INTRODUCTION TO SOILS - A LABORATORY MANUAL

SOIL 252L - Soils Laboratory

Department of Agronomy and Horticulture

New Mexico State University

7th Edition, 1993

Contributors:

B. Buchanan

S. Khresat

W. Lindemann

M. Pelletier-Hunyadi

P. Purves

P. Shelton

J. Vinson

B. Williams

T. Jones

B. McCaslin

TABLE OF CONTENTS

Chapter Page

1. Rocks and Minerals . . . . . . . . . . . . . . . . 2

2. Field Trip: Soil Formation . . . . . . . . . . . 7

3. Soil Sampling and Subsampling . . . . . . . . . . 12

4. Soil Texture . . . . . . . . . . . . . . . . . . 20

5. Soil Bulk Density and Particle Density . . . . . . 33

6. Field Trip: Soil Morphology . . . . . . . . . . . 40

7. Soil Colloids . . . . . . . . . . . . . . . . . . 48

8. Soil pH and Soluble Salts . . . . . . . . . . . . 57

9. Soil Water . . . . . . . . . . . . . . . . . . . 66

10. Water Movement in Soil . . . . . . . . . . . . . . 76

11. Soil Biology and Microbiology . . . . . . . . . . 82

12. Field Trip: Land use . . . . . . . . . . . . . . 87

13. Soil Fertility . . . . . . . . . . . . . . . . . 93

Chapter 1

ROCKS AND MINERALS

I. ROCKS

Rocks are aggregates of one or more minerals. There are three classes of rocks based upon their formation mode. These are; Igneous, Sedimentary, and Metamorphic.

IGNEOUS ROCKS: These rocks are formed from the cooling and solidification of molten magma (volcanoes). They are the most abundant rocks in the earth's crust. Igneous rocks are classified on their chemical composition and mode of occurrence. According to their chemical composition igneous rocks are classified as follows. Acidic igneous rocks contain more than 65 % silica and are usually light in color (e.g. Granite). Basic igneous rocks contain less than 50% silica and are usually dark colored (e.g. Basalt). Neutral igneous rocks contain between 50 and 65% silica (e.g. Andesite).

According to their mode of occurrence (location of the rocks when they were formed) igneous rocks are classified as follows.

Intrusive rocks have formed deep within the earth's crust under a relatively slow cooling process and have coarse textured grains (e.g. Granite, Gabbro). Extrusive rocks have formed near the earth's surface under a relatively rapid cooling process and have fine textured grains (e.g. Rhyolite, Basalt).

SEDIMENTARY ROCKS. Sedimentary rocks are classified as either clastics or precipitates. Clastics are formed through physical means where fragments or sediments of decaying igneous rocks are consolidated into solid masses by pressure or cementing agents. The cementing agents range from clay to calcium carbonate.(e.g. Limestone). The precipitates are produced either by chemical or biochemical precipitation of soluble ions from solution.

METAMORPHIC ROCKS. Metamorphic rocks are classified on the basis of their structure and parent rock. Foliated or banded metamorphic rocks have parallel layers of different minerals. The color may be uniform or alternate between light and dark colored layers (e.g. Slate). The non-foliated or non-banded types have random patterns of minerals and colors (e.g. Marble).

II. MINERALS

Minerals are naturally occurring inorganic substances which have a certain chemical composition, distinctive physical properties, and are classified according to these characteristics. Based upon origin there are primary and secondary minerals.

Primary minerals are formed at elevated temperatures from igneous and metamorphic rocks. They are present in the original rock form from which the soil was formed. (e.g. Feldspars).

Secondary minerals are formed in soil by decomposition of the primary minerals and their recomposition into new ones (e.g. Calcite).

Physical properties used in the identification of minerals are cleavage or fracture, color, streak, and hardness. Cleavage is the property of breaking along one or more smooth, parallel planes on the crystal axes of the mineral. Cleavage is described by the number of directions and the angle between cleavage surfaces. Some minerals break in directions other than along crystal axes forming regular patterns. This property is called fracture.

Streak is the color of the finely powdered mineral and is determined by rubbing the mineral on a piece of unglazed porcelain.

Hardness is the resistance that a mineral offers to scratching or abrasion. Harder minerals weather more slowly than softer minerals (see Moh's Scale of hardness of Minerals).

Lab Exercise:

- Examine the samples of igneous rocks in the lab and determine their properties--acidic or basic, extrusive or intrusive.

- Examine the samples of the sedimentary rocks and determine their classification (clastics or precipitates).

- Examine the samples of the metamorphic rocks and determine their structure.

- Examine the selected minerals in the lab. and determine their physical properties, and chemical composition. (Use the information in Table 1.1 to help you in studying the minerals).

MOH'S SCALE OF HARDNESS OF MINERALS

Study the selected minerals on the desks and determine their physical properties, chemical composition, origin, and potential supply of plant nutrient. Use the information in Table 1.1 your text, and the mineralogy texts available in the laboratory to assist in your study.

TABLE 1.1 CLASSIFICATION OF MINERALS