Dry soils

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). 

Heavy Clay Soils. Heavy clay soils show an extreme form of the behaviour of water and nitrate in aggregated soils. Water cannot move through the matrix of such soils, except when it is imbibed by the dry soil. However, many of these soils... 

Table 16.17 Guidelines for classification of contaminated soils suggested range of values (ppm) on air dried soils...

Bulk density, soil - Mass of dry soil per unit bulk volume (combined volume of soil solids and pore space). 

Fig. 7. Root and shoot dry weight of wheat after 22 days of growth (5-leaf Stage) at various soil penetrometer resistances. Variations in penetrometer resistance were obtained by varying soil bulk density and water content. Symbols are as follows. Shape refers to bulk density (g cm ) 0,1.17 A, 1.29 , 1.37 <0, 1.41 V, 1.45. Shade refers to water content (g g dry soil) open symbols, 0.22 or 0.23 half-shaded, 0.25 closed, 0.27. Points are means s.E. (n = 6). Modified from Masle Passioura (1988).

Fig. 12. Relationships between root ABA content and bulk soil water content for maize plants growing in drying soil columns. Data are from Fig. 11, but do not include soil water contents less than 0.1 g cm in which many roots were non-living. Modified from Zhang Davies (1989).

Sharp, R.E. Davies, W.J. (1985). Root growth and water uptake by maize plants in drying soil. Journal of Experimental Botany, 36, 1441-56. 

Turning to the acute toxicity of PAH, terrestrial organisms will be dealt with before considering aquatic organisms, to which somewhat different considerations apply. The acute toxicity of PAHs to mammals is relatively low. Naphthalene, for example, has a mean oral LD50 of 2700 mg/kg to the rat. Similar values have been found with other PAHs. LC50 values of 150 mg/kg and 170-210 mg/kg have been reported, for phenanthrene and fluorene, respectively, in the earthworm. The NOEL level for survival and reproduction in the earthworm was estimated to be 180 mg/ kg dry soil for benzo[a]pyrene, chrysene, and benzoMfluoranthene (Enviromnental Health Criteria 202). 

Several facts have emerged from our studies with 2,7-DCDD and 2,3,7,8-TCDD. They are not biosynthesized by condensation of chloro-phenols in soils, and they are not photoproducts of 2,4-dichlorophenol. They do not leach into the soil profile and consequently pose no threat to groundwater, and they are not taken up by plants from minute residues likely to occur in soils. Photodecomposition is insignificant on dry soil surfaces but is probably important in water. Dichlorodibenzo-p-dioxin is lost by volatilization, but TCDD is probably involatile. These compounds are not translocated within the plant from foliar application, and they are degraded in the soil. 

Electroosmosis is used to remove liquid (moisture) from different porous solids (e.g., in drying soil for building purposes, which improves the bond between the foundations and the soil). A combination of electrophoresis and electroosmosis is sometimes used to dry peat or clay. In this way, the water content of peat can be reduced from 90% to 55-60%. Unfortunately, the energy required for a further reduction of the water content is very high. 

Drought stress increases the soil mechanical impedance on plant roots, which in turn can stimulate root exudation (1,4,5). lncrea.sed release of mucilage may contribute to the maintainance of Zn uptake in dry soils by facilitating Zn transport to the root surface in mucilage-embedded soil particles (264). This effect might be supported by water transfer from the subsoil in the roots, which is... 

In the laboratory, soil samples collected in the held are mixed thoroughly and reduced in size to laboratory samples. The air-dried soils are passed through a 2-mm sieve in order to remove stones and roots, then the water content of the soil is calculated after drying at 105 °C for 5h. If the analytical samples cannot be analyzed immediately after drying and sieving, they should be stored at about —20 °C in glass or Teflon bottles fltted with screw-caps. 

Air-dried soil samples were screened through a 2-mm sieve, then the water content in the soil was calculated after holding the soil samples for 5h at 105 °C. 

Residual dinitroaniline herbicides are generally extracted from 10-25 g of air-dried soil samples using organic solvents such as ethyl acetate, acetonitrile, methylene chloride and acetone by sonication, mechanical shaking or Soxhlet extraction. If necessary, the extract is then cleaned by a Florisil column or SPE. The extract is allowed to evaporate completely to dryness and the residue is dissolved in an appropriate volume of the solvent for GC or HPLC analysis. 

A 20-g sample of air-dried soil is extracted with 100 mL of ethyl acetate in a flask shaker for 45 min. After shaking, the extract is decanted and separated. The soil is re-extracted with 100 mL of ethyl acetate for 45 min. The combined soil extracts are filtered through a Whatman No 1 filter paper and the filter cake is washed with an additional 20 mL of ethyl acetate. The extracts are evaporated nearly to dryness, under vacuum, using a rotary evaporator. The residue is dissolved in an appropriate volume before GC analysis. ... 

For a soil sample, weigh 30 g (dry soil) of the sample into a 300-mL Erlenmeyer Aask and add 150 mL of water-acetoniAile (1 9, v/v). Sonicate the mixture for 30 min. Filter the exAact through a Alter paper overlaid with 20 g of Celite in a Buchner funnel into a 1-L round-bottom Aask with suction. Rinse the beaker and the Alter cake twice with 50 mL of acetoniAile. Combine the AlAates and concenAate to approximately... 

Weigh 50 g (dry soil base) of the sample into a 500-mL round-bottom flask and add 120 mL of methanol and 40 mL of water. Attach a condenser to the flask and reflux at 75 °C for 1 h. Filter the mixture through a filter paper by suction and collect the filtrate in a 500-mL round-bottom flask. Wash the residue and the flask with 80 mL of methanol and filter and collect the washings in the same manner. Combine the filtrates in a 500-mL separatory funnel. 

The air-dried soils (50 g) are processed similarly to the plant materials. 

Weigh 40 g (dry soil weight) of the sample into a 500-mL round-bottom flask and add 200 irL of acetone-water (3 1, v/v). Reflux the sample at 80 °C for 1 h after attaching a condenser. After cooling the extract, add 5 g of Cellite to the flask, mix the contents well and filter the extract. Conduct the subsequent procedures in a similar manner as for rice grains and concentrate the filtrates to approximately 50 mL. 

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