How often do you have to haul new dirt into the field to replenish the topsoil on the sod farm?
1) Many people have a notion that there is a thick layer of soil that is harvested along with the sod. The actual thickness of soil shipped with sod is only about 1/3". There is a significant percentage of that 1/3" that is root mass. Let's say that works out to the equivalent of 1/6" of actual soil loss. The other part of the equation is the remaining root system that is left in the soil after harvest. The bushy part of the root system extends about 10" into the soil. Those roots break down and add mass to the soil.
2) Sod is a soil conserving crop. When the wind blows, row crop fields tend to lose top soil. Sod fields "catch" airborn dirt particles. Over time, this adds up to a significant amount of topdressing.
3) Sod requires 18 months to reach maturity. It takes 2 years to go through the cycle of field preparation, seeding, growth and harvesting. Many people are suprised to hear this. Remember, it's not enough to just have sufficient turf cover, you have to be able to harvest and handle it without it falling apart. As a result, the frequency of harvesting a particular field is low.
In summary, sod growers don't gain or lose a significant amount of topsoil. It should not be seen as a form of strip mining. It is actually a conservative form of farming.
Information on the installation and care of sod can be found on The Information Page.