CH 3 N N and 3-p-chlorophenyl-5- methyl rhodanine
C. Soil Treatment with Heat
The cooking of soil to kill soil pathogens is probably the oldest known method of soil treatment. It was discovered first in all likelihood by neolithic man, who may well have observed salutary responses of plants growing in old fire sites. Sixty years ago (and perhaps even earlier) the practice suggested by this observation was put into effect. Newhall
(1955) mentions that the roasting of tobacco seedbed soil by surface burning was a common practice when seedbeds were scattered about in wooded areas. This worked very well until the tobacco growers ran out of wood.
The use of heat sterilization of soil by steam was first used on a practical scale by Rudd in 1893, following Kuhn's experimental studies in 1880 and Frank's research in 1888 (Johnson, 1946; Baker and Roistacher, 1957).
There are three ways of disinfesting soil with heat: these are, by use of ( 1 ) dry heat, ( 2 ) steam, and ( 3 ) hot water. Baker and Roistacher
11. S O I L T R E A T M E N T 467 (1957) have evaluated the merits of each method. They conclude that the use of steam is the most effective as well as the most efficient method of heat disinfestation. Dry heat does not conduct well, and hot water not only tends to puddle soil, but it penetrates soil poorly.
Where steam is available (as in greenhouses) its use is preferable to that of 'chemicals for soil disinfestation. Steam is fast, effective, and leaves no toxic residues. There is little advantage in using steam under pressure over free-flowing steam. The principle problem is to confine the steam to prevent excess heat loss and for a long enough period to heat the soil mass sufficiently to kill pathogens.
Most pathogenic fungi and nematodes can be killed by exposures for a few minutes to temperatures above 50° C. Baker and Roistacher (1957) have pointed out that hot water treatments will kill species of Pythium, Botrytis, Rhizoctonia, Sclerotium, Sclerotinia, and Fusarium in tissues at temperatures of 46-57° C , in time periods of exposure from 5 to 40 minutes. Nematodes such as Aphelenchoides and Meloidogyne are killed by hot water at temperatures of 48-53° C. at exposure times of 10-11 minutes.
Under ideal conditions, heating a soil mass to 60° C. for 30 minutes should be sufficient to destroy all plant pathogenic fungi and nema-todes. Because of uneven distribution of heat in a soil mass, Baker and Roistacher (1957) recommend that a soil temperature of at least 80° C.
be maintained for 30 minutes. In practice, however, these workers feel that to be on the safe side the temperature should be raised to 100° C.
for 30 minutes.
Steamed soil is even more likely to be reinvaded by soil-inhabiting fungi than is chemically treated soil. This is because steamed soil usually is more completely disinfested. Well-cooked soil is a fine growth medium for most fungal soil inhabitants. It follows that extreme care should be taken with steamed soil to avoid recontamination.
Many ingenious devices have been developed over the years to apply heat to soil. Newhall (1955) and Baker and Roistacher (1957) have described application equipment and its uses in considerable detail.
Stationary soil masses are treated by steam in mobile bins, steam boxes, vaults, cabinets, and autoclaves. Steam can be applied also to soil in benches by the use of inverted pans, perforated pipes, and steam rakes.
Steam does a good job of cleaning up soil. To make the use of steam practical beyond the greenhouse, however, we need cheaper fuels and a lot of engineering. Newhall (1955) estimates that it would require forty tons of coal to supply sufficient heat to disinfest an acre of soil 1 ft.
deep. This is a lot of fuel, and the equipment and manpower required
468 W . A. KREUTZER
for such an application would be very costly. This discouraging picture might be changed by clever engineering and the use of atomic power.
Soil disinfestation with heat may get out of the greenhouse yet.
IV. S U M M A R Y — N o w AND T H E F U T U R E
In this chapter an attempt has been made to set forth the basic principles of soil treatment primarily with nematocides and fungicides.
Heat treatment is included only for the sake of completeness.
Between the plant and its environment there are a series of complex interactions. In agricultural soils, under a given set of conditions, the living phase (biophase) is in a state of dynamic equilibrium. This equi-librium state may be upset by chemical treatment, environmental changes, or cultural practices—causing a succession of transitory states which rapidly proceed toward a new equilibrium.
Since the micropopulation at all times is conceived to consist of organisms which either favor or harm the crop plant (sustainers or in-hibitors), the new equilibrium formed is either good or bad for the crop plant.
On a prevalence basis the most important of the obvious soil-borne diseases are those that involve seeds and seedlings. On a broader and less obvious level, however, all plants probably suffer from some degree of root pathogenesis.
When a biocide is added to the soil, interactions will occur which result in changes in both the soil and biocide. The biocide may be hydrolyzed or otherwise degraded by the chemicals as well as by the microorganisms in the soil. The phenomenon of sorption of the biocide to soil colloids is broadly viewed as a type of bonding, whether it be of the covalent, hydrogen, ionic, or van der Waal's variety. The degree and type of sorption encountered, as well as the potential dispersibility of a biocide, will depend upon the properties of both the biocide and the soil.
The soil is affected to a lesser extent from contact with the biocide.
Small changes in soil chemistry can occur, whereas there is little to no effect on the physical constitution of the soil. Profound local effects on the soil biophase do occur, with resultant favorable or unfavorable effects reflected by the growing crop plant.
Soil treatments with nematocides and fungicides have given control of diseases of known as well as of unknown etiology. Control responses in diseases of obscure etiology are believed to be the result of correction of unfavorable microbiological balances, control of unknown and un-described facultative or obligate parasites, or control of toxin-forming organisms.
11. SOIL T R E A T M E N T 469 In looking toward future developments in the chemical control of soil pathogens, we may expect the following:
( 1 ) Greater emphasis will be placed on control of minor and uni
versal root pathogens rather than those causing more spectacular and sporadic lethal maladies.
( 2 ) Research on the organisms of the biophase, particularly the microflora of the rhizosphere, will segregate the inhibitors from the sustainers. Conditions favoring or hindering their development will be learned. By appropriate soil amendments and enlightened cultural prac
tices, the microbiological balance will be shifted toward the sustainer side.
( 3 ) There will be a trend toward the use of more specific and highly efficient fungicides and nematocides. Such materials will be relatively nonphytotoxic and have a long residual action. Increased emphasis will be placed on treatment of critical soil zones at the time of planting and during the growth and development of the plants. Less effort will be made to treat large masses of soil.
( 4 ) Systemically acting chemicals eventually will be found which will be applied by seed treatment, foliage sprays, in fertilizers, or irriga
tion water. They will be active at very low concentrations. Root and stem infections will be warded off by translocated chemicals, which will act by increasing the resistance of the plant or by inhibiting the develop
ing pathogen.
We will eventually control fungus and nematode attack of stems and roots by not one, but by a combination of these future practices.
Not now; but some day.
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