be an essential
nutrient for plants. Ni-deficient conditions, barley plants fail
to produce viable grain because of a disruption of the normal
grain-filling and maturation processes.
.
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Micronutrients
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Correcting pH
problems for most
soils will correct
micronutrient
deficiencies
Most common
deficiency is Iron on
alkaline soils for those
plants that like acid
conditions.
Fe Deficiency on rhododendron,
Fe deficiency on pin oak
Nutrient Management
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Need to manage
nutrients
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Plant growth
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Feed the world
Economic
Aesthetic
Environmental
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N in drinking water
Hypoxia Gulf of Mexico
Eutrophication of fresh
waters
Jubilees (oxygen-deficient waters) occur in
coastal areas around the world, as a result of
excessive nutrients (particularly nitrogen)
flowing from rivers into oceans.
Justus von Liebig’s Law of Minimum
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Plant production
can be no greater
than that level
allowed by the
growth factor
present in the
lowest amount
relative to the
optimum amount
for that factor
Tools for detecting nutrient deficiency
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1) Tissue testing -involves a complete
and detailed laboratory analysis of
nutrient elements in the plant leaves.
This is a very accurate way of
assessing how much nutrient the plant
has actually taken up from the soil.
Recommendations are made on the
basis of these test results:
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Backed by research
Dependent on plant growth stage and
plant part.
Soil testing
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Collecting a soil
sample to determine
the current nutrient
status of the soil.
Soil Sampling
Sufficiency Method of Nutrient Needs
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Uses soil testing to predict fertilizer needs.
Based on green house and field research.
Soil test is a predictive tool.
Gives soil credit for it’s nutrient providing ability.
Correlation
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Process used to determine if
 a soil nutrient, as
extracted by a soil test,
 and crop response to
added nutrient, are so
related that one directly
implies the other.
Exploratory fertilization trial
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Greenhouse – a
controlled environment
with soil homogeneity.
Trials in field with
selected soils not as
controlled but needed
to verify greenhouse
trials.
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Determine
percentage yield
values for each
fertilizer rate trial.
Determine soil test
values for nutrient
being studied.
Plot percentage yield
vs soil test value.
120
110
Relative corn yield (% )
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100
90
80
70
60
50
40
0
20
40
Bray-P (ppm)
60
80
Soil test categories
Very low
Chance of
response
90 %
Low
75 %
Medium
50 %
High
30 %
Very high
10 %
Probability of
Profitable Response
Category
1.00
0.85
0.60
0.40
0.15
0.00
V. Low
Low
Med.
High
Level of soil fertility
V. High
Improving soil fertility the 'green' way
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Importance of
expanding the use of
agricultural production
methods that are both
agronomically and
economically
sustainable.
Legume intercropped with maize in
Wenchi, Ghana, Africa