135. Read the following passage, then answer the question below.

The Amazon rainforest covers about five and a half million square kilometers across nine countries, an area nearly as large as the continental United States. It holds roughly one in ten of the world's known species, and its river carries more water than any other. Its influence, however, extends far beyond its borders. Through evapotranspiration, the combined release of water from soil and through the leaves of trees, the forest sends enormous amounts of moisture into the air. Some scientists call these air currents 'flying rivers,' since they carry rain to farmland hundreds of kilometers away, including the agricultural regions of southern Brazil.

The forest's productivity is something of a paradox, because its soils are poor. Heavy rains wash nutrients out of the ground, so most nutrients are stored in living plants and in a thin layer of fallen leaves, which decompose and are absorbed so quickly that very little accumulates. This explains why clearing the forest for farming often disappoints: after the trees are burned, crops exhaust the soil in a few years, and farmers move on. Interestingly, archaeologists have found patches of unusually dark, fertile soil called terra preta, created by ancient inhabitants who mixed charcoal, pottery fragments, and waste into the ground. These findings suggest that parts of the Amazon supported dense populations long ago and that the forest was shaped by people more than once believed.

Today, about a fifth of the original forest has been lost or degraded, mainly to cattle ranching and soy farming. Some climate scientists warn of a tipping point: if too much forest disappears, the reduced rainfall might cause large areas to turn into drier, savanna-like vegetation, which would release stored carbon and further reduce rain. Where exactly this threshold lies is uncertain, but the possibility has made protecting the remaining forest more urgent.

Why does clearing the forest for crops often disappoint?