Agriculture and Natural Resources
FSA2118
Understanding the Numbers
on Your Soil Test Report
Leo Espinoza
Assistant Professor and
Agronomist - Soils
Nathan Slaton
Associate Professor,
Soil Testing
Morteza Mozaffari
Research Assistant
Professor, Soil Testing
Arkansas Is
Our Campus
Visit our web site at:
http://www.uaex.edu
A routine soil test provides an
index describing the availability of
nutrients for plant uptake. Routine
soil tests measure only a portion of
the total pool of nutrients in the soil.
Soils have large amounts of most
plant-essential nutrients, but only
a small fraction (often less than 1%)
are in a form that can be taken up by
plants. The release of native soil
nutrients and the “tie-up” of nutrients
added from manures, fertilizers,
compost and plant residues involve
complex soil chemical, microbiological
and physical processes.
In January 2006, a number of
changes were implemented in the
University of Arkansas soil testing
and fertilizer recommendations
program. The information presented
in this fact sheet will help the reader
understand the numbers in the soil
test report.
Fertilization Philosophy
Due to variations in soil proper-
ties from one geographic region to
another, soil testing laboratories may
use different extractant solutions. Soil
testing labs use these solutions to
extract plant-available nutrients from
soil and apply different philosophies
to interpret the results and estimate
the amount of nutrients required to
optimize plant growth and yield
potential. The University of Arkansas
uses the Mehlich-3 soil test method
and recommends fertilizer rates that
optimize plant growth and yield and
replace the macronutrients removed
by the harvested portion of a crop. For
some soils, additional fertilizer will be
recommended to build or maintain
the soil levels near a “Medium” range
for P (phosphorus) and K (potassium).
The amount of P and K needed to
raise the soil test level to “Medium”
may not be economically or agronomi-
cally practical in a single application
or growing season, particularly for
soils with very low nutrient levels.
Therefore, the University of Arkansas’
recommendations normally use an
eight-year period to build nutrient-
deficient soils to the “Medium” level.
The recommendations assume that, on
average, 15 lb P
2
O
5
/acre are required
to raise the soil-test P level by 1 ppm
(2 lb/acre), and 8 lb K
2
O/acre are
needed to raise the soil-test K level by
1 ppm. Fertilizer and lime recom-
mendations are also based on crop
rotations, soil texture, plant variety
and yield goal when appropriate.
Nutrient Availability Index
The concentrations of soil
nutrients appear in the Nutrient
Availability Index section of the
University of Arkansas soil test report
and are reported with units of ppm
(parts per million) and pounds per
acre (lb/acre). One part per million
equals approximately 2 pounds per
acre (when the sample is taken from
the top 6 inches). In addition to
reporting the concentration of each
nutrient, there is also an availability
index or soil test level associated with
the P (phosphorus), K (potassium) and
Zn (zinc) concentrations. This level is
related to the expected crop yield that
would be produced without additional
fertilization. A nominal fertilizer rate
may be recommended for selected
University of Arkansas, United States Department of Agriculture, and County Governments Cooperating
crops on soils with “Optimum” soil nutrient levels to
compensate for nutrients removed by the harvested
portion of the crop. Variables other than fertilization
(e.g., water stress, insects, hardpans, etc.) can also
affect yield potential, even if plants are properly
fertilized. Table 1 shows the general interpretation of
soil-nutrient concentrations and levels for most
agronomic crops. Because plant species often have
different nutrient requirements, the defined soil-
nutrient concentrations that accompany the soil test
levels are general in nature.
The interpretations provided in Table 1
apply only to routine tests conducted by the
University of Arkansas soil testing laboratory
and should not be used to interpret information
provided by other laboratories. Contact your
county Extension office for additional information or
other publications.
