Department of Soils Mimeograph Report 55-1
SOIL TESTING: AN AID IN BETTER SOIL MANAGEMENT
William L. Pritchett
Introduction- ------------------- ---- 1
Sampling the Soil- -------------------- 2
Soil Conditions and Fertilizer Materials - - - - 4
Soil reaction (pH) and lime requirement - - -
Nitrogen-- -------- ----.-. -------.---- 7
Phosphorus- --------.-------------- 9
Potassium ---- ------------------ 1 12
Minor elements- ----------------- -- 13
Total soluble salts ---------------- li
Interpretation of Soil Test Results- ---------- - 1
"Threshold" values- ----------------- 1
Plant food content of crops - ------------- 16
Fertilizer efficiency - - - ---------- 17
Discussion and Summary ------------------ 19
University of Florida
Agricultural Experiment Station
Willard M. Fifield, Director
SOIL TESTING: An Aid in Soil Management
For over a century agriculturists have sought chemical means of deter-
mining the quantities of available plant nutrients in soils. In recent years
their efforts have been rewarded with a certain degree of success, and soil
testing has become quite popular as a means for better characterization of
soil fertility and the prediction of the needs of soils for fertilizers and 2B
soil amendments. The more than one and a half million soil samples analyzed
in the United States in 1953 attest to the widespread popularity of soil
testing as does the increasing number of articles published covering its merits.
In view of the absence of a coordinated soil testing service in Florida
and the criticism leveled at its usefulness, under our conditions, by some
of our scientists, one might well ask if this popularity among farmers was
justified, or is it after all only a psychological extension tool?
Not even its most enthusiastic supporters will deny that soil testing
has a number of inherent weaknesses that limit its value. It is very improb-
able that any chemical means can be devised to measure the amount of plant
nutrients available, or that will become available to a crop during a growing
period. And while it is a relatively simple matter to measure the total
plant nutrient resources of a soil by an ultimate chemicalanalysis, it is
well known that no close relationship has been found between the total quan-
tities of the various soil constituents and the supply available to the plants.
A number of factors affect the ability of the plant to take up the
"available" plant nutrient in the soil. Some of these are (1) extremes in
temperature, (2) drought, (3) disease, (4) insect damage, (5) poor drainage
or lack of aeration, (6) soil texture and (7) antagonism of one element upon
the uptake of another. A soil test will not reveal these difficulties nor
will the addition of fertilizers be of much value until problems of these
types are first corrected.
In fact, it is generally recognized that soil test values per se are of
limited value for diagnostic purposes. To provide an appropriate scale of
values, the results of laboratory tests must be calibrated in terms of crop
response to fertilizer applications. Obviously. the ideal method of determining
the fertility level of soil is by conducting field experiments with different
levels of fertilizers with all crops on all soil types. However, as this is
a tremendous task, correlations between laboratory analyses and field results
on the major soil areas is an obvious compromise. The correlations thus ob-
tained can ordinarily be used only in a qualitative manner. Therefore, the
soil-test value must be interpreted in arriving at the recommended fertilizer
application. This interpretation takes into account not only the soil test
value and correlations with fertilizer trials, but all other factors that
may affect the results.
However, the fact that these problems in the use of a chemical soil test
exist does not necessarily mean that the test has no value nor that it has
no place in Florida agriculture. It does mean that its limitations must be
recognized and the test results used accordingly. Certainly, the test for
the more mobile nutrients such as nitrate nitrogen and potash, are less re-
liable for our sandy soils than is a similar test for heavy soils. Neverthe-
less, the test for available phosphorus, calcium and magnesium and the pH
determination can be as useful on Florida soils as elsewhere. A chemical
test of a soil sample, carefully taken to represent the area, interpreted
in the light of calibrated field trials and knowledge of the soil and past
cropping, can be an invaluable guide to better soil management, as well as
a "trouble-shooting" and educational tool.
Soil Testing Laboratories:
Some type of soil testing service is available from state agencies in
every state. Commercial laboratories also render a testing service in many
states. Florida probably has a wider variety of testing facilities avail-
able to growers than any other state even though little coordination exists
between these various laboratories. These facilities can be grouped into the
(a) Commercial laboratories, such as Thornton Laboratories, Tampaj
Southern Analytical Laboratories, Jacksonville; Chatelier Laboratories, St.
Petersburg; and others, will analyze soil samples and base a recommendation
upon the results for a fee.
(b) Agricultural consulting services which maintain laboratories and
test soils of their clients as a part of their integrated program of crop
(c) Fertilizer companies which have soil testing laboratories and
analyze samples collected by their agents as a service to their customers.
(d) State supported institutions which maintain facilities for soil
testing on a service basis and conduct some research on methods of analyses
and correlation of laboratory and field results include the Central Florida
Experiment Station, Sanford; Everglades Experiment Station, Belle Glade; Sub-
Tropical Experiment Station, Homestead; and the Department of Soils, Agricul-
tural Experiment Station, Gainesville.
Sampling the Soil:
Proper sampling is most important. A sample collected from a field or
area for which information is desired must be as representative of the field
as possible; otherwise tests may give misleading information. To aid in
collecting satisfactory samples the following steps should be followed:
(a) Each sample must be a composite of twelve to fifteen plugs taken
at various locations in the field.
(b) Obtain at least two composite samples from each area to be tested.
(c) Soils that are distinctly different in appearance, crop growth, or
past treatment should be sampled separately, provided the areas are of such
size and nature that they can be fertilized individually. For example, take
separate composite samples of light and dark colored areas and of areas vary-
ing in slope, drainage, soil type, or past management. Small areas which
differ from the main body of soil such as sink-holes and fence row.n should be
(d) Individual cores can be taken with soil tubes or augers, or with
a trowel or shovel if care is taken to take uniform slices, to a depth of
six inches (plow depth), except for pastures and turfs which should be
sampled only to a depth of three inches.
(e) The soil should be placed in a suitable container such as a clean
bucket, mixed well, then about one pint removed for testing.
(f) A map should be drawn and the location of each field and composite
sample taken, marked.
(g) The samples should be allowed to air-dry, do not heat, before placing
in clean ice cream cartons or double paper bags for mailing. Each sample
should be marked with your name and sample number.
(h) Answer as fully as possible the questions on the "Information Sheet"(1)
as to crops to be grown, past history of field, etc~
Fall and early spring is recommended as the best time for prefertilizae-
tion sampling* Sufficient time should elapse before sampling for the top soil
to reach equilibrium following heavy applications of lime or fertilizer. For
prefertilization samples this time interval should be at least six months.
In a recent field sampling study conducted by the Experiment Station to
isolate the percent error associated with each step in determining the chem-
ical properties of the soil, the variation due to laboratory analyses and
subsampling was found to be quite small in comparison to the location-to-
location variation in the field. Drawing of pint subsamples from a larger
composite sample of sandy soil did not constitute a large error under the
conditions studied. The error in field sampling can probably be most easily
reduced by taking two or more composite samples from different sections of
the field. In the above study there appeared to be little correlation between
locations. In other words, the variation of fertility level of soils within
a small area was almost as great as between areas of the field where no
gradient was evident. Cultivated areas were more uniform than pasture areas,
probably due to urine spots. The number of borings required for a composite
sample was found to be less for pH and phosphorus than for potassium and
calcium, within specified limits of accuracy. Since the former are of more
importance in service sampling, some accuracy may be sacrificed in the deter-
mination of the latter for the sake of economy by reducing location borings
in a given area.
