Sforno on Deuteronomy 26:2/ספורנו על דברים כ״ו:ב׳

1מראשית כל פרי האדמה אשר תביא מארצך, the choicest of each category. [the word ראשית is not understood as “first.” Ed] We find the term ראשית used in this sense in Amos 6,6 וראשית שמנים ימשחו, “they anoint themselves with the choicest oils.” In verse 1 of the same chapter we read נקובי ראשית הגויים, “the ones known as the choicest of the nations. ” These “choicest” fruit are the seven types of produce for which the Land of Israel is famous, and this is why we have been commanded already in Exodus 23,19 to offer samples of these in the Temple. The gift known as ביכורים is not literally the fist ripe produce of each of these seven species, but the ones that grow on the best soil the farmer has so that it is truly also his choicest. The seven categorise of produce have been enumerated in Deut. 8,8-9.