TECHNICAL NOTES

> **US DEPARTMENT OF AGRICULTURE NATURAL RESOURCES CONSERVATION
> SERVICE**

**AGRONOMY - 57 Albuquerque, NM**

###  September, 2000

**PHOSPHORUS ASSESSMENT TOOL**

**For New Mexico**

**Robert Flynn Mike Sporcic Linda Scheffe**

**Asst. Professor, Agronomy State Agronomist Water Quality Specialist**

**New Mexico State Univ USDA-NRCS USDA-NRCS**

**Agricultural Science Center Albuquerque, NM Albuquerque, NM**

# Artesia, NM

##### BACKGROUND

Water quality problems associated with phosphorus are generally confined
to surface water. Phosphorus (P) in most NM soils is tightly held to
soil particles and does not leach. However, the P held in organic phases
from residues such as manure can dissolve in water and be lost if
improperly managed. Adsorbed P on soil particles can cause surface water
contamination as P containing sediments move off the land in
agricultural runoff.

P is the second major element utilized by actively growing plants but
differs considerably from nitrate in its water solubility and mobility.
Soil solution P levels are typically less than 0.01 ppm in most soils,
and ground water levels seldom exceed 0.05 ppm*.* Between 20 and 80% of
the total P in soils is held in organically combined forms with a large
amount of the organic-P held by the active microbial biomass. Much P
fertilizer applied to soils is retained in the near-surface layer in
various inorganic precipitates and organically combined forms that
prevent it from leaching. Sandy or peat soils may not retain or bind P
to the same extent as previously discussed, but P migration downward to
ground water is still generally minimal. The highly calcareous nature of
our soils causes P to be very unavailable.

While the risk of ground water contamination by P from crop production
systems can be assumed to be limited, the solid forms of P that
accumulate in surface soil are subject to loss via erosion. Runoff
losses to surface waters are the major water quality risk from P.
Increased public and regulatory concern over the use and application of
P to agricultural lands is based mainly upon the fact that increased P
loading to surface waters can cause eutrophication. Algal and aquatic
weed growth in most inland surface water systems is P-limited and
elevated P loading leads to algal blooms and mats, heavy growth of
aquatic plants and weeds, deoxygenation, and occasional problems with
drinking water taste and odor.

P runoff from permanently vegetated areas such as hayland, pasture,
rangeland or forest can be significant, and largely occurs as traces of
orthophosphate ions in solution. Organic P additions from riparian leaf
and stem inputs are also possible. Where erosion risk increases, such as
for annual crops with conventional tillage, the total-P loss increases
greatly as the P is moved in solid particulate form from the eroding
soil. Water-soluble P is immediately available for biological uptake
when the sediment-bound or particulate P forms are released over longer
periods and it is referred to as \"bioavailable particulate P\". The
overall impact of a given production system on P loadings to local
surface waters will therefore be primarily dependent upon relative rates
of sediment loss and the system\'s influence on P levels in eroding soil
surfaces.

P can easily enter surface water through dislocation and erosion of soil
particles that maintain this tightly bound nutrient. Surface erosion can
remove soil particles containing P. Surface soils, which are the most
susceptible to erosion, generally have much higher P levels than deeper
soil horizons due to applications of fertilizers, manure, roots, residue
and sludge that contain this nutrient. The higher the P content of the
soil, the more P will erode per ton of soil lost. Once into the surface
water system, P is a major contributor to excessive algae growth which
can have detrimental environmental and aesthetic consequences. Little P
is lost by leaching, though it moves more freely in sandy than in clay
soils. Erosion and crop removal are the primary pathways for P removal
for most soils in NM. Phosphorus dissolved in runoff water may be an
additional P loss pathway for very high P amended soils and
surface-applied organic material.

The interaction between the particulate and dissolved P in the runoff is
very dynamic and the mechanism of transport is complex. Therefore, it is
difficult to predict the transformation and ultimate fate of P as it
moves through the landscape.

# PURPOSE

The purpose of the Phosphorus Index is to provide field staffs,
watershed planners, and land users with a tool to assess the various
landforms and management practices for potential risk of phosphorus
movement to water bodies. The Phosphorus Index ranks sites where the
risk of phosphorus movement may be relatively higher than that of other
sites. When the parameters of the index are analyzed, it is apparent
that an individual parameter or parameters may be influencing the index
disproportionately. These identified parameters are the basis for
planning corrective soil and water conservation practices and management
techniques.

This index is used as a tool for understanding the relative contribution
that individual landform and management parameters have toward risk of
phosphorus movement and will provide a method for developing management
guidelines for phosphorus at the site to lessen their impact on water
quality.

