Titus (Living in an Ivory Basement) blogs about the about of sequencing coverage required for various NGS applications and explains how he gets the number (via an this the spreadsheet )
Fig 2 which shows the coverage required (in non log Y scale) is really his main point of how crazy a lot of coverage that will make Illumina a very happy company if everyone started doing soil metagenomes.
I must say frankly I am surprised that marine samples or even human gut samples have such a vast difference in species diversity compared to soil.
Definitely thought provoking when you think about the search for extra terrestrial life perhaps it's easier assume we will find an alien bacteria first than anything else .. (X-files will be terribly less exciting though if it's all about sending space soil probes to retrieve alien bacteria that eats your flesh)
haha sorry random thought on a Monday ...