Phosphorus (P) and Potassium (K)
Phosphorus and potassium are two of the three
macronutrients (the other being nitrogen) required by
plants for optimum growth. They are required in
larger amounts compared to the micronutrients (e.g.,
zinc, iron, boron, etc.). Yield response to P fertilization
is not likely when the soil P is ≥36 ppm (72 lb/acre)
for row and forage crops, above 25 ppm (50 lb/acre)
for fruit crops and above 75 ppm (150 lb/acre) for
vegetable production. Responses to potassium
fertilization are not likely when the soil tests above
175 ppm (350 lb/acre) for vegetables, row and forage
crops and above 90 ppm (180 lb/acre) for fruit crops.
Calcium (Ca) and Magnesium (Mg)
Most sandy soils have calcium concentrations
below 400 to 500 parts per million (800 to 1,000
lb/acre), while clayey soils usually test above
2,500 ppm. Normally, the higher the calcium level, the
greater the soil clay content. Recent limestone
applications may result in higher calcium levels. If
the soil pH is maintained in the recommended range
for the crop grown, calcium deficiency is very unlikely.
In general, the higher the clay content, the more lime
will be required to raise soil pH to the desired level.
Limited information is available on the crop
response to magnesium fertilization in Arkansas, but
if the soil tests below 31 ppm (62 lb/acre), the soil test
report will suggest an application of magnesium.
Most soils low in magnesium are often acidic and low
in calcium.
Iron (Fe), Manganese (Mn), Copper
(Cu), Zinc (Zn) and Boron (B)
The extractable levels of these micronutrients are
printed on the soil test report; however, with the
exception of zinc, their levels do not currently affect
the fertilizer recommendations. Soil-test zinc levels
below 4 ppm (8 lb/acre) coupled with pH above 6.0
may trigger a zinc fertilizer recommendation. Plant
tissue and soil analyses should be used together to
assess the need for application of the other micro-
nutrients. A very high level of any micronutrient does
not necessarily indicate that a plant nutrient toxicity
will develop. For example, soil-test iron values above
TABLE 1. Interpretation of soil-nutrient concentration ranges and soil test levels of surface soil samples for
most row crops and forages. The interpretation for vegetable crops and other plants may vary.
Soil Test Level
Expected
Yield
Potential†
Mehlich-3 Nutrient Concentrations
P K
[Most Crops]
K
[Turf Codes]
Ca‡ Mg‡
SO
4
-S‡
Mn‡ Cu‡ Zn
- - - - - - - - - - - - - - - - - - - - - - - mg/kg (or ppm) - - - - - - - - - - - - - - - - - - - - - - -
Very Low§ <65% <16 <61 <21 <1.6
Low§ 65 - 85% 16 - 25 61 - 90 21 - 40 ≤400 ≤30 ≤10 <40 <1.0 1.6 - 3.0
Medium§ 85 - 95% 26 - 35 91 - 130 41 - 60 3.1 - 4.0
Optimum 100% 36 - 50 131 - 175 61 - 100 4.0 - 8.0
Above Optimum
(High)
100% >50 >175 >100 >8.0
†Expected yield potential without fertilization.
‡Recommendations are not provided for these nutrients. The listed values represent general guidelines for interpretation.
§The soil test levels of “Very Low,” “Low” and “Medium” are considered “Sub-Optimum” levels.
200 ppm (400 lb/acre) and zinc values above 40 ppm
(80 lb/acre) are sometimes observed, but rarely are
these concentrations toxic to plants. In contrast,
manganese levels exceeding 200 ppm (400 lb/acre),
coupled with a soil pH below 5.2, may result in
manganese toxicity. This particular problem is easily
corrected by applying recommended rates of lime to
the soil. Soil-test Mn values <40 ppm (80 lb/acre) are
considered low. Although Mn fertilizer is not currently
recommended for agronomic crops in Arkansas,
manganese deficiencies are sometimes observed on
soil with pH >6.5 and soil-test Mn concentrations
below 20 ppm (40 lb/acre) and may require
application of Mn fertilizer.
