A Practical Course in Agricultural Chemistry

163 Pages·1967·2.435 MB·English

by  D. W. Gilchrist Shirlaw and J. E. Nichols (Auth.)

A PRACTICAL
COURSE IN
AGRICULTURAL
CHEMISTRY
by
D. W. GILCHRIST SHIRLAW, M.Sc.
φ
PERGAMON PRESS
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First edition 1967
Reprinted 1969
Library of Congress Catalog Card No. 67-21278
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AUTHOR'S PREFACE
THIS book presents practical methods in agricultural
chemistry, which it is hoped will be suitable for university
students taking agricultural chemistry as a subsidiary course,
for students in agricultural colleges, and for students in farm
institutes. It is realized that the needs of these different
groups of students vary considerably, but by judicious selec-
tion the lecturer in charge of each course should find the
material in this book adequate.
In the interest of simplicity the theory behind the deter-
minations has often been omitted; emphasis is placed on
straightforward methods giving reasonably precise results.
In a few cases methods have been shortened, possibly at the
expense of a high degree of accuracy, though this may not be
very serious when it is remembered that most agricultural
analysis is subject to at least a 10 per cent sampling error. The
object is to cater for the type of student for whom the results
and the subsequent interpretation of the results are of
greater importance than the underlying chemistry of the
analysis.
The scope of a practical course must necessarily be deter-
mined in part by the availability of equipment; many of the
methods here described may be ruled out by the initial cost
of the necessary equipment; where this could be the case,
alternative methods have often been given. Nevertheless,
any laboratory that is to run a specialist course in agricultural
chemistry ought to have some specialist equipment. For
example, a flame photometer is today almost essential for the
determination of sodium and potassium, either in the analysis
of soils or of feeding stuffs; some form of colorimeter for the
IX
X AUTHOR'S PREFACE
determination of phosphate is also desirable, though this can
be replaced, either by a comparatively simple colour com-
parator employing glass standards, or by standard solutions
in which the colour is developed at the same time as the
determination is made. Examples of specialist equipment that
can be regarded as essential are: a Kjeldahl digestion rack,
some form of soil shaker, a Soxhlet extraction apparatus, and
at least one water bath that can be thermostatically controlled.
In addition, the laboratory should, of course, possess the
normal range of glassware.
No textbook that deals with a rapidly developing field can
claim to be truly up to date, but it is hoped that the majority
of modern methods, which are suitable for inclusion as
student exercises, have been considered in the text. Two
major developments in agricultural analysis have been made
during the past ten years. The first is the atomic absorption
technique —extremely useful in the determination of metallic
cations; the second is gas chromatography, rapidly becoming
a standard technique in biochemical analysis. Before atomic
absorption analysis can be made an outlay of at least £1200 is
required for equipment. Gas chromatography, however, can
often be carried out with an outlay of little more than £50
though more elaborate apparatus is, of course, desirable.
Methods employing these apparatuses are described, but
alternative methods for the determinations are also given.
The laboratory instructions for the methods are in each case
preceded by a short discussion and instructions for pre-
paring the necessary reagents; only chemicals of an appro-
priate standard of purity should, of course, be used. Follow-
ing the laboratory instructions the data required for calculating
the results are presented; usually a simple formula is given
together with the details of how this formula is derived; in
some cases a worked example is used to illustrate the method.
I wish to express my thanks to Professor Nichols, who read
the manuscript and made many helpful suggestions. My
thanks are also due to my colleague, Mr. A. A. Millar, who
AUTHOR'S PREFACE xi
worked with me in testing many of the newer methods,
especially those for amino acids. I am also grateful to Miss
G. Gibson, who redrew the diagrams from my rough sketches
and to Mrs. D. Grugan who typed and retyped the manuscript.
CHAPTER 1
THE ANALYSIS OF SOILS
1.1. SOIL SAMPLING
Where possible the student should take his own sample of
soil from a field or part of a field known to him and should
use this sample for all the soil analysis with which he is con-
cerned. This is readily possible where a farm is attached to the
institute, college or university. In this case each student can
be allocated one field or a part of the field. Where the fields
are large, or where they show great variability, or where
different areas of the field have been cropped or treated
differently in the past, each of these areas should be treated
separately. Differences in topography are sufficient to justify
the taking of more than one sample. It must be remembered
that if the analysis is to give results truly representative of
the conditions in the field, then the sample itself must be
truly representative of the field. Even when great care is
exercised the standard error arising from sampling has been
shown to be up to ten times as great as the error arising from
the analytical procedures.
The best instrument to use in soil sampling is a cheese-type
auger. When pushed into the ground and turned round
these augers will withdraw a core of soil; if a number of cores
of soil are taken from a given area and well mixed, the soil
sample ought to be representative of that area. At least ten
and preferably thirty such cores should be taken for each
sample, though this number must depend on the size of the
field being sampled. The most satisfactory method of obtain-
ing a number of samples is to walk across the field two or
1
2 A PRACTICAL COURSE IN AGRICULTURAL CHEMISTRY
three times in a zigzag pattern and take one soil core every
fifteen to twenty steps. In some cases where only small areas
of land are being sampled, this may not give a sufficient
number of soil cores; in other cases too large a volume of
soil may result. In the first case more frequent sampling
may be necessary, possibly one sample to every five or ten
paces; in the second case subsequent quartering of the soil
sample will reduce the bulk quantity. By this technique the
