Precision applications of nutrients - Dr. Josh McGrath, University of Kentucky, from the 2020 Conservation Tillage and Technology Conference, held March 3-4, 2020, Ada, OH, USA.

1. UNIVERSITY OF KENTUCKY
Precision management of P and K:
What do we need and what do we have?
Josh McGrath
Follow @NPK_Professor

2. Soil Testing
•Four separate activities:
1. Soil sampling
2. Soil analysis
3. Interpretation of results
4. Recommendations
2

3. Soil Testing
•Four separate activities:
1. Soil sampling
2. Soil analysis
3. Interpretation of results
4. Recommendations
3
• Traditionally viewed as limiting function when
managing field average
• When planning VR we often grid sample and
interpolate between points

4. Soil Testing
•Four separate activities:
1. Soil sampling
2. Soil analysis
3. Interpretation of results
4. Recommendations
4
• Traditionally viewed as limiting function
when managing field average
• When planning VR we often grid sample and
interpolate between points
• We’re pretty good at soil analysis

5. Soil Testing
•Four separate activities:
1. Soil sampling
2. Soil analysis
3. Interpretation of results
4. Recommendations
5
• Traditionally viewed as limiting function
when managing field average
• When planning VR we often grid sample and
interpolate between points
• We’re pretty good at soil analysis
• We have a long way to go here

6. What do we need and what do we have for VR?
•~40 - 70% report VR nutrient
management– what’s the
agronomic basis?
•High resolution
characterization of spatially
variable nutrient need
• Spatial distribution of nutrient
availability (soil testing)
•Interpretation of soil test
results with matching precision
•Recommendations developed
for VR application
•We can vary fertilizer at a
pretty fine resolution
•We can’t (precisely) map need
at the same resolution
•We don’t develop
recommendations at that
resolution
6

7. Soil Testing
•Four separate activities:
1. Soil sampling
2. Soil analysis
3. Interpretation of results
4. Recommendations
7

8. Soil Sampling for Precision Ag
10 13 11 22
7
9 15 3512
45 13 18
?
10 13 11 22
• Precision ag breaks a field into
smaller management zones
• Grid Sampling (grid point and grid
cell):
• Typically involves interpolation of grid
sample data
• Previous management has altered soil
nutrient levels
• Combined small fields into large field
• Directed sampling:
• Break field into zones and collect
“average” sample for each zone
• Requires other data: yield maps, remotely
sensed images, or other sources of spatial
data
• Requires experience with field

9. Most VRP likely based on interpolated grid soil data
•Interpolation estimates
unknown value between two
sample points
•Samples must be collected
close enough that they are
correlated
•r > 0.3
•Requires samples on ¼ acre
grid or less
9
Lauzon, J.D., I.P. O’Halloran, D.J. Fallow, A.P. von Bertoldi, and D. Aspinall. 2005. Spatial Variability of Soil Test
Phosphorus, Potassium, and pH of Ontario Soils. Agronomy Journal 97(2): 524–532.

10. How does interpolation perform?
•Often field average is closer to true value
than coarse sampling (>1/4 acre)
•At small scale soil properties tend to be
stochastic
•Random such that they can be predicted
accurately, but not necessarily precisely
•On average the estimated values are right,
at each spot they are off further than the
field average
10
¼ acre grid
1 acre grid
Courtesy John Spargo, Penn State Soil Testing Lab

11. High Precision
Low Accuracy
Low Precision
High Accuracy
Reminder: Accuracy v. Precision
http://en.wikipedia.org/wiki/File:High_precision_Low_accuracy.svg
11

12. Soil Testing
•Four separate activities:
1. Soil sampling
2. Soil analysis
3. Interpretation of results
4. Recommendations
12

13. Soil analysis
•Soil testing should
provide
•Nutrient in soil solution
(intensity) and in stored
pools (quantity)
•Buffer capacity (Q/I).
13

14. Soil analysis
•Soil testing should
provide
•Nutrient in soil solution
(intensity) and in stored
pools (quantity)
•Buffer capacity (Q/I).
14
•Multiple factors affect Q/I
•Soil testing can only
provide an index of
nutrient supplying capacity
of a soil.

15. Soil Chemical Analysis
•There are multiple soil tests
that use different procedures
and chemicals
15

16. Soil Chemical Analysis
•There are multiple soil tests
that use different procedures
and chemicals
•Each extracts different
nutrient amounts and forms
from soil pools
•Require local correlation and
calibration to be useful
16

17. Soil Testing
•Four separate activities:
1. Soil sampling
2. Soil analysis
3. Interpretation of results
4. Recommendations
17

18. Soil test interpretation &
Fertilizer recommendations
•Correlation
•Relative yield versus soil test
value
•Plant response to application
of element
• Requires check plot and sufficient plot
•Conduct experiment at
multiple sites (multiple soil
concentrations)
18
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 50 100 150 200
RelativeYield
Soil Test Result
Soil critical concentration
Relative Yield = 0.95

19. Soil test interpretation &
Fertilizer recommendations
•Calibration
•Amount of applied nutrient
versus soil test value
•Multiple fertilizer rates at one
site (one soil test value)
•Conduct at multiple sites
•Build fertilizer
recommendations for different
soil test values
19
0
20
40
60
80
100
120
140
160
180
200
0 50 100 150
Yield(bu/acre)
Fertilizer Rate (lbs/acre)
Yield response to fertilizer rate at different
sites
Soil Test = 30
Soil Test = 20
Soil Test = 10

20. Variability in correlation and calibration results?
20

21. Soil Testing
•Four separate activities:
1. Soil sampling
2. Soil analysis
3. Interpretation of results
4. Recommendations
21

