Bare soil studies

Size of test site B Depends on study design. The minimum area required for a typical large-plot design is about 0.25 ha Test site must allow for test design plus sufficient buffer zone around perimeter of field to protect against external disturbance For bare-soil studies, shady sites should be avoided Continued overleaf)... 

Plot maintenance B Expertise must be available to maintain the test site and, if cropped, to take care of the crop For bare-soil studies, the soil surface must be carefully prepared prior to test substance application and kept weed-free without disturbing the test areas. If the test is cropped, the crop should be treated according to Good Agricultural Practice. In case of a soil accumulation study, the field may be cultivated and cropped each season for up to 6 years... 

For compounds applied to annual crops, another approach is to apply the compound to bare soil prior to crop emergence and follow the soil dissipation of the compound as the crop emerges and grows throughout its normal growing season.This is the appropriate use pattern for pre-emergence compounds and represents another approach that may used to study the soil dissipation of foliar-applied compounds. 

The guiding principles in test plot maintenance are to (1) minimize soil surface disturbance at all times, (2) ensure that control and treated plots are similarly maintained, (3) avoid applying other agrochemicals that may interfere with sample analysis or that are otherwise contrary to the purpose of the study, (4) follow the prescribed irrigation policy determined for the study site, and (5) keep bare-soil test plots free of vegetation, as follows. 

In studies conducted in Texas, Adams (1965) found that the soil surface and mulch-soil interface temperatures were in the following order on fallow plots bare > 1-inch gravel > 2-inch gravel > 2-inch straw. The daily soil temperatures at 1.30 p.m. from early June to mid-August varied from 90° to 95°F at a depth of 3 inches under bare soil, but were near 80° F under a 3-inch straw mulch. 

Studies of Nebraska soils, made by Duley (1939), showed that the condition of the soil surface is the main factor involved in the intake of water. He states that the thin layer that forms on the surface of bare soils during rains had a greater effect on water intake than did soil type, slope, moisture content, or profile characteristics. A high rate of intake of water was attained by the maintenance of a cover of crop residue on the surface. 

Gilmour and Miller [324] have studied the fate of a mercurous-mercuric fungicide (calochlor) added to turf grass and to bare soil and found that about half of the total mercury added was lost in 57 days. Plant uptake, however, did not account for this loss and these authors concluded that it was mainly due to... 

As with other factors, no direct statements can be made relating the reaction of a soil to its corrosive properties. Extremely acid soils (pH 4 0 and lower) can cause rapid corrosion of bare metals of most types. This degree of acidity is not common, being limited to certain-bog soils and soils made acid by large accumulations of acidic plant materials such as needles in a coniferous forest. Most soils range from pH5 0 to pH8 0, and corrosion rates are apt to depend on many other environmental factors rather than soil reaction per se. The 45-year study of underground corrosion conducted by the United States Bureau of Standards included study of the effect of soils of varying pH on different metals, and extensive data were reported. 

When trifluralin was released in the atmosphere on a sunny day, it was rapidly converted to the photochemical 2,6-dinitro-7V-propyl-a,a,a-trifluoro-/ -toluidine. The estimated half-life is 20 min (Woodrow et ah, 1978). The vapor-phase photolysis of trifluralin was studied in the laboratory using a photoreactor, which simulated sunlight conditions (Soderquist et al., 1975). Vapor-phase photoproducts of trifluralin were identified as 2,6-dinitro-Wpropyl-a,a,a-trifluoro-p-toluidine, 2,6 dinitro-a,a,a-trifluoro-jo toluidine, 2-ethyT7-nitro-l-propyl-5-(trifluoromethyl)benzimidaz-ole, 2 ethyT7-nitro-5-(trifluoromethyl)benzimidazole, and four benzimidazole precursors, reported by Leitis and Crosby (1974). Similar photoproducts were also identified in air above both bare surface treated soil and soil incorporated fields (Soderquist et al., 1975). 

The possibility of nitrate formation in soils by strictly chemical oxidation is seldom mentioned, or barely so, in texts or review articles. This is not surprising since studies of this subject are limited and few quantitative data are available. The oxidation of ammonia to nitric acid is a long-established commercial process used in the production of this acid for fertilizers or other purposes. It is of interest to consider the main steps involved since some of the same reactions occur in soils. Krase (1932) outlined the commercial process. 

Studies reported by Army et al. (1961) showed that the soil moisture content below 2 inches was not materially increased by surface residues. Evaporation losses during the summer months from both stubble-mulched and bare fallow soils constitute a high percentage of the total precipitation. A summary of many experiments conducted in the Great Plains showed losses during the fallow period of 76% of the precipitation. In other tests 83% of the rainfall was lost between the harvest of one crop and the seeding of the next (Mathews and Army, 1960). 

One of the best known studies of allelopathy was done in the California chaparral, in an area where two of the most important shrubs are Salvia leucophylla and Artemisia cali-fornica. Bare zones of soil from 1 to 2 m in width surround each shrub. Beyond the bare zones are areas of stunted growth where a few herbs show limited development. Muller and Chou (1972) have postulated that these zones are pro-... 

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