GARDENERS FACTSHEET NO. 26
INTERPRETING YOUR SOIL TESTING RESULTS
CVI Cooperative Extension Service
Now that you have your soil test results in your hand, what
do they mean and how can they benefit you? Soil test results will
give you a good indication of your soil fertility. This publication
describes the soil report form and defines the terms found on the
WHAT IS A SOIL TEST?
A soil test is a scientific method for obtaining information
about the fertility level of a soil. Soil samples are analyzed by
chemical means to determine soil reaction (pH), organic matter,
soluble salts, nitrogen, phosphorus, potassium, calcium,
magnesium, sodium, sulfur, copper, zinc, iron, and manganese
contents. Results along with information about the soil, cropping
history and treatment are used to determine fertilizer and other soil
amendments needed to meet the plant nutrient requirements of
each individual crop.
VALUE OF SOIL TEST RESULTS
Individual soil test results serve as a basis for proper and
profitable fertilization. Over a period of time under intense
cultivation (3 years for field crops, 2 years for vegetables, and 5
years for ornamental crops) it will be necessary to retest the
fertility of your soil. Sampling and testing procedures will be
identical but interpretation of the results will be more accurate.
Results from soil tests performed after the original soil test along
with records of all soil amendments made since the original soil
sample was collected, will give an accurate account of the effect
that you and your crops have had on the soil in that time between
sample collections. With this information, soil amendments can be
added with even greater accuracy than with the single soil test
Fertilizer only insures high yields when other conditions
affecting crop yields are favorable. A good stand, adequate
moisture, control of insects, diseases, and weeds, as well as good
physical condition of the soil are important in obtaining profitable
returns from money invested in fertilizers.
REPORTING RESULTS OF SOIL TESTS
At this point, it is important to consider the soil report form.
The nutrient level of each soil is rated "Below", "Optimum", or
"Above." The dotted line in the "Below" region indicates the
critical level at which the nutrient could seriously weaken or limit
plant growth. If the nutrient is below critical level, a definite plant
response would be expected by adding the element either in the
form of organic or chemical fertilizer. When the nutrient level falls
in the "Optimum" region the nutrient is sufficient to allow for
optimum plant growth. For most crops, a complete fertilizer such
as (10, 10, 10) is recommended to maintain optimum levels of
nitrogen, phosphorus, and potassium in the soil, since these
elements are needed in relatively large amounts by the plant. If
the nutrient levels falls in the "Above" region, no additions of
the nutrient are recommended and problems might result due to
toxic quantities of the element or because of the imbalance of
other nutrients relative to this one. To rid the soil of a toxic
element, it is either recommended to leach the soil with large
quantities of cistern or rain water, or to add an element to replace
the toxic element on the exchange sites of the soil.
DESCRIPTION OF TERMS ON THE SOIL REPORT FORM
Table 1 defines the various terms and units found on the
Soil Report Form. How do these terms indicate soil fertility? This
section discusses the relevance of these terms and their
importance in understanding the status of your soil.
Table 1. Terms on the Soil Report Form
pH indicates whether the soil is acid or alkaline. 7.0
indicates a neutral reaction; less than 7.0 is acid and
greater than 7.0 is alkaline.
OM Organic Matter. The organic fraction of the soil that
includes plant and animal residues at various stages of
ppm parts per million
Sol. Salts salinity orthe amount of soluble salts in a soil as
expressed in ppm.
meq./100ml. milliequivalent per 100 milliliter of soil.
Act.C.E.C. Active Cation Exchange Capacity.
C.E.C. of 0-15 indicates a sandy soil
C.E.C. of 15-25 indicates a loam soil
C.E.C. above 25-indicates a clay soil
The soil reaction (pH) given on the report form indicates
whether the soil is acid or alkaline. A pH of 7.0 indicates a neutral
reaction, whereas less than 7.0 is acid and greater than 7.0 is
alkaline. Most field crops grow satisfactorily in the range from 5.5
to 8.3. The pH measurement is the single most informative reading
on the soil test report form. It indicates which nutrients might be
unavailable due to the formation of insoluble compounds. For
example, at relatively alkaline pH*s (above 8.0), the micronutrients
iron (Fe), zinc (Zn), manganese (Mn), and copper (Cu) are fixed
and the plant is unable to extract them from the soil. In this case,
corrective measures might be achieved more readily by applying
foliar sprays of these trace metals directly to the foliage instead of
application to the soil. At this pH, applying these elements
directly to the soil would result in the formation of insoluble
compounds that plant roots cannot extract.
