It is determined from soil sampling data and ranges from 0 (very low) to 100 (very high).
Additions or deductions in this value to take account of variations in yield due to terrain and climate (e.g. average annual temperature) give the field value (Ackerzahl).
[1] The following colour scheme is used referring to the soil quality for certain ranges of values:[1] The values exhibited by different types of soil are:[2] As a yardstick for soil quality, a "Reich Standard Farm" was set up during the Third Reich in the parish of Eickendorf in the Magdeburger Börde, an exceptionally rich arable region.
It was the basis of comparison for the tax rating of farms in Germany.
Another measure, the field value (Ackerzahl), is derived from the soil value, taking account of other factors such as climate and location e.g. along the edge of woodland.