Rashi: When Yisrael speak Emes, and Hashem looks at the Tzedakah that they do in the land, "also Hashem gives..." (verse 13).

Radak: When they do good deeds in the land, Hashem reflects Tzedek on them from Shamayim. Decrees from above are according to people's deeds on the land.

Malbim: It compares the matter to sprouting of grain, in which are joined Divine influence with human effort. If people will not plow, seed and prepare the land to sprout, Divine influence of rain and dew will not help. Only after the land is prepared and seeded, then Shamayim empties on it dew, rain and Hashem's Brachah. So Emes sprouts from the ground, i.e. people's merit; this prepares them to receive Hashem's Chesed. Then Tzedek is reflected from Shamayim, i.e. Chesed on their deeds, which is called Tzedek.