22. Soil Test Calibration: How much fertilizer?
1. Sufficiency approach
• When soil test level is below optimum,
apply only enough nutrients to meet
crop needs
2. Buildup and maintenance
approach
• Rapidly build low soil test
concentrations to optimum level
• Replace nutrients removed by crop at
higher soil test levels where response is
not expected
3. Hybrid Approach
22What about BCSR?
-100
0
100
200
300
400
500
600
700
0 10 20 30 40 50 60 70
FertilizerPrate(lb-P2O5/a)
Initial soil test P (lb/a)
Fertilizer P rate to move STP10 units CornP2O5 Rec

23. Calibration: Fertilizer Recommendation Systems
23
0
50
100
150
200
250
0 5 10 15 20 25 30
Fertilizerrate(lb-P2O5/a)
Mehlich 3 Soil P (mg/kg)
Sufficiency only recommendations
Sufficiency
0
50
100
150
200
250
0 5 10 15 20 25 30
Fertilizerrate(lb-P2O5/a)
Mehlich 3 Soil P (mg/kg)
Example of Build and Maintain
Recommendations
Buildup
Maintenance
ActualCriticalLevel
ActualCriticalLevel

24. How should we make precise recommendations?
•Build and maintain ignores
soil buffer capacity
•To move STP 1 lb/acre
•Low STP required 10 – 25 lb-
P2O5/acre
•Optimum STP or above
required ~5 lb-P2O5/acre
•Initial STP <6
•600 – 200 lb/a P2O5 to move
soil test +10 lb/a
•Soils don’t pay interest
(Thom and Dollarhide, 2002)
0
5
10
15
20
25
0 50 100 150 200 250 300
Lb-P2O5added/lb-STPchange
Initial Soil Test P (lb/acre)

25. Soil nutrient storage – buffer capacity
25
This Photo by Unknown Author is licensed under CC BY-NC-ND

26. How should we make precise recommendations?
•Precision ag -- frequent soil
testing and sufficiency rates
•We need to know the yield
maximizing (sufficiency) rate
•Sufficiency rate < build &
maintain
•Sufficiency probably < crop
removal
•buffer capacity makes up
difference
(Thom and Dollarhide, 2002)
0
50
100
150
200
250
0 5 10 15 20 25 30
Fertilizerrate(lb-P2O5/a)
Mehlich 3 Soil P (mg/kg)
Example of Build and Maintain
Recommendations
Buildup
Maintenance
Sufficiency

27. Designing soil testing for precision
•Four separate activities:
1. Soil sampling
2. Soil analysis
3. Interpretation of results
4. Recommendations
•How do we spatially
characterize response
potential?
•Do we need additional
points of information
besides current soil test?
•What would this data look
like?
•Philosophical approach to
precision ag?
27

28. How do we move forward?
•I would argue we need new
research design and analysis
methods to get precise
•Five P rates, randomized in
five blocks, at four locations,
for three years…
28
We don’t need more
data; we need different
data!

29. Spatial variability in soil test correlation
•Reduce plot size to limit
variability
•Two treatments: sufficient or
none
•One phosphorus application
rate using APP in 2x2
•29 kg ha-1 P (60 lb/acre P2O5)
•56 kg ha-1 N (50 lb/acre)
balanced using UAN
29

30. Early growth response: All sites
• 𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 =
)*+,-../
)*+,-..0
×100%
•Biomass in kg/ha
•Red line indicates University
critical level
•Green line indicates 95%
Relative Biomass
•Majority of the time starter P
increased biomass V4-V6
30

31. Delta Yield Paired T-Test: Princeton
•Delta Yield = YP-Y0
•Found to be highly
significant
•Mean difference 9.4 bu/a
31

32. On average soil test correctly predicted yield response
32
Regardless of soil test we had a yield
response in about 50% of the plots
0
20
40
60
80
100
120
140
0 5 10 15 20 25 30 35
RelativeYield
Mehlich 3 Phosphorus (mg kg-1)

33. Variance decreases as soil P increases
33

34. Soil testing for SSM: New challenges
34
0%
20%
40%
60%
80%
100%
120%
0 10 20 30 40 50 60
RelativeYield
Soil test phosphorus
Where do we place the critical level? •We have focused on mapping
soil P status spatially
•Correlation and calibration
were designed to make
accurate recommendations
•What if in addition to soil P
concentration varying, the
critical level varies?
•spatially and temporally?

35. Grid sampling
•Interpolated soil sample maps (>1/4 acre
grid) are unreliable AT BEST.
•There is nothing wrong with the grid
approach, the problem is interpolation of the
data
•More frequent sampling is better use of
money
•If you insist on grid sampling, then shift grid
over time to get denser sample map
•Look at soil test range, median, average, and
deviation – just don’t interpolate
35

36. Zone sampling
•Intensively sampled zones
might work
•Use topography, soil texture,
or even historic grid samples
• Look at summary statistics from grid
data by zone
•Use yield to test zones – but
not necessarily to create zones
•We’re working on guidance
for zone development using
free software (e.g. QGIS,
MZA)
36

37. Future opportunities
•Even with a decent soil test
map (grid or zone) our
recommendations are very
coarse and were intended to
be an average
• Use your technology to insert
check strips (High, Low, None)
within your prescription and
evaluate recommendations
yourself
•To truly practice precision ag
you need to be closer to
sufficiency rates
37
0
50
100
150
200
250
0 5 10 15 20 25 30
Fertilizerrate(lb-P2O5/a)
Mehlich 3 Soil P (mg/kg)
Example of Build and Maintain
Recommendations
Buildup
Maintenance
Sufficiency

38. 38
Josh.mcgrath@uky.edu
Follow @NPK_Professor