Correct sampling of lawn and greenhouse soils is no less important than
that of field soils. The majority of the samples are from small areas which
often are artificial soils. Even the natural soil used for these purposes
will have had large applications of amendments. Most lawn and greenhouse
samples are taken because of poor plant growth. Since much of the trouble
may be due to over-fertilization, disease or insect damage, or other environ-
mental factors, more detailed information should be given on past management
than for field samples. 'here good and poor plant growth exist under similar
(1) A sample of Information Sheet is given in Appendix.
environmental conditions, samples of soil from both areas should be submitted
with information on the trouble encountered.
Most laboratories ask that only the upper three inches of lanms and pas-
ture soils be sampled since a major portion of the added fertilizer is fixed
near the surface. Two samples taken in the same spot, one to three inches
and the other to six inches, will contain different amounts of plant nutrients.
That amount found in the top three inches appears to more nearly correlate
with the actual fertilizer needs of plants on undisturbed soils than samples
taken to greater depths. Depth of sampling should be noted if it is other
than six inches.
SOIL CONDITIONS AND FERTILIZER MATERIALS
The method used by the Soils Department of the Agricultural Experiment
Station, like most rapid chemical soil tests, employs an extracting solution
to remove the exchangeable or "available" plant nutrients. The test procedures
are given in Appendix A. The extracting solutions used by different labora-
tories vary considerably in acidity, but they all have a similar goal, i.e.,
to extract a proportionate part of the nutrients available to growing plants
during a growing period. Since this can not be an exact measure, it must be
correlated with the response of crops to added fertilizers and interpreted
in the light of the kind of soil, crop requirement, climatic conditions, etc.
The following discussion of pH and plant nutrients is given to aid in the
interpretation of soil test results and to assist in making recommendations
based upon the results,
A. Soil reaction (pH) and lime requirement:
Soil reaction refers to the acidity or alkalinity of a soil and tests
express reaction in terms of pH. The scale is from 0 to 14, with 7 as the
dividing point or neutrality. Figures below 7 represent increasing acidity
(sourness), while higher ones indicate increasing alkalinity (sweetness).
Each figure differs by a multiple of 10, so that pH 5.0 is ten times and pH
4.0 is one hundred times more acid than pH 6.0, However, this does not mean
to say that it takes ten times more limestone to correct soil acidity from
pH 4.0 to pH 5$0 than to correct from pH 5.0 to pH 60,. Within the limits
normally encountered in cultivated soils, the relationship between pH and
base saturation of the soil approaches linearity.
Virgin Florida soils range from pH 3.8 (very acid) to pH 8.2 (alkaline),
but cultivated soils more commonly fall into the narrower limits of 4.8 to
7.8. The desirable range under most conditions is between 5.5 and 6.2.
The acidity determination is probably the most accurate and satisfactory
of all rapid tests made on soils. It is a valuable measure of soil condition.
It serves as a rough guide to the calcium status of the soil and may be an
indicator of the availability of 9th4r nutrients. Leaching of potassium, ammo-
nium, nitrogen, magnesium and calcium following fertilizer applications is,
in general, more severe from strongly acid soils than from those that are
slightly acid to neutral.
The availability of residual phosphorus (1) in some soils markedly in-
creases with a rise in pH from strongly acid to slightly acid. However, care
should be taken that the reaction of most Florida soils is not raised above
the slightly acid range (5.8 to 6.2) because the availability of elements
such as copper, zinc, boron, and manganese may become critical at high pH
levels* This is particularly true with sandy soils with low organic content.
As a rule, the colorimetric tests for pH in general use are sufficiently
accurate for most field crops, pastures and lawns. However, for making living
recommendations for crops which grow well only within a narrow pH range and
for pH determinations on light sands, the accuracy of a potentiometer is
Soil pH is an expression of its active acidity and cannot always be used
as a measure of the lime requirement. While it can conveniently be used as a
guide to lime requirement on our heavier soils, it should be used in conjunc-
tion with calcium and magnesium determinations on light soils. In other words,
some sandy soils may have a pH sufficiently high for normal plant growth and
soil microbial activity and yet not contain sufficient calcium or magnesium
to satisfy plant requirements. Consequently, two factors must be considered
in making liming recommendations: first, the nature of the soil and second,
the requirement of the crop to be grown.
The pH level varies considerably for the sane field depending upon the
time of year that the sample is taken. For example, under a program of heavy
fertilization of shade tobacco, the pH dropped from 5.2 in January to around
4.4 in June. Even with moderately fertilized field crops this mid-summer re-
duction in soil reaction often amounts to one half pH unit when physiologically
neutral fertilizers are used.
To prevent overliming, especially on sandy soils of low humus content,
the following table gives the maximum amounts of ground limestone which should
be applied at one time to obtain a pH unit raise. For example, if the pH of
a well drained sandy loam is to be raised from pH 5.5 to 6o0, or one half pH
unit, then half of the indicated quantity or 1000 pounds of high grade ground
calcic limestone would be required.
Fla. Agr. Exp. Sta. Press Bulletin 606w
Rate in pounds per acre to raise pH 1.0 unit
Soil Group High Calcic lime* Hydrated lime
1. White sands 500 375
2. Well drained fine sands and
light gray flatwoods sands 1000 750
3. Well drained loamy fine sands
and medium gray flatwoods sands 1500 1125
4. Well drained sandy loams and
dark gray flatwoods sands and
sandy loams 2000 1500
5. Clay loams, clays and black
mineral soils, depending on
humus content 2000 6000 1500 4500
6. Peat and mucks 6000 L400
* Testing 90 percent CaC03 equivalent, ground so that 50
60-mesh sieve. (Fla, Agr. Exp. Sta. Circular S-39.)
Hydrated lime reacts faster with the soil than high calcic agricultural
limestone and should, therefore, be used with care to prevent temporary over-
liming. One half of the required amount may be applied at one time and the
remainder after two to three months,
On extremely sandy soils frequent, small applications of lime may be
needed to maintain an adequate supply of calcium and magnesium for crop
growth, while heavier and less frequent applications may be satisfactory on
the heavier textured soils.
Soils which test low in magnesium should be treated with limestone con-
taining appreciable amounts of magnesium carbonate. Liming material contain-
ing a rather high percentage of magnesium carbonate (usually 36 to 48 percent)
is called dolomite. Dolomite is somewhat slower in correcting soil acidity
than is high calcic limestone although its power to neutralize soil acidity
is slightly greater. Dolomite is sometimes favored over high calcic lime-
stone on light sand to prevent "overliming", since it will not raise the pH
to as high a level as the latter.
Basic slag, which is used as a source of phosphorus, also has value as a
liming material. A 1,000 pound application is equal to 700 pounds of limestone,
In special cases when the soil reaction needs to be lowered, such as for
growing azaleas in areas where liming materials used in house construction
have raised the pH unduly, sulfur may be used. Sulfur equal to one third of
the ground limestone requirement listed in the previous table will produce
a pH drop of approximately 1.0 in the absence of free lime. Sulfur should
be used with caution to prevent undue lowering of pH and-injury to plants.
As discussed above, soil reaction (pH) can not always be used as a measure
of calcium requirement as sandy soils may be sufficiently high in reaction
and yet not contain enough calcium or magnesium for best crop growth. For
this reason the "available" calcium and magnesium are tested. Plants differ
greatly in their requirements for these nutrients, as do soils in this ability
to supply them. Minimum soil levels for normal plant growth are given in
Appendix B for some crops.