##### SITE CHARACTERISTICS

A number of soil, hydrology, and land management site characteristics
describe the landform. The **Phosphorus Index Rating for New Mexico**
(Table 1) uses parameters that can have an influence on phosphorus
availability, retention, management, and movement. These include:

1.  Available phosphorus soil test levels, given in soil laboratory test
    units. (Usually the Olsen-P method (NaHCO~3~ extraction) for NM
    soils, neutral to calcareous soils).

2.  Phosphorus fertilizer (both organic and inorganic) application
    rates, in pounds available phosphate (P~2~O~5~) per acre.

3.  Organic phosphorus source application methods.

4.  Phosphorus fertilizer application methods.

5.  Proximity of nearest field edge to named stream or lake measured in
    feet.

6.  The erosion rate, in tons per acre per year.

7.  Potential Runoff using permeability and slope.

8.  Irrigation erosion potential, based on slope (S) in percent and flow
    rate (Q) in gallons/min.

9.  Grazing management, including imported feeds.

10. Field edge buffers.

Field specific data for the ten site characteristics of the Phosphorus
Index are readily available at the field level. Some analytic testing of
the soil and organic material is required to determine the rating
levels. This soil and material analysis is considered essential as a
basis for the assessment.

The P Index is a simple 10 by 5 matrix that relates site characteristics
with a range of value categories. The ten characteristics are:

> 1\) Soil Test P Level
>
> 2\) P Application Rate
>
> 3\) Organic P Source Application Method
>
> 4\) Fertilizer P Application Method
>
> 5\) Proximity of Nearest Field Edge to Named Stream or Lake

6\) Soil Erosion

> 7\) Runoff Class
>
> 8\) Irrigation Erosion
>
> 9\) Grazing Management

10\) Conservation Buffers

The five value categories are:

> Very low
>
> Low
>
> Medium
>
> High
>
> Very high

Each site characteristic is rated VERY LOW, LOW, MEDIUM, HIGH, or VERY
HIGH, by determining the range rating for each value category. For
example: Soil test P ranges of \<8 ppm for very low, 8-14 ppm for low,
15-22 ppm for medium, 23-30 ppm for high, and \>30 ppm for very high
were assigned to each of the value categories.

##### DEFINITIONS

####### Soil Test P

> New Mexico Soils are usually low in plant available phosphorus because
> phosphorus is quickly tied up in calcareous soils. The bicarbonate P
> soils test (also know as Olsen-P soil test or Sodium bicarbonate-P
> test), it measures water soluble P, highly soluble calcium P, and
> organic P. This type of test should be specified for most soils in New
> Mexico, except if the soil is on the acid side (pH \< 7). Low pH soils
> should use a Bray test for P.
>
> For cropland, take soil samples from the top 12 inches to assess the
> level of \"available P" in the surface layer of the soil. For pasture
> or hayland, the sample should be 4 to 6 inches. At least 10
> subs-samples should be taken in the field of concern. The "available
> P" is the level customarily given in a soil test interpretation by the
> Cooperative Extension Service or commercial soil test laboratories.
> The soil test P range in each value category are: Very Low, \<8 ppm;
> Low, 8-15 ppm; Medium, 15-23 ppm; High, 23-30 ppm; and Very High, \>30
> ppm. (See NMSU CES Guide No. A-122).
>
> The soil test level for \"available P" does not ascertain the total P
> in the surface soil. It does however, give an indication of the amount
> of total P that may be present because of the general relationship
> between the forms of P (organic, adsorbed, and labile P) and the
> solution P available for crop uptake.

**[P Application Rate]{.underline}**

> The P application rate is the amount, in pounds per acre (lbs/ac), of
> phosphate (P~2~O~5~) from all sources that is applied to the soil. The
> rate ranges in each value category ate: Very Low, none applied; Low,
> 1-30 lbs/ac; Medium, 31-90 lbs/ac; High, 91-150 lbs/ac; and Very High,
> \>150 lbs/ac.

**[Organic P Source Application Method]{.underline}**

> The manner in which organic P material is applied to the soil and the
> time that the organic material is exposed on the soil surface until
> crop utilization can determine potential P movement. Incorporation
> implies that the organic P material is buried below the soil surface
> at a minimum of three to six inches. The value categories of
> increasing severity, ranging from no application to surface applied
> more than 3 months before planting, and depicts the longer surface
> exposure time between organic P material application, incorporation,
> and crop utilization. The longer the material sits on the soil surface
> the greater the chance for surface runoff.

**[Fertilizer P Application Method]{.underline}**

> The manner in which P fertilizer is applied to the soil and the amount
> of time that the fertilizer is exposed on the soil surface until crop
> utilization effects potential P movement. Incorporation implies that
> the fertilizer P is buried below the soil surface at 3 to 6 inches.
> The value categories of increasing severity, ranging from no
> application to surface applied more than 3 months before planting,
> depict the longer surface exposure time between fertilizer
> application, incorporation, and crop utilization. The longer the
> material sits on the surface the greater the potential for surface
> runoff.

