What caused the many clashes between the Powhatan Confederacy and the Settlers of Jamestown?

The answer to this question can be summed by the want of profit by the colony council. Instead of putting all of their agricultural resources into the growth of plants for survival, they instead chose to plant and harvest tobacco.

http://www.historyisfun.org/gardens.htm

The main reason that the colony of Jamestown was established was to exploit for profit, the resources that the New World had to offer. (http://www.britannica.com/EBchecked/topic/300134/Jamestown-Colony/247837/First-years-1607-09?anchor=ref849028) Because the settlers had little success in finding precious metals such as gold and silver as was their original intent, they had to find other profitable means in order to fund the continued expedition into the wilderness of America. In 1613, John Rolfe provided the answer to the question of profits. Rolfe imported from the West Indies the seeds that were to become the first crop of tobacco in Virginia. (http://www.ushistory.org/us/2d.asp)

John Rolfe

Because tobacco was so profitable (selling for at least four shillings per pound) to those in England, even against the opposition of King James I, the settlers of Jamestown chose to plant and harvest tobacco in large fields. Tobacco can only be planted in one field for about three seasons because the crop kills the land by sapping all of the nutrients that the soil has. After the fields have been used after this three year cycle, the settlers had to seek other land plots to plant more tobacco. The only place to get the land needed for the next growing cycle was from the Indians who inhabited the areas surrounding the boundaries of Jamestown Colony. This need for more planting space began what was to be a long “war” with the Powhatan Confederacy.  (http://www.ushistory.org/us/2d.asp)