Peat, perlite, vermiculite, calcined clay, bark, composted materials, and similar materials are mixed in many combinations to provide the plant growth media now used in greenhouses.
Some mixes still contain field soil but in rather small proportions.
These lightweight mixes are easier to handle and offer improved aeration and moisture-holding properties, but have little ability to hold nutrients. Hence, nutrient availability in most greenhouse growth media depends upon the intensity factor. Analytical procedures used have been modifications of the Spurway test procedures. These worked well for testing greenhouse soil mixes when soil was the base material. However, as the composition of greenhouse growth media has changed to include peat and processed materials, the field-soil testing procedures have become inadequate.