The gemara in Shabbos (31a) says that we learn the six sedarim from the pasuk in Yeshaya (33:6), "V'Haya emunas Itecha Chosen Yeshuos...". Seder Zra'im is referenced by the word "emunas". Tosfos brings a Yerushalmi that explains that someone who takes a seed and throws it below the ground to let them rot, expecting life sustaining produce to grow, shows that he is Maamin in Hashem.
Rav Chaim ben Betzalel the brother of the Maharal MiPrague, in his classic Sefer HaChaim asks, is there any fool in the world who doesn't beleive in planting? What will he then eat? What is the Yerushalmi saying? He offers a different interpretation of this Yerushalmi. Seder Zra'im, says the Sefer HaChaim is about Trumos and Maasros. A person plants and then takes off part and gives it to the Kohen. There is no police ensuring he give this, and no one to measure that he gave the required amount. Yet the person does it because he believes in Hashem.
Just like it seems odd that a person throws a seed in the ground expecting great returns, so too taking your hard earned labor and sweat and giving it to the Kohen and Levi seems a strange way to make money. Yet the same perfectly natural phenomenon that allows a a disposed seed to flourish into food, will take money given away and return it many times over. This is the emunah of Seder Zra'im.
We must believe and know that tzedaka is no different than an any other investment in this world. Just like we believe in planting and investing, we need to believe that a return on Tzedaka is perfectly natural and in line with the way the world works. If you choose to eat more food and leave over, remember you are trading great wealth for a few extra crumbs.