How did the agriculture in America lead to introduction of slavery?

In North America, at least, agriculture led to the
introduction of slavery because the Southern colonies' agricultural sector was based on
the production of staple crops.  When agriculture is done like this, a plantation will
grow basically just one crop that it sells instead of growing lots of different kinds of
crops for its own use.

In the South, plantations started
off growing tobacco and rice and indigo, depending on where they were.  This was hard
work and free people did not really want to do it since they could go off and get farms
for themselves on all the "free" land that was around.  Because of this, the plantation
owners needed a source of labor that was not free.  They started with indentured
servants but soon came to realize that slaves would be a more reliable source of
labor.