The text's theme is thanksgiving: in stanza 1 for the natural harvest and in stanza 3 for the spiritual harvest. That thanksgiving tone, however, functions as a frame around stanza 2, which reminds us that thanksgiving must also be shown in our deeds of sharing God's bounty with those in need. Although the text is a modern one, it expresses the same message as did the Old Testament prophets: offerings of thanksgiving are acceptable to God only if "the orphans and the widows" have received loving care (see Isa. 1:10-17; Amos 5:21-24; Micah 6:6-8). That message is so necessary at North American harvest feasts!
Psalter Hymnal Handbook
For the source of the harvest, God’s children are called to look to God who with his powerful hand upholds all things, even “...leaf and blade, rain and drought, fruitful and lean years, food and drink...” (Heidelberg Catechism, Lord’s Day 10, Question and Answer 27). As a response for the harvest, God’s people are called to give him thanks.
The fourth petition of the Lord’s Prayer also expresses this dependence on God as provider (Heidelberg Catechism, Lord’s Day 50, Question and Answer 125).