Using the Missouri P index John A. Lory, Ph.D. Division of Plant Sciences Commercial Agriculture Program University of Missouri

Many streams and lakes are phosphorus limited. Jones and Knowlton 1993 Water Clarity as measured by Secchi depth (m) Missouri reservoirs

Challenge: Where, when, and how to apply manure to minimize P loss to surface water.

Nutrient planners are required to assess phosphorus loss on all fields receiving manure.

Low N-based Applications Medium N-based Applications High P-based Applications Very High No Manure Applications Phosphorus Loss Assessment

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?

Missouri’s 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?

Soil testing objective Determine crop needs for phosphorus (P) to maximize yield. Soil Test P (lb acre -1 ) Dry Matter (g) Critical value = 43 lb/ac

Soil test P Yield Low Medium HighVery High Critical Value Soil testing objective The “critical value” is the lowest soil test level where no response to fertilizer is expected.

Soil test P Low Medium HighVery High Critical Value Soil testing objective Critical soil test P = 40 or 45 lbs P/acre Yield

Soil testing objective Critical soil test P = 40 or 45 lbs P/acre Soil test P Yield Low Medium HighVery High Critical Value Buildup Maintain None Fertilizer Rec.:

Low N-based Applications Medium N-based Applications High P-based Applications Very High No Manure Applications

Missouri’s 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?

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

P indexes are state specific. Index Target: - Runoff - Irrigation - Runoff - Irrigation Long-term Runoff - Runoff - Leaching

P indexes are state specific. Approach: Multiplicative tabular factors Simplified model Additive tabular factors Simplified model

Do Differences Between States Make Sense? ?

P index developed cooperatively by MU and NRCS. Based on best professional judgment. Available as a spreadsheet at:

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.

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

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.

Higher standard in the clear-water region of the state.

P index rating system RatingClear-water region Other Low<1 <3 Medium>1 and 3 and <7 High>5 and 7 and <12 Very High>10 >12

Higher standard in the clear-water region of the state.

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

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