There are several techniques to identify microbial diversity in soil such as 16S rRNA, PCR-DGGE, pyrosequencing etc. Which one is the best and economic technique and least time consuming.
Illumina sequencing is cheap as you can do many samples in a run. There are also many university core facilities and commercial labs that can do the sequencing for you. And there are open access programs available such as Qiime in order to analyze the sequences obtained.
The Basic method to study the soil biodiversity is to calculate mannually the Diversity indices.. like simpson index, shannon wiener indices..etc..these methods are classical and not needed so much of instruments..after isolation and enumeration of microbes by classical microbiological methods..
You can study one of my paper attached on my BIOdata on research gate...