In addition to a favorable soil reaction and light, air, water and tem-
perature, the plant requires a number of chemical elements for normal growth
and development. These elements may be thought of as the raw materials from
which the plant manufactures straw, fruit and grain. Elements used in the
largest quantities by plants are those in most abundant supply. These elements,
carbon, hydrogen and oxygen, are fortunately supplied in the air and water.
The remainder, often called plant nutrients, must be derived from the soil.
These can be grouped as follows:
Primary: Nitrogen, phosphorus and potassium
Secondary: Calcium, magnesium and sulfur
Minor: Manganese, iron, zinc, copper and boron.
(Other elements that may be required by plants but seldom found
limiting in soils are chlorine, sodium, silicon, cobalt, fluorine
The primary plant foods, so-called because they are elements most often
limiting plant growth, are the main constituents of our fertilizers. A 100
pound bag of 4-8-8 analysis fertilizer contains four pounds of nitrogen (N),
eight pounds of available phosphoric acid (P205) and eight pounds of water-
soluble potash (K20). These primary nutrients are also the ones given the
most emphasis in soil testing.
Nitrogen is the plant nutrient most generally limiting in Florida soils,
With the exception of peats and mucks, all Florida soils need some additional
source of nitrogen for maximum plant growth of non-legumes. Although the
atmosphere over each acre of land contains over 72,000 tons of nitrogen, it
is in a form unavailable to plants. Most of the soil nitrogen is contained
in the organic matter. Except for that fixed by microorganisms living in
the soil and on the roots of leguminous plants, the large amounts of nitrogen
neededito produce a crop must be derived from the decomposition of organic
material or from added fertilizer.
Plants can use neither nitrogen nor organic matter assuch. Through the
agency of bacteria these materials are readily changed into chemical compounds
which become available sources of nitrogenous plant food. The plant available
forms are nitrate and ammonia nitrogen. Of these two the nitrate form is
the most available and because of its solubility is the most readily leached
from the soil by heavy rains. Ammonia nitrogen does not leach from the soil
as readily as the nitrate form. However, this form of nitrogen is readily
changed into the nitrate form by the action of microorganisms. Organic nitro-
gen is slowly changed into the nitrate form and results in less nitrogen loss
Although a test for nitrate nitrogen is often applied to soil samples,
the results have limited significance for field soils due to its transient
nature. In other words, many factors such as rainfall, temperatures, and
large amounts of undecomposed organic material influence the nitrate level
in the soil to such an extent that a test for this nutrient is difficult to
interpret. A test for the organic matter content can probably be more closely
related to the fertility level of a soil than can the nitrate test, but no
rapid test exists for determining the rate in which organic matter will become
available to plants. Since most mineral soils of Florida are very low in
nitrogen, the amount of this element tobe applied as commercial fertilizer
depends primarily on the nature of the crop and the soil moisture condition.
The need for nitrogen can often be judged by the behavior of the plant. Poor
color and slow growth are good indicators.
The nitrogenous materials used in commercial fertilizers are normally
grouped as organic or inorganic, depending on source or form. The nitrogen
in inorganic forms is generally rapidly available to plants while that.in the
organic form is not water soluble# therefore not available for plant growth
until it has been made so by chemical or bacterial action. Animal sources
of organic nitrogen are tankage, hoof meal, fish waste, and guanos. Vegetable
sources provide cottonseed meal, castor pomace, soybean meal, and other
The more common sources of nitrogenous fertilizer nutrients:
Organic Ammonium Nitrate Amide
Material nitrogen nitrogen nitrogen nitrogen
Sulfate of ammonia 20.5 -
Nitrate of soda 16.0 -
Nitrate of soda potash 15.0 -
Ammonium nitrate 16.5 16.5 -
Calcium nitrate 15.5 -
Anhydrous liquid ammonia 82.2 --
Calcium cyanamid 20.6
Cottonseed meal 6a0-7.5 --
Castor pomace 6o0-70O -
Sewage sludge, activated 50-75 -
Boneneals 3,-0-a3 ---
Soybean meal 740-8,0 -
Guano 5O0-90. -
* Urea and cyanamid are water
in the soil,
soluble organic and react similar to inorganics
The plant itself makes little distinction between the different sources,
as the element nitrogen itself ultimately is the same regardless of source.
Practical considerations will influence the farmer's choice. The cost per
unit of nitrogen, and the speed with which the nitrogen becomes available
are major factors to consider. The cheapest sources may be manure, green
cover crops and similar vegetation produced on the farm. However, such organic
material must be completely decomposed in the soil before it can release
nitrogen to the soil solution, and this takes time.
Some sources of nitrogen are acid forming while others develop a basic
residue in the soil. The nitrogen which is recovered in the plants and
harvested has little effect on the lime content of the soil. However, nitro-
gen not used by plants and that taken up and returned to the soil in crop
residues is changed to ammonia on decomposition of the residues and then into
nitrate nitrogen. The nitrate nitrogen leaches from the soil taking with it
calcium, magnesium, potassium, and sodium. This action results in increased
soil acidity. For instance, for every unit of nitrogen furnished by sulfate
of ammonia, there is required the equivalent of 107 pounds of lime as calcium
carbonate to neutralize the residual acidity. However, for every unit of
nitrogen from sodium nitrate a basicity equivalent to that of 36 pounds of
calcium carbonate is developed. The ammonium fertilizers such as ammonium
sulfate, ammonium nitrate, ammonia solutions and urea are acid forming; while
cyanamid, calcium nitrate and other carriers of nitrogen that have either
calcium, magnesium, sodium or potassium combined, are alkaline in reaction.
The lower prices of ammonium sources are generally enough to offset the cost
of the additional limestone needed to overcome the acids formed by their use
in a soil.
The phosphorus level in Florida cultivated soils varies greatly between
soil areas and even within soil areas* These variations have been brought
about to a large extent by differences in the past fertilization practices,
since most virgin soils of Florida are low in available phosphorus. Under
heavy fertilization the level of phosphorus may gradually increase since more
phosphorus is usually added in fertilizers than is removed by crops or lost
by leaching or erosion. Some 85 percent of the 140 million pounds of P20g
applied to crops in the state last year was used on four crops: commercial
vegetables, citrus, potatoes and tobacco. It is estimated that 51 million
pounds were applied to vegetable crops. However, these same crops removed
from the soil only about 1. million pounds of P20g, About 58 million
pounds were applied to citrus crops in fertilizers, while only 10 million
pounds were removed by the crop. About L.1 million pounds of P205 were
applied to potatoes and tobacco while only about 2.8 million pounds were
removed in the crops. Thus the total amount of phosphorus applied to these
four crops exceeded the amount removed in the product by some 81.8 million
pounds of P205 annually.
Phosphorus, like nitrogen, nevertheless must be applied as fertilizer
for normal plant growth on most Florida soils. Phosphorus promotes rapid
plant growth and development, hastens maturity and improves quality of
vegetation. A deficiency of this nutrient results in small, abnormally shaped
leaves, with a reddish purple or purple discoloration on the under-side. The
plant will have a poorly developed root system with but few lateral branches.
The determination of available soil phosphorus has been one of the most
popular tests in rapid soil analyses. The amount of phosphorus removed by the
extracting solution is most commonly determined by the ammonium molybdate-
stannous chloride method (Appendix A). While the actual determination is
fairly standard in all laboratories, the extracting solutions vary widely.
