Developing A New Approach To Soil Phosphorus Testing And Recommendations - Dr. Chad Penn, USDA-ARS, from the 2020 Conservation Tillage and Technology Conference, held March 3-4, 2020, Ada, OH, USA.

1. Developing a New Approach
to Soil Phosphorus Testing
and Recommendations
Chad Penn
USDA Agricultural Research Service
National Soil Erosion Research
Laboratory
Jim Camberato and Matthew Wiethorn
Purdue University

2. Improving P Soil Testing and
Recommendations
• Precise P recommendations
– Increase agronomic production efficiency
– Reduce non-point losses of P

3. Traditional soil P testing
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25 30 35 40 45
Y-Values
?
??

4. Empirical vs. mechanistic soil
testing and fertilizer recs
“The most commonly used tests, extract
some portion of the labile soil P pool.
However, when different soils are used,
solution P concentration, buffer capacity,
and diffusion rate may not be correlated,
therefore any one of the values would not be
correlated with predicted P uptake and the
simpler soil test would not be more reliable”
Stanley Barber: Soil Nutrient Bioavailability

5. Critical Soil P Levels
Lins, Cox, and Nicholaides, 1985. “Optimizing P
fertilization rates for soybeans grown on oxisols and
associated entisols”. SSSAJ
9 lb/acre 27 lb/acre18 lb/acre
(lb/acre)

6. Soil-Plant Nutrient Dynamics
WATER IN
NO3
-
PO4
3-
Soil-bound
nutrients
NO3
-
NH4
+
PO4
3-
PO4
3-
PO4
3-
NH4
+
PO4
3-
Solution
Labile Non-labile
Soil-bound nutrients
Plant Uptake of Nutrient

7. Uptake of solution P by plants
• Function of solution P concentration
• Replenishment by soil solids (quantity)
• Three ways of obtaining solution P
– All three depend on root architecture,
solubility, and location of P near roots
• Root interception
• Bulk flow
• Diffusion: highly dependent on location of P!
𝐽𝑟 = 𝑣 𝑜 𝐶𝑙

8. P movement to roots: Diffusion
P
P
P
P P
P
P
PP P
P
P
Higher P
Concentration
Lower P
Concentration
Higher P
Concentration
Gradient
Dominant mechanism, but SLOW and short range
𝐽𝑟 = 𝐷𝑒 𝑏
𝜕𝐶𝑙
𝜕𝑟

9. Diffusion followed by root
uptake:
P P
P
P
P
P
P P
P
P
P
P
P
P
P
Plant
uptake
𝐼 𝑁 =
𝐼 𝑚𝑎𝑥 (𝐶𝑙 − 𝐶 𝑚)
𝐾 𝑚 + 𝐶𝑙 − 𝐶 𝑚

10. Overall equation describing P
uptake with P movement:
Ion
movement via
diffusion
Ion
movement via
mass flow
Root uptake
kinetics
𝐷𝑒 𝑏
𝜕𝐶𝑙
𝜕𝑟
+ 𝑣 𝑜 𝐶𝑙 =
𝐼 𝑚𝑎𝑥 (𝐶𝑙 − 𝐶 𝑚)
𝐾 𝑚 + 𝐶𝑙 − 𝐶 𝑚
Solved transient-state equation using the
Crank-Nicholson method by Dr. John
Cushman in 1980

11. Different plants require different
concentrations in solution in order
to meet the required P mass…..
“External P Requirement”
But it’s a moving target!
critical solution P
concentration
1970’s soil
scientists & me

12. We need to be multi-
dimensional
• Let’s utilize the Barber-Cushman model to
pinpoint the location of the moving target
– Mechanistic
– Mass-balance
• Consider that a finite mass of P must be taken up
for a plant to attain maximum yield
– Mass of P uptake is universal
» i.e. not a moving target

13. Soil P Requirement
• Mass of solution P uptake is universal
across soil properties
– But that required mass of P can be provided
dynamically at different solution concentrations
– Example: 550 mg P/corn plant over 120 days
0.8 mg P/L
Soil bound-P Solution-P
19
times/day
0.2 mg P/L
76
times/day
Soil bound-P Solution-P
*depends on root
surface area

14. Soil P Requirement
• So how much soil-bound P do we
need?
–It depends on the ability of the soil to
supply the solution P
• Quantity-Intensity relationship
– Soil properties: mineralogy, texture, OM, pH,
etc.
– Total P content and P forms
–Also depends on physical location of P
and ability to move (diffusion)

15. Current fertilizer
recommendations
• Based on results from soil test
– 0 to 6 inches
• Not representative of No-till conditions
– i.e. P location
• No consideration for soil type or conditions
• Crop varieties?

