Soil extract serves as a source of nutrition for many soil microbes and is used in laboratory studies. Some organisms can be stimulated by components of soil organic matter as well as the mineral nutrients. Humic materials are used in culturing actinomycetes for example. Early work in culturing microbes used undefined substrates in more cases than not. This particular article is quite old coming from the 1950's. Many undefined media are favored by workers in the field a good example is potato dextrose agar. Neither the extract nature of the media nor the reasoning for their use or inclusion is not always clear or based on detailed study but on experience and history of their use. Potato dextrose agar is a common media for plant pathologists as general media. probably because potatoes were cheap and available and dextrose also. Interesting the standardized Difco potato dextrose agar is very consistent but many plant pathologists find making it fresh gives better culture expression.  You can always experiment with media but when you do make sure you have the suggested reference media according to their recipes.  

Follw https://doi.org/10.1016/j.micres.2013.09.003

Soil extract acts as a source of nutrients as found in the soil and also,acts as buffer for soil microflora 

Thanks very much for your  answers. So the soil is an important part of the composition of the media? I just have a second question, once I have filtered the soil after it has been autoclaved, do I use the filtrate or the solid material for the composition. One paper says 250ml soil extract while the other says 250g soil extract. 

Use the Filtrate without the solid material so you can observe the media reactions better! If it the media too opaque you will not be able to see cleared zones well from the action of solubilizing agents presumably enzymes or organic acid production. 

Kindly provide me detailed procedure for extraction of ZNSB in soil because by reading the different research articles i came to know the isolation method but i want to know the base line procedure for its extraction