to provide air and water and humus needed by the germs which build up the soluble nitrogen compounds， and (4) to kill the weeds which would use the space and plant foods needed by the growing crops and would choke them out. Proper tillage probably has more to do with thrifty and productive farming than any other one thing. By careful till- age much expense for fertilizers can be saved and the value of the crop produced greatly increased.
GOOD SOIL.
A truck farm.
49.Value of Soils. -Many different factors enter into the deter- mination of the value of a soil. Soils which in one locality would be of great value are almost valueless in other localities. Light sandy soil far from a market， unless transportation facilities are exceptionally good， is almost worthless； the same soil near a city where fertilizers can be easily procured and where early vegetables find a ready market is of great value.
Different soils are adapted to different crops， and where a soil， although not good for many crops， is adapted for raising a crop which in its locality is valuable， the soil is called good. Thus the soil in many parts of Florida， although unsuited for raising most crops， is suited for orange trees and early vegetables， and so is a good soil. The stony soil in certain of the orange regions of California would be an exceedingly poor soil for most crops， but it is good for oranges and therefore it is most valuable.
Summary. -Only a little more than a quarter of the earth"s surface is land， and only a little over a twentieth of this land belongs to the United States. Though there are depths in the ocean greater than our highest mountains， most sea animals live near the surface， while land animals are distributed over hills and valleys. This gives greater variety to life on land.
Water is simply a compound of oxygen and hydrogen. It has many valuable and interesting properties： it is practically incompressible； it is the greatest dissolver of substances that there is； it evaporates readily， giving us rain， and， when put under pressure， our steam power； it has great power of taking up heat， thus regulating the climate of the land near which large bodies of it lie.
The land is made up of various animal， mineral and vegetable substances. But four fifths of it consists of sedimentary rocks， that is， those that were deposited by water， as distinguished from igneous or rocks formed from melted materials within the earth， and metamorphic or rocks that have been changed from sedimentary or igneous rocks.
The soil comes chiefly from the decaying and weathering of rocks. It is divided into local or sedentary soil， that which is formed from the rocks directly beneath it， and transported soil， that which is generally brought down and deposited by water， ice and wind. Soils are classed roughly as gravel， sand， clay， and loam.
The fertility of the soil depends largely upon composition， air and water， ventilation and drainage. Fertile soil must contain nitrogen， potash and lime. The roots of plants must have air to breathe， and water must dissolve the nourishing substances in the soil and bring them to the roots to be absorbed. For this reason the soil should not be packed hard like clay， nor should it be loose like coarse gravel， as clay does not let the water soak through readily， while gravel lets it seep down too fast.
To maintain its fertility， the soil must be frequently cultivated todecrease seepage and evaporation. It must be supplied with freshnourishment by manures or other fertilizers. In some districts where the rain supply is inadequate， irrigation and dry farming are practiced. Different soils are suited to different crops. Without the fertility of the soil， life on the earth would cease.
QUESTIONS
For what would you look if trying to determine whether a land surface had ever been under the sea？
What are the characteristic differences between land and water surfaces and between the conditions to which the animals and plants of each are subjected？
What reasons can you suggest for likening the earth to a water engine？ To what great classes do the rocks in your neighborhood belong？ What examples of rock weathering have you ever seen？ Describe.
Is the soil in your neighborhood local or transported？ Does its character vary much in different places？ Does the fertility vary？
What would you suggest as the causes of these variations？ What is necessary to produce a fertile soil？
How can the fertility of a soil be maintained？
How can the supply of soil water be maintained in dry regions and at times of drought？
What determines the value of a soil？
【中文阅读】
36.陆地与海洋--地球的表面积为197000000平方英里，其中28%是陆 地。它的广阔无垠与绵延无尽，似乎已超出我们的感知范围，但尽管如此，它 还是让我们对美国本土的面积有了一些概念，除去阿拉斯加及其附属岛屿，美 国的陆地面积是它的1/18。
我们可以把地球表面划分为两个半球，一个囊括了大部分的陆地，另一 个则涵盖了大部分的海洋。大陆板块的这种紧凑分布，让不同地域的人们更 加相互靠近，促进了他们的相互往来与沟通交流，因为大陆为他们的交通运 输提供了极大便利。在地球的气候条件下，大部分陆地分布在北极附近而不是在南极附近，也对地球上的居民们极为有利。我们之后还将看到，陆地板 块的不规则分布状况，以及它们被海洋分割开来而不是连在一起，同样对地 球居民很有好处。
37.沧海桑田的变换--人们通过大量的研究考察发现，大陆与海洋的各 自分布并不是一成不变的。一个地方曾经是陆地，后来也可以变成海洋；而那 些曾经是海洋的地方，现在也可能是陆地了。在远离海洋的内陆，人们发现了 嵌在岩石中的海洋贝壳；通过深水探测，一些河流流经的峡谷与河床也被发现 曾经处于海底，而这些地方距离它们现在的入海口非常遥远。一些曾经是海滩 的地方，现在也被发现已经远远高出海平面数百英尺了。
从一些类似的迹象看，瑞典的北部海岸似乎在过去的150年里升高了大约7 英尺，而荷兰的陆地却在逐渐下沉。对马萨诸塞州海岸线的长期观测表明，它 似乎也在以很慢的速度下沉。类似的陆地与海洋的相对升降运动的现象，在全 世界各个地方都有被发现。
在加利福尼亚的靠近洛杉矶港口的圣佩罗，一些古老的海滩也被发现正 在逐渐抬升，依次错层并形成斜坡丘陵。苏斯，这位伟大的奥地利地质学家认 为，海洋与大陆之间显著的相对运动，是由于海洋自身的地质升降运动，而不 是由大陆的地质升降运动造成的。尽管很可能如此，但是海洋与大陆之间的升 降变化也有很多显著的实例，二者之间的海岸线也始终是一条变化中的曲线。
38.陆地表面的特征--相比于海洋表面，陆地表面与之大不相同，因为 陆地表面至少是固定的。它凹凸不平、粗糙而不规则，由一些不同种类的岩石 与土壤构成。它的大部分表面都高出海平面，但也有一小部分地方下沉到了低 于海平面的高度，比如在靠近死海的加利福尼亚帝王谷，有一个叫沙顿海的地 方就是如此。陆地表面长期经受风霜雨雪的侵蚀，长此以往便慢慢改变了最初 的模样。要想轻松地走遍地球大陆的每一个角落，真的很难，原因便是它千差 万别的参差嶙峋。
陆地表面的特征状况即使在很小的区域内也呈现着丰富多样的变化。白天 黑夜，春秋冬夏，温度在陆地上变化很大。陆地上的动物必须尽可能地发挥它 们的肌肉力量，四处游走，到处觅食。因此它们必须高度有组织地群居，以维 持它们在陆地上的生存。水生动物则完全没有这样困难的生存环境，与陆地上 的动物相比，二者之间真可谓天壤之别。
39.水的特性
