SOIL SCIENCE 45
rootstocks they grew only one-third to one-half as high. For foundations
and other plantings in restricted areas, the suppressed growth
from dwarfing understock is frequently desirable.
Important research contributions were made during the year in the
fields of soil physics, soil chemistry, basic soil-plant relationships, and
soil microbiology. Since it is not possible to review all of the research
in these fields, that of microbiology is selected for major emphasis because
of its timely significance. Other areas of soil science and plant
nutrition will be reviewed in subsequent years.
The use of organic chemical compounds for the large-scale control of
weeds, insects, and diseases brought a new problem into crop production,
namely: What effect would these materials have upon soil productivity?
If the new herbicides, insecticides, and fungicides have
any effect upon crop growth, would this effect take the form of adverse
action on the micro-organisms that normally inhabit fertile soils ?
Experiments seeking answers to these questions have been conducted
during the past 5 or 6 years. Their purpose was to learn what effect
organic chemicals have on such important biological processes as the
decomposition of organic matter, nitrification, and nitrogen fixation,
all of which are intimately associated with crop production. The findings
of some representative experiments along these lines are here
summarized, as are certain of those in related fields of soils microbiology.
Effects of Herbicides and Insecticides on Soils
The Pennsylvania station found that 2,4-D has little, if any, effect
on the process of nitrification in the soil as long as the rate of application
is not excessively high. The same thing was true for phenyl
mercuric acetate, another chemical used for weed control. Applications
of 2,4-D, either in the form of the sodium salt or in the amine
form, resulted in only temporary depressions in nitrification. The
rate of nitrification returned to normal within a period of 2 weeks,
although it was approximately 2 weeks longer before the last traces
of the herbicide disappeared from the soil. The amine form had a
slightly greater effect than the sodium salt in depressing nitrification.
A second application of either of the 2,4-D preparations reduced
nitrification in much the same manner as the first, but recovery of the
nitrification process took place in about the same length of time. The
herbicide disappeared more rapidly, however, after the second treatment.
When 25 parts per million of phenyl mercuric acetate was
applied, complete recovery of nitrification occurred in less than 30
days, whereas with applications of 50 parts per million only partial
recovery resulted in this period of time. When 100 parts per million
were applied, there was little evidence of nitrification at the end of 30
Nitrification studies at the Kansas station showed that applications
of 3 and 10 pounds per acre of the sodium salt of 2,4-D reduced nitrate
accumulation up to 4 weeks on a sandy loam soil. Applications of 25
and 50 pounds likewise reduced nitrates during the 4-week period.
At the end of 8 weeks, however, the amount of nitrate nitrogen in
U.S. Department of Agriculture. Agricultural Research Administration. Office of Experiment Stations. Report on the Agricultural Experiment Stations, 1952. Washington, D. C.. UNT Digital Library. http://digital.library.unt.edu/ark:/67531/metadc5990/. Accessed December 7, 2013.