Most extracting solutions are satisfactory over a wide range of soil types
as long as they have been correlated with field trials; nevertheless, some
solutions are more satisfactory than others on certain soils. A good ex-
tractant should permit the measurement of small differences of available
phosphorus and these differences should correlate rather well with the actual
level of available soil phosphorus as shown by field experimentation. Many
laboratories, for convenience, use a single extracting solution for all
determinations made. However, some laboratories find it desirable to use a
different solution for extracting phosphorus in alkaline soils than that used
on acid soils, and a different extractant for each nutrient. The Soil Testing
Laboratory of the Soils Department, Agricultural Experiment Station, uses a
single extractant based on Morgan s solution. Ammonium acetate (buffered at
pH 4.8) is used instead of sodium acetate in.order to minimize the interference
with the potassium determination on the flame photometer. This solution
appears to work very satisfactorily for extracting phosphorus on sandy to
loamy fine sands. It is less satisfactory on heavier soils since the small
quantities of phosphorus extracted cannot easily be correlated with the actual
levels of available phosphorus in the soil. However, ammonium acetate is
probably as satisfactory as any single extractant for the widely varying soil
conditions found in Florida,
A comparison was made between several widely used extracting solutions
and ammonium acetate (pH 4.8). Soils from established experiments at the
North Florida and West Florida Stations were extracted by the (1) normal
laboratory procedure as well as with the (2) strong Bray solution (0.03 N
NH4F in 0.1 N HC1), and (3) weak Bray solution (0.03 N NH4F in 0.025 N HC1)
both in a 1:10 ratio of soil to extractant, and (4) Truog solution (0.002 N
H2SO4 buffered to pH 3 with (NH0)2SO) in a 5:200 ratio of soil to solution.
Previous studies on certain Florida soils had indicated that the strong
Bray solution extracted about 50 percent more than the weak Bray solution,
and about five times as much phosphorus as the Truog extractant. Unfortunate-
ly, these ratios do not hold under all conditions, since strong acid solutions
tend to extract a larger percentage of phosphorus than the weak solutions from
soils containing large amounts of this nutrient. The Truog extractant more
nearly characterized the available soil phosphorus in sandy soils of low
phosphorus fixing capacity than the Bray solutions. However, with red and
yellow soils of West Florida, the weak Bray solution was best when the soil
contained a low quantity of residual phosphorus (around 50 pounds P205 per
acre). When the residual phosphorus was over 200 pounds P205 per acre, the
strong Bray extractant gave best results on these soils.
On a Ruston fine sand planted to pasture with varying levels of residual
phosphorus, the ammonium acetate extracted about one third as much as the
weak Bray and about one sixth as much phosphorus as the strong Bray from soils
containing less than 50 pounds per acre of ammonium acetate extractable phos-
phorus. Vhen the amount of residual phosphorus exceeded 50 pounds per acre,
the proportions extracted by the Bray solutions were much greater.
In other studies using radioactive phosphorus, in which the "A" value
was calculated as the level of available soil phosphorus, the ability of the
several extracting solutions to remove amounts of residual phosphorus equiva-
lent to the "A" value was determined. The "A" value is calculated from the
equation: percent of phosphorus taken up by the crop that is supplied from
the tagged phosphorus, is to that coming from the soil reserve, as the amount
of tagged phosphorus applied to the soil is to the amount of available soil
phosphorus. (In other words, if 150 pounds of radioactive phosphorus are added
to the soil and it is determined that 75 percent of all of the phosphorus taken
up by the crop came from the tagged phosphorus, then 25 percent must come from
the soil. Since the 75% was derived from 150 pounds, then the 25 percent must
be derived from 50 pounds of available soil phosphorus.)
As will be pointed out later, only about one sixth of the added phosphorus
actually becomes available to the first crop. Consequently, it seems logical
that the "A" value should be divided by the fertilizer efficiency in order to
get the true picture of the ability of a soil to supply phosphorus. Some
typical comparisons on Greenville fine sandy loam are given:
"Pounds/acre P20O in soil :
S : Strong: Weak : : Ammonium : "A" Value
: Total : Bray: Bray : Truog : Acetate :
: 85 : 327: 191 : 39 : 6.0 : 63
:102 : 56 : 29 : 58 : 9.3 : 81
:1260 : 900: 553 : 123 : 15.3 : 130
:2290 : 1482 : 1050 : 207 : 27.3 : 167
The data given above would indicate a closer correlation between the
Truog extractant with the modified "A" value; however, on sandy soil the
ammonium acetate extractant gives a rather close correlation with this value.
"Threshold" values for the ammonium acetate extractant will be discussed
in a later section; however, some values for interpreting the level of phos-
phorus extracted from Florida soils with other solutions have been reported.
For instance, for pastures there is an indicated need for additional phosphorus
fertilizer when the test shows less than 23 pounds of P205 per acre extracted
with dilute H2SO4 (Truog) or less than 80 pounds per acre of P205 by the
weak Bray method. On the acid flatwood fine sands with a sodium acetate
extractant the value of 12 pounds of P205 indicates a definite need for addi-
tional fertilizer phosphorus on grass, while a value of 28 or above indicates
no need for additional phosphorus.
One might conclude that if one extracting solution is to be used for the
determination of all nutrients on the variety of soils that exists in the state,
ammonium acetate is probably the best solution. However, if one is dealing
with only the heavier mineral soils, then perhaps the Truog extractant wuld
be more satisfactory for phosphorus. The ammonium acetate could still be
used for the extraction of cations on these soils.
Fertilizer materials may contain one or more of the following chemical forms
of the calcium compounds of phosphoric acid:
1. Mono-calcium phosphate (CaHL(PO4)g) is soluble in water and readily
available to growing plants. About 80 percent of the phosphoric acid (P205)
in 20 percent superphosphate is in this form.
2. Di-calcium phosphate (Ca2H2(PO4)2) is classed as available to plants,
although it is not soluble in pure water. It is soluble in weak acids and
is often referred to as "reverted" or "precipitated."
3. Tri-calcium phosphate (Ca3(P04)2) is found in bones and is insoluble
in water. It can not be used readily by growing crops. In acid soils it
slowly breaks down by chemical action to a form that may be used by plants.
Rock phosphate, also known as floats, is adapted for use on acid soils and
because of its slow availability, its use is largely confined to pastures,
All forms of phosphoric acid are strongly held by the soil against losses
by leaching, unless the soil is one of the strongly acid white or gray flat-
wood sands. Moderate liming largely corrects this condition. This retention
of phosphoric acid has resulted in a build-up of available phosphorus on some
soils after years of heavy fertilization. Liming of acid soils of West Florida
may increase the availability of the residual phosphorus, due to conversion
of relatively insoluble iron and aluminum phosphates in the soil into more
available calcium phosphates.
Sulfur is often deficient in Florida soils and would be one of the criti-
cal elements for plant growth if it were not supplied in superphosphate.
Continued use of rock phosphate or high analysis triple superphosphate, which
contains little gypsum (CaS04), will eventually result in a sulfur deficiency
unless sulfur is otherwise supplied to.replace that removed by crops.
The supply of potassium in soils is usually higher than nitrogen or phos-
phorus; although the available potassium in cultivated soils is generally low.
The potassium content of soils is normally more closely related to soil type
than to past fertilization* However, potassium is known to accumulate when
high levels of fertilization are practiced. Potash is held in the soil in
much the same way as ammoniacal nitrogen. In sandy soils it may move down
out of the root zone of shallow rooted crops to some extent, but apparently
much of it can be recovered by deep rooted plants. The higher the clay con-
tent of the soil the less likelihood there is of leaching of potash.
Potash deficiency symptoms of crops have been fairly well established.
