Mayank,

You can use 10 ml HNO3 and 5 ml of HClO3 for 1 g of sample according to AOAC (1990) and Hseun (2004).

AOAC (1990).  AOAC official methods of analysis. 15th ed. Association of Official  Analytical Chemists, Arlington, Virginia. Pp. 84–85

Hseu, Z. Y. (2004). Evaluating heavy metal content in nine compost using four digestion  methods. Bioresource Technology, 95: 53-59

Article Evaluating Heavy Metal Contents in Nine Composts Using Four ...

Please, follow the precautionary measures indicated as HCO3 can be hazardous, especially when adding it to the solution already containing HNO3. So, allow digest to cool before adding HCLO3. 

In my laboratory we first use 10 mL of HNO3 on 1g of soil on a hot plate in beakers partially covered with watch glasses. When all the HNO3 is evaporated we then add 5 more mL of HNO3 together with 10mL of HClO4. Then we heat it up until formation of persistent dense white fumes. The mixture is diluted to a known volume with distilled water and heavy metals measured..

You can use :

1.10 ml HNO3 and 5 ml of HClO4 for 1 g of soil ( for Cd 4g of soil)

2. 10 ml demineralised water + 5ml HClO4

3. 25 ml demineralised water + 5ml HNO3 (1:1)+ 1ml 30 % H2O2

Whatever ratio you use you must remember that the method is not strictly one for total metals but an extraction, all be it a very aggressive one. The metals tied up in the silicates etc will not be dissolved but most people can live with that. However since the method is an extraction procedure the result will be defined by the method and the performance can vary from soil type to soil type.  The more critical issue is what final analytical step you will use. If you are using ICP then destroying all the dissolved organic matter will not be critical but if you use an electrochemical method such as ASV it is much more important. The key is to check your method against a verified reference material as similar as possible to your own samples and to come to a realistic decision as to what recovery you need to achieve.

It depends which way of digestion you will apply. If you digest soil in open system amounts of acids are like mentioned above, but if you use microwave oven you must follow instructions given in digestive book for your oven.  I use the second way and for heavy metals in soil I use 5 ml HNO3 and 1 ml of HClO4 for 0.5 g of soil + 1 ml H2O2.  If you have very small amounts of Cd in your soil I suggest applying open digestion with higher amount of soil, jut like Hanna Jaworska said.

It all depends on the type of soil. But as a researcher, you can try different ratio of acid mixture, and select the most appropriate. It is, however, always very important to use reference materials

Ithink 40 ml in ratio of 5:1:1  to 40 ml including H2So4

In accordance with standard procedures, aqua regia digestion in a microwave system was used to extract all heavy metals in soil, including Cd

The most used ratio is 9:1.

5:1