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Soil Formation and Composition — Reading Comprehension

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MS-ESS2-1

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This comprehensive middle school science passage explores soil formation and composition, perfectly aligned to NGSS standard MS-ESS2-1. Students will learn how soil forms through the weathering of bedrock and the accumulation of organic matter over time. The passage explains the main factors influencing soil formation, such as climate, organisms, parent material, topography, and time, and details the quantitative composition of soil: minerals (45%), water (25%), air (25%), and organic matter (5%). Students will also discover the different soil horizons (O, A, B, C, and R), their characteristics, and how soil types vary across climates and ecosystems. The text emphasizes soil's role as a living ecosystem, housing billions of organisms, and highlights its essential functions—supporting plant life, filtering water, storing carbon, and providing habitat. With integrated audio, a Spanish translation, a differentiated reading version, glossary, comprehension activities, and graphic organizers, this resource offers a complete, standards-based learning experience for grades 6–8.

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Soil Composition and Structure.
Wikimedia Commons (Public domain).

Soil is a vital natural resource that supports all terrestrial life on Earth. Every handful of soil is the result of a complex, centuries-long process that transforms solid rock and decaying organic material into a mixture rich with life. Scientists study soil to understand how it forms, what it contains, and how it sustains ecosystems. Without healthy soil, plants could not grow, water could not be filtered, and most land animals—including humans—would not survive.

How Soil Forms: The Mechanisms Behind Soil Creation

The process of soil formation begins with bedrock, also known as parent material. Over time, weathering—caused by wind, water, temperature changes, and living organisms—breaks the bedrock into smaller particles. Meanwhile, plants, animals, and microbes add organic matter by dying and decomposing, enriching the developing soil. Soil formation is influenced by five main factors: parent material, climate, living organisms, topography (the landscape’s shape), and time. For example, soil in a rainy forest develops faster and contains more organic matter than soil in a dry desert. The process can take hundreds to thousands of years, making soil a non-renewable resource on a human timescale.

Soil Composition and Structure: Layers and Life

Soil is not just dirt. It is a mixture of minerals (45%), water (25%), air (25%), and organic matter (5%). Each part plays a role: minerals provide nutrients, water and air support life, and organic matter fuels decomposition and fertility. Soil is organized into horizons or layers. The O horizon is the top layer, full of organic material like leaves and twigs. Beneath it, the A horizon (topsoil) is richest in nutrients and supports most plant roots. The B horizon (subsoil) accumulates minerals washed down from above, while the C horizon consists of weathered parent material. The deepest layer, the R horizon, is solid bedrock. Each horizon has unique properties that affect water movement, root growth, and soil fertility.

Soil as an Ecosystem: Diversity and Functions

Soil is a living ecosystem teeming with life. A single handful can contain billions of bacteria, fungi, insects, and other tiny organisms. These organisms help break down organic matter, recycle nutrients, and maintain soil structure. Soil types vary depending on climate and vegetation; for example, prairie soils are dark and fertile, while tropical soils may be red and low in nutrients. Healthy soil supports plant growth, filters water, stores carbon, and provides habitat for countless species. Degraded or eroded soil can lead to food shortages, polluted water, and loss of biodiversity.

Understanding soil formation and composition helps us protect this essential resource for farming, clean water, and climate regulation. Scientists use soil studies to develop better land management practices and address environmental challenges.

Interesting Fact: A single teaspoon of healthy soil can contain more living organisms than there are people on Earth!

What is the main process that breaks down bedrock to form soil?

WeatheringErosionCondensationPhotosynthesis

Which of the following is NOT a factor that influences soil formation?

Parent materialClimateOcean tidesOrganisms

According to the passage, what percentage of typical soil is made up of minerals?

45%25%60%5%

What is found in the O horizon of soil?

Organic material like leaves and twigsSolid bedrockOnly mineralsWater and air only

What is a main function of soil mentioned in the passage?

It stores carbonIt creates rainIt produces sunlightIt forms mountains

What does the word 'topography' mean as used in the passage?

The shape and features of the landThe color of soilThe climate of an areaThe number of plants living in an area

What does the passage suggest about soil in a rainy forest compared to a desert?

It forms faster and contains more organic matterIt is always sandierIt does not support plant lifeIt has less water

Why is soil considered a non-renewable resource on a human timescale?

Because it forms very slowly over hundreds or thousands of yearsBecause it is always being replacedBecause it is made by machinesBecause it cannot be damaged

True or False: Soil is made up mostly of organic matter.

TrueFalse

True or False: A single handful of soil can contain billions of living organisms.

TrueFalse