Soil sample
A soil sample or soil culture was any dirt collected for scientific purposes. Soil samples could be collected for analysis of nutrients and other properties which were important to plant health.
The Earth probe Nomad was originally programmed to secure and sterilize soil samples from other planets. Following its fusion with Tan Ru as of 2267, it mistakenly believed it was to seek out new life (the alien probe's original mission) and sterilize (i.e. eliminate) it. (TOS: "The Changeling")
In 2268, Leonard McCoy remarked that soil cultures and roots taken from Neural had the potential to be a medical treasure house, as Starfleet had previously suggested. (TOS: "A Private Little War")