1. Spatial Variability in Precision Agriculture
What is it?
Precision n. The quality or state of being precise.
Used or intended for precise measurement.
Made for the least variation from a set standard. (Webster, 1995)
Precise adj. Capable of, caused by, or designating an action, performance, or process carried out or successively repeated within close specified limits (Webster, 1995).

2. Precision Agriculture
What is it?
Precision in management?
Knowing more precisely the
size of fields,
level of inputs (rates),
yields,
$ costs, and
$ returns?

3. Precision Agriculture
What is it?
Management of production inputs in relation to more precisely delineated needs (Johnson, 1/18/01).
Recognizes spatial variability of production needs within a population
of production units, where production units are smaller than they used to be.

4. Spatial variability among production units.
What is the size of a production unit?
Depends on the enterprise.
Small dairy = single dairy animal.
Wagoner Ranch, TX = 7,000 – 8,000 acre wheat field.
Agronomic units = “fields” “zones”

5. Production Unit Size?
By Farm?

6. Production Unit Size?
2-4 acre grids?

7. Production Unit Size? By plant?

8. Spatial variability among production units.
What causes field delineation.
Natural boundaries.
Rivers
Rock out-crops
Political boundaries.
Roads
Survey units
Land ownership
Consolidation

9. Spatial variability among production units.
What causes field delineation.
Soil productivity appropriate to the crop (e.g. bottom land for alfalfa).
Size determined by land use
Government acreage restrictions (CRP)
Tees, fairways, greens
Size that is “convenient” to the operation for administering production inputs.
Cultivation
Planting
Harvesting (mowing)
Fertilizing
Irrigation
Etc.

10. Spatial variability (macro) for agronomic land use.
Inherent (natural).
Related to soil productivity and soil forming factors
Time
Parent material
Climate
Vegetation
Slope

11. Soil acidity and Oklahoma rainfall
                                                                                                                      
Usually not acidic
Usually acidic

12. Spatial variability (macro) for agronomic land use.
Acquired (use induced).
Influence of historical crop production on soil properties.
Alfalfa vs. wheat for acidification and soil organic matter.
Fertilizer use and change in soil fertility (Garfield County).

13. “Cow Pocks” in wheat pasture
Acquired spatial variability (micro).
“Cow Pocks” in wheat pasture

14. Acquired spatial variability (micro).
Ave = 47; CV = 30

15. 1x1 (60-acre cell)

16. 6x4 (2.5 acre/cell)

17. 12x8 (0.625 acre/cell)

18. 25x16 (0.15 acre/cell)

19. 50x32 ( acre/cell)

20. 100x64 (45 yd2/cell)

21. 200x127 (11 yd2/cell)

22. 472x300 (2 yd2/cell)

23. Management Zones A C B

24. Open SST file

25. Fundamentals of Nutrient Management

26. Plant Growth and Soil Nutrient Supply Relationships
Mitscherlich (1909)
“…increase in yield of a crop as a result of increasing a single
growth factor is proportional to the decrement from the
maximum yield obtainable by increasing the particular growth
factor.”
dy/dx = (A - y) c
Law of
“diminishing
returns”
A-y for x1 and Dy1
Dy2
Yield (y)
Dy1
x1 x2
Increasing level of growth factor (nutrient, x)

27. Plant Growth and Soil Nutrient Supply Relationships Mitscherlich
Soil deficiency levels could be expressed as a “percent
sufficiency”
100
% of
Maximum
Yield or
“Yield
Possibility” 75 50
Soil Phosphate (P) or Potassium (K) Supply
(soil test index)

28. Plant Growth and Soil Nutrient Supply Relationships
Mitscherlich
Soil Test Correlation and Calibration
Soil Test Percent Fertilizer
P Index Sufficiency P2O5

