Rain Forest Soils
On viewing the lush plant growth of a tropical rain forest, most people would conclude that the soil beneath it is rich in nutrients. However, although rain forest soils are highly variable, they have in common the fact that abundant rainfall washes mineral nutrients out of them and into streams. This process is known as leaching. Because of rain leaching, most tropical rain forest soils have low to very low mineral nutrient content, in dramatic contrast to mineral-rich grassland soils. Tropical forest soils also often contain particular types of clays that, unlike the mineral-binding clays of temperate forest soils, do not bind mineral ions well. Aluminum is the dominant cation (positively charged ion) present in tropical soils; but plants do not require this element, and it is moderately toxic to a wide range of plants. Aluminum also reduces the availability of phosphorus, an element in high demand by plants.
Select the TWO answer choices that, according to paragraph 1, help explain the relatively low mineral nutrient content of most tropical rain forest soils. To receive credit, you must select TWO answers.
∙The water in the streams flowing through tropical rain forests generally has relatively low mineral content.
∙The range of plants that grow in tropical rain forests is so wide that almost every kind of mineral in the soil gets used up.
∙The clays contained in tropical rain forest soils do not bind mineral ions well.
∙Abundant rainfall causes minerals to leach out of the soil.
According to paragraph 1, one reason that the relatively high levels of aluminum in rain forest soils present a problem for plants is that aluminum
∙reduces the mineral-binding capacity of soils
∙is somewhat toxic to plants
∙makes too much phosphorus available to plants
editordoesnotcontainamaintype∙is present as a positively charged ion in tropical rain forest soils
High moisture and temperatures speed the growth of soil microbes that decompose organic compounds, so tropical soils typically contain far lower amounts of organic materials (humus) than do other forest or grassland soils. Because organic compounds help loosen compact clay soils, hold water, and bind mineral nutrients, the relative lack of organic materials in tropical soils is deleterious to plants. Plant roots cannot penetrate far into hard clay soils, and during dry periods, the soil cannot hold enough water to supply plant needs. Because the concentration of dark-colored organic materials is low in tropical soils, they are often colored red or yellow by the presence of iron, aluminum, and manganese oxides; when dry, these soils become rock hard. The famous Cambodian temples of Angkor Wat , which have survived for many centuries, were constructed from blocks of such hard rain forest soils.
According to paragraph 2, clay soils that contain relatively low amounts of organic materials have all of the following disadvantages for plants EXCEPT
∙poor water retention
∙poor root penetrability
∙few soil microbes
∙low levels of mineral nutrients
Paragraph 2 supports the idea that, as compared with rain forest soils, grassland soils
∙contain a greater variety of soil microbes
∙contain less organic material
∙are able to hold more water
∙are generally lighter colored
Why does the author mention “Angkor Wat” ?
∙To show that rain forest soils are essentially the same today as they were many centuries ago
∙To make the point that rain forest soils have certain advantages over other types of soils
∙To illustrate how colorful rain forest soils can sometimes be
∙To emphasize how hard rain forest soils can become
Paragraph 2 suggests the idea that compared with grassland soils, tropical rain forest soils have all of the following characteristics EXCEPT
∙soil microbes grow more slowly
∙contain lower amounts of organic materials
∙hold less water
∙contain iron, aluminum and manganese oxides
Given such poor soils, how can lush tropical forests exist? The answer is that the forest`s minerals are held in its living biomass-the trees and other plants and the animals. In contra
st to grasslands, where a large proportion of plant biomass is produced underground, that of tropical forests is nearly all aboveground. Dead leaves, branches, and other plant parts, as well as the wastes and bodies of rain forest animals, barely reach the forest floor before they are rapidly decayed by abundant decomposers-bacterial and fungal. Minerals released by decay are quickly absorbed by multitudinous shallow, fine tree feeder roots and stored in plant tissues. Many tropical rain forest plants (like those in other forests) have mycorrhizal (fungus-root) partners whose delicate hyphae spread through great volumes of soil, from which they release and absorb minerals and ferry them back to the host plant in exchange for needed organic compounds. The fungal hyphae are able to absorb phosphorus that plant roots could not themselves obtain from the very dilute soil solutions, and fungal hyphae can transfer mineral nutrients from one forest plant to another. Consequently , tropical rain forests typically have what are known as closed nutrient systems, in which minerals are handed off from one organism to another with little leaking through to the soil. When mineral nutrients do not spend much time in the soil, they cannot be leached into streams. Closed nutrient systems have evolved in response to the leaching
effects of heavy tropical rainfall. Evidence for this conclusion is that nutrient systems are more open in the richest tropical soils and tightest in the poorest soils.

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