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THE INFLUENCE OF HEATED SOILS ON SEED GER-
MINATION AND SOIL GROWTH



JAMES JOHNSON

UNIVERSITY- C^
] HESiS {Oii±



Department of HortictiUiire, W'/.STons/?i Agricultural Experirnnit Station



Reprinted from

Soil Science, Vol. VII, No. 1, January, 1919



n. of »>•

JAN 10 1920



.of



Reprinted from Soil Science,
Vol. VII, No. 1, January, 1919



THE INFLUENCE OF HEATED SOILS ON SEED GERMINATION
AND PLANT GROWTH

JAMES JOHNSON

Department of Horticulture, Wisconsin Agricultural Experiment Station

Received for publication January 6, 1918

INTRODUCTION

The use of heat-sterilized soils in methods of research in soil biology and
plant pathology as well as in various phases of practical agriculture is rapidly
increasing in value. For purposes of research especially, the sterilization of
soil by heat is practically the only method which can be relied upon for pur-
poses of rendering the soil sterile as regards any particular organism. Since
the application of heat to the soil may, and usually does, result in important
changes aside from sterilization which may greatly influence plant growth, it
is evident that a knowledge of the alteration's which occur or may be expected
to occur in the soil is essential in order to draw reliable conclusions from cer-
tain types of research conducted with heated soils. The problem is by no
means a new one. A great variety of literature exists upon the subject of soil
sterilization from various angles of attack. Since the subject may be treated
from the standpoint of the soil itself as chemistry, biology, or physics, or from
the standpoint of plants grown in these soils as physiology or pathology, the
field has been a very productive one. At the same time, its complexity has
led to quite widely varying results and conclusions. Although it is evident
that investigations in this field should take into account all factors which may
be involved before drawing far-reaching conclusions, it is also evident that it
is exceedingly difficult to grasp the significance of, and lay proportionate
weight upon all the factors concerned. The results presented in this paper
were first undertaken from a phytopathological point of view. The bearing
of the conclusions drawn on phytopathological research are contemplated in
another paper. It is the purpose of this paper to present the data obtained
on the changes produced in heated soils as measured by their influence on
seed germination and plant growth, and to discuss the probable nature of
these changes.

A clear conception of the subject of soil sterilization requires at the outset
a good understanding of the methods by which soils may be sterilized. These
are: (a) Sterilization by heat, and (b) sterilization by chemicals. The for-
mer may again be divided into methods using (a) live steam, and (b) dry
heat; the latter into methods using (a) volatile antiseptics, and (b) non-
volatile antiseptics. Clearly these methods may be expected to yield funda-

1

SOIL SCIENCE, VOL. VII, NO. 1



2 JAMES JOHNSON

mentally different results, not only in regard to the method employed, but
also in regard to the intensity with which it is applied. Technically the term
sterilization means the complete destruction of all living matter in a medium,
and implies the continued sterility of this medium if desired. This term as
used in connection with the treatment of the soil, however, does not
necessarily mean the complete destruction of all living matter in the soil nor
its continued sterihty. Ordinarily it refers to the destruction of one or
more specific organisms which are not desired in the soil with no special pre-
cautions to prevent reinfestation. In a broader sense, however, the object
of soil sterilization is the production of a more favorable medium for the
growth of cultivated plants by means of heat or chemical disinfection. This
may be brought about as much by the improvement of the chemical, physical,
or biological relationships in the soil, as by the simple destruction of one or
more forms of undesirable organisms. In view of these facts the terms
"partial sterilization" and "pasteurization" have come to be used by some
investigators.

The object of soil sterilization in practical agriculture is, therefore, usually
either the destruction of some particular plant parasite harbored in the soil,
or the improvement of the fertihty of the soil by affecting its chemical,
biological or physical condition. Where it is practiced for the former reason,
the latter results are naturally also secured as secondary effects. Many
farmers are, for instance, coming to regard the process as valuable from the
standpoint of destruction of weed seeds in plant beds alone.

