Is Zakat obligatory upon a merchant brother who lost his money and entered a cycle of debt due to a failed import deal, knowing that he owns a shared house and a productive agricultural land that has not yet yielded? And if it is obligatory, would the Zakat be substantial?

If your indebted brother is unable to repay his debt, and he does not possess anything beyond his basic needs with which to settle it, then he is eligible for zakat, and giving it to him is more deserving than giving it to others. However, if he is able to repay the debt, even if by selling possessions that exceed his basic needs, then he is not eligible for zakat. And if he is eligible, give him according to his need, even if that constitutes the majority of the zakat.