However, these symptoms cannot always be relied on as a guide to the fertility
status of the soil since other environmental factors, such as drought and
insect and disease damage, may mask the symptom. Deficiency of potash causes
a burned effect along the edges of leaves which spreads from the outer edge
inward to the center of the leaf, On soft leaved plants such as tobacco,
tomatoes and potatoes, it appears as brown spots in the leaf and on legumes
it begins as yellowish-colored spots along the edges of the leave and then
spreads to other parts of the leaf. The lower or older leaves of a plant are
first to exhibit these symptoms as the potassium is transferred from the older
to the young tissue.
While deficiency symptoms and tissue tests may be useful in diagnosing
potassium needs of particular crops especially perennial plants they can-
not replace the function of a soil test. Actually the hunger sign may not
show up until it is too late to correct the cause. A tissue test conducted
in a central laboratory may encounter the same difficulty due to time con-
sumed in transit.
The chemical soil test for potash can be a valuable aid in soil management
when calibrated with field trials. However, due to the mobility of this
nutrient in sandy soils, the test can be more easily correlated with crop
yields on heavier soils than on.light soils. In other words, the relation
of quantity of potash extracted by a solvent to that taken up by a plant may
vary considerably with the kind of soil. Furthermore, the potash requirement
for different kinds of crops is particularly variable. With heavy applications,
such as with vegetable crops, luxury consumption of potash may occur.
Until the recent adaptation of the flame photometer for measuring the
amount of potassium in the soil extracts, the sodium cobaltinitrite method
was widely used for this test. The latter method has many limitations while
the flame photometer is rapid, simple and the potassium may be determined
directly in the ammonium acetate extracts. This extracting solution appears
particularly suited for measuring exchangeable potassium in soils as the am-
monium ion depresses the conversion of nonexchangeable potassium into the
exchangeable form and affords a more accurate measure of the latter.
The potash salts used in fertilizers are water soluble and are, therefore,
quickly available. The important sources of potash are as follows:
Source Percent K20
Muriate of potash (KC1) 48 to 62
Sulfate of potash 48 to 52
Sulfate of potash-magnesia 22 to 27
Manure salts 22 to 32
Nitrate of soda potash 12 to 14
Nitrate of potash Cir. 45
Cottonseed meal Cir. 2
Tankage i to 15
Tobacco stems 5 to 9
There is little difference in the various sources ofsalts, although the
last three materials listed are organic and may be more expensive on the unit
basis than the inorganic forms. Muriate is the cheapest source available
because of its high potassium content, but the sulfate and other forms are
just as effective. There is some objection to the use of muriate because of
chloride effects on quality, especially on tobacco. The effect on other crops
is not considered serious. Sulfate of potash-magnesia is often used when
readily available source of magnesium is desired in addition to the potash*
Other plant nutrients, essential to plant growth but not usually found
lacking in soils are copper, manganese, zinc, boron, iron and molybdenum.
One or more of these elements may be found limiting to plant growth in cer-
tain Florida soils, such as the highly leached sands, peats and mucks. How-
ever, they are needed in very small quantities and in limited areas. No re-
liable rapid test now exists for measuring the available level of these plant
nutrients in the soil. Nevertheless, as the use of minor elements becomes
widespread it is probable that rapid chemical tests will be devised for these
nutrients. Excesses as well as deficiencies of certain of these elements
will probably be tested for as a standard procedure when a need for this
service becomes generally recognized.
As previously mentioned, deficiency of certain minor elements may be
avoided by exercising restraint in liming to prevent the pH of the soil rising
above 6.0 to 6.2.
Total Soluble Salts:
Severe plant damage can result from high concentrations of soluble salts,
usually chlorides and sulfates. Troubles are most common in spots where
underground water or sources of irrigation water with a high content of
chlorides or sulfates have resulted in salt accumulations. High concentrations
of salts from over-fertilizing sometimes occur in greenhouses and vegetable
fields and on lawns and shrubs. When this happens, plants have trouble
absorbing sufficient water, especially in hot weather when rapid transpira-
tion and high concentrations of soluble salts are often encountered, resulting
in physiological drought. Failure of seed to germinate is one of the first
The test for soluble salts works on the principle that the higher the
concentration of salts in a given solution, the greater will be the conduction
of an electrical current between two poles submerged in the soil solution.
The determination of total soluble salts is commonly made with a solu-bridge
(see Appendix A). The reading, like that from the potentiometer, must be
corrected for temperature.
Most general crops will make normal growth, with sufficient water and
nutrients, etc., with a total soluble salt concentration in the soil up to
500 ppm. The more salt tolerant plants such as cotton, alfalfa, sugar beets,
cereals, sorghums, Hubam clover, Bermuda grass and celery will make normal
growth in concentration of soluble salts up to 1000 ppm. Above this level
restricted plant growth, poor germination and yields can be expected for all
species. These values are for the heavier sands. For light sands low in
organic matter, less salt can be tolerated. The limit for salt tolerance is
also lowered considerably when the soil moisture content is decreased to near
the wilting percentage.
INTERPRETATION OF SOIL TEST RESULTS
The three major problems connected with rapid chemical analyses of soils
are (1) that of securing an adequate sample representative of the area
(2) measuring the "available quantities of the plant nutrients, and (3) in-
terpreting the results of the chemical analysis of the sample. The first is
primarily a matter of education of the person taking the sample to the pre-
cautions that must be observed to insure that the pint of soil sent to the
laboratory truly represents the fertility conditions of the field or lawn
sampled. The second phase becomes routine after reasonable laboratory tech-
niques have been set up to insure that the methods adopted will result in
minimum errors with the equipment available* The third step, interpretation
of results, is the most difficult and is the one that some scientists feel
has not been adequately solved to make rapid soil testing a worth-while tool
in soil management. The interpretation of results involves (a) the calibration
of laboratory test data in terms of crop response to fertilization in the
field and (b) their interpretation considering not only the soil test values
but all other factors that may affect the results.
In calibrating laboratory results with yield response to fertilizers at
this Station all fertility experiments conducted in recent years by the Soils
Department, in which prefertilization soil samples were taken, have been sum-
marized. The data from these trials have been calibrated with laboratory
tests where possible. A number of factors limited the usefulness of these
data for this purpose. They included (a) poor yield due to adverse climate
or disease and insect problems, (b) lack of adequate samples, (c) changes in
analytical procedures, and (d) plant growth limited by deficiencies of nutri-
ents other than the one under study. In order to include a wider range of
soil types and crops than was represented by these past experiments, a series
of cooperative fertilizer trials was initiated in 1954 with field crops in
North and West Florida, specifically to calibrate soil tests. These 64 trials
with corn, peanuts, flue-cured and shade-grown tobacco were conducted on
privately-owned farms cooperatively with the Extension Service*
One of the objectives of summarizing the results of past experiments
was to set up "threshold" value as a base for interpreting the soil test data.
Tentative values have been established for many conditions and are given in
Appendix B. These values will be modified as more complete information in-
dicates the need,
"Threshold" or base values, as used herein, refers to the level of the
available plant nutrient, extracted with ammonium acetate (pH 4.8), present
in those pre-fertilized soils producing maximum economic yields when using
current area fertilizer recommendations. In other words, it is the level
of nutrients in the average soil of the area, before fertilization and planting,
upon which the general recommendations are based. This assumes that area
recommendations are based upon average conditions of fertility, moisture and
growth factors for the region; and that such recommendations are designed to
produce the maximum economic yields under those conditions. These applications
could be called maintenance amounts, since they are generally not aimed at
depleting nor building up the fertility level of the soil, but rather to add
approximately the quantity of nutrients needed to sustain good yields.