Nitrate-Nitrogen (NO
3
-N) and
Sulfate-Sulfur (SO
4
-S)
Nitrogen is normally the most limiting nutrient
for optimum plant growth. Soil tests that estimate
soil N availability are not currently used because soil
N exists in many forms which may change with time
and influence plant availability. Soil nitrogen (N) and
sulfur (S) are measured in nitrate-nitrogen (NO
3
-N)
and sulfate-sulfur (SO
4
-S) forms. For most crops
grown in Arkansas, nitrogen fertilizer recommen-
dations are developed from research trials and are
based on previous crop, soil texture, yield goal and
sometimes cultivar. Analysis for soil nitrate-N,
however, is done routinely only for a few selected
crops, and it is used to refine their N-fertilizer
recommendations. Soil samples can be analyzed for
nitrate-N if requested for other crops, but fertilizer-N
recommendations, particularly for lawns and forages,
are not adjusted.
Sulfate-sulfur and nitrate-nitrogen can leach in
sandy soils and typically accumulate above the dense
or clayey soil horizons. For this reason, positive crop
responses to sulfur fertilization are not common in
clayey soils. As organic matter decomposes, sulfur and
nitrogen are released into the soil solution. As a
consequence of these complex reactions, the concen-
trations of these nutrients may vary considerably
with time, environmental conditions and soil depth.
Recommendations for sulfur fertilization are based on
cropping history and, to some extent, on soil test
level, especially for corn, cotton, wheat and forages.
pH or Soil Reaction
The soil reaction, or pH, is a measure of the
acidity or alkalinity of the soil. A pH of 7.0 is neutral.
Soil pH values below 7.0 are acid, while those above
7.0 are basic or alkaline. Each whole unit (e.g., 1.0)
change in pH represents a ten-fold difference in
acidity or alkalinity. For example, a pH of 5.2 is
10 times more acidic than a pH of 6.2. For most
vegetable and row crops, a pH of 5.8 to 6.5 is optimal.
A pH range of 5.5 to 5.8 is desirable for roses,
turfgrasses, fruits and nuts. Certain shrubs and
blueberries thrive in soils with a pH below 5.5. Most
plants suffer visually when soil pH is below 4.8. Lime
is recommended to neutralize soil acidity, with clayey
soils requiring more lime than soils having a sandy or
silty texture. Elemental sulfur (S) or aluminum
sulfate (Al
2
SO
4
) is recommended to acidify the soil
(lower the soil pH) for acid-loving plants. Soil pH
values (measured in water) may vary by 1.0 pH unit
or more during a growing season. In general, soil pH
values are highest in the cool, wet winter months and
lowest during the hot, dry summer months.
1 2 3 4 5 6 7 8 9 10 11 12
pH of Common Household Items
Salt Content (also referred to as
electrical conductivity, or EC)
The electrical conductivity of a soil is used to
measure the potential risk of salt injury to plants,
and it is currently measured with a 1:2 soil:water
mixture. This measurement includes all soluble salts,
not just sodium chloride that most people are familiar
with. Electrical conductivity readings can vary
dramatically within fields and across time and are
greatly affected by environmental conditions (e.g.,
rainfall). For this reason, soil EC is no longer
measured on all routine soil samples, but is available
(free of charge) by request. Measurement of soil EC
can be useful when diagnosing crop growth problems,
but has limited use in Arkansas for predicting fields
that will experience salinity injury due to salt
accumulation from hot, dry conditions, over-
fertilization or salts deposited by irrigation water.
Electrical conductivity values for soil samples
collected during the winter months are commonly
<100 µmhos/cm and are considered normal.
Depending on the salt sensitivity of the plant species
(rice, roses and strawberries are more sensitive than
cotton or bermudagrass), salt injury symptoms may
occur when EC values are >500 µmhos/cm.
Estimated Cation Exchange
Capacity (ECEC)
Cation exchange capacity (CEC) refers to the
ability of negatively charged soil particles to attract
and retain positively charged ions [calcium (Ca
++
),
magnesium (Mg
++
), potassium (K
+
), sodium (Na
+
),
ammonium (NH
4
+
), aluminum (Al
+++
) and hydrogen
(H
+
)]. Cation exchange capacity is expressed in units
of centimoles per kilogram (cmol/kg). Soil CEC on the
University of Arkansas soil test report is termed
“estimated cation exchange capacity,” or ECEC,
because this property is calculated (rather than
determined analytically) by summing the basic cation
(Ca, Mg, Na and K) charges and estimating the acidic
cation charges from soil pH. Soil ECEC is also an
indication of soil texture and organic matter content.