**[Nearest Field Edge to Named Stream or Lake]{.underline}**

This factor considers the potential flow distance from the edge of the
field closest to the water body to the water body. The closer the water
body to the edge of the field, the higher potential risk of P
contamination. These values consider the local topography, existing
setback, and buffer regulations for application of nutrient sources.
Local or state guidelines should be used where available.

**[Soil Erosion]{.underline}**

> Soil erosion is defined as the loss of soil along the slope or
> unsheltered distance caused by the processes of water and wind. Soil
> erosion is estimated from erosion prediction models including the
> Revised Universal Soil Loss Equation (RUSLE), for water erosion and
> Wind Erosion Equation (WEQ), for wind erosion. Erosion induced by
> irrigation is calculated by other convenient methods. The value
> category is given in tons of soil loss per acre per year
> (ton/acre/year). These soil loss prediction models do not predict
> sediment transport and delivery to a water body. The prediction models
> are used in this index to indicate a movement of soil, thus potential
> for sediment and attached phosphorus movement across the slope or
> unsheltered distance and toward a water body.

**[Runoff Class]{.underline}**

> The runoff class is the runoff potential of soluble P moving from
> locations of placement. The runoff class of the site can be determined
> from soil survey data and slope measurements in the field. Guidance in
> determining the runoff class is based on soil permeability classes and
> the percent slope of the site (**Table 2** -- Adapted from the
> USDA-NRCS National Soil Survey Handbook). The result of using the
> matrix relating soil permeability class and slope provides the value
> categories: NEGLIGIBLE, VERY LOW, LOW, MEDIUM, HIGH, and VERY HIGH.
> Note NEGLIBLE and VERY LOW are combine so that a 5 factor rating for
> the matrix can be maintained.

**[Surface Irrigation Erosion]{.underline}**

Potential P loss resulting from furrow irrigation-induced erosion is
considered by inclusion of a rating system based on soil susceptibility
to particle detachment by hydraulic shear and flow rate of water in the
furrow. The susceptibility to detachment is given by a relative ranking
of soil erodibility classes under furrow irrigation (**Table 3**). These
classes are an initial attempt at a relative ranking based on inherent
stable and static soil properties (i.e., texture and clay mineralogy).
There are temporal variations in the relative erodibility and actual
amount of erosion with furrow erosion. These changes in erodibility are
a function of soil properties and management. However, no attempt is
made to consider temporal soil properties or management factors in the
rating. The introduced flow rate in the furrow (Q) is given by the
irrigation water management plan and recorded as gallons per minute
(gal/min). The furrow slope (S) of the site is given as a percentage
(feet per 100 feet). (See USDA-NRCS National Engineering Handbook 15,
chapter 5). The product of flow rate (Q) and slope (S) is used to
determine the value category.

**[Grazing Management]{.underline}**

Grazing management relates to the recycling of phosphorus nutrients by
grazing fields that are also manure application fields. Supplemental
feeding in the application field imports additional P with feed and
concentrates in animals, increasing the rating. There are 5 value
categories based on how grazing is done. They are Not grazed, Grazed
Crop Residues, Pasture with less than 30% of the feed needed brought in,
Pasture with 30 to 80% of the feed needed brought in, and Pasture with
80 to 100% of the feed needed brought in.

## Conservation Buffers

Conservation buffers are areas or strips of land maintained in permanent
vegetation to help control pollutants and manage other environmental
problems. Any grass or legume strip between the watercourse and the
cropland field can act as a buffer. With buffers, wider is better.
Points are assigned based on the buffer width. Conservation buffers
clean runoff, by helping stop sediment, and adsorb P. Contour Buffer
Strips, Field Borders, Filter Strips, Grass Waterways with Vegetated
Filters, and Riparian Forest Buffers are examples of conservation
buffers. Range and pasture in fair to good condition that could slow
runoff and trap sediment can serve as a buffer.

##### 

##### Procedures for Making an Assessment

Each site characteristic has been assigned a weighting value based on
reasoning that one particular site characteristic may be more prominent
than another at allowing potential phosphorus movement from the site.
There is scientific basis for concluding that these relative differences
exist; however, the absolute weighting factors given are currently based
on professional judgment. The site characteristic weighting factors are:

**Site Characteristics Weighting Factors**

> Soil Test P Level 1
>
> Phosphorus Application Rate 1
>
> Organic Phosphorus Source Application Method 1
>
> Phosphorus Fertilizer Application Method 1
>
> Proximity of Field Edge to Named Stream or Lake 1.5
>
> Soil Erosion 1.5
>
> Runoff Class 1.5
>
> Irrigation Erosion 1.5
>
> Grazing Management 0.5
>
> Conservation Buffers 1.5

The value categories are rated using a log base of 2. The greater the
ratings, the proportionally higher are the values. The higher the value,
the higher potential for significant problems related to phosphorus
movement.