Area recommendations are based upon broad average conditions and have
their greatest usefulness in localities where research results are insuffici-
ent to meet the need of the individual grower, instead of the average grower.
While the field-to-field variation within an area, due to differences in soil
type, past management, etc., limits the value of general area recommendations,
it is under these conditions that soil testing proves particularly valuable.
If all soils within an area were uniform, soil testing would have little ap-
peal, However, both systems have their strong points and until more accurate
calibrations of the soil test are obtained the soil test results for the
major nutrients may be used satisfactorily to modify the general area ferti-
lizer recommendations on individual fields. When a particular soil tests
higher or lower in a nutrient than the average for the area (threshold level),
then those results should serve as a basis for modifying the area recommendation
to assure maximum yields. It can also be used to correct the soil back to
that "average" condition, when practical, so that the area recommendation can
again be followed.
The so-called "threshold" value serves as a basis for comparison, indicating
the need to raise or lower the application of plant food. It does not indicate
the magnitude of the change. Amount of added fertilizer needed for maximum
yields depends not only upon the level in the soil, but also upon (a) the
requirement of the particular crop and (b) the efficiency of the fertilizer
material in that particular soil.
Plant Food Content of Crops:
A look at the plant food contained in an acre of an average-yielding crop
gives an insight into the ability of present applications to meet the needs of
soil to sustain good crop yields. In other words, if the fertilizer application
does not supply as much plant food as that removed by the harvested crop and
lost by leaching, then the soil is being depleted and will eventually result in
lowered yield unless corrected. The approximate quantities of major plant foods
contained in an acre of several crops are listed below (above ground portion
: : : Pounds per acre :
: Crop : Yield per acre : N : P205 : Kp0 :
Corn 65 bu. 95 35 70
Tobacco 1500 lbs. 80 20 115
Cotton 500 Ibs. (lint) 65 25 50
Soy beans 25 bu. 125 hO 60
Peanuts 2000 Ibs. 85 15 50
Oats 50 bu. 50 20 45
Potatoes 300 bu. 125 35 170
Tomatoes 10 tons 100 35 175
Celery 350 crates 80 65 235
Cabbage 15 tons 100 25 100
Oranges 600 boxes 90 30 130
Sweet clover 5 tons 185 45 165
Ladino clover 2 tons 145 21 73
These data consider the nutrients contained in the whole plant, not just
those removed from the soil in the harvested crop, since it is important to
know how much of the nutrients must be available to the crop if good yields
are to be obtained. They also help to point out why different crops growing
on the same soil must be fertilized differently for best results. Other
reasons besides variations in plant food content are differences in the feed-
ing power of the plant for the nutrients in the soil and the market value of
the crop. (It might be pointed out that heavy fertilization promotes luxury
consumption of some nutrients.)
For example, a 65-bushel corn crop contains an average nutrient content
of 95 pounds of nitrogen, 35 pounds of phosphorus, and 70 pounds of potash*
A recommended fertilizer application for corn of 600 pounds of L-12-12 plus
66 pounds of nitrogen contains 90-72-72 pounds of these nutrients. It is
plain that more phosphorus is being applied to the soil than is taken up by
the plants. This is necessary since a large part of it is "fixed" in the
soil. Consequently, as mentioned earlier, phosphorus tends to build up in
soils under continuous fertilization. (Since all crops remove about the same
amount of phosphorus on an acre basis, this build-up of residual phosphorus
is particularly important in vegetable, fruit and other heavily fertilized
crops.) On the other hand, the application of nitrogen and potassium barely
meets the needs of the crop. The organic fraction of the soil must be counted
upon to make up any difference between that needed for plant growth and that
contained in the fertilizer. However, the reserves of soil potash are depleted
after a few years of cropping and since present recommended applications
normally supply only about that amount taken up by the crop, it is little
wonder that our soils need more and more potash for maximum plant growth.
As interesting as the plant food content of crops may be, it is unfortu-
nate that the difference between the available nutrients of the soil and that
taken up by the crop can not be used alone as a guide for fertilizer applica-
tion. A large part of the added fertilizer material is lost by leaching or
erosion or fixed into a form not immediately available, before it can be
taken up by the crop.
The phosphorus content on the basis of acre-yield of most crops does not
vary greatly and is about 20 to 40 pounds per acre. However, their content
of nitrogen and especially of potassium varies widely. The high valued and
intensively cultivated crops contain 115 to 200 pounds of K20 per acre of
crop while the extensively cultivated crops generally contain 40 to 70 pounds.
of K20 per acre,
The percentage of fertilizer material added to a soil that is recovered
or taken up by the current crop is the measure of efficiency of that material.
This measure may be used as a guide to the amount of material that must be
added to correct a soil deficiency. More specifically, it may be expressed
as the pounds of plant food that must be supplied to a soil to bring it up
to the threshold value,
Nitrogen fertilizers applied in the drill or broadcast on sod crops are
fairly efficient on all soil types. Placed so that it can be readily taken
up by the plant, little nitrogen may be lost by leaching. It is true that
organic sources of nitrogen are slowly available and are less likely to be
lost by leaching during heavy rains than are inorganic sources, just as
mineral nitrogen is more subject to leaching on light soils than on heavy soil.
However, under a system of applying the material when needed by the plant in
the drill and as a side-dressing an average efficiency of 60 to 70 percent
may be expected under most conditions. In other words, for every pound of
nitrogen needed by the crop, above that supplied by the soil, about 1.5 pounds
of nitrogen must be applied as fertilizer,
The percentage uptake of phosphorus by the crop from a current application
of fertilizer is generally much lower than that of nitrogen. Here the picture
is complicated by fixation or reversion of the phosphorus.
Through the use of radioactive pho shorus applied to soils containing
varying levels of residual phosphorus (1, the efficiency of superphosphate
applied to Marlboro fine sandy loam was found to `be influenced by the follow-
ing factors: (a) level of the residual phosphorus in the soil, (b) rate of
application of superphosphate, and (c) to a certain extent the type of crop,
i.e., sod or row crop. The higher the level of residual phosphorus the lower
was the percentage of uptake of the applied phosphorus by the first year's
crop. However, the differences in efficiencies were not significant at the
lower levels of residual phosphorus, (levels normally encountered in those
The first cutting of fescue grass contained 18, 14., and 12 percent of
the superphosphate applied at the rates of 50, 100, and 150 pounds per acre
of P205. The second cutting contained only slightly less than the first
The percentage of phosphorus in Ladino clover from a current application
of 50 pounds of P205 was about 25 on Marlboro fine sandy loam. Liming with
1000, 2000 and 3000 pounds per acre of dolomite did not appreciably affect
the uptake of superphosphate. Rye grass took up about 25 percent of a 30
pound application when the residual level was low (250 pounds of P205 applied
the previous year) but decreased to 19.4 when the residual was from 500 pounds
of P205 applied the previous year.
The data on the influence of soil texture on the efficiency of fertilizer
phosphorus are not- conclusives However, it appears that these differences in
utilization of phosphorus applied to soils low in residual phosphorus but
varying in texture is not great during the first year.
Superphosphate was about four times as available as rock phosphate when
equivalent amounts of P205 were applied to soils of Western Florida as well
as flatwood sands
Oats, grown in the greenhouse on different soils, received from 22 per-
cent (on fine sands) to 48 percent (on fine sandy loams) of their phosphorus
from current applications of superphosphate at the first cutting but only
18 percent on the light soils and 16 percent on the heavy soils by the third
(1) Soil Science 74: L09-411 (1952).
