Water extracts of soil

This type of extraction can be carried out in two different ways. A soil sample can be brought into the laboratory and extracted with relatively large amounts of water to try to determine its inorganic composition. The water-to-soil ratio can be either on a mass-to-mass basis or a volume-to-mass ratio. A one-to-one ratio is commonly used, although other ratios have been used. After a designated extraction time, with or without shaking, water is filtered from the soil and analyzed. A typical water extraction of soil is given in Procedure 11.1. 

Figure 14.2. UV-Vis spectrum of simple deionized water extract of soil. Absorption maximum is indicated by an arrow. Note that the absorbance is above 1, and therefore the solution should be diluted before attempting to interpret the spectrum.

A method for the determination of chlorate in water extracts of soil is based on its conversion to free chlorine upon reaction with hydrochloric acid, followed by spectrophotometric evaluation of chlorine at 448 nm by the spectrophoto-metric o-toluidine method [9]. A correction is made for interference by iron (III), nitrite, free chloride derived from hypochlorites and strong oxidising agents by subtracting the absorbance of a modified blank, containing a lower concentration of hydrochloric acid, from that obtained in the test. 

Bradfield and Cooke [45] have described an ion-chromatographic method employing a UV detector for the determination of nitrate sulfate and phosphate in water extracts of soils (see Fig. 6.4). Soils are leached with water and Dowex 50-X4 ion exchange resin added to the aqueous extract, which is then passed... 

However, in irrigation water and water extract of soil interfering ions are negligible and can be neglected). 

Hot water extraction of soil boron (B) (Berger Truog 1939) has been used widely to obtain an index of soil B status. The method presents difficulties for routine analysis as it is often difficult to prepare water extracts free of colloidal material. Dilute salt solutions have largely overcome this limitation, two examples being the use of 0.01 M CaCl2-0.05 M mannitol (Cartwright etal. 1983) and hot 0.01 M CaCl2 (Aitken etal. 1987). The empirical method described is based on the use of hot water reflux for soil extraction, with alternative analytical finishes based on a colour reaction and ICPAES. 

Table I. Diagnostic Criteria for Boron Nutrition of Field and Vegetable Crops Based upon Hot Water Extraction of Soil Boron (10)...

Martin, J.K. (1970) Organic phosphate compounds in water extracts of soils. Soil Science 109, 362-375. 

Several electrochemical tests have been used successfully over the years to estimate the corrosion behavior of a given metal in soil or to determine the soil corrosivity. These tests can be carried out in water extract of soil and/or in water saturated soil, when all dissolved salts (ions) are present. The electrochemical test methods and practices are described in several ASTM Tests Methods and Practice ... 

Jakubowska et al. [91] have presented a differential pulse anodic stripping voltammetric method associated with FIA for determination of thallium in water extract of soil samples. 

WHITEHEAD D.C., DIBB H. and HARTLEY R.D. 1983. Bound phenolic compounds in water extracts of soils, plant roots and leaf litter. Soil Biology and Biochemistry,... 

Analysis of water directly extraction of soil with methanol followed by dilution of extract with water... 

Transfer the concentrate from Section 6.1 to a 200-mL separatory funnel with 50 mL of water and add 10 mL of saturated aqueous sodium chloride solution. Extract twice with 100 mL of n-hexane. Dry the n-hexane extract by passing through about 80 g of anhydrous sodium sulfate on a glass funnel into a 500-mL separatory funnel for the rice samples and into a 500-mL round-bottom flask for the soil sample. Wash the anhydrous sodium sulfate with 30 mL of n-hexane and combine the washings into the vessel. The n-hexane extract of soil sample is evaporated to dryness under reduced pressure, then the soil residue is processed as described in Section 6.3.2. 

The extraction of soil with water at pH 7 would seem to be a good way to study the soil inorganic chemistry. The most common solvent that soils are in contact with is water in the form of rain. However, rain is not neutral but acidic. Rainwater pH ranges from 3.8 to 5.6, depending on the air in which it forms. Acid rain that contains H2S04 and HN03 created by the reaction of gases in the atmosphere with water can have a pH as low as 2.0 [2],... 

L Explain why the extraction of soil with distilled water with pH 7 will not duplicate the loss of inorganic components from soil due to percolation of rainwater though soil. 

It is important to keep in mind that any extraction of organic matter from soil will include both naturally occurring organic matter and organic contaminants. Separating the two at some later stage of analysis is thus an essential analytical step. For example, extraction of soil with hexane or dichloromethane will extract both l,l,l-trichloro-2,2-di(4-dicholorphenyl)ethane (DDT), a contaminant, and octadecanoic acid, a natural fatty acid. Also, the herbicide 2,4-dichlorophenoxy acetic acid, a contaminant, and indole-3-acetic acid, a natural plant hormone, are both extracted by water (see Figure 12.3). These... 

The procedure [56] involving subcritical water extraction of polychlorobiphenyls from soils described in section 5.6.1.4 has also been applied to sediments. 

Neutral and basic herbicides were extracted from water made alkaline with sodium hydroxide or from soil, with chloroform extracts of soil were cleaned up on basic alumina containing 15% of water. Acidic herbicides were extracted with ethyl ether from water acidified with hydrochloric acid or from an aqueous extract of soil prepared by treatment with 10% aqueous potassium chloride that was 0.05M in... 

For most analyses, it is necessary to separate the analytes of interest from the matrix (i.e., soil, sediment, and water). Extraction of analytes can be performed using one or more of the following methods (1) extracting the analytes into a solvent (2) heating the sample, as may be necessary to remove the solvent and for the analysis of volatile compounds and (3) purging the sample with an inert gas, as is also used in the analyses of volatile compounds. 

Mahler, R.L., Naylor, D.V. and Frederickson, M.K. (1984) Hot water extraction of boron from soils using sealed plastic pouches. Communications in Soil Science and Plant Analysis 15, 479-A92. 

Extraction of soil with hexane acetone (1 1), centrifugation, separation of hexane from acetone/water layer. Extraction of acetone/water phase with chlorofornrdiethyl ether (1 1), solvent exchanged to methanol. Hexane layer contained diazinon, chloroform/diethyl ether fraction contained 2-isopropyl-4-methyl-6-hydroxy-pyrimidine. 

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