16. P mass
requirement for
max yield
Soil
extractable P
requirement
Crop type
(cultivar?)
Quantity/Intensity
Fertilizer/soil-P
relationship
Soil
properties
Fertilizer rate
recommendation
P location and
movement
Soil properties and
management
Buffered
solution P
requirement
Empirical

17. At the end of the day….
• The plant demands a finite mass of P to
be taken up by the roots, to reach max
grain yield
• Question: based on the specific conditions
and management of that soil, what
concentration of Soil Test P is necessary
to meet the mass demand of the plant?

18. Current P Recommendations
• Not bad
• Get us in the ballpark
– Not precise
– Lots of room for improvement
• Save $
• Improve production efficiency
• Reduce P losses

19. Long Term Goal
• Utilize and improve the Barber-Cushman
model for developing more precise and
condition-specific fertility recommendations
– Required P mass for various cultivars
– Plant uptake kinetics curves
– Incorporate root modelling
• Continue to use STP extractants, but vary
the optimum level depending on soils, crop,
and conditions

20. Required P uptake mass and P
utilization efficiency: sand culture
hydroponics
Supply all non-P nutrients at adequate concentrations
• Add P at a range of concentrations
• Determine the minimum mass of P uptake at
maximum yield
• Compare cultivars

21. Time Lapse:

22. Corn Results
• No significant interaction between P
treatment and variety for yield components
• Increased P significantly increased yield and
growth to a point
• At max yield, P use efficiency was similar
among varieties
– But, in approaching max yield, Dekalb < Dyna-
grow < Pioneer
• Differences in overall production:
– Max grain yield and total biomass: Dekalb <
Dyna-grow < Pioneer

23. After 0.55 to 0.63 g
P uptake, no further
increase in grain
yield
But total biomass
continues to
increase until
uptake of ~ 0.8 g
0
50
100
150
200
250
300
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Grainyieldperplant(g)
P uptake per plant (g)
0
100
200
300
400
500
600
700
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Totalbiomassperplant
(g)
P uptake per plant (g)
Luxury Consumption

24. P distribution in plant
30 50 55 90 94 158
19 47 56
164 216
315
13 34 44
79
87
119
236 436 536
577 591
591
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
298 566 690 909 987 1183
%massofnutrient
Total P uptake (mg)
values on bar indicate uptake mass in mg
Grain
Roots
Stem Cobb Tassel
Leaves Husk

25. Near
optimum
Medium Poor
Near
optimum Medium Poor
Near
optimum Medium Poor
Billy-
Bob’s
Seed Co.
Jim’s
Seeds,
LLC
Mom’s
Seeds
Inc.

26. Comparison to field data:

27. P uptake vs. yield: Avg across
varieties
0
50
100
150
200
250
300
0 0.5 1 1.5 2
Grainyieldperplant(g)
P uptake per plant (g)
Experiment Data

28. P uptake vs. yield: Avg across
varieties
0
50
100
150
200
250
300
0 0.5 1 1.5 2
Grainyieldperplant(g)
P uptake per plant (g)
Experiment Data
Lin-Plat
Lin-Lin

30. Conclusion
• Easy part is done:
– We need to get around 0.6 g of P into a corn
plant to reach max yield
• Beyond that is luxury consumption and “quality”
• Hard part is next: what is the target soil P
level?
– It depends…….moving target

32. Questions?
Chad.penn@ars.usda.gov
Twitter Handle: House of Phos

33. Improved soil quality does not
“make” more P
• P obviously cannot be fixed like N
• “Humus Theory”
– Plant nutrients assimilated by the vis vitalis
• “Gleiches kann nur aus Gleichem entstehen”
• Soil health can only improve P accessibility,
but that does not defeat the mass
requirement
• Deeper roots
• This will help, but consider P is very low below 15 cm
– Subsoil P is even less for no-till due to stratification

34. Improved soil quality does not
“make” more P
• Organisms
– Free-living rhizosphere microorganisms
• Mostly negligible effect compared to sterile soils
• Can possibly reduce P availability
• Amount of P in microbial biomass in rhizosphere is
less than 3% of daily requirement
– VA mycorrhizae
• Effectively increases root surface area
• But, it accesses same soluble P pool as the roots
– Thus, this can be accounted for in Barber-Cushman model
• Changes to Q/I relationship
– For better or for worse?

35. “I don’t need to fertilize with P
anymore”
• You can get away with this for a few years
– Due to the nature of P
• Highly buffered by the soil
• But eventually, the soil bound P (quantity)
is mined to the point where it can no longer
supply a sufficient concentration of solution
P (intensity) to meet the P mass demand
– Or at a sufficient speed
• Remember that the plant takes up a finite
amount of P from soil that must be replaced