Soil.
土壤。
Soil is a mixture of weathered rock, minerals, humus, air, and water.
土壤是风化岩石、矿物质、腐殖质、空气和水的混合物。
When rocks are weathered, they break down into smaller pieces.
岩石风化后，会分解成更小的碎片。
Sand is a grain of rock.
沙是一粒岩石。
Grains can be carried by water or wind.
谷物可以被水或风携带。
Grains collect in a pile, and when mixed with humus, they become soil.
谷物堆积在一起，当与腐殖质混合时，它们就变成了土壤。
When plants or animals die, they decompose into humus.
当植物或动物死亡时，它们分解成腐殖质。
Humus is rich in nutrients that plants use to grow.
腐殖质富含植物用来生长的营养物质。
There are three main layers of soil.
土壤主要有三层。
The top layer has mostly humus in it.
顶层大部分是腐殖质。
Most plant roots are in the top layer of soil.
大多数植物的根都在土壤的顶层。
The middle layer has less humus and more small rocks and minerals in it.
中间层腐殖质较少，里面的小岩石和矿物较多。
The bottom layer is mostly rock.
底层大部分是岩石。
Some animals live in soil.
有些动物生活在土壤里。
When they dig into the soil, they move humus down.
当它们深入土壤时，腐殖质向下移动。
This makes the soil richer in nutrients.
这使得土壤的养分更丰富。