Various methods of appHcation have been devised with the result that the
practice has been increasing rapidly in certain forms of intensive plant culture.
Up to the present time, soil sterilization has been found most useful and has
received its greatest impetus in the culture of plants under glass. The vegetable
forcing house industry particularly, has used the method on a large scale.
Florists use it less commonly, but find it advantageous for certain plants and
for starting seed. In the culture of tobacco, ginseng, coniferous seedlings,
and most seedlings for the market gardener, it has been found well worthy of
use.

BRIEF SUMMARY OF LITERATURE

The beneficial action of heated soils upon plant growth was observed long
before the sterilizing value of heat was known. According to Sir Humphrey
Davy (12) the improvement of "sterile" lands by burning was known to the
ancient Romans, the custom having been mentioned by Virgil in his first book
of the Georgics. It is well known that the burning of various types of soil
became quite general in Europe during the eighteenth century. This method
was accompanied by a considerable amount of scientific investigation upon the
subject, until the practice fell into disuse about the middle of the last century.

The actual use of sterilizing agents upon the soil for the primary purpose of
destroying injurious forms of hving organisms present in the soil is largely a



INFLUENCE OF HEATED SOILS ON GERMINATION .\ND GROWTH 6

development of the past three or four decades. Accompanying these studies
and practices of soil sterilization for the destruction of soil organisms, there
has accumulated a great deal of literature dealing with the scientific principles
involved. These publications have been especially stimulated by a desire to
explain the reason for the increased growth of plants on sterilized soils. The
investigations undertaken from quite different angles of attack and with great
variation in type of soil, kind and intensity of sterilizing agents, as well as in
type of plants used, have quite naturally resulted in widely varying con-
clusions.

A detailed review of the literature concerning soil sterihzation would be too
voluminous to present here. For present purposes it may suffice to present
in brief form an outline of the principal changes produced in and by steri-
lized soils, and to mention briefly certain theories concerning the nature of
the action of sterilized soils on plant growth, leaving the discussion of the
literature more directly concerned with this investigation to be treated under
the separate phases of the subject as they are taken up.

Principal changes produced in and by sterilized soils.

I. Destruction of life.

A. Normal soil flora and fauna, desirable and undesirable forms of bacteria, fungi, pro-

tozoa, and higher animals.

B. Plant parasites, especially pathogenic bacteria, fungi, nematodes, and injurious

soil infesting insects.

C. Propagalive organs of higher plants, especially weed seeds.

II. Immediate chemical action (formation of toxic and beneficial compounds).

A. Decomposition of organic material resulting in formation of ammonia, carbon diox-

ide and various new and complek organic compounds.

B. Decomposition of inorganic material, reduction of nitrates and nitrites to ammonia

and increased solubility of potassium, phosphorus and other salts.

III. Bio-chemical action.

A. Increased ammonification particularly and modified nitrification, denitrification,
and nitrogen fixation.

IV. Physical action.

A. Absorptive capacity of soil modified for water, gases, and salts.

B. Increased concentration of soil soltdion.

C. Modified capillarity, colloidal state, and mechanical condition.

D. Modified color and odor.

V. Action on organisms growing in sterilized soils.

A. Lower organisms.

1. Increased development due to reduced competition, increased food supply,

destruction of "bacterio- toxins," "stimulation" by products added or
formed, or other causes.

2. Retardation in growth in rare cases due to injurious conditions produced.

B. Green plants.

1 . Injurious action as indicated by retarded rate and percentage of seed germi-

nation and by retarded rate of plant growth.

2. Beneficial action as shown by increased rate and percentage of seed germina-

tion and increased rate and amount of plant growth.

3. Modification in form, color, and other "qualitative" changes.



4 JAMES JOHNSON

The theories which have been promulgated by various investigators rest
primarily upon the explanation of the beneficial and injurious action of ster-
ilized soils upon plant growth. These theories are partially interrelated and
do not permit of satisfactory classification, but the main ones may be grouped
as follows:

1. Stimulation hypothesis. Koch's (31) theory formulated in 1899 was
probably the first attempt at scientific explanation of observed facts. He
believed the sterilizing agent or its products directly ''stimulated" the growth
of the plants or soil bacteria, which in turn influenced plant growth.

