Dirt. Plain old dirt.

Well, not really.  This is compost, 40 yards of it, composed primarily of manure from the farm and large amounts of wood chips.  The wood chips capture the nitrogen in the manure, and the combination makes this lovely black gold.  This particular pile will go back into the greenhouse and be the basis for next years greenhouse crops.

Here's the first step.  A layer of new, clean chips that we'll use to catch the liquids from the pig food. 

If you look in the upper left hand corner you can see some fruit and vegetables that came in that truckload.  The pigs eat what they want, and what they don't eat gets pushed into windrows -- you can see on in the middle, partially obscuring the two white domes.  There are still bits of food that the pigs find tasty in the windrows, and they'll dig and root through the pile several times.  Once a week or so I'll go out with the tractor and push it back into a mound, and this stirring of the compost allows for complete composting of the material.  I pile it up with the tractor so that it gets completely turned over; if I don't, I've found that it doesn't compost as completely as quickly.  The two pigs in the upper right hand corner of the picture are actually digging into the pile and turning it over as I took this picture.  Good pigs! 

When I windrow I'm careful to put a few inches of new chips on top of the compost.  This helps to insulate the pile a little, to get to the magic 160 degrees for sterilizing the compost, and to keep any odors down.  It really doesn't smell when you use enough carbon.  You can compost animals in wood chips as well, but I don't do that in the pig pasture.  The pigs would dig up the carcass and eat it. 

Pigs are the only earthmoving equipment that increases in value as you use it.  As Joe Salatin has said "Pigs go around with this sign on their heads:  Will work for food!"

After a couple move months the smaller wood chips and all of the food has completely composted away.  Larger wood fragments still remain and provide a carbon sink for any excess nitrogen in the manure or urine. 
The pigs enjoy sleeping on the compost piles because of the heat generated, especially in the winter.  The larger piles will maintain a working temperature inside even when covered with snow -- they look a bit like volcanoes, with a steaming plume.  The pigs lay in the steam. 

When this soil goes into the greenhouse you get some really lovely results.  Like this, or this.