As Mr. Webster indicates, scarify in agricultural use means to break up the surface of the soil. A device such as a harrow or disker is used, usually pulled behind a tractor. This allows seeding to take place. Later in the season, after germination, it may be done anew to allow water to penetrate the soil. Harrowing/disking is an important activity during hot, dry summers (like summer 2002)
if the infrequent rains are to get into the soil insted of running off the fields.
We also scarify certain types of hard shelled seed. In this case, it means to abrade the coats of the seeds to enable water intake and therefore germination. This simulates the digestive system of birds and animals which might naturally have distributed this kind of seed via ingesting and excreting the seed.
For the urban dweller, scarify means to remove the top inch or so of pavement / asphalt / chippings in order to resurface the roadbed. This results in the parallel grooves in the roadway that make your tires hum and your teeth rattle. Some types of road resurfacing heat the surface, scarify, lift and remix the material, and lay it back down in a single operation. This costs about as much as laying down a new surface would -- but of course it stops
the roadbed from rising year after year. More commonly scarified material is hauled away and fresh surface is laid down later. Usually several weeks later, after driving on the scarified surface has driven you to gibbering lunacy.
(Country gravel roads are much easier to deal with. They grade them, and then they pour new gravel on top.)