2. Modified bacterial activity. Hiltner and Stormer (25) in 1903 showed by
soil bacterial counts that sterilization resulted in an initial decrease in numbers
of organisms followed by a marked increase in numbers, and hence an increase
in the productiveness of the soil.

3. Protozoan theory. Russell and Hutchinson (62) believe that the benefit
from "partial sterilization" may result from the destruction of the larger
phagocytic microorganisms (mostly protozoans) which inhibit the develop-
ment of the beneficial organisms. The protozoa being destroyed, the am-
monifying bacteria, for instance, increase rapidly producing an increase in
nitrogen and hence plant growth.

4. "Bacierio-toxin" theory. Grieg-Smith (22) considers that certain sub-
stances which he calls "bacterio-toxins" and which exist naturally in soils,
inhibiting bacterial growth, are destroyed by sterilizing agents.

5. Modified organic soil compounds. Schreiner and Lathrop (65) believed
that certain complex organic soil constituents produced as a result of sterili-
zation by heat, may increase plant growth while others inhibit it. That or-
ganic soil constituents of an indefinite nature were produced which were in-
jurious to plant growth in heated soils had also been suggested by Struckman
(73), Dietrich (13) and Pickering (52).

6. Modified inorganic soil compounds. This theory though supported by
no investigators in particular should be added here since it has been repeat-
edly shown since the time of Struckman (73) that increase in inorganic plant
food constituents occurs in heated soils. The theory is perhaps best supported
by Liebscher (40) who believed that sterilization was essentially nitrogenous
fertilization.

7. Plant parasite theories. The fact that in certain soils the benefit of soil
sterilization may be due largely to the destruction of parasitic organisms is un-
questionable. The wide appHcation of this theory, however, to the subject
in question, as supported perhaps most energetically by Bolley (5), serves to
place this type of response to sterilization among the theories explaining the
action of sterilized soils.

8. Physical theories. These theories are not subscribed to by any author
in particular at the present time, although it was quite generally believed at
one time that all the benefit derived from burning the soil was due to purely
physical changes. Some of the physical factors which play a part in soil fer-



INFLUENCE OF HEATED SOILS ON GERMINATION AND GROWTH 5

tility are, however, coming to be regarded as very influential in conjunction
with chemical factors. Seaver and Clark's (67) "concentration theory" may,
for instance, properly be placed here.

For a more detailed summary of the subject of soil sterilization especially
as regards the relation of protozoa to sterilized soils, the recent review of
Kopelofif and Coleman (34) should be consulted.

EXPERIMENTAL WORK

The investigations upon the subject of soil sterilization were begun by
the writer in 1909 with the object of studying methods for the control of
the damping-oflf disease of plants, caused by Pythium debaryanum, and
Rhizoctonia. At the outset, it became evident that when soils were treated
either with heat or chemicals, various "secondary effects" of sterilization oc-
curred, which modified the soil and plant growth aside from the control of dis-
ease. Following a publication upon the control of damping-off in plant beds
(27), the study of these "secondary effects" was undertaken. The primary
object of this study was to attempt to find an explanation for the injurious
action followed by the beneficial action of steriHzed soils on plant growth. On
account of the size and complexity of the problem, it became necessary to
limit the investigation primarily to the effects of sterilization by heat. The
studies up to date have been mainly concerned with the action of heated soils
upon seed germination and plant growth; although various other phases have
been taken up from time to time with the general idea of rounding out the
problem in such a way as to make it of value also in the plant pathological
investigations carried on simultaneously. In carrying on this work the soil has
in many cases been heated to a degree far above that which is used in ordinary
sterilization. While no apology may be necessary for such procedure from a
chemical standpoint, the general idea has been to increase the action so that
there would be produced a sufficient magnification of effect to obtain a good
theoretical working basis for the explanation of similar results secured at or-
dinary temperatures of sterilization. While comparisons cannot readily be
made with the "partial sterilization" of Russell and his associates, (62, 64),
yet it is hoped that some evidence of value in this connection may be gathered
from the results. The work presented here is perhaps more comparable to
that of Pickering (50-53), or Seaver and Clark (67, 68), than to that of other
investigators. On the other hand the writer has gone into the subject with a
desire to bring together the various phases of the subject as presented in the
literature with the idea of rounding out the problem and attempting to clear
up some of the obscure points.

