Agricultural Science Digest
Vol. 11, July - August, 1969
PURDUE UNIVERSITY

PESTICIDES IN SOILS

The leaching and movement of pesticides
in soil is primarily affected by adsorption,
physical properties of the soil and climatic
conditions, according to a comprehensive re-
view and synthesis of current research on the
interaction of soils and pesticides by a Purdue
University agronomist and a USDA chemist.

They observed that while seven factors
are known to influence the fate and behavior
of pesticides in soil systems, adsorption ap-
pears to be the major factor affecting the in-
teractions occurring between pesticides and
soil colloids.

The other six factors affecting pesticide
action are chemical decomposition, photo-
chemical decomposition, microbial decompo-
sition, volatilization, movement and organism
uptake.

Near-universal agreement that rainfall
frequency and intensity have differing effects
on pesticide leaching in soils was reported.
Greater rainfall frequencies but lower inten-
sities increased the amount and depth of
leaching.

Low soil moisture content was reported
to cause reduced pesticide movement because
of the lowered water solubility of the pesti-
cide, and the enhanced ability of the pesticide
to compete for adsorption sites.

Also, in tests, pesticides leached more
in light than in dark-textured soils.

Variations in the extent of surface move-
ment of pesticides was reported. Findings
pointed to a direct relationship between
2, 4-D application rates and washoff, but the
movement was greatest in the earlier part of
a rain storm. Soil bioassays indicated that
most of the 2, 4-D remained in the top three
inches of the soil layer. Loss of 2, 4, 5-T,
dicamba and picloram was influenced more by
cultural practices, the nature and rate of ap-
plication, and the slope of the field.

The iso, octyl, butyl and ether ester for-
mulations are far more susceptible to removal
by washoff than amine salts.

Pesticide transportation was reported to
be dependent principally on the relative diffu-
sion of the airspace and water in the soil, and
by the upward or downward flowing of water.

Soil porosity and airspace has been found
to be one of the more important factors affect-
ing the diffusion of soil fumigants. The lateral
movement of capillary water under furrow ir-
rigation is believed to be a prime mover of
herbicides.

Non-volatile pesticides are moved more
by percolating water, although also affected
by soil water moving small distances.
Dicamba and diphenamid moved upward
under sub-irrigation conditions, and also
when surface-applied water was allowed to
evaporate freely from the soil surface.

Journal paper 3656, G. W. Bailey and
Joe L. White, Department of Agronomy.

EFFECTS OF ROW SPACING ON EROSION

The trend to narrow row spacing of corn
and soybeans caused Purdue researchers to
compare the effects of row spacing on the
amount of crop cover and erosion and mois-
ture infiltration.

Six treatments were compared on Russell
silt loam soils with an average slope of 5 per
cent. To conduct the test under controlled
conditions, the researchers simu-
lated a series of rainstorms, and
measured water infiltration and soil
loss at four crop stages; two weeks

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