Absorption by plants.--Herbicides are absorbed by plant roots and are usually 
translocated to the aerial parts. Within the plant the herbicide molecules are subjected 
to various physical and chemical processes. Crop plants may be removed from the land 
or they may be returned to the soil along with weed growth. Most plant roots remain in 
the soil. Therefore a portion of herbicides absorbed by plants and the metabolic products 
of herbicides in plant tissues may eventually be returned to the soil. 
It has been stressed previously that soils reduce the initial effectiveness of herbi- 
cides and that the degree of effect varied among soil types and herbicides. Unpublished 
data showed that at least four times as much simazine was required in a clay loam soil 
as in solution culture to produce the same weight reductions of seedling oats. In another 
experiment the dry-weight increase of oat tops following simazine treatment through the 
roots was reduced 50 percent by 7.2 mg. of the herbicide (C!4 expressed as simazine) 
per gram of, dry tissue at harvest 9 days after initial exposure (49). The amount of 
simazine (C 4 expressed as simazine) required in seedling oat plants to reduce plant 
weight was less than 2 percent of that present in 400 ml. of the 0.05 p.p.m. by weight 
culture solution initially. Although the conditions of this experiment were markedly dif- 
ferent from those which occur in the field, plants probably absorb only a small fraction 
of the total amount of an herbicide applied to the soil. 
CONCLUSIONS 
Although considerable progress has been made, much additional information is 
needed on the fate of herbicides in soils. Weed research scientists need to know more 
about the persistence of herbicides in soils under varying environmental conditions so 
that they can establish safe rotational practices. Mammalism toxicity of some soil deg- 
radation products should be determined, because these products can also be absorbed by 
plants. Information on adsorption-desorption relationships; on the interrelationships of 
adsorption, volatility, solubility, and leaching of herbicides; on the nature and extent of 
microbial and chemical inactivation; on the importance of photodecomposition; and on the 
influence of various environmental factors on these processes is essential to an under- 
standing of the behavior of herbicides in soils. 
Weed scientists should determine the component or components of the soil from 
which dosage requirements of soil-applied herbicides can be predicted. Some of this 
information is available (48, 56), but more is necessary. Eventually specific recom- 
mendations of rates, times, and methods of application of herbicides may be based on 
weather forecasts and analyses of soil samples from farmers’ fields (25). 
One of the most urgent needs for research on the fate of herbicides in soils is 
methods of isolation and identification of herbicides and breakdown products. Biological 
and chemical assays must be improved and new ones devised. Radioactive isotopes have 
been used very little to study the decomposition of soil-applied herbicides. Soil samples 
could be treated with different lots of an herbicide with each lot tagged with C!4 at dif- 
ferent positions in the molecule. By known analytical techniques the unchanged herbicide 
and many reaction products could be separated and identified. Radioisotopes should be 
most useful tools in future research of this nature. 
As new herbicides are developed, their behavior in soils in response to variable 
soil characteristics, weather conditions, and cultural practices must be investigated 
concurrently with some of the more fundamental aspects prior to recommendations for 
use by farmers. 
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