Generally, in Arkansas, sandy-textured soils have an
ECEC <9 cmol/kg, loamy soils have an ECEC of 9 to
20 cmol/kg and clayey soils have an ECEC above
20 cmol/kg. Soil clay content, clay type and organic
matter content influence the soil CEC. In general
ECEC increases as the soil clay and/or organic matter
content increase.
Organic Matter (O.M.)
Organic matter is no longer a routine test, but
can be determined for a fee. Check with your county
Extension office to determine the current fee. Soil
organic matter content in Arkansas soils typically
ranges from 0.5% to 5.0%. Soil organic matter
contents <0.5% are low, and values >2.0% are
desirable.
Estimated Soil Texture
The soil textural class designation for submitted
soil samples was previously provided by the client.
However, inconsistent and erroneous textural class
designations often resulted in lime and nitrogen
fertilizer recommendations that were not appropriate
for the intended crop and soil properties. Therefore,
the laboratory now estimates the soil textural class
based on soil pH and soil-test calcium. While the
relationship works well in most cases, continuous
animal manure applications, recent liming and long-
term application of alkaline well water can
dramatically increase soil calcium and may lead to an
erroneous estimate of soil textural class. Texture
influences the recommended N and lime rates.
Therefore, if the estimated soil texture is not correct,
contact the county Extension agent.
Percent Base Saturation
(% Base Saturation)
Base saturation represents the percentage of soil
cation exchange sites occupied by the basic ions Ca,
Mg, Na and K. The difference between this number
and 100 is the percentage of cation exchange sites
occupied by acidic cations: Al and H. Under most
conditions, a relatively high base saturation (>60%) is
desirable. Soil pH increases as percent base
saturation increases, with base saturations of 70% to
80% representing soils having pH >6.0.
Sodium (Na) is not an essential element for plant
growth, but is important for diagnosing problem soils
that may contain high amounts of Na. In soils with
high soil sodium levels, irrigation water may also be
high in sodium or the soil may contain natural
deposits of this element. Soil-test concentrations are
not given, but are expressed as exchangeable Na
percentage on the ECEC. When the estimated
exchangeable sodium exceeds 15%, the soil is
considered “sodic,” but crop production problems may
occur at lower levels. Exchangeable sodium
percentages <5% usually cause few production
problems.
Fertilizer and Lime
Recommendations
The amount of fertilizer and lime recommended
may be given in pounds per acre (lb/acre), pounds per
1,000 square feet (lb/1000 ft
2
) or pounds per 100 feet
of row (lb/100 ft row), depending on the crop selected.
The Crop Notes section in the soil test report
includes instructions on how and when to apply the
recommended fertilizer. The notes apply only to the
respective crop code (e.g., Crop 1 Notes apply only to
Crop 1). Precautionary notes or recommendations for
other nutrients may also appear in this section. The
user is encouraged to obtain publication FSA2153,
The Soil Test Report, for further information.
Printed by University of Arkansas Cooperative Extension Service Printing Services.
DR. LEO ESPINOZA is assistant professor and agronomist - Issued in furtherance of Cooperative Extension work, Acts of May 8
soils, Cooperative Extension Service, Little Rock. DR. NATHAN and June 30, 1914, in cooperation with the U.S. Department of
SLATON is associate professor, soil testing, University of Agriculture, Director, Cooperative Extension Service, University of
Arkansas, Fayetteville. DR. MORTEZA MOZAFFARI is research Arkansas. The Arkansas Cooperative Extension Service offers its
assistant professor, soil testing, Soil Testing and Research programs to all eligible persons regardless of race, color, national
Laboratory, Marianna. They are employees of the University of origin, religion, gender, age, disability, marital or veteran status,
Arkansas Division of Agriculture. or any other legally protected status, and is an Affirmative
FSA2118-PD-2-07R Action/Equal Opportunity Employer.