The value ratings are:

None or very low = 0

low = 1

medium = 2

> high = 4
>
> very high = 8

The P Index Worksheet for New Mexico can be used to record the values
from the index for a specific field. To make an assessment using the P
Index, use **Table 1** (P Index Worksheet for New Mexico), select a
rating value for each site characteristic using the categories NONE or
VERY LOW, LOW, MEDIUM, HIGH, or VERY HIGH. The value in the table is the
result of multiplying the site characteristic weighting factor by the
rating value to get the weighted value for that characteristic (see
index value). Proceed to rate and factor each characteristic of the
index. Sum the values for all ten characteristics, and compare the total
using the Phosphorus Index Rating for NM, Weighted Factor Chart (**Table
4**). A description of site vulnerability by the Hazard Class Rating is
given to describe the potential loss of P for a given field.

A Microsoft Excel spreadsheet is available (PI-NM.xls) to automate the
evaluation. The file is posted on the NRCS NM web site. The file name is
**PI-NM.xls**. The location of the file on the web is
**www.nm.usda.gov/techserv/techres1.htm**. If unable to down load,
contact the state agronomist.

INTERPRETATIONS OF SITE VULNERABLITY RATINGS (P HAZARD CLASS) FOR THE P
INDEX

######## 

######## **[P Hazard Class Rating Described]{.underline}**

**VERY LOW OR LOW** -- A field that has a VERY LOW OR LOW potential for
P movement offsite. If farming practices were maintained at current
levels, the probability of an adverse impact to surface water resources
from P losses from the field would be very low or low. **Nutrient
application can be based on nitrogen for all sources.**

**MEDIUM** -- A field that has a MEDIUM potential for P movement
offsite. The probability for an adverse impact to surface water
resources is greater than that from a LOW vulnerability rated site. Some
remedial action should be taken to lessen the probability of P movement.
**Nutrient application can be based on nitrogen for all sources.**

**HIGH**- This site has a HIGH potential for P movement from the site.
There is a high probability for an adverse impact to surface water
resources unless remedial action is taken. Soil and water conservation
as well as phosphorus management practices are necessary to reduce the
risk of P movement and probable water quality degradation. **Nutrient
application must be P based at 1.5 times crop removal when manure or
other organic by-products are applied.** When inorganic fertilizer is
applied, its rate must follow the Land Grant University's P
recommendation for crop production.

**VERY HIGH** - This site has a VERY HIGH potential for P movement from
the site. The probability for an adverse impact to surface water
resources is very high. Remedial action is required to reduce the risk
of P movement. All necessary soil and water conservation practices plus
a phosphorus management plan must be put in place to reduce the
potential of water quality degradation. **Nutrient application must be P
based at crop removal when manure or other organic by-products are
applied.**

**EXCESSIVE**- This site has a VERY, VERY HIGH potential for P movement
from the site. The probability for an adverse impact to surface water
resources is extreme. Remedial action is required to reduce the risk of
P movement. All necessary soil and water conservation practices plus a
phosphorus management plan must be put in place to reduce the potential
of water quality degradation. **No application of P is permitted.**

## Precautions in the Use of the Phosphorus Index

The Phosphorus Index is an assessment tool intended to be used by
planners and land users to assess the risk that exists for phosphorus
leaving the landform site and travelling toward a water body. It also
can be used to identify the critical parameters of soil, topography, and
management that most influence the movement. Using these parameters, the
index can then help select in the selection of management alternatives
that would significantly address the potential impact and reduce the
risk. The index is intended to be part of the planning process that
takes place between the land user and resource planner. It can be used
to communicate the concept, process, and results that can be expected if
various alternatives are used in the management of the natural resources
at the site. THE [PHOSPHORUS INDEX IS NOT INTENDED TO BE AN EVALUATION
SCALE FOR DETERMINING WHETHER LANDUSERS ARE ABIDING WITHIN
WATER]{.underline} [QUALITY OR NUTRIENT MANAGEMENT STANDARDS THAT HAVE
BEEN ESTABLISHED BY LOCAL, STATE, OR FEDERAL AGENCIES]{.underline}. Any
attempt to use this index as a regulatory scale would be grossly beyond
the intent of the assessment tool and the concept and philosophy of the
working group that developed it. As discussed in this technical note,
this Phosphorus Index has been adapted to local conditions by a process
of regional adaptations of the site characteristic parameters. This
local development involves those local and state agencies and resource
groups that are concerned with the management of phosphorus. After this
index was adapted to this locality, it was tested by the development
group to assure that the assessments are giving valid and reasonable
results for the region. Field testing of the index was used to assess
the value of the index.