It would appear that with recommended applications of 50 to 70 pounds of
P205 on field crops and pastures growing on soils with the normal phosphate
reserves, that about one fourth of the phosphorus taken up by the plants comes
from the current application. This amounts to about 10 pounds per acre for
most crops and gives an efficiency of about &8 percent. This figure agrees
with results at the North Florida Station k where it was found that about
six pounds of P205 as superphosphate was needed to raise the level of available
phosphorus in the soil by one pounds
The uptake of potassium from normal applications of potash fertilizers
has been found to approach 100 percent when applied to pasture crops. However,
its efficiency under row crops probably more nearly equals that of nitrogen.
DISCUSSION AND SUMMARY
As previously pointed out, the "threshold" values and fertilizer effi-
ciencies are tentative and subject to change as more complete data are
obtained. The search for improvement of these values is a continuing one*
However, it would be unrise to fail to make the fullest use of these present
values on the basis that future values will be more accurate.
The results of soil tests may be used in making fertilizer recommendations
for individual farms if they are obtained from testing adequate samples and
are interpreted with the aid of calibrated experimental data. The data needed
for interpreting the test results are (a) "threshold" values, (b) plant food
requirements of crops and (c) efficiencies of the fertilizer material. These
values have been explained and in the following example will serve to demon-
strate their application: the analysis of a sample of sandy loam soil from
a field to be planted to corn indicated that it contained 7 pounds of "avail-
able" P205 and 60 pounds of K20 per acre and low nitrate nitrogen.
1. Since the phosphorus level in the soil is 5 pounds of P205 below
the "threshold" level for that nutrient, an application of six times this
amount, or 30 pounds of P205, would be required in addition to the recommended
application. The additional application of 150 pounds of 20 percent super-
phosphate would have to be made to bring the soil up to the "threshold" level.
This amount of superphosphate is calculated on the basis of an efficiency of
18 percent for the added phosphate (6 x x 100 150). Soils containing
(2) Report of Progress, Project 28, 95
(2) Report of Progress, Project 428, 1954*
phosphorus at (or within experimental limits of) the "threshold" level are
fertilized on the basis of the general area recommendation. Consequently,
the soil in this example should receive 150 pounds of superphosphate, in
addition to the regular fertilizer, in order to raise its phosphorus level
up to that value.
2. The potassium in this example is also lower than the "threshold"
level. An application of 60 pounds of K20 or 100 pounds of KC1 would be needed
in addition to that called for in the area recommendations. This assumes
that 1.5 pounds of K20 must be added for each pound needed by the crop, and
therefore: 15, x 0 100 100.
60 x 100 100*
The application of nitrogen should follow the area recommendation, unless
the crop follows a good green manure crop.
Moisture is often a limiting factor in crop production in Florida. Since
fertilizer recommendations for field crops are made for non-irrigated fields,
the fertilizer application should be increased when irrigation is contemplated.
The same method of calculation may also be used in determining fertilizer
applications for soils testing higher than the threshold level. (However,
care should be taken in reducing the potash application unless the fertiliza-
tion history of the field indicates a true reserve has been built up.) For
soils testing high in only one nutrient, it is probably more practical to
change fertilizer ratios. This method may also be used for soils testing
low in only one nutrient. Some suggested changes in ratios are given in
Appendix "C". Of course, if a soil tests higher or lower in both phosphorus
and potassium, it may be practical to just lower or increase the rate of
the normally recommended ratio
Herman L. Breland
The following procedures are presently used by the Soil Testing Laboratory,
Gainesville, for the determination of pH and the available nutrients.
The hydrogen ion concentration is determined with a glass electrode and
potentiometer. Particular care must be taken with light sandy soils and other
weakly buffered soils to avoid alteration of the pH during the preparation of
the sample and suspension.
The soil pH is determined by placing a 50 ml. beaker full of air-dry soil
in a 150 ml. beaker, adding 100 ml. of distilled water, and stirring thoroughly,
allowing it to stand for one hour (two hours for peat), stir again and immediately
determine the pH by means of a glass electrode.
A single leaching solution of ammonium acetate (pH 4.8) is being used at
the present time for all the routine chemical determinations.
A five gram soil sample is weighed and placed in one of the bottles in
the extraction rack; then 25 ml. of ammonium acetate, pH 4.8, is added and the
rack is placed on the shaking machine for 30 minutes. At the end of the 30
minutes shaking period the samples are filtered and the leachate saved for the
chemical analysis. Each extraction rack holds 10 bottles and the shaking machine
holds 5 racks; therefore, the soil samples are usually analyzed 50 at a time.
Add 1271.3 ml, concentrated acetic acid, 8606 ml. concentrated ammonium
hydroxide, and dilute to 18 liters. This should give a pH of approximately
4.8. This pH is carefully maintained at this value.
A 5 ml. aliquot of the leachate is transferred to a 50 ml. volumetric and
diluted with about 25 ml. of distilled water. Then add 5 ml. of the ammonium
molybdate, 1 ml, of the dilute stannous chloride, and enough distilled water to
make up the volume. Shake immediately. The resulting blue color is then read
using a Cenco Colorimeter with a red filter.
Ammonium Molybdate Sulfuric Acid Solution
Dissolve 25 grams of ammonium molybdate in 200 ml. of distilled water heated
to 600GC and filter. Dilute 280 ml. of arsenic and phosphorus free concentrated
sulfuric acid (H2SO, approximately 36 N) to 800 ml. (acid to water). After
both solutions have cooled, add the ammonium molybdate solution slowly, with
shaking, to the sulfuric acid solution. After the combined solution has cooled
to room temperature make up with water to exactly 1000 ml. This is a 10 N
sulfuric acid solution containing 2.5 grams of ammonium molybdate per 100 ml*
Stannous Chloride (Stock Solution)
Dissolve 25 grams of stannous chloride (SnCl2*2H20) in concentrated hydro-
chloric acid (HC1) and dilute to 100 ml. with concentrated HC1. This stock
solution is stable for a considerable length of time when stored in a dark place.
Stannous Chloride (Dilute Solution)
Take a 5 ml aliquot of the stock solution, add 5 ml. of concentrated HC1,
and dilute to 100 ml. with distilled water. Use exactly 1.00 ml. of this
solution in 50 ml. of the solution in which the color is to be developed.
A standard phosphorus stock solution can be made by drying KHPPO4 in an
oven at 105oC., weighing out 2.196149 grams of the material, and placing it into
a liter volumetric flask. The flask is then made to volume with distilled
water. This solution then contains 500 parts per million of phosphorus. Then,
by proper dilution, standards of varying concentrations can be made for a
suitable standard curve as follows:
PPM of M1. of 500 PPM Final
Standard Stock Solution Volume Ml.
0 0 500
Calculations: PPM of P found X dilution factor X 2 X 2.2912 = lbs. P205
per acre (assuming a mineral soil weighs 2,000,000 lbs, per acre six inches).
The potassium is determined by means of a Beckman Model B. Flame Photometer.
The values obtained from running a series of potassium standards of known
concentration, at a wavelength of 7680 angstroms, are plotted to make a standard
curve. The potassium content of an unknown sample can then be determined by
measuring the percent of light transmitted and then reading the potassium concen-
tration from the standard curve*
The potassium standards are made with potassium chloride dried at 1050C
A concentrated standard containing 1000 parts per million of potassium is made
by weighing 1.9069 grams of dried potassium chloride into a 1000 ml. volumetric
flask and making to volume with the ammonium acetate (pH 4.8) leaching solution*
The standards for the working curve are then made by proper dilution as
I1. of 1000 PPM
Calculations: PPM of K in sample X dilution factor X 2 X 1.2046-lbs. K
as K20 per acre six inches (assuming 2,000,000 lbs. of soil per acre six inches).
