Field Specific Decisions:N vs P

CNMP Core CurriculumSection 5 – Nutrient Management

CNMP Development Core Training Curriculum

These course materials have been developed as a cooperative effort between five land-grant universities and The Natural Resources Conservation Service.

Ames, Iowa 50011, (515) 294-4111.

Copyright © 1995-2006, Iowa State University of Science and Technology. All rights reserved.

Copyright Information

Objectives

• To gain an appreciation for the appropriateness of when and how to nitrogen or phosphorus base a manure application plan.

• To have a knowledge of basic decision tools such as manure and soil tests. A phosphorus index of some sort is generally used to aid in these decisions, so students will gain an understanding of both the transport and source factors that make up a P Index.

Required Materials and Supplemental Resources

• Required Materials– NRCS 590 Nutrient Management Standard– NRCS 633 Waste Utilization Standard – Any state specific P Index materials.

• Supplemental Resources– Land Grant University bulletins on soil and

manure sampling.

Nutrient Management

• Managing the amount, form, placement, and timing of application of nutrients to crops

• Supply plant nutrients for optimum yields

• Minimize entry of nutrients to surface and

groundwater, and maintain or improve

condition of soil

Decision Tools• Soil Sampling

• Manure Sampling

• Indexes

• Thresholds

Manure Sampling

• Why Sample?• Determine nutrients and their availability to crops• Enable better management of soil nutrients with

manure and commercial fertilizers

• Book values vs. Manure analysis• Book values can be significantly different than an

analysis of your manure• Manure analysis reflects dilution from rain, runoff,

watering systems, storage type• Manure analysis is only as good as the sample

taken

Sampling Manure

• When to Sample– Annual, seasonal sampling – Consistent sampling over time is best– Sample when the BULK of manure is applied– According to permit or plan requirements

• How to Sample– Many extension bulletins are available, use

methods recommended in your state

Manure Sampling

• The most representative samples are taken immediately before application

• Take samples when mixing and loading or after agitation

• Collect samples over different depths (stages) of hauling out to check for stratification, especially when not agitating

• Sample different systems differently• Book values may be better for straw packs• KEEP RECORDS

Manure Sample Collection

Manure Sample Collection

Soil Sampling

• Essential part of nutrient management

• University recommendations

Soil Sampling Methods

• Random

• Grid

• Zone

Random Method• Zig-zag pattern across the field

• Take a composite sample of at least 10 cores representing no more than 10-15 acres

Grid Method

• Divide field into equal grids

• Take composite sample of at least 10 cores at each georeferenced point

Zone Method• Divide field in 10-15 acre zones

according to specific characteristics

• Soil map, management history, topography, yield maps, nutrient history

Loam

Sand

Muck

Never limed

Limed 5 years ago

Formerly 2 fields now all 1 field

Soil Test Recommendations

• Select a test that is appropriate to the state and soil characteristics (Mehlich 1, Mehlich 3, Bray, Olsen etc.)

• Apply nutrients based on university recommendations

• In general, each field should be sampled once every 3-5 years

– Follow NRCS 590 Standards

Soil Test Recommendations

• Recommendations and reporting varies by state – University of Tennessee: – low = 0 - 18 lb P / acre– medium = 18 to 30 lb P / acre– high = 31 to 120 lb P / acre– very high = > 120 lb P / acre

• Phosphorus applications usually recommended only on low or medium testing soils

Soil Thresholds

• State sets upper limit of soil test phosphorus

• Above threshold, reduced or no P applications allowed

Phosphorus Index

• Identifies sites where the risk of phosphorus movement is higher or lower compared to other sites

• More flexibility: manure or litter can be applied at N rates where the risk of P movement is low, and limits applications where risks are high.

Phosphorus Index Facts

• Early versions were additive

• Multiplicative or combination versions are more common now

• Used by NRCS field staff, watershed planners, and land users/owners

Multiplicative vs. Additive

• Risk is an interaction of site factors and transport factors; multiplicative better represents reality

• If site factors are high but transport factors are very low, then risk is low as transport is needed for risk; with multiplicative, it is possible to get a low-risk rating

• With additive, factor values are added

Source Factors

• Soil test phosphorus• Quantity of P being

added• Type of P being

added: organic and/or inorganic

• Application method/timing

Tennessee Index Source Factors

Transport Factors

• Erosion

• Runoff

• Subsurface drainage

• Flooding frequency

• Proximity to surface water, drainage channels

• Irrigation

• Conservation practices

Tennessee Transport Factors

Index Interpretation (TN)

In-class Example

• Calculate a P-Index for field 4

Assumptions

• Soil type: DeD2, Dewey Silt Loam• Erosion Potential: Medium• Soil Test P: 68 lbs, High• Manure P application: 150 lbs per acre

surface applied in March with winter cover• Tillage Practices: no-till, heavy residue• No buffer, surface water is at the west edge

of the field and the crop is planted adjacent to the stream

Questions

• Using the information provided, calculate the index for field 4.

• Determine whether manure application can be applied on a Nitrogen or Phosphorus basis.

• What practices could be implemented on this field to reduce the P Index value?

Answer

• Hydrologic group B

• 12-20% slope

Index rating: 638 Very High

• Phosphorus-based application is necessary according to TN requirements

Index Interpretation (TN)

How can we lower the index?

• Example 1: add a vegetative buffer and non-application area

• Index rating = 232, still P-based application

• Example 2: example 1 plus reduce manure application to 100 lbs

• Index rating= 192, N-based is now acceptable

Summary

• Many tools are available to aid in nutrient planning decisions

• Consult the state specific index, threshold, and extension publications for information

• Follow your state regulatory and permitting requirements

Questions?