Narrator: Listen to part of a lecture in the geology class.

旁白：请听一段地质学讲座的节选片段。

Professor: So, we all know soil.

教授：所以呢，我们都认识土壤。

It's important to plant growth, right?

土壤对于植物生长很重要，对吗？

And we know that there're different types of soil in different places and that some soils are more fertile than others.

我们还知道，在不同地方有不同种类的土壤，一些土壤比另一些土壤更加肥沃。

But what is soil?

但是土壤到底是什么呢？

And how's it formed?

土壤是怎么形成的呢？

Well, we're going to go into this in some depth, but for now let's just lay down the basics.

我们马上要来深度探讨这个问题，但是现在我们先要看一看一些基础的问题。

Soil is composed of two kinds of material: inorganic material, basically small pieces of rock, and organic material, which is animal and plant matter.

土壤是由两种东西构成的：无机物质，基本上是小的岩石颗粒，还有有机物质，就是动物和植物的尸体。

OK. So, what do you think?

好的。所以你们觉得呢？

If I mix bits of rock with composted vegetables, will I get soil?

如果我把堆肥的植物和岩石混合起来，我能得到土壤吗？

The answer is no, because the formation of soil is a dynamic process.

答案是不能，因为土壤的形成是一个动态的过程。

It involves not only the initial inputs, the raw materials, but also the transformation of those materials, and the movement of some of the materials and the loss of others.

它涉及到不仅仅是开始的物质输入，不仅是原材料，还有这些原材料的转化，其中一些材料的迁移以及一些物质的丧失。

So, the inputs are bits of rock and organic matter.

所以，前期的输入过程只是冰山一角。

Now, the bits of rock, the inorganic input to soil, uh...they come from the breakdown of rocks on Earth's surface through a process called weathering.

现在说到岩石的碎片，那些形成土壤的无机物，嗯……它们来源于地表岩石的破碎，这个过程叫做风化作用。

Weathering can be either physical or chemical.

风化作用既可以是物理的，也可以是化学的。

Physical weathering, uh...that's when exposure to the elements over time causes a rock to break up and eventually disintegrate.

物理风化作用，嗯……就是岩石长时间暴露在某些物质里，导致其分裂最终彻底分解。

Uh... of course, some rocks are more resistant to physical weathering than others.

嗯……当然了，一些岩石比另一些岩石更能抗风化作用。

If you think of the sand particles in soil, those are the result of physical weathering, and they have the same chemical composition as the original rock.

如果你想到土壤里的沙粒，它们就是物理风化作用的结果，它们和原来的岩石的化学成分是一样的。

Now, chemical weathering, uh... that's the chemical breakup of rocks.

现在说说化学风化作用，嗯……这就是岩石的化学分解。

It differs from physical weathering in that the chemical properties of the minerals are actually changed.

这个过程和物理风化作用不一样，因为化学风化作用使得矿石的化学成分发生了改变。

The clay minerals you find in soil are the result of chemical weathering.

我们在土壤里发现的黏土矿物质就是化学风化作用的结果。

Clay minerals are called secondary minerals, because their composition has been altered.

黏土矿物质是第二等的矿物质，因为它的成分发生了改变。

Okay. So we have weathered rock, which needs to be combined with organic matter.

好的。所以我们有风化的岩石，风化的岩石需要和有机物来结合（以形成土壤）。

So what does the organic input consist of?

所以有机物的输入都是由什么构成的呢？

It's the remains of plants and animals, but mostly plants.

那就是动植物的尸体，但是主要还是植物的尸体。

Now, just as rocks are broken down by weathering, the animal and plant residues are broken down, too.

就像岩石会在风化作用中分解一样，动植物的尸体也会分解。

They're reduced to simple chemicals by microorganisms in a process called mineralization.

它们被微生物分解了，这个过程叫做矿化。

And just as some rocks are more resistant than others to weathering, the compounds found in the soil's organic input resist mineralization at different rates.

就像一些岩石比另一些岩石更加不容易被风化一样，土壤的有机物输入里的不同的有机物的矿化速率不同。

The compound cellulose is the major constituent of most plant tissue.

纤维素化合物是大多数植物组织的主要成分。

It mineralizes relatively quickly.

它矿化地非常快。

But there are woody substances in certain plants that strengthen the cell walls.

但是在某些植物里也有木质的物质，这种物质可以加强细胞壁。

They are found in smaller concentrations and their mineralization can take several years.

它们的浓度更低，矿化可以花费好几年。

Weathering and mineralization transform the inorganic and organic inputs in a number of ways.

风化和矿化在多年间逐渐转化着土壤的无机物和有机物输入。

And it's partly from these transformations that soil gets its unique properties.

也是部分因为这些转化过程，土壤就获得了它独特的成分。

How does it work?

所以它是怎么工作的呢？

Uh... take the dark brown color of soil.

嗯……举棕黑色土壤作为例子。

After microorganisms have broken down the cellulose, we're left with two things: the microbe's waste and the more resistant plant material that microorganisms can't break down easily.

在微生物分解完纤维素之后，我们可以得到两种东西：细菌的垃圾和更加难以腐蚀的植物组织，这些组织更难被微生物分解。

These materials ultimately get transformed into a new material called humus.

这些材料最终会被转化成一种新的叫做腐殖质的物质。

And when humus is combined with the clay minerals in soil, that's what gives soil its dark brown color.

当腐殖质和土壤中的黏土矿物质结合，这就使得土壤变成了棕黑色。

So now, if we've got clay and humus, these transformed materials, and we mix them together, we've got something very close to soil.

所以，如果我们有了黏土和腐殖质，这些已经经过了转化的材料，我们把它们混合到一起，就可以得到一种类似于土壤的东西。

But soil isn't static, and there're still other processes that go into the formation of soil: the movement and loss of materials.

但是土壤不是一成不变的，还有其他的过程会融入进土壤的形成：物质的流动和流失。

The soil in any location isn't a uniform mixture.

任何地点的土壤都不是一个统一的混合物。

Its composition varies with depth.

它的组成随着深度的不同而不同。

You see, mineral and organic materials move through soil vertically.

你看，矿物质的和有机物的材料可以在土壤内部垂直地移动。

Some materials move more easily than others.

一些物质比另一些物质移动更慢。

Water carries the more mobile materials from the upper level of soil to the lower levels.

水流将更多的移动物质从高层土壤带入到了低层的土壤。

So the upper levels of the soil eventually get depleted of these materials, while the lower levels get enriched with them.

所以高层的土壤最终流失掉了这些物质，低层的土壤则储存了更多的这些物质。

And that creates distinct layers of soil as far down as the rock underlying the soil.

这就创造了不同层次的土壤，土壤一直分层到触碰到最底部的岩石。

And the materials that dissolve easily in water can get lost completely if the water carries them horizontally out of the soil and into rivers.

容易溶解在水中的物质则会在水流把它们从土里带走，流入河流的时候流失掉。

Now, of course, new mineral and organic material will be deposited at the surface and become incorporated into the soil, but you see how the processes of movement and loss contribute to the formation of soil.

当然了，在土壤表面会继续沉积新的矿物质和有机物，这些物质被吸收进土壤，这样我们就明白了土壤物质的移动和流失在土壤形成中的作用。