The same instrument and procedure used in the potassium determination is
used for calcium. A wavelength of 4227 angstroms is used for the calcium
place into a 1000
PPM of calcium.
The calcium s
grams of calcium carbonate that has been dried at 1050C and
ml. volumetric flask. The flask is made to volume with the
(pH 4.8) leaching solution. This stock solution contains 1000
standards for a working curve are then made by proper dilution
Calculations: PPM Ca in sample X dilution factor X 2 X 1.3992 = Ibs. of Ca
as CaO per acre six inches (assuming 2,000,000 lbs. of soil per acre six inches).
The same instrument and procedure used in the potassium determination is
employed for sodium. A wavelength of 5890 angstroms is used for the sodium
Weigh 2.~418 grams of sodium chloride that has been dried at 105oC. and
place into a 1000 ml. volumetric flask. The flask is then made to volume with
the ammonium acetate (pH L.8) leaching solution. This solution contains 1000
PPM of sodium.
The sodium standards for a working curve are then made by proper dilutions
MI. Stock Solution Final Volume Standard
0 0 PPM 250 ml O PPM
2.5 1000 10
Calculations: PPM Na in sample X dilution factor X 2 X 2.5418 = Ibs. Na
as NaCl per acre (assuming 2,000,000 lbs, of soil per acre six inches).
A simple qualitative spot test for magnesium is being used by the laboratory
at present while the Flame Photometer method is being perfected. This is made
by placing .5 drops of the soil extract in a depression of the spot plate,
then adding one drop of the titan yellow solution and five drops of the sodium
hydroxide solution. The resultant color is then compared with that of known
Dissolve 0.15 grams of Titan yellow in a mixture of 50 ml. of methyl alcohol
and 50 ml. of distilled water.
Sodium Hydroxide Reagent
Dissolve 15 grams of sodium hydroxide in 100 ml. of distilled water*
Dissolve 1.766 grams of magnesium acetate, Mg(C2H302)2.H20, in the ammonium
acetate leaching solution and dilute to 100 ml. The following standard magnesium
(equivalent to MgO) solutions are then made by proper dilution: 66 (L), 99 (M),
660 (H), 1992 (VH) pounds per acre.
Since nitrates are considered to be mobile, or easily leached, the amount
present in the soil will vary considerably from time to time. Therefore, a spot
test is used for this determination.
Transfer one drop of the soil extract to the spot plate. Add four drops
of the diphenylamine reagent; let stand for two minutes, stir with a glass rod
and compare the intensity of the resultant blue color with that of a known
Dissolve 0.05 grams of diphenylamine in 25 ml. of concentrated hydrochloric
Dissolve 0.607 grams of sodium nitrate (NaN03) diluted to 100 ml. with the
extracting solution. By proper dilution make standards of the following concen-
trations: 8, 26 and 88 lbs. per acre of nitrate nitrogen.
Total Soluble Salts
The total soluble salts are determined by means of a solu-bridge. Add
200 mls. of distilled water to 100 grams of soil and stir several times during
a 24 hour period. Filter out about 60 ml. into a 100 ml. graduate cylinder
and determine the conductivity.
As the soluble salts of one gram of soil will be in two grams of solution,
the concentration of the solution in PPM will,have to be multiplied by two to
give PPM of salt in the soil.
Purity of Limestone
The sample is ground in a porcelain mortar to pass a 60 mesh seive. Place
0.5 gm. of material in 250 ml. beaker; add 50 ml. of 0.5 N HC1, cover with watch-
glass, and heat slowly just to boiling. Cool, filter and titrate the excess of
acid with the 0.5 N NaOH using phenolphthalein indicator. Calculate and report
the results as percentage of calcium carbonate equivalent. One ml. of 0.5 N HC1
equals 0.025 gm. equivalent of calcium carbonate. The ml. of HC1 x 5 equals the
calcium carbonate equivalent in percent.
Determination of Magnesium in Limestone
After the percent purity of the limestone is determined, make the solution
acid by adding 1 ml. concentrated HC1. Then add 5 gms, ammonium chloride and 10
mls. of 10 percent oxalic acid. Heat to boiling add three drops of brom cresol
green and then add ammonia. to a bluish-green. Digest on hot plate, not boiling,
for one hour and allow to stand over-night, then filter. Wash the filter with hot
water until all trace of the oxalate is removed. Wash precipate into clean beaker
with hot water. Add water to make about 100 mlso and then add.10 nl. of 1:1
H2SO4. Titrate with 0.2 N K hnO4 until purple color is developed and then add the
filter paper and finish the titration. One ml. of 0.2 N K MnO4 equals 0.01 gms.
of CaCO3. Therefore, the number of nis. of 0.2 N K MhnO equals the amount of
CaC03 present. Subtract the quantity of CaC03 from the CaC03 equivalent. The
difference multiplied by 0.843 equals the amount of Mg C03 present. The
quantities of CaCO3 and Mg CO3 may be expressed on a percentage basis.
Tentative "Threshold" Values of Soil Nutrients Extracted by
Ammonium Acetate (pH 468) from Prefertilization Samples.
I. Extensively Cultivated:
General field crops;
Corn, peanuts, small grain,
II. Intensively Cultivated:
Ornamentals, Greens, etc,
Soil Nutrients, Lbs. Per
Pp20 Kp0 CaO
): o20 50 200
: )20 70 300
)12 90 400
: )70 100 t00
: )70 10 500
)50 200 600
General Area Fertilizer Recommendations and Suggested Ratio Changes
for Soils Testing Higher or Lower Than "Threshold" in K20 and Pg05
eral Area Rec Uen&a ion resold erilzr Grdes ertiior Grads
Pounds Per Acre Values for Soils Testing for Soils Testing
Crop Soil Soils ( ls at or Near Threshold Level) PA05-K20 HiGh in P205 or Low in K20 Low in Pg05 or HiIh in K20
I. Extensive Crops: Rates: Grades: Side Dressing Per Acre
as Non-legumes;(Corn, millet, (Sandy loans 5-600# 4-12-12 60# 12 90 **35--12 or 4-10-7 or
(Cotton, and (Drjk sands 4-500# 4-12-12 45# 20 70 4-8-12 or 3-15-9 or
(Small grain (Licht sands 3-400# 4-12-12 50# 20 50 5-9-18 5-18-6
b. Legumes: (Soya, peanuts 35500# 0-14-14 --- 18 90 0-10-20 or 0-16-8 or
(Lupine, indigo 0-8-24 0-15-5
(Sunuy lolsst- 6-800#1 8-8-8 60# 12 90 10-5-10 or 6-12-6
Grass (Dark sands 4-600# 8-8-8 45j 20 70 10-0-10
(Light sands 3-400# 8-8-8 50# 20 50
(Sandy loa:s 7-1000# 0-14-14 --- 12 90 0-10-20 or 0-15-8 or
Legume-grass (Dark ands 5-700 0-14-14 or --- 20 70 0-8-24 0-15-5
(Light sands 5-500# 0-10-20 -- 20 50
testing higher or lower in both P205 and KZ0 would receive correspondingly lowef or higer amounts of the generally recomnaended Cr-de.
low only in P205 or in Kg0 may also be corrected by applications of superphospnate or potash in addition to generally recommended grade,
would depend on degree of deficiency, Rate calculated on nitrogen equivalent,