在上一篇文章中，青岛朗阁智美的老师为大家留下了一道托福阅读题目，那么接下来，我们就来看这道托福阅读最后一题的正确解题方法，托福考生们快跟着小编来看~

　　首先，总结段落大意。大部分托福阅读文章的段落大意都是每段话首尾句结合在一起即可。如果首尾句结合在一起之后，依然无法判断，可以扫读一下段落中间部分内容(不要看for example, for instance, such as, like, 破折号，冒号之后的内容以及括号里边的内容，因为都不是中心思想)。大家可以看下面一段话：

　　Paragraph 1: Research has shown that certain minerals are required by plants for normal growth and development. The soil is the source of these minerals, which are absorbed by the plant with the water from the soil. Even nitrogen, which is a gas in its elemental state, in normally absorbed from the soil as nitrate ions. Some soils are notoriously deficient in micro nutrients and are therefore unable to support most plant life. So-called serpentine soils, for example, are deficient in calcium, and only plants able to tolerate low levels of this mineral can survive. In modern agriculture, mineral depletion of soils is a major concern, since harvesting crops interrupts the recycling of nutrients back to the soil.

　　通过上面的方法，我们可以判断出来，托福阅读文章中的这段话主旨应该是第一句和最后一句，即：

　　“Research has shown that certain minerals are required by plants for normal growth and development.”和“In modern agriculture, mineral depletion of soils is a major concern, since harvesting crops interrupts the recycling of nutrients back to the soil.”

　　通过阅读这两句话，我们发现，本段话的主旨应该是：植物需要某些矿物质进行正常的生长和发展。再阅读本段话中间部分内容，发现是在讲土壤和矿物质之间的关系，印证了这个托福阅读段落大意的总结是没有问题的。

　　Paragraph 2: Mineral deficiencies can often be detected by specific symptoms such as chlorosis (loss of chlorophyll resulting in yellow or white leaf issue), necrosis (isolated dead patches), anthocyanin formation (development of deep red pigmentation of leaves or stem), stunted growth, and development of woody tissue in an herbaceous plant. Soils are most commonly deficient in nitrogen and phosphorus. Nitrogen-deficient plants exhibit many of the symptoms just described. Leaves develop chlorosis; stems are short and slender, and anthocyanin discoloration occurs on stems, petioles, and lower leaf surfaces. Phosphorus-deficient plants are often stunted, with leaves turning a characteristic dark green, often with the accumulation of anthocyanin. Typically, older leaves are affected first as the phosphorus is mobilized to young growing tissue. Iron deficiency is characterized by chlorosis between veins in young leaves.

　　用同样的方法判断出，这个托福阅读段落的主旨(首尾句)：植物缺乏某些矿物质，会产生一些症状。

　　Paragraph 3: Much of the research on nutrient deficiencies is based on growing plants hydroponically, that is, in soilless liquid nutrient solutions. This technique allows researchers to create solutions that selectively omit certain nutrients and then observe the resulting effects on the plants. Hydroponics has applications beyond basic research, since it facilitates the growing of greenhouse vegetables during winter. Acroponics, a technique in which plants are suspended and the roots misted with a nutrient solution, is another method for growing plants without soil.

　　本托福阅读段主旨(首尾句)：我们可以用水培法来检测矿物质缺乏的症状。

　　Paragraph 4: While mineral deficiencies can limit the growth of plants, an overabundance of certain minerals can be toxic and can also limit growth. Saline soils, which have high concentrations of sodium chloride and other salts, limit plant growth, and research continues to focus on developing salt-tolerant varieties of agricultural crops. Research has focused on the toxic effects of heavy metals such as lead, cadmium, mercury, and aluminum; however, even copper and zinc, which are essential elements, can become toxic in high concentrations. Although most plants cannot survive in these soils, certain plants have the ability to tolerate high levels of these minerals.