TABLE 1. PHOSPHORUS INDEX WORKSHEET FOR NEW MEXICO

<table>
<colgroup>
<col style="width: 20%" />
<col style="width: 2%" />
<col style="width: 11%" />
<col style="width: 1%" />
<col style="width: 2%" />
<col style="width: 10%" />
<col style="width: 0%" />
<col style="width: 0%" />
<col style="width: 8%" />
<col style="width: 2%" />
<col style="width: 3%" />
<col style="width: 7%" />
<col style="width: 1%" />
<col style="width: 2%" />
<col style="width: 0%" />
<col style="width: 8%" />
<col style="width: 0%" />
<col style="width: 5%" />
<col style="width: 8%" />
<col style="width: 0%" />
<col style="width: 0%" />
</colgroup>
<tbody>
<tr class="odd">
<td colspan="19"><p id="phosphorus-index-worksheet-for-new-mexico"
class="heading">PHOSPHORUS INDEX WORKSHEET for New Mexico</p></td>
<td></td>
<td></td>
</tr>
<tr class="even">
<td><strong>Client Name:</strong></td>
<td colspan="4"></td>
<td colspan="3"><strong>Field(s):</strong></td>
<td colspan="3"></td>
<td><strong>Date:</strong></td>
<td colspan="5"></td>
<td colspan="2"></td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td><strong>Planner:</strong></td>
<td colspan="4"></td>
<td colspan="3"><strong>Location:</strong></td>
<td colspan="3"></td>
<td><strong>Crop:</strong></td>
<td colspan="5"></td>
<td colspan="2"></td>
<td></td>
<td></td>
</tr>
<tr class="even">
<td colspan="2"><strong>Permeability (in/hr):</strong></td>
<td colspan="2"></td>
<td colspan="2"><strong>Slope (%):</strong></td>
<td colspan="3"></td>
<td colspan="6"><strong>Planned/Exist.:</strong></td>
<td></td>
<td colspan="3"></td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td colspan="2"><strong>Site Characteristic</strong></td>
<td colspan="16"><strong>Place an X in the appropriate box for each of
the Site Characteristic listed below.</strong></td>
<td><strong>Sub Total</strong></td>
<td></td>
<td></td>
</tr>
<tr class="even">
<td colspan="2"><strong>Soil Test P Level</strong></td>
<td>Very Low &lt;8 ppm</td>
<td colspan="4">Low 8-15 ppm</td>
<td colspan="3">Moderate &gt;15-23 ppm</td>
<td colspan="4">High &gt;23-30 ppm</td>
<td colspan="4">Very High &gt;30 ppm</td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td colspan="2"></td>
<td></td>
<td colspan="4"></td>
<td colspan="3"></td>
<td colspan="4"></td>
<td colspan="4"></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="even">
<td colspan="2"><strong>Phosphorus (P<sub>2</sub>O<sub>5</sub>)
Application Rate</strong></td>
<td>None Applied</td>
<td colspan="4">&lt;30 lbs/ac P<sub>2</sub>O<sub>5</sub></td>
<td colspan="3">30-90 lbs/ac P<sub>2</sub>O<sub>5</sub></td>
<td colspan="4">&gt;90-150 lbs/ac P<sub>2</sub>O<sub>5</sub></td>
<td colspan="4">&gt;150 lbs/ac P<sub>2</sub>O<sub>5</sub></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td colspan="2"></td>
<td></td>
<td colspan="4"></td>
<td colspan="3"></td>
<td colspan="4"></td>
<td colspan="4"></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="even">
<td colspan="2"><strong>Organic Phosphorus Source Application
Method</strong></td>
<td>None Applied</td>
<td colspan="4">Placed with Planter Deeper than 2 in.</td>
<td colspan="3">Incorporated Immediately before Planting</td>
<td colspan="4">Incorp. &gt;3 Mo. Before Planting <strong>or</strong>
Surface Applied &lt;3 Mo. before Planting</td>
<td colspan="4">Surface Applied &gt;3 Months Before Planting</td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td colspan="2"></td>
<td></td>
<td colspan="4"></td>
<td colspan="3"></td>
<td colspan="4"></td>
<td colspan="4"></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="even">
<td colspan="2"><strong>Phosphorus Fertilizer Application
Method</strong></td>
<td>None Applied</td>
<td colspan="4">Placed with Planter Deeper than 2 in.</td>
<td colspan="3">Incorporated Immediately before Planting</td>
<td colspan="4">Incorp. &gt;3 Mo. Before Planting <strong>or</strong>
Surface Applied &lt;3 Mo. before Planting</td>
<td colspan="4">Surface Applied &gt;3 Months Before Planting</td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td colspan="2"></td>
<td></td>
<td colspan="4"></td>
<td colspan="3"></td>
<td colspan="4"></td>
<td colspan="4"></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="even">
<td colspan="2"><strong>Proximity of Nearest Field Edge to Named Stream
or Lake</strong></td>
<td>Very Low &gt;1000 feet</td>
<td colspan="4">Low &gt;500-1000 feet</td>
<td colspan="3">Medium &gt;200-500 feet</td>
<td colspan="4">High 30-200 feet</td>
<td colspan="4">Very High &lt;30 feet</td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td colspan="2"></td>
<td></td>
<td colspan="4"></td>
<td colspan="3"></td>
<td colspan="4"></td>
<td colspan="4"></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="even">
<td colspan="2"><strong>Soil Erosion (WEQ &amp; RUSLE)</strong></td>
<td>Very Low &lt;1 t/ac</td>
<td colspan="4">Low 1-3 t/ac</td>
<td colspan="3">Medium &gt;3-5 t/ac</td>
<td colspan="4">High &gt;5-15 t/ac</td>
<td colspan="4">Very High &gt;15 t/ac</td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td colspan="2"></td>
<td></td>
<td colspan="4"></td>
<td colspan="3"></td>
<td colspan="4"></td>
<td colspan="4"></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="even">
<td colspan="2"><strong>Runoff Class (Runoff Class Table
2)</strong></td>
<td>Very Low</td>
<td colspan="4">Low</td>
<td colspan="3">Medium</td>
<td colspan="4">High</td>
<td colspan="4">Very High</td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td colspan="2"></td>
<td></td>
<td colspan="4"></td>
<td colspan="3"></td>
<td colspan="4"></td>
<td colspan="4"></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="even">
<td colspan="2"><strong>Irrigation Erosion (furrow)</strong></td>
<td>Not Irrigated or No Furrow Irrigation</td>
<td colspan="4">Tailwater Recover <strong>or</strong> QS&lt;6 for very
erodible soils <strong>or</strong> QS&lt;10 for resistant soils</td>
<td colspan="3">QS&gt;10 for erosion resistant soils</td>
<td colspan="4">QS&gt;10 for erodible soils</td>
<td colspan="4">QS&gt;6 for very erodible soils</td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td colspan="2"></td>
<td></td>
<td colspan="4"></td>
<td colspan="3"></td>
<td colspan="4"></td>
<td colspan="4"></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="even">
<td colspan="2" rowspan="2"><strong>Grazing Management</strong></td>
<td>Not Grazed</td>
<td colspan="4">Graze Crop Residues</td>
<td colspan="3">Pasture &lt;30% Dry Matter as Supplemental Feed</td>
<td colspan="4">Pasture 30 to 80% Dry Matter as Supplemental Feed</td>
<td colspan="4">Pasture 80 to 100% Dry Matter as Supplemental Feed</td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td></td>
<td colspan="4"></td>
<td colspan="3"></td>
<td colspan="4"></td>
<td colspan="4"></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="even">
<td colspan="2"><strong>Vegetative Buffers</strong></td>
<td>&gt;100 ft wide</td>
<td colspan="4">&gt;65-100 ft wide</td>
<td colspan="3">20-65 ft wide</td>
<td colspan="4">&lt;20 ft wide</td>
<td colspan="4">No buffer</td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td colspan="2"></td>
<td></td>
<td colspan="4"></td>
<td colspan="3"></td>
<td colspan="4"></td>
<td colspan="4"></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="even">
<td colspan="2"><strong>P Hazard Class:</strong></td>
<td></td>
<td colspan="4"></td>
<td colspan="3"></td>
<td colspan="8"><strong>Total Index Points:</strong></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td colspan="10"><strong>Phosphorus Application
Classification:</strong></td>
<td colspan="3"></td>
<td colspan="5"></td>
<td></td>
<td></td>
<td></td>
</tr>
</tbody>
</table>

