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 2022. 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.
degradation: loss of quality
51. According to the passage, all of the following contribute to soil degradation EXCEPT
(A) poor agricultural management
(B) rotation of annual crops
(C) accumulation of salts
(D) lack of water drainage
52. The word deplete in paragraph 2 is closest in meaning to
53. Which sentence below best expresses the essential information in the highlighted sentence in paragraph 3? Incorrect choices change the meaning in important ways or leave out essential information.
(A) The scientific study of soils is improving soil quality in many regions worldwide.
(B) Few people would disagree that soil quality is as diminished as scientists say it is.
(C) The number of scientists who specialize in the study of soils has been decreasing.
(D) Most scientists agree that soil degradation is occurring in many parts of the world.
54. What can be inferred from paragraph 3 about annual crops and common property lands?
(A) They have been part of agriculture for only the past fifty years.
(B) They are not considered to be significant causes of soil degradation.
(C) They produce more income than perennial crops and private lands do.
(D) They may decrease soil fertility and agricultural productivity.
55. According to the passage, which of the following is a serious consequence of irrigation?
(A) Loss of fresh water for drinking
(B) Decrease in the level of rivers
(C) Excessive buildup of salts in the soil
(D) Increase in the rate of soil erosion
56. In what way is salinization a serious problem for agriculture?
(A) Soils with high salt concentrations are destructive to crop plants.
(B) Salinization can lead to soil compaction or to loss of soil cover.
(C) Excessive amounts of water are required to prevent salinization,
(D) Adding salt to fertilizer raises the operating costs of agriculture.
57. The word stunt in paragraph 5 is closest in meaning to
58. The term waterlogging in paragraph 6 means that
(A) logs are floating in water
(B) water is not available
(C) the soil holds too much water
(D) salts are removed from water
59. How are waterlogging and salinization related?
(A) Salinization occurs only where waterlogging is a problem.
(B) Waterlogging raises and concentrates salts near the surface,
(C) Farmers add salts to irrigation water as a way to fertilize soil.
(D) Both waterlogging and salinization contribute to soil erosion.
60. The word which in paragraph 7 refers to
61. Why does the author mention com and wheat yields in paragraph 7?
(A) To illustrate the effect of a small amount of erosion
(B) To explain why erosion affects only certain crops
(C) To describe the result of a shift from traditional practices
(D) To show how poor management influences crop yields
62. Look at the four squares, [A],[B],[C],[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.
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. [A] As the water flows over and through the ground, it dissolves salts, increasing the salinity of the water. [B] 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. [C] As the water evaporates, high concentrations of salts such as sodium chloride are left behind in the topsoil. [D] Salt buildup can stunt crop growth, decrease yields, kill crop plants, and eventually make the land unproductive.
63. Read the first sentence of a summary of the passage. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
Several factors are responsible for the degradation of the world’s soils.
(A) Soils are created through the processes of erosion, organic decay, and accumulation,
(B) Contamination of the soil from air and water pollution is a growing threat.
(C) Agriculture and poor land management practices decrease the quality of soils over time.
(D) Irrigation can diminish soil productivity by causing salts and water to accumulate.
(E) The soils in the San Joaquin Valley contain a clay layer that contributes to waterlogging.
(F) The erosion of topsoil on agricultural lands is a growing problem that affects crop yields.