　　本托福阅读段主旨(首尾句)：矿物质缺乏或过多都会限制植物生长。

　　Paragraph 5: Scientists have known for some time that certain plants, called hyper accumulators, can concentrate minerals at levels a hundredfold or greater than normal. A survey of known hyper accumulators identified that 75 percent of them amassed nickel, cobalt, copper, zinc, manganese, lead, and cadmium are other minerals of choice. Hyper accumulators run the entire range of the plant world. They may be herbs, shrubs, or trees. Many members of the mustard family, spurge family, legume family, and grass family are top hyper accumulators. Many are found in tropical and subtropical areas of the world, where accumulation of high concentrations of metals may afford some protection against plant-eating insects and microbial pathogens.

　　本托福阅读段主旨(首尾句)：科学家们知道某些植物(超积累物种)可以吸收高含量的矿物质。

　　Paragraph 6: Only recently have investigators considered using these plants to clean up soil and waste sites that have been contaminated by toxic levels of heavy metals – an environmentally friendly approach known as phytoremediation. This scenario begins with the planting of hyper accumulating species in the target area, such as an abandoned mine or an irrigation pond contaminated by runoff. Toxic minerals would first be absorbed by roots but later relocated to the stem and leaves. A harvest of the shoots would remove the toxic compounds off site to be burned or composted to recover the metal for industrial uses. After several years of cultivation and harvest, the site would be restored at a cost much lower than the price of excavation and reburial, the standard practice for remediation of contaminated soils. For examples, in field trials, the plant alpine pennycress removed zinc and cadmium from soils near a zinc smelter, and Indian mustard, native to Pakistan and India, has been effective in reducing levels of selenium salts by 50 percent in contaminated soils.

　　本托福阅读段主旨(首尾句)：科学家们开始用这些超积累物种治理土壤污染，该方法叫植物修复。

　　接下来，根据上面总结的中心思想，排除错误选项：

　　A. Some plants are able to accumulate extremely high levels of certain minerals and thus can be used to clean up soils contaminated with toxic levels of these minerals. 与paragraph 5的主旨一致，选择。

　　B. Though beneficial in lower levels, high levels of salts, other minerals, and heavy metals can be harmful to plants. 与paragraph 4主旨一致，选择。

　　C. When plants do not absorb sufficient amounts of essential minerals, characteristic abnormalities result. 与paragraph 1主旨一致，选择。

　　D. Because high concentrations of sodium chloride and other salts limit growth in most plants, much research has been done in an effort to develop salt-tolerant agricultural crops.

　　该托福阅读选项的重心在sodium chloride and other salts, 我们用scanning的方法，定位到paragraph 4中间部分，即“Saline soils, which have high concentrations of sodium chloride and other salts, limit plant growth, and research continues to focus on developing salt-tolerant varieties of agricultural crops.”该句是第四段第二句，是个并列句，并没有提到任何选项中的因果关系和意思，排除。

　　E. Some plants can tolerate comparatively low levels of certain minerals, but such plants are of little use for recycling nutrients back into depleted soils. 本选项的意思是即便植物可以忍受较低含量的某种矿物质，但是这种植物对将营养循环到土壤里没有用。用scanning定位到第一段“So-called serpentine soils, for example, are deficient in calcium, and only plants able to tolerate low levels of this mineral can survive. In modern agriculture, mineral depletion of soils is a major concern, since harvesting crops interrupts the recycling of nutrients back to the soil.” 这两句话和托福阅读选项的意思完全不一样，并没有转折的意思，排除。

　　F. Mineral deficiencies in many plants can be cured by misting their roots with a nutrient solution or by transferring the plants to a soilless nutrient solution.根据选项的mineral deficiency可以定位到托福阅读原文第三段“Much of the research on nutrient deficiencies is based on growing plants hydroponically, that is, in soilless liquid nutrient solutions. This technique allows researchers to create solutions that selectively omit certain nutrients and then observe the resulting effects on the plants.” 这两句话是为了证明用水耕方法观测矿物质缺乏的影响，并没有说矿物质缺乏可以被cured,排除。

　　因此，正确答案为：A, B, C。

　　好了，关于这道托福阅读题目的讲解到这里就结束了，相信通过青岛朗阁智美老师的讲解，大家应该明白如何做这托福阅读最后一题了吧，希望大家以后能都不用再怕这最后一题了~青岛朗阁智美专注于托福培训，还有问题可以咨询我们的在线客服哦~

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