1. Lecture 13a Soil Fertility – N-P-K
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.

2. Fertilizers Fertilizer Plant
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,
5% P2O5,
5% K2O.
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
10 ÷ 5=2, so the ratio of this fertilizer is

3. 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 elemental analysis =
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

4. Converting P2O5 & K2O (oxide from) into P and K (elemental form)
% 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 ( ) , 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

5. 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

6. Soil Fertility
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

7. Nitrogen NH4+ and NO3- forms 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

8. Nitrogen Cycle

10. Fertilizer Nitrogen
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

11. N Fertilizer Rate and Application
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 nitrate-N concentration in the subsurface, drainage water by
2.6 mg/L (14%)
and decreased corn yield by 25 bu/A (16%).

12. On the other hand, increasing the N rate by 33% above the fall-applied 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;

13. Alfalfa and grass CRP effectively filter tile drain water
Randall et al., 1997

14. Nitrogen Pollution from Manure

15. Nitrogen soil testing 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.

16. Nitrogen Credits
Once the amount of N needed for the plant is determined -
N credits need to be taken for:
Previous crop
Previous manure applications or sludge
2nd year after alfalfa

17. Cow Manure Green Manure
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.
Cow Manure
Green Manure

18. Price of N P K
NH $/lb N
Urea $/lb N
P2O $/lb P
K2O $/lb K
Source-

19. 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

20. To control P availability :
Phosphorus Fertility
To control P availability :
1) control pH (lime acid soils)
2) Place fertilizer in bands near the seed,
3) Maintain organic matter levels for P release and to avoid P tie-up by microorganisms.
Root
Fertilizer

21. Soybean Yields with P fertilizer
_______________________Yield bu/acre___
Lbs P fertilizer added Waseca Lamber.
Ridge Till
Soybeans

22. P P deficient tomato Soil test for P (Bray pH<7.4 of soil)
0-5 ppm = very low
6-10 ppm = LOW
11-15 ppm = med
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

23. Soil P
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
P deficiency reduces root growth
Solid P Phase Solution Phase Root Hair

24. John P. Crumrine

27. Potassium Fertility (Potash)
Potassium (K+) is a problem on
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

28. K Different crops have different K requirements Soil test :
0-40 ppm = very low – 80 ppm = low ppm = medium – 160 ppm = high
> 160 ppm = very high
( > 160 = no K needed)
K deficient soybeans

29. Potassium 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
_______________________________________

30. Ca, Mg
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

31. S
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.

32. Sample Fertilizer Problem- For your notes
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% 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.

33. 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= lb.K

34. Micronutrients 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)

35. Role of Micronutrients
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.

36. Micronutrients
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

37. Nutrient Management Need to manage nutrients Plant growth
Feed the world
Economic
Aesthetic
Environmental
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.

38. Justus von Liebig’s Law of Minimum
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

39. Tools for detecting nutrient deficiency
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:
Backed by research
Dependent on plant growth stage and plant part.

40. Soil testing
Collecting a soil sample to determine the current nutrient status of the soil.

41. Soil Sampling

42. Sufficiency Method of Nutrient Needs
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.

43. Correlation 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.

44. Exploratory fertilization trial
Greenhouse – a controlled environment with soil homogeneity.
Trials in field with selected soils not as controlled but needed to verify greenhouse trials.

45. Determine percentage yield values for each fertilizer rate trial.
Determine soil test values for nutrient being studied.
Plot percentage yield vs soil test value.

46. Soil test categories Category Very low 90 % Low 75 % Medium 50 % High
Chance of response
Very low
90 %
Low
75 %
Medium
50 %
High
30 %
Very high
10 %

47. Improving soil fertility the 'green' way
Importance of expanding the use of agricultural production methods that are both agronomically and economically sustainable.
Legume intercropped with maize in Wenchi, Ghana, Africa

48. What is the earth showing us?
The End