Tafsiran Ulangan 12:19: Rashi, Chizkuni, Da'at Zekenim & Rashbam

השמר לך TAKE HEED TO THYSELF [LEST THOU FORSAKE THE LEVITE] — This is intended in addition to the positive command expressed in the previous verse, to attach to it (the neglect of the Levite) a negative command, also.

כל ימיך, “as long as you live.” This includes even the years of sh’mittah and Jubilee years when no crops are harvested. (Sifri)

כל ימיך על אדמתך, “as long as you live on your land.” According to the plain meaning of the verse, the choice of the word אדמה, “soil, earth,” here instead of ארצך “your land,” may have been dictated by the fact that the Levites did not get any ancestral land, and the other tribes therefore had to see to it that they could have a livelihood, i.e. you who have soil have to look after your brethren who have not been given soil.

על אדמתך [TAKE HEED TO THYSELF LEST THOU FORSAKE THE LEVITE] UPON THY GROUND — but in exile (outside the Holy Land) you are not admonished as regards him more than as regards the poor of the ordinary Israelites (Sifrei Devarim 74:9; Talmud Yerushalmi Horayot end).