Report on the Agricultural Experiment Stations, 1952 Page: 46
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46 REPORT ON EXPERIMENT STATIONS, 1952
every case was, for all practical purposes, the same as in the untreated
controls. On a silt loam soil 3 pounds of 2,4-D did not cause a significant
nitrate reduction at any time during a 16-week period. Applications
of 10, 25, and 50 pounds per acre reduced the rate of nitrification
during the first 8 weeks, but only the 25- and 50-pound rates gave
significant reductions during the second 8-week period.
The total number of bacteria in soils was not altered to any appreciable
extent by applications of 2,4-D at ordinary field rates. On the
other hand, applications of 25 and 50 pounds per acre on the silt loam
soil produced temporary increases in the number of bacteria, lasting
up to 8 weeks. The rate of decomposition of organic matter was not
affected by the 2,4-D.
Maleic hydrazide, recommended as an herbicide under certain conditions,
was found to be toxic to plant growth on several major soil
types in California. Among the 11 soils studied, toxicity was highest
and inactivation slowest in Arbuckle clay loam, 15 parts per million
being toxic originally and 340 parts per million still causing complete
sterilization 5 months later. Sterility to plant growth was obtained
at 140 parts per million in 4 soils. Inactivation was fastest in Aiken
clay loam, where 680 parts per million were not toxic 3 months after
application. In Yolo fine sandy loam, 5,000 parts per million still
inhibited all growth after 7 months.
Maleic hydrazide was held by the clay component of the Aiken clay
loam soil, but moved freely with the soil solution in all of the other
soils. As a result of these experiments, the California station concluded
that maleic hydrazide in the amounts used would not constitute
a hazard on most soils during warm growing weather. Although it
can sterilize soil against plant growth when used in large enough
quantities, it decomposes too readily under normal conditions to be
considered an effective sterilant.
Three different phenyl mercuric compounds (the acetate, hydroxide,
and triethanol ammonium lactate) were also studied at the California
station. Applications of these compounds at the rate of 680 parts
per million inhibited plant growth in the four soils used, and at 220
parts per million the reduction in yield was considerable. Under
greenhouse conditions the chemicals had decomposed by the time of
second cropping to such an extent in all instances that little toxicity
was left in the soil, even at the highest concentration (680 parts per
million). Leaching experiments showed, however, that 20 centimeters
of water did not displace the thriethanol ammonium salt from the top
layer of the soil.
No significant difference was found between the toxicity of pentachlorophenol
(PCP) and its sodium salt. Over a period of 12 months
neither of these compounds broke down appreciably in the soil under
greenhouse conditions. The California scientists point out that this
is in decided contrast to other organic chemicals. Because PCP is a
potent fungicide, this failure of decomposition in warm moist soils
may inhibit microbial activity or produce partial sterilization over a
long period of time.
Leaching could possibly be relied upon to rid the soil of the toxic
principle of PCP since the chemical is not fixed by the clay fraction.
Large quantities of PCP, in the range of 600 to 1,200 pounds per acre
of soil distributed to a depth of 1 foot, were found necessary to in
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United States. Office of Experiment Stations. Report on the Agricultural Experiment Stations, 1952, book, January 1953; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc5990/m1/48/: accessed November 23, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.