  ----------------- ------------------ ----------- ------------------ ------------- ------------
                    **Phosphorus Index                                              
                    Classification**                                                

                    **Index Pts.**     **P Haz.    **P Application                  
                                       Class**     Classification**                 

                    0-10               Very Low    N Based                          

                    10-17              Low         N Based                          

                    17-27              Medium      N Based                          

                    27-37              High        P Based (1.5 x                   
                                                   crop removal)                    

                    37-47              Very High   P Based (at crop                 
                                                   removal)                         

                    \>47               Excessive   No P application                 
                                                   allowed                          
  ----------------- ------------------ ----------- ------------------ ------------- ------------

**TABLE 2. RUNOFF CLASS BASED ON FIELD SLOPE AND PERMEABILITY CLASS**

  ------------------ --------- ----------------- -------------- ------------ -------------- ------------ --------------- ---------------
  **Runoff Class                                                                                                         
  Based on Field                                                                                                         
  Slope and                                                                                                              
  Permeability                                                                                                           
  Class^1^**                                                                                                             

  **Slope %**        **Very    **Rapid 20-6**    **Moderately   **Moderate   **Moderately   **Slow       **Very Slow     **Impermeable
                     Rapid                       Rapid 6-2**    2-0.6**      Slow 0.6-0.2** 0.2-0.06**   0.06-0.0015**   \<0.0015**
                     \>20**                                                                                              

