Soil, surface wetness

Because many pesticides are appHed to the soil surface, the transport of pesticide during water infiltration is important. Water infiltration is characterized by high initial infiltration rates which decrease rapidly to a nearly constant rate. Dry soils have greater rates of infiltration than wet soils during the initial appHcation of water. Thus, perfluridone movement after appHcation of 3.8 cm of water was considerably greater in soil at a water content of <1% of field capacity than at 50% of field capacity (62). Fluometuron moved deeper into the soil in response to greater rainfall intensity or after rainfall onto a dry rather than a moist soil (63). 

Considerable work has been done on the behavior of pollutant species at air-water and air-soil interfaces. For example, wet and diy deposition measurements of various gaseous and particulate species have been made over a wide range of atmospheric and land-cover conditions. Still, the problem is of such complexity that species-dependent and particle-size-dependent rates of transfer from the atmosphere to water and soil surfaces are not completely understood. There is much to be learned about pollutant transfer at water-soil interfaces. Concern about groundwater contamination by mineral... 

The methyl parathion released to the atmosphere can be transported back to surface water and soil by wet deposition. Methyl parathion that is released to the atmosphere can also be transformed by indirect photolysis to its oxygen analog, methyl paraoxon, by oxidation with photochemically produced oxygen radicals. However, methyl parathion is not expected to undergo significant transformation to methyl paraoxon. 

Saprophytic, dark pigmented fungi such as Alternaria spp. can infect a wide range of plant species, especially tissues that are exposed to other biotic or abiotic stressors and older and senescing plant tissues. Also, wet weather conditions favour attack by Alternaria spp. Inoculum of Alternaria and potentially production of altemariol is further enhanced when cereal straw and stubble is left on the soil surface and not sufficiently incorporated into the soil after harvest (direct seeding and minimum tillage systems). 

Another possibility for plants to influence the P cycle is the hydraulic redistribution of water. This is the redistribution of water from wet to dry soil areas via the roots, which has been suggested to have an impact on the availability of P due to better mobility of inorganic P in wet soil (Lambers et al. 2006). McCulley et al. (2004) found that the concentration of extractable P was greater at depth than in the top meter of the soil in several arid and semi-arid systems in the southwestern USA and that nutrients were uplifted from this depth. They proposed that hydraulic redistribution of water from the soil surface to depths up to 10 m by roots was the mechanism by which P and other nutrients were mobilized and could be taken up by plants. 

Compaction occurs when soils are regularly walked on or cultivated in wet conditions. It is a particular problem in heavy soil. Avoid it by creating paths that follow the routes you want to take around the garden, and planting beds that are narrow enough to be worked on from paths. Regular use of a mechanical cultivator can also create compacted hard pan below the soil surface that plant roots cannot penetrate. 

The percent pesticide volatilized in one day from wet soil correlated positively with the factor [vapor pressure/(water solubility X binding constant)]. This factor has been reported to be linearly related to the volatilization rate of chemicals from soil surfaces (27). For pesticides with Henry s law constants and soil binding constants within the range studied, the factor is also approximately proportional to the fraction of chemical in soil air at equilibrium (28). In the present study, it was found that four of the pesticides had low factors, and less than 1% volatilized in 1 day (Table III). Diazinon, on the other hand, had a higher factor, and 2% of it volatilized. The use of this factor therefore does seem to have some merit for qualitative prediction. 

Fig. 6.11. Deposition of Lycopodium spores to soil A, dry soil B, wet soil C, theoretical values assuming surface is a perfect sink.

Future quantification of sulfur gas emissions from S. excelsum roots should avoid possible mechanical damage to roots caused by their excavation from soils as in the present study. Instead, plants need to be grown in soil in containers to which simulated rainfall additions are made while sampling for sulfur gases above the soil surface. Once the relations of sulfur gas emission rates to the frequency and amounts of soil wetting are determined, these data can be coupled with a rainfall frequency and quantity model to simulate potential sulfur gas emission for S. excelsum on an annual basis. 

Environmental Fate. CDDs are subject to atmospheric transport and both wet and dry deposition (Kieatiwong et al. 1990). They are partitioned to air, water, sediment, and soil, and they accumulate in both aquatic and terrestrial biota. CDDs can volatilize to the atmosphere from water and soil surfaces. They adsorb strongly to soils and are not likely to leach into groundwater (Eduljee 1987b). In the aquatic environment, CDDs partition to sediment or suspended particulates. TCDD, HpCDD, and OCDD are subject to photolysis in air, water, and soil (Plimmer et al. 1973). 2,3,7,8-TCDD is biodegraded very slowly in soil and thus is likely to persist in the soil. A better understanding of environmental behavior of CDDs is needed with respect to the importance of vapor-phase versus particulate transport, the... 

The second effect of water on evaporation, requiring only the presence of water, arises from the fact that many clay surfaces (and some organic ones) are strongly hydrophilic but also capable of adsorbing other molecules. The pesticide may therefore be held sufficiently strongly on a dry soil for its evaporation to be greatly reduced. When the soil is wetted, however, the stronger affinity of the water displaces the pesticide. 

Over most of the United States the potential evaporation is higher. That it may not in fact be higher only serves to emphasize the fact that the soil surface is wet only for a small part of the summer. Only by one mechanism (wick evaporation) does evaporation of water assist that of the pesticide. If the pesticide is on the surface and water is not, the... 

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