Materials and methods

The different soils used were selected largely for their variations in general
type, especially in physical structure, rather than for differences in their chemi-



JAMES JOHNSON



cal properties or their productivity. Their names will, therefore, quite satis-
factorily indicate the general nature of these soils. No complete mechanical
analyses have been made, but a determination of the loss on ignition indicates
the range of organic matter present in the various soils as well as the general
character of each soil in regard to this constituent. The determinations of
total nitrogen, phosphorus, and potassium serve to give an idea of the state of
fertility of these soils as far as these elements are concerned. The general
character of the soils used is summarized in table 1.^

TABLE 1

General character of sails used in the experimental work





LOSS ON












NAME


IGNI-
TION


N


P2O5


K2O


SOURCE


GENERAL DESCRIPTION




per cent


percent


percent


Percent






Waukesha silt loam


5.4


0.26


0.056


1.65


Milton, Wis-
consin


Dark, tillable, fairly
productive


Muck


14.8


0.65


0.14


1.52


Station farm,
Madison


From drained cropped




land, fairly fertile














but responding to














nitrogen


Virgin sandy loam


5.6


0.26


0.054


0.66


Station farm,
Madison


Dark, good texture,
not fertile from
wooded pasture land


Miami silt loam




0.20


0.07


1.69


Station farm,
Madison


From field cropped 10
yekrs to tobacco but
heavily fertilized


Fine sandy loam


4.3


0.14


0.068


1.20


Station or-
chard, Mad-
ison


A medium light soil
containing consider-
able silt, not very
productive


Peat


68.1


2.82






Station farm,
Madison


A thoroughly decom-




posed peat, previ-














ously cropped and














quite fertile


Norfolk sand


1.9


0.05






Upper Marl-


A very light sand, rel-












boro, Mary-


atively low in pro-












land


ductivity


Red clay


5.5


02






Ashland, Wis-


Very heavy red clay










consin


and non-productive



The soils were taken from the fields during the fall and allowed to air dry for
most of the work where dry heat was used. The methods of heating the soils
were usually of two kinds : First by steam in the autoclave at which a temper-
ature of 114°-116°C. was maintained for one and one-half hour, or second by
dry heat, in gas, electric, or hot water ovens. For temperatures of 500°C. or

1 The writer is indebted to the Soils Department, University of Wisconsin, for the deter-
minations of nitrogen, potassium, and phosphorus.



INFLUENCE OF HEATED SOILS ON GERMINATION AND GROWTH 7

over the electric furnace was used. The time of heating was usually brought
as near to two hours as possible.

When the work was first started and in fact during the greater part of this
investigation, it was supposed that the time of heating the soil was of com-
paratively small consequence. The soil was usually, therefore, kept at the
desired temperature for only a comparatively short period of time, the heat
being allowed to increase gradually in the dry ovens for the first hour. Later
on it became evident that a source of experimental error lay in the length of
time of heating and the lack of uniformity of heat distribution in large
samples of soil. Where large quantities of soil were heated, they were usually
spread out in the ovens in a layer about one inch thick. This was also true
when heating was done in the autoclave for short periods of time. The
temperatures were taken either by means of a mercury or electrical ther-
mometer for low temperatures, or with a Pyrometer for temperatures above
250°C. In the later experiments, a mercury thermometer was also used
for temperatures up to 500°C. It was deemed important to know ap-
proximately the rate of rise of the temperature of the soils in the autoclave.
An arrangement was made by means of a packing valve and electrical re-
sistance thermometers by which this could be done. The rise in temperature
of substances placed in the autoclave was relatively rapid at first but reached
the maximum slowly. According to these preliminary tests, 35 to 60 minutes,
depending on the container used, should be sufficient to thoroughly heat the
soil throughout in the autoclave to a temperature of 115°C. at 15 to 20 pounds
of pressure in bulks as large as 2 to 3 kgm. One hour and thirty minutes was,
however, always allowed for sterilization in the autoclave except where
otherwise stated.