                     (in/hr)   (in/hr)           (in/hr)        (in/hr)      (in/hr)        (in/hr)      (in/hr)         (in/hr)

  **Level or         **N**     **N**             **N**          **N**        **N**          **N**        **N**           **VH**
  Concave**                                                                                                              

  **\>0 to 1**       **N**     **N**             **N**          **N**        **L**          **M**        **H**           **VH**

  **1 to \<5**       **N**     **N**             **VL**         **L**        **M**          **H**        **VH**          **VH**

  **5-\<10**         **VL**    **VL**            **L**          **M**        **H**          **VH**       **VH**          **VH**

  **10-\<20**        **VL**    **VL**            **L**          **M**        **H**          **VH**       **VH**          **VH**

  **\>20**           **L**     **L**             **M**          **H**        **VH**         **VH**       **VH**          **VH**

                                                                                                                         

  **Note:** Adapted                                                                                                      
  from the National                                                                                                      
  Soil Survey                                                                                                            
  Handbook.                                                                                                              

                                                                                                                         

  ^1^Based on the                                                                                                        
  most restrictive                                                                                                       
  horizon above 20                                                                                                       
  inches. If the                                                                                                         
  most restrictive                                                                                                       
  horizon is between                                                                                                     
  20 and                                                                                                                 

  40 inches. The                                                                                                         
  runoff estimate                                                                                                        
  should be reduced                                                                                                      
  by one class                                                                                                           
  (e.g., medium to                                                                                                       
  low). If the most                                                                                                      
  restrictive                                                                                                            

  layer in the soil                                                                                                      
  is below 40                                                                                                            
  inches, use the                                                                                                        
  lowers class that                                                                                                      
  occurs above 40                                                                                                        
  inches.                                                                                                                

                                                                                                                         

  **Runoff Classes:                                                                                                      
  N-negligible,                                                                                                          
  VL-very low,                                                                                                           
  L-low, M-medium,                                                                                                       
  H-high, VH-very                                                                                                        
  high**                                                                                                                 

                                                                                                                         

  **Special Rule                                                                                                         
  1** - A soil                                                                                                           
  horizon that has a                                                                                                     
  seasonal water                                                                                                         
  table is assumed                                                                                                       
  to have very slow                                                                                                      
  permeability.                                                                                                          

                                                                                                                         

  **Special Rule                                                                                                         
  2** - Runoff is                                                                                                        
  rated as                                                                                                               
  \"negligible\" (N)                                                                                                     
  if the soil is in                                                                                                      
  a depression,                                                                                                          
  regardless of the                                                                                                      
  permeability.                                                                                                          

                                                                                                                         

  **Assumptions:**             **1.** Bare soil                                                                          
                               surface.                                                                                  

                               **2.** Low water                                                                          
                               retention due to                                                                          
                               ground surface                                                                            
                               irregularities.                                                                           

                               **3.** Steady                                                                             
                               ponded                                                                                    
                               infiltration                                                                              
                               rate.                                                                                     

                               **4.** Bulk                                                                               
                               density of upper                                                                          
                               10\" is within                                                                            
                               normal range for                                                                          
                               the soil.                                                                                 

                                                                                                                         
  ------------------ --------- ----------------- -------------- ------------ -------------- ------------ --------------- ---------------

**TABLE 3. Furrow IRRIGATION EROSION site characteristics**

**I.** **QS value**

Q = flow rate of water introduced into the furrow (in gallons per
minute, GPM).

> S = furrow slope (in feet per 100 feet, percent).
>
> Example: For a 5 gpm flow rate and a 2% furrow grade:

QS = 5 gpm \* 2% grade = 10

**II. Relative ranking of soil erodibility under furrow irrigation**

Use local criteria to determine the relative erodibility of the soil in
question. If no local criteria are established, use the following for
guidance:

> **A. [Very Erodible Soils]{.underline}**
>
> Soils in which the surface layer texture is silt, or silt loam with \<
> 15% nonmontmorillonitic clay, or fine and very fine sandy loam with \<
> 15% nonmontmorillonitic clay, or loamy fine sand, or loamy very fine
> sand. Contact a soil scientist for clay content and mineralogy.
>
> **B. [Erosion-Resistant Soils]{.underline}**
>
> Soils that have the following characteristics in the upper 5 cm of the
> surface layer:
>
> silty clay, clay, or sandy clay texture, weak or massive structure,
> and mixed or montmorillonitic clay mineralogy.
>
> other soils that have medium or coarse blocky structure or coarse
> granular structure (i.e. natural aggregates \> 10 mm) and very firm or
> firmer rupture resistance class in the moist state (i.e. requires at
> least strong force between thumb and forefinger to cause failure of a
> moist soil aggregate).
>
> See the Soil Survey Manual (1993), chapter 3 for description of soil
> structural aggregates (peds), and table 3-14 for soil
> rupture-resistance classes.
>
> **C. [Erodible Soils]{.underline}**
>
> Soils that have a surface layer not fitting any of the above criteria.