soil sample is placed on a sheet of paper, well mixed and then
divided into four sections. Two opposite quarters are dis-
carded and the other two quarters mixed, placed back on the
sheet and quartered again; this is repeated until the volume
of the soil has been reduced to that required.
1.2. THE PREPARATION OF THE SAMPLE
The soil sample should be taken back to the laboratory,
carefully labelled, spread on to a tin box and allowed to dry.
The drying process may be speeded up by placing the soil
sample in an oven at not more than 40°C. It should be
remembered, however, that even air drying will result in some
changes in the sample. This applies in particular to the levels
of nitrate nitrogen, available phosphate and, to a lesser
extent, exchangeable ions.
After drying, the sample is gently crushed in a mortar with
a pestle, care being taken to avoid breaking any stone which
may have been included in the sample. The soil is now sieved
through a 2-mm sieve and that portion passing through the
sieve is bottled, labelled and used for all analysis. This
sample is termed fine earth and for most purposes about
250 g will be sufficient. Careful mixing is essential since only
comparatively small weights will be used in the analysis. An
estimate may be made of the percentage of stone in the
sample; stone being defined as particles over 2 mm in diameter.
THE ANALYSIS OF SOILS 3
1.3. THE MECHANICAL ANALYSIS OF SOILS
A number of different methods for the mechanical analysis
of soils have from time to time been advocated. The simplest
of these, requiring little equipment, is the hydrometer
method. This depends on the fact that the specific gravity of
the soil suspension is proportional to the weight of soil in
the suspension and that the different soil particles will settle
out of suspension at a rate proportional to their size. Thus
the heavier coarse sand particles settle first followed by the
fine sand particles, then the silt and finally the clay.
Hydrometer readings are taken at two different times;
firstly, when it is calculated that all the coarse sand and all
the fine sand will have settled below the level at which they
will affect the hydrometer reading, and secondly, when all
the silt has so settled. The apparatus required depends upon
the number of soil samples to be handled. Where only one
or two samples are to be analysed a soil mixer of the type
supplied by Klaxon Ltd. will prove satisfactory. If a number
of samples are to be handled a reciprocating shaker is essen-
tial. A litre-measuring cylinder is required for each soil
sample and a Bouyoucos hydrometer graduated in terms of
grams of soil per litre of solution is the only other equipment
required.
Reagent required
The soil is dispersed with a 5 per cent solution of Calgon
(sodium hexametaphosphate) buffered to a pH of 8. This is
made by dissolving 50 g of Calgon and 5-724 g of sodium
carbonate in a litre of distilled water.
Procedure
1. 50 g of the fine earth fraction are placed in a shaking
bottle and 25 ml of 5 per cent Calgon together with 200 ml
of water are added.
2. The soil is dispersed either by agitating for 20 min in the
Klaxon mixer or by shaking in a reciprocating shaker for
4 A PRACTICAL COURSE IN AGRICULTURAL CHEMISTRY
8 hr. At the end of this time all the soil aggregates will have
been broken down; i.e. the soil particles will be present as
discrete units.
3. The soil suspension is now decanted into a litre-measur-
ing cylinder and water added to the litre mark. The measuring
cylinder is shaken end over end for 1 min and the time care-
fully noted. If necessary, one or two drops of amyl alcohol
may now be added to break the froth and to make the reading
of the hydrometer easier to take.
4. After standing for 4 min 30 sec, the hydrometer is
carefully placed in the suspension; the reading on the hydro-
meter is noted at exactly 5 min. At this time the silt and the
clay will still be in suspension, but the coarse and the fine sand
will have settled below the level at which they would affect
the hydrometer reading. The reading on the hydrometer is
therefore the weight of silt and clay per litre. A hydrometer
reading is affected by the temperature of the solution and, if
the temperature of the solution deviates from 20°C, it must
be corrected: for every one degree above 20°C, 0-3 of a unit
should be added, and for every one degree below 20°C, 0-3 of
a unit should be deducted.
5. The suspension is now allowed to stand undisturbed for
5 hr. About half a minute before the end of the 5 hr the hydro-
meter is again carefully placed in the suspension and the
reading carefully noted at exactly 5 hr. By this time all the
silt will have settled below the level at which it will affect the
hydrometer reading and the reading will therefore be the
weight of clay per litre. A similar correction for temperature
should be made as before.
6. Most of the suspension is now poured off, care being
exercised to retain all the sand fractions. The remaining sus-
pension together with the sand is carefully washed into a
400-ml beaker which has been marked at the 10-cm level,
i.e. when the beaker is filled to a depth of 10 cm with liquid.
7. Water is added to the 10-cm level and the soil deposit
stirred up; the beaker is set to one side for a time long enough
THE ANALYSIS OF SOILS 5
for the coarse and fine sand fractions to settle. This depends
on the temperature of the solution and is given in Table 1.
For example, at 20°C, 4 min 48 sec must be allowed. After
this time the suspension is decanted, care being taken to
retain the sediment. A complete separation at the first
decantation will not be possible, since the liquid will be found
to be too cloudy.
8. The beaker is again filled with water, the sediment being
carefully stirred up, set aside and allowed to stand for the
requisite time as before.
9. The decantation is repeated until at the end of the
decantation time, the liquid above the sediment is quite clear.
10. The sediment now consists entirely of coarse and fine
sand which may be washed on to a filter paper dried and
weighed.
TABLE 1. THE TIME OF SEDIMENTATION OF FINE SAND AT DIFFERENT
TEMPERATURES
Temperature Sedimentation time Temperature Sedimentation time
(°C) through 10 cm (°C) through 10 cm
Min See Min See
8 6 40 21 4 40
9 6 30 22 4 30
10 6 20 23 4 30
11 6 10 24 4 20
12 6 0 25 4 15
13 5 50 26 4 10
14 5 40 27 4 5
15 5 30 28 4 0
16 5 20 29 3 55
17 5 10 30 3 50
18 5 0 31 3 45
19 5 0 32 3 40
20 4 48 33 3 35
The results so far obtained are the weights of silt plus clay,
clay and sand recovered from the original sample. To obtain
the percentage of silt plus clay, the corrected hydrometer