In the germination tests the soils were heated in the Petri dishes used in the
tests at the lower temperatures, but in clay dishes at the higher temperatures.
Fifty grams of air-dry soil was weighed out and used in all cases in the germi-
nation tests. As soon as cool after heating, water was added to the soils bring-
ing them all as near as possible up to the same percentage of moisture, which
was practically but not quite up to saturation. As soon as all the soils were
evenly moistened, 100 seeds, which had previously been counted out, were
placed in each Petri dish, merely being scattered over the surface of the soil.
The tests were all run in duplicate. The Petri dishes retained a fairly con-
stant supply of moisture from 10 to 12 days, and it was rarely found necessary
to add more water to complete the tests. The seeds were picked out with
tweezers and the number that germinated at certain intervals, usually of 12,
18, or 24 hours, were recorded. Checks on unheated soil or filter paper were
always used.

In the case of heating large quantities of soil with dry heat for plant growth
studies, more difficulty was encountered in getting uniformity of heating. In
some of the earlier tests the soil was heated directly in the pots usedj but later
it was found more satisfactory to heat the soil in shallow pans which would



8 JAMES JOHNSON

permit of one or two stirrings during the heating process as well as a
thorough mixing afterward. After being thoroughly cooled, the soils were
watered and sown to seed or seedlings transplanted on the same day or the
day following heating. No special attempt was made in the ordinary tests
to prevent reinfestation of the pots with organisms.

Extraction of the soil with water was made by allowing equal weights of
water and soil to remain in contact for 24 hours with frequent shaking or stir-
ring. The extract was then filtered off through filter paper, usually with re-
duced pressure when filtering was slow. These extracts were used for germi-
nation tests by saturating three or four layers of filter paper in Petri dishes
with the solution, using water on filter paper as checks. Extracts obtained
in this way were also used for the freezing-point determinations and in the
temperature studies with Dewar flasks.

Freezing-point determinations were made with a Beckmann thermometer
in the ordinary way, the method differing, however, from that described by
Boyoucos (6), in that extracts were used instead of the soil itself. The read-
ings are not intended to represent the actual concentration of the soil solution
in the soil as they are no doubt too low, but they do give a fairly good measure
of the comparative concentrations of the soil solutions used.

Ammonia determinations were made by the ordinary magnesium oxide dis-
tillation method, which again is to be regarded as giving comparative rather
than actual amounts of ammonia present.

Seed germination on heated soils

The earlier workers on heated soils did not apparently note any particular
effect of heated soil on seed germination. This is to be expected since it is
usually only in comparative germination tests that this fact becomes most
evident. Stone and Smith (71) were probably the first to carry on experi-
ments along this line. They found an apparent acceleration of germination
in heated soils and noted further that different kinds of seeds behaved differ-
ently in this respect. Pickering (50) started a fairly extensive study of the sub-
ject of the toxic action of heated soils as measured by seed germination. Un-
fortunately his first papers, especially, are marked by considerable lack of
uniformity in results and although most of the general conclusions drawn are
quite correct in principle, the data are in many cases meager and confusing.
He found, for instance, in one case that heating soil to 100°C. prevented seed
germination altogether, while heating to 250°C. had little effect. In reality
the opposite is more likely to be the case. On the whole, however, he con-
cludes that the time of incubation increases considerably as the temperature
to which the soil is heated is increased, whereas the percentage of seeds ger-
minating decreases. In some cases acceleration was noted on soils heated
to 60° or 80°C. Heating to 200° produced the maximum retarding effect.
Seeds varied in their response, mustard for instance being more affected by
heating than rye. The length of time of heating soil at a certain temperature



INFLUENCE OF HEATED SOILS ON GERMINATION AND GROWTH 9

had no special eflFect, but increased moisture content on heating increased the
toxicity. Pickering also noted that the toxic property was retained in stor-
age of heated soils, and that its retention was influenced by moisture content
and temperature during storage.

In a second paper Pickering (51) reports that the formation of the inhibitory
substance to seed germination begins at temperatures as low as 30° C. and


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