29. Plant Growth and Soil Nutrient Supply Relationships Bray
“…as the mobility of a nutrient in the soil decreases, the amount of that
nutrient needed in the soil to produce a maximum yield (the soil nutrient
requirement) increases from a value determined by the magnitude of the yield
and the optimum percentage composition of the crop, to a constant value.”
Bray mobile nutrient
100
% of
Maximum
Yield or
“Yield
Possibility” 75 50
Soil Phosphate (P) or Potassium (K) Supply
(soil test index)

30. Plant Growth and Soil Nutrient Supply Relationships
Bray
For a nutrient that is 100 % mobile in the soil (NO3-N ?)
Soil nutrient
supply requirement = Yield X % nutrient in tissue
(Input requirement = harvest output or removal)
Idealized situation would be hydroponics nutrient
supplying system (no soil-nutrient interaction)

31. What Happens to Applied Nitrogen Fertilizer?
SOIL REACTIONS
AMMONIUM
FERTILIZERS
NITRATE
NITROGEN
AMMONIUM
NITROGEN
SOIL M ICROORGANISMS
SOIL ORGANIC
MATTER

32. What Happens to Applied Nitrogen Fertilizer?
CROP UPTAKE
NH3
AMMONIUM
FERTILIZERS
NITRATE
NITROGEN
AMMONIUM
NITROGEN
SOIL REACTIONS
SOIL
MICROORGANISMS
SOIL ORGANIC
MATTER

33. Plant Growth and Soil Nutrient Supply Relationships
Wheat response to fertilizer N
Bray mobile nutrient

34. Plant Growth and Soil Nutrient Supply Relationships
Bray
Soil nutrient
supply requirement = Yield X % nutrient in tissue
Bushel Wheat Requirement = (lb/bu) * % N
= 60 * 2.2 % N (13 % C.P.)
= 1.33 lb N/bushel
Assumes
100 % efficiency in converting soil N to wheat grain N.
relatively constant N content
At 70 % efficiency, requirement is 1.33/.70 = 1.9 lb N/bu

35. Plant Growth and Soil Nutrient Supply Relationships Bray
Bushel Wheat Requirement = (lb/bu) * % N
= 60 * 2.2 % N (13 % C.P.)
= 1.33 lb N/bushel
At 70 % efficiency and 13 % C.P., requirement is 1.33/.70 = 1.9 lb N/bu
At 50 % efficiency and 15 % C.P., requirement is 1.53/.50 = 3.1 lb N/bu
At 100 % efficiency and 11 % C.P., requirement is 1.1/1 = 1.1 lb N/bu

36. Fate of Inorganic N in Soils

37. Nitrogen soil availability
Source and fate of nitrate (NO3-).
NO3-
Denitrification
N2O and N2
- O2
Rainfall
NO3-
O2 + NH4+
Nitrification
H+ +
Leaching
NO3-

38. Nitrogen soil availability
Source and fate of ammonium (NH4+).
Rainfall
Leaching
NO3-
Denitrification
N2O and N2
- O2
Volatilization
H+ +
NO3-
O2 +
CEC (-)
NH4+
Soil Organic
Matter-N
Mineralization +
OH-
NH3 + H2O

39. Nitrogen soil availability
Rainfall
Leaching
NO3-
Denitrification
N2O and N2
- O2
Nitrogen soil availability
Source and fate of ammonium (NH4+)
CEC (-)
Mineralization +
OH-
NH3 + H2O
H+ +
O2 +
Volatilization
Soil Organic
Matter-N
NH3
NO3-
NO3-
NH4+
immobilization

40. Plant Growth and Soil Nutrient Supply Relationships
Bray
Current Oklahoma field practice
Estimated yield in bu/acre (YG) * 2 lb N/bu =
Estimated N requirement
Estimated N requirement - soil test NO3-N =
Fertilizer N requirement.
Estimated topdress N = est.(Yield * %N) -
preplant and soil N supplied
sensor based goal
Estimated topdress N =k sensed yield and
sensed % N

41. Plant Growth and Soil Nutrient Supply Relationships
N use efficiency = [100 (Nx - N0) grain N] / Nx applied