Please follow the link given below.

http://link.springer.com/article/10.1007%2Fs10661-013-3354-5#page-1

Dr.Aravind Singh,the reference provided by you is pertinent and provides information on the relative performance of US EPA methods 3051A and 3050B. These methods have wider acceptability.But the amounts of heavy metals extracted by these methods may represent the total amounts and may not correlate well with plant or microbial extraction of heavy metals from the same soils.Some of the soil test methods used for extracting micronutrient cations may extract the plant-available or phyto-available heavy metals also and they may also be less time consuming and rapid.The commonly advocated methods are EDTA ,DTPA,Mehlich1 and Mehlich 3 methods.Neutral salt extractants(0.01MCaCl2 and 0.1MNaNO3) are also being tested with some promising results.Thank you Dr.(Ms.)Barbara for providing interesting information on phyto-extracted heavy metal disposal, especially recovering them through pyrolysis.

Dr Barbara's phyto extraction process is the best one considering the environmental impact assessment of heavy metals.

 Ms.Singh,we always relate the soil test for heavy metal with plant growth and performance.Concentration of a particular heavy metal in soil  extract is   related to  its concentration in plant.Soil test screening levels of heavy metals will normally be determined through three different approaches,namely,phytotoxicity,food contamination and soil microbial activity diminution.The following paper provides the details of these approaches  and limits for some heavy metals.

Screening of soil for assessment of toxicity of heavy metals to organisms.By Saha,J .K. and Panwar,N. Current Science ,vol,106,no.25,pages 300-304 January 2014.Available online.

Dear colleagues, I provide here under two more useful references pertaining to determination of concentration limits for heavy metals in soils using soil and plant tests.

Evaluation of extractants for estimation of the phytoavailable trace metals in soils.By Menzies,N.W et al. Environmental Pollution 145(2007)121-130. Available online at www.sciencedirect.com.

Determination of lead and cadmium concentration limits in agricultural soil and municipal solid waste compost through an approach of zero tolerance to food contamination. By Saha,J.K. et al. Environ Monit Assess  Published online :07 August 2009 .DOI 10.1007/s10661-009-1122-3

Dear Colleagues

All references are very useful but the last one : 

Screening of soil for assessment of toxicity of heavy metals to organisms.By Saha,J .K. and Panwar,N. Current Science ,vol,106,no.25,pages 300-304 January 2014.Available online.

seems more rational and acceptable.

Best regards

I would recommend the following methods:

The total content of heavy metals was determined after mineralization of soil in the mixture of acids (HF andHClO4) using Crock and Severson’s procedure (1980) and the content of available metals- after extraction with DTPA solution according to Lindsay and Norvell method (1978).

and according to the method of microwave mineralization

H. Dąbkowska- Naskręt, H. Jaworska , M. Kobierski, 2000:  A microwave oven digestion method for the determination of metals in soils. Macro and Trace Elements, Menge- und Spuren Elemente, 20, 21-26, Fridrich- Schller- Univ. Jena.

Dear colleagues Ziarati and Jaworska,thank you for your  good comments.Both the total concentration of heavy metals and plant- available form of heavy metals are important.When one plans for remediation of the contaminated soils, the knowledge of total  contents is important.However, all the total amount of an element is not plant -available.So when we desire to interpret the data in relation to plant and microbial availability and in turn the entry into animal and human system, the available form of heavy metals like DTPA,Mehlich1 and 3, 0.01M CaCl2 and 0.1M NaNO3 extractable are important.

Dear Rao sir

Heavy metal assessment is depend on type of soil, crop, industries near by, plants, and  consequently in animal and human bodies, fertilizer and pesticides.  as per my observation the soil samples were dried at a room temperature for 7 days and sieved (2 mm sieve). The dried samples  were subjected to a cool extraction with 0.5 M HNO3 for  30 minutes. Fe,Mg,Cu,Zn,Pb and Ca were  determined by  acetylene-air flame atomic absorption  spectrophotomectry and Cd was determined by graphite

furnace atomic absorption spectrophotometry. The pH levels of soil layers were also measured. Samples of grasses, mosses and pine needles were dried for 14 days at room temperature. Experimental plant material should not washed. Although organic carbon play important roll in soil.

Best Regards

Arvind

 Mr.Swarnakar,I appreciate your  comments.I have come across in literature the use of 0.5M HNO3,1M HNO3 and 2M HNO3 extraction procedures for estimating heavy metals accumulated in soil.We should have a common method across many countries or at least in a particular country.This will help to impose standard limits for comparison or regulation and monitoring of soil contamination.

Different methods (heavy metals in pore water (PW), diffusive gradients in thin films (DGT), diethylene triamine pentaacetic acid (DTPA) extraction, and total heavy metals (THM) in soil) for the assessment of heavy metal bioavailability from soils having various properties and heavy metal contents have been studied by various workers. See following paper:

http://link.springer.com/article/10.1007/s11270-010-0400-6

Dr.Malhotra, thank you for the reference.

Good question I like that