6 DEPARTMENT OF AGRICULTURE

antagonistic to plant growth through these methods usually results i*
remarkable increases in plant growth; the higher yields thus proving
quite conclusively the really detrimental effect of nematodes on croq
production. But here again, past and current conceptions have beer
slow to acknowledge the beneficial effects as being mainly the result q
parasite and disease control.
 That the sterilization of soil by steam makes for better growth A
plants, frequently to such an extent that fertilizer applications may be
omitted, has long been known. For an explanation of this phenomenon
it was assumed, however, that such sterilization made more nitrogen
available and that this condition rather than parasite and disease conq
trol, was responsible for the growth and yield increase. With the advent
of efficacious soil fumigants such as carbon bisulphide (CS2), chloropicri I
(C ClaNO2), methyl bromide (CHaBr) and others, including the most
recent DD mixture (1,3-Dichloro-propene 1,2-Dichloropropane), tha
same explanation was still promulgated. Here too, an increase of avail,
able nitrogen was assumed and thought to cause growth "stimulation."
Peculiarly enough the question of where this nitrogen originated, par-
ticularly with fumigants not containing it, did not arise. It was argued
that both steam and chemical sterilization will kill most, if not all
organisms in the soil, and thus make more nitrogen available through
the decomposition of the bodies of these organisms. Such an argument
appears fallacious. If it were correct, then man too would have his
living space affected only through decomposition of his body after deatl!
Life is action and it is the living biological factor in the soil that exerts
the more extended, the more intensive, and the more significant action
on plant growth rather than the dead one. That is to say it is the living
plant-parasitic and plant-pathogenic biotic factors in a given soil whicl
are responsible for reducing the growth and production of crops grow-
ing therein. We are convinced that the striking increases in growth ani
yield that frequently follow sterilization, either by steam, hot water, or
fumigants, are largely the result of parasite and disease control ani
more particularly, nematode control. The results of soil fumigation in
our opinion thus emphasize the very great importance of plant-parasiti 
nematodes in crop production.
 Thus the growing recognition of the significance of plant and soil
nematodes as a factor in crop production is well founded. It is hoped
that it will lead to helpful developments for the control of these cropq
antagonistic biotic factors through the use of more efficacious fumigants,
and better methods of application. Here the way is open for significant
increases in crop production combined with a possible reduction of
acreage and a lesser need for fertilizers. Let us provide healthy rooq
systems for our crop plants through control of the many nematodes and
other agents now causing root lesions, root decay, root amputations
blind root tips, abnormal matted growth, etc., and surprising yield in-
creases will result. A healthy root system will more effectively utilize


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