小站备考
托福
托福阅读
托福Delta阅读

托福红delta-test5阅读Look at the four squares[■],A, B, C, and D, which indicate where the following sentence could be a答案解析

展开
Soil Quality
Tip:单击查看句义;划选/双击查生词
Soil is a renewable resource, but only on a very long time scale, as it takes hundreds or thousands of years for the natural processes of erosion, organic decay, and accumulation to create soils. Soil quality and the potential to produce crops can vary enormously from region to region and among various soil types.

One important factor affecting the productivity of soils over time is agriculture. Top- quality lands are brought into production earlier because of their higher potential to produce food. As more and more land has been brought under agricultural production, the average quality of land has decreased, reducing potential productivity per hectare. Crops deplete soil fertility by consuming nutrients, and this eventually reduces crop yields. Poor management practices lead to soil compaction and soil pollution as well as loss of soil cover. Without proper management and the constant addition of nutrients and energy in the form of fertilizers and irrigation, crop production falls over time.

Within the scientific community, there is little doubt that soil quality is diminishing in many areas around the world. Scientists have found that the quality of one quarter of the world’s soils has experienced some degradation, and the pace of degradation has accelerated over the past 50 years. The loss of soil fertility has caused a slowing in the growth of agricultural productivity. Annual crops tend to degrade soils more than perennial crops, and common property lands generally suffer more degradation than private lands.

Today, irrigated cropland produces about one-third of the world’s food. Approximately 18 percent of the world’s cropland is irrigated, and scientists project this amount to double by 2020. Irrigation can increase crop yields per hectare to two or three times the yields of land watered only by rain. However, there are also some harmful side effects. Besides increasing crop productivity in the short run, irrigation can lead to sharp drops in crop productivity in the long run by causing excessive salt buildup and rising water tables.

One of the most critical soil quality problems related to irrigation is the increase in concentration of dissolved salts. This process, salinization, affects an estimated one-fourth of the world’s irrigated cropland. In some places, irrigation water contains as much as 3.5 tons of salt per 1,000 cubic meters. [■]As the water flows over and through the ground, it dissolves salts, increasing the salinity of the water. [■]Since some crops require 6,000 to 10,000 cubic meters of water per hectare, land can receive tens of tons of salt per hectare. [■]As the water evaporates, high concentrations of salts such as sodium chloride are left behind in the topsoil. [■]Salt buildup can stunt crop growth, decrease yields, kill crop plants, and eventually make the land unproductive.

A problem that often accompanies salinization in dry regions is waterlogging, which often occurs when farmers apply heavy amounts of irrigation water in an attempt to prevent salts from accumulating. However, unless the water drains properly, it collects underground and gradually raises the water table closer to the surface, thereby bringing salts to the surface and concentrating them. Saltwater then envelops the fragile root systems of plants, killing the plants and converting fertile fields to wet deserts. This is a particularly serious problem in California’s heavily irrigated San Joaquin Valley, where soils contain a clay layer that prevents water from flowing through the ground. Worldwide, at least one-tenth of all irrigated land is subject to waterlogging.

Another serious soil problem is erosion, the loss of soils from water and wind action.Soil erosion occurs on agricultural land without vegetative cover for protection or because of poor agricultural management. Scientists estimate that topsoil on cultivated land is being lost 16 to 300 times faster than it is being replaced. In many parts of the world, a shift away from traditional agricultural practices is placing greater pressure on the soil, which in turn is causing an increase in soil erosion rates. Studies in the United States suggest that the loss of just 2.5 centimeters of topsoil reduces corn and wheat yields by 6 percent.

Glossary:

degradation: loss of quality


12.Look at the four squares[■],A, B, C, and D, which indicate where the following sentence could be added to the passage. Where would the sentence best fit?

Unless they are flushed or drained from the soil, their buildup may cause a number of serious problems. .

你的答案:
正确答案:D
题目解析:
 后才能查看题目解析,还没有账号? 马上注册
在添加的句子中,they 和 their 指代了上一句中的salts such as sodium chloride。增加的句子介绍了由盐积累引起a number of serious problems的话题 ,下一句话通过实例展开:...stunt crop growth, decrease yields, kill crop plants, and eventually make the land unproductive.

学习页面

Medi

terr

anean

加强 + 政府 + 名词后缀

加强的政府——管理

原文例句

加入生词

本文生词 0

色块区域是你收藏过的生词;

查询次数越多,颜色越深哦~

显示文中生词

登录后才能收藏生词哦,现在登录注册>

本文重点词 45

文中加粗单词为本文重点词;

根据词频与核心词范围精心挑选,托福考试必掌握词汇。

显示文中重点词
学习本文词汇

文中划选/双击的生词、加粗重点词已收纳至词盒

可随时点击词盒查看哦~

只有在词句精学模式下才能开启词盒功能哦~

我知道了

词盒
收藏
笔记
我的笔记
5000
保存
反馈