What kind of Ma'aser is the Pasuk talking about?
Rashi: It is talking about Ma'aser Sheini (which the Torah calls 'Kodesh') 1
See Sifsei Chachamim.
What are "mi'Zera ha'Aretz" and "mi'Peri ha'Eitz" referring to?
Rashi: "mi'Zera ha'Aretz" is referring to Dagan (the five species of grain), 1 and "mi'Peri ha'Eitz", to wine and olive-oil - exclusively
Wheat, barley, rye, oats and spelt.
And what are the connotations of "la'Hashem hu"?