Using the Missouri P index

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Using the Missouri P index

• John A. Lory, Ph.D.
• Division of Plant Sciences
• Commercial Agriculture Program
• University of Missouri

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Many streams and lakes are phosphorus limited.
Missouri reservoirs
Water Clarity as measured by Secchi depth (m)
Jones and Knowlton 1993
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Challenge Where, when, and how to
apply manure to minimize P loss to surface
water.
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Nutrient planners are required to assess
phosphorus loss on all fields receiving manure.
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Phosphorus Loss Assessment
No Manure Applications
Very High
High
P-based Applications
N-based Applications
Medium
N-based Applications
Low
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You choose the method

• Agronomic soil test recommendation
• Objective does the crop need P?
• Phosphorus threshold
• Phosphorus Index
• Objective will applied P be vulnerable to
  losses?

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Missouris approach 2 choices

• Agronomic soil test recommendation
• Objective does the crop need P?
• Phosphorus threshold
• Phosphorus Index
• Objective will applied P be vulnerable to
  losses?

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Soil testing objective
Determine crop needs for phosphorus (P) to
maximize yield.
Dry Matter (g)
Critical value 43 lb/ac
Soil Test P (lb acre-1)
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Soil testing objective
The critical value is the lowest soil test
level where no response to fertilizer is expected.
Critical Value
High
Very High
Low
Medium
Yield
Soil test P
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Soil testing objective
Critical soil test P 40 or 45 lbs P/acre
Critical Value
High
Very High
Low
Medium
Yield
Soil test P
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Soil testing objective
Critical soil test P 40 or 45 lbs P/acre
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No Manure Applications
Very High
High
P-based Applications
N-based Applications
Medium
N-based Applications
Low
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Missouris approach 2 choices

• Agronomic soil test recommendation
• Objective does the crop need P?
• Phosphorus threshold
• Phosphorus Index
• Objective will applied P be vulnerable to
  losses?

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Factors affecting runoff of P

• Runoff volume
• Method of application
• Time since application
• Application rate
• Soil test P
• Soil type
• Cultivation
• Vegetative cover

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P indexes are state specific.
Index Target

• Runoff
• Leaching

• Runoff
• Irrigation

• Runoff
• Irrigation

Long-term Runoff
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P indexes are state specific.
Approach
Simplified model
Multiplicative tabular factors
Additive tabular factors
Simplified model
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Do Differences Between StatesMake Sense?
?
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• P index developed cooperatively by MU and NRCS.
• Based on best professional judgment.
• Available as a spreadsheet at http//nmplanner.mi
  ssouri.edu/

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Missouri P index objectives

• Strategic planning tool for use in writing a
  5-year nutrient management plan.
• Identify fields where field characteristics are
  likely to result in high P loss in runoff.
• Simple to use with readily available information
  for people with an NRCS conservation plan.

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6 Factors in the Missouri P index

• Soil test result
• include units and sampling depth
• Tillage
• Tilled vs. notill and forages
• RUSLE estimate of erosion
• Land cover
• Soil hydrologic group
• A, B, C or D
• Distance from water feature

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Simple model approach

• Sediment P
• Function of RUSLE-2 erosion, soil test P and
  sediment delivery ratio.
• Soluble P
• Function of runoff volume (curve number approach)
  and soil test P.

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Higher standard in the clear-water region of the
state.
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P index rating system
Rating Clear-water region
Other Low lt1 lt3 Medium gt1 and lt5
gt3 and lt7 High gt5 and lt10 gt7 and lt12 Very
High gt10 gt12
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Higher standard in the clear-water region of the
state.
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Surface applications require a tactical tool.

• Educational tools being developed to identify
  times of year runoff least likely
• Inputs
• Ground cover - County
• Soil hydrologic group - Soil condition
• Soil moisture
• Output
• Evaluation of runoff risk (low, medium, high)
• Predicted rainfall amount needed to initiate
  runoff

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Future

• P index integrated into RUSLE 2
• Email notification system to help evaluate timing
  of manure applications.