As the pH is lowered (by addition of sulfur or other acid
forming materials), these micronutrients (Fe, Zn, Mn, Cu) would
become more soluble and therefore available to the plant. In areas
where acid soils dominate, limestone (CaCO3) is often added to
raise the pH. However, in the Virgin Islands, alkaline soils
dominate and therefore sulfur is added to neutralize the soil and
help bring trace elements into the optimum region.
Organic matter is a very good indicator of the fertility of soil.
Tropical regions tend to be relatively low in organic matter
because of rapid breakdown due to high temperatures and
humidity. Most arable soils have organic matter ranging from 2-
8%. Organic matter levels below 2% indicate the need to add
manure, compost, or grow a green manure crop, such as perennial
soybeans, and plough it under. Organic matter is of crucial
importance in supplying nitrogen, phosphorus, and sulfur to the
The level of soluble salts can be an important factor in the
fertility of your soil. This is particularly true where irrigation is
practiced or in areas close to the ocean. Soluble salts indicate the
salt level of the soil. Levels above 1400 ppm are considered
problems. Plants differ in their tolerance to salt, so that plants
such as coconuts thrive at salt levels that would injure plants
such as beans.
Not only must nutrients be at adequate levels to grow
healthy crops, but the levels must be balanced in regard to each
other. An imbalance in the calcium to magnesium ratio (Ca/Mg)
or the magnesium to potassium ratio (Mg/K) would result in
significant reductions in yield. These ratios are as important as
the individual levels of nutrients and for this reason they are
included on the report form.
The remaining terms on the soil report form are the
elements essential in crop production. These elements are listed
in Table II along with their function and deficiency symptoms.
WHERE TO SEND SOIL SAMPLES
Soil samples are tested at the Cooperative Extension
Service, St. Thomas campus of the College of the Virgin Islands
in the New House Building. Soil samples may be taken to the
Extension office on St. Croix where they will be sent to St.
Thomas for analysis. Drying, processing, testing, interpretation
and return of results normally require a minimum of 15 to 25 days
to complete. There is no charge for this service to V.I. residents
at present. For more information call 774-0210 or 778-0246.
Table 2. Elements Diagnosed by the Soil Testing Laboratory
Indispensable constituent of ammo acids and proteins Provides for leafy, green
Essential for energy transfer and fruit and seed formation
Helps resist drought and lodging Activates enzyme systems
Essential component in some ammo acids, also present in enzymes, vitamins, and
other essential organic compounds
Important in cell elongation and cell division
Part of chlorophyll molecule necessary for photosynthesis
Essential for proper cell development and normal growth, flowering, and quality
Indispensable for chlorophyll synthesis Component of enzymes and carriers
operating in the respiratory system
Involved in enzymatic activity, chlorophyll synthesis, carbohydrate transformations,
and growth promoting substances
Associated with chlorophyll synthesis, nitrogen metabolism and
General yellowing of leaves and slow growth, occurring in older
Dark blue-green leaves possibly with red or purple veins Slow
growth Appears in older leaves
Margins of lower leaves appear yellow and may develop brown spots
Yellow, chlorotic leaves Appears in lower leaves first Resembles
Failure of terminal buds to develop
Yellow blotching beginning at tips of leaf and on lower leaves first
Cessation of growth of terminal bud, followed by younger leaves
turning pale green at the base (rather than the tip)
General yellowing, appearing first in younger leaves Leaftissue
between veins turns yellow while veins remain green
Small yellow leaves that may turn brown and drop
General chlorosis of young leaves between the veins.
Often gray spots or streak on leaves and upper system.
Products and suppliers mentioned by name in this publication are used as examples and in no way imply endorsement or recommendation of these products or suppliers
Issued in furtherance of Cooperative Extension work acts of Congress of May 8 and June 30, 1914 (as amended), in cooperation with the US Department of Agriculture, DS
Padda, Director, College of the Virgin Islands Cooperative Extension Service The College of the Virgin Islands Cooperative Extension Service is an Equal Opportunity/Affirmative
Action organization, providing educational services in the field of agriculture, home economics, rural development, 4-H youth development and related subjects to all persons
regardless of race, color, religion, sex or national origin