The mitzva of shemitah commands the Jewish People to stop working their fields every seventh year and promises that, miraculously, Hashem will provide for their needs. When the Jewish People had a high level of bitachon , the amount of food that was harvested in the sixth year was no different from any other year - however, it was able to nourish for three years instead of one. When the level of the Jewish People's trust in Hashem was low, however, then the fields yielded, in actual terms, three times the amount of a normal year. The first way was through a hidden miracle, the second through an open miracle. An open miracle is always a 'second-best' in Hashem's plan.
Man is the creature who is designed specifically to have freedom of choice.
Open miracles are so compelling that they limit Man's freedom of choice. Rav Chaim of Volozhin once asked the Vilna Gaon what the Talmud means when it says that one of Hashem's attributes is 'to be satisfied with His lot'. The Vilna Gaon replied that Hashem's lot is the Jewish People. He would like us to be on a higher level, but nevertheless He is content with us at whatever level He finds us. One of the effects of the mitzva of shemitah was to plant in the hearts of the Jewish People the idea that Hashem, for all His Transcendence, nevertheless supervises every last detail of this world.