What is the meaning of "Lasu'ach ba'Sadeh"?
Rashbam: Yitzchak went into the field to plant trees and to tend to his servants (who were working there). In fact "Lasu'ach " means (not to pray, but) to see to his plants - from the word "Si'ach" (vegetation).
Ha'Emek Davar: He went to pray, but not literally in the field. One may not pray in an open place! Rather, he was (e.g.) under a tree, and put his eye and heart towards Har ha'Moriyah.
QUESTIONS ON RASHI
Rashi writes that Yitzchak went to pray. Why did he go into the field to pray?
Seforno: So that passers-by should not interrupt him, even though he had already prayed by the well of Lachai Ro'i. 1