Need to remove septic leach trenches?

The property I'm soon to own has a mobile home on it I intend to sell; I intend to have the septic tank pumped out and removed, and I'm wondering if the leach lines, trenches and so forth need to be taken out as well before the plot can be opened up for farming. Anyone?

i'm not sure about farming..

when we had our housefire..we had the old house removed, and they broke in and filled the old tank area..but didn't remove the lines..of course it isn't PLOWED as we are a no till area..and it isn't really farmed..but it is planted to mixed beds of trees, shrubs, perennials, vines..etc

The area isn't going to be regularly plowed, per se. The plan is to chisel plow the entire property once (it's a completely flat plot, so no leveling or extensive dirt moving will be required) before putting in permanent "low-till" beds. My reasoning is that after the tank is removed and the leach field is planted to a legume for a season or two, it should be safe enough to plant it to food crops.. maybe waiting an additional couple of seasons before trying to pull any root veg from it; however, I've got no experience dealing with this sort of thing, and I'm wondering if there is anything obvious I've missed.

In our area, (NE Washington) leach fields have to be at least 18 inches to 3 feet deep.  I am not an expert but I don't think most food plants (except for trees, maybe) have roots that go that deep. 

If there is a real concern, maybe put some non food but beneficial plants in that area?  Most leach fields are under 100 feet long and can vary in widths.  We have ours planted with oats right now.  We only planted that as it was a disturbed area, we had the seed, and just wanted to get something on it right away.  Later that area will be used for plants for our future nursery operation, with non food plants on it for root stock.  Shallow rooted plants though, so they will not interfere too much with the leach field.

Farmers that use sludge from treatment plants have to plant crops that are not for direct human use.  They can be for growing animal fodder, and then the animal is considered the food source.  They apply the sludge by spraying it directly on the ground.  Stinky mess, when driving through farmland having it applied.

Good thing I suppose, otherwise cities would be swimming in their own wastes pretty quickly.