# TABLE 4. PHOSPHORUS INDEX RATING FOR NEW MEXICO: WEIGHTING FACTOR CHART

  ------------------ ---------- ------------- ------------ -------------- -------------- ----------
  **Phosphorus Index                                                                     
  Rating for New                                                                         
  Mexico**                                                                               

                                **Weighting                                              
                                Factor Times                                             
                                the Column                                               
                                Factor**                                                 

  **Site             **Wt.      **None or     **Low**      **Medium**     **High**       **Very
  Characteristic**   Factor**   Very Low**                                               High**

                                **0**         **1**        **2**          **4**          **8**

  **Soil Test P      **1**      Very Low \<8  Low 8-15 ppm Moderate       High \>23-30   Very High
  Level**                       ppm                        \>15-23 ppm    ppm            \>30 ppm

                                **0**         **1**        **2**          **4**          **8**

  **Phosphorus       **1**      None Applied  \<30 lbs/ac  30-90 lbs/ac   \>90-150       \>150
  (P~2~O~5~)                                  P~2~O~5~     P~2~O~5~       lbs/ac         lbs/ac
  Application Rate**                                                      P~2~O~5~       P~2~O~5~

                                **0**         **1**        **2**          **4**          **8**

  **Organic          **1**      None Applied  Injected 3-6 Incorporated   Incorporated   Surface
  Phosphorus Source                           inch below   Immediately    \>3 Months     Applied
  Application                                 surface      before         Before         
  Method**                                                 Planting       Planting       
                                                                          **or** Surface 
                                                                          Applied \<3    
                                                                          Months before  
                                                                          Planting       

                                **0**         **1**        **2**          **4**          **8**

  **Phosphorus       **1**      None Applied  Placed with  Incorporated   Incorporated   Surface
  Fertilizer                                  Planter      Immediately    \>3 Months     Applied
  Application                                 Deeper than  before         Before         
  Method**                                    2 in.        Planting       Planting       
                                                                          **or** Surface 
                                                                          Applied \<3    
                                                                          Months before  
                                                                          Planting       

                                **0**         **1**        **2**          **4**          **8**

  **Proximity of     **1.5**    Very Low      Low          Medium         High 30-200    Very High
  Nearest Field Edge            \>1000 feet   \>500-1000   \>200-500 feet feet           \<30 feet
  to Named Stream or                          feet                                       
  Lake**                                                                                 

                                **0**         **1.5**      **3**          **6**          **12**

  **Soil Erosion     **1.5**    Very Low \<1  Low 1-3 t/ac Medium \>3-5   High \>5-15    Very High
  (WEQ & RUSLE)**               t/ac                       t/ac           t/ac           \>15 t/ac

                                **0**         **1**        **2**          **4**          **8**

  **Runoff Class     **1.5**    Negligible &  Low          Medium         High           Very High
  (Runoff Class                 Very Low                                                 
  Table 2)**                                                                             

                                **0**         **1.5**      **3**          **6**          **12**

  **Irrigation       **1.5**    Not Irrigated Tailwater    QS\>10 for     QS\>10 for     QS\>6 for
  Erosion**                     **or** No     Recover      erosion        erodible soils very
                                Furrow        **or** QS\<6 resistant                     erodible
                                Irrigation    for very     soils                         soils
                                              erodible                                   
                                              soils **or**                               
                                              QS\<10 for                                 
                                              other soils                                

                                **0**         **1.5**      **3**          **6**          **12**

  **Grazing          **0.5**    Not Grazed    Only Graze   Pasture \<30%  Pasture 30 to  Pasture 80
  Management**                                Crop         Dry Matter as  80% Dry Matter to 100%
                                              Residues     Supp. Feed     as             Dry Matter
                                                                          Supplemental   as Supp.
                                                                          Feed           Feed

                                **0**         **0.5**      **1**          **2**          **4**

  **Vegetative       **1.5**    \>100 ft wide \>65-100 ft  20-65 ft wide  \<20 ft wide   No Buffer
  Buffer**                                    wide                                       

                                **0**         **1.5**      **3**          **6**          **12**
  ------------------ ---------- ------------- ------------ -------------- -------------- ----------

  ----- --------- ------------------ ------------------ ---------------------------
                  **Phosphorus Index                    
                  Classification**                      

                  **Index Pts.**     **P Hazard Class** **P Application
                                                        Classification**

                  0-10               Very Low           N Based

                  10-17              Low                N Based

                  17-27              Medium             N Based

                  27-37              High               P Based (1.5 x crop
                                                        removal)

                  37-47              Very High          P Based (at crop removal)

                  \>47               Excessive          No P application allowed
  ----- --------- ------------------ ------------------ ---------------------------