I would avoid damaging a natural grassland or prairie site to use as potting soil without some specific reason that has research review and look for alternatives or on site research if appropriate.  If done on very limited basis, consult with soil scientist, archeologist and ecologist on what would be a suitable sample and location, not mix top soil with  subsoil and try to limit extent of damage, mitigate as possible. 

If soil mixing is no issue, perhaps a already damaged site may be used as a sample with less impact.

In my opinion, in order that homogenicity ensured, the use of mixture of top and subsoils would be most appropriate for you experiment. you may avoid disturbing the structure by limited disturbance before, during and after collecting the samples. This opinion of mine is based on my assumption that you are only collecting some sample from the natural grassland site for use under greenhouse condition. Wishing you the best of luck as you proceed with your experiment.

Please see link mentioned above by Dr. Raj

Potting soil, also known as potting mix orpotting compost, is a medium in which to grow plants, herbs and vegetables in a pot or other durable container. The first recorded use of the term is from an 1861 issue of theAmerican Agriculturist.[1]

Some common ingredients used in potting soil are peat, composted bark, sand, perliteand recycled mushroom compost, although many others are used and the proportions vary hugely. Most commercially available brands have their pH fine-tuned with ground limestone; some contain small amounts of fertilizer and slow-release nutrients.[2] Despite its name, little or no soil is used in potting soil because it is considered too heavy for growing houseplants.[3]

Some plants require potting soil that is specific for their environment. For example, an African violet would grow better in potting soil containing extra peat moss, while a cactus requires sharp (i.e. plenty of) drainage, most commonly perlite or sand.[4] But potting soil is not ideal for all contained gardening. Insectivorous plants, such as the Venus flytrap and the pitcher plant, prefer nutrient-poor soils common to bogs and fens,[5] while water-based plants thrive in a heavier topsoil mix.[6]

Commercially available potting soil is sterilized, in order to avoid the spread of weeds and plant-borne diseases. It is possible to reuse commercial potting soil, provided that the remnants of plant roots, fungus, weeds and insects are removed from the mixture through heating before new planting can take place.[7] Packaged potting soil is sold in bags ranging from 5 to 50 pounds (2.3–22.7 kg).[8]

As with garden soil, potting soil can attract insects. For example, the fungus gnat is often found around houseplants because it lays eggs in moist potting soil.[9]

Infections due to potting mix have been reported in Australia,[10] New Zealand,[11] theNetherlands[12] and the United States.[13]

On June 13, 2000, the U.S. Centers for Disease Control and Prevention (CDC) reported that a woman in Washington was hospitalized with pneumonia that was triggered by Legionella longbeachae, the bacterium associated with Legionnaires' Disease. The CDC also confirmed the presence of Legionella longbeachae in soil in Australia and Japan.

Thats a really intelligent question , must appreciate it..

I strongly feel, soil from natural grass could effectively sued in developing the potting mixture , since it contains lot of siderophores  that can effectively ward off any potential Fe deficiency , besides  soil possess good microbial load...worth using fro potting mixture..