CHAPTER 12

SOILS AND SOIL DEVELOPMENT

 

INTRODUCTION

Soils are indispensable resources, without which life could not exist.

 

Soil is a dynamic resource capable of supporting a vegetative cover.

 

Soil act as a buffer zone where the lithosphere, atmosphere, hydrosphere and biosphere can interact.

 

PRINCIPLE SOIL COMPONENTS

Soil is made up of both insoluble mineral material and soluble chemicals in solution.

 

The most common minerals found in soils are combinations of the most common elements of the earth’s crust, namely silicon, aluminum, oxygen, and iron.

 

INORGANIC MATERIALS

These elements and chemicals that make up soil come from many sources.

Some are derived from the weathering of the underlying rocks or sediment.

Some are added or moved by chemicals in solution.

Other parts come from the air or are derived from organic activity, some of which helps disintegrate rocks.

 

SOIL WATER

Precipitation brings water to the soil. When rain hits the surface, some of it runs off and some gets absorbed into the ground.

 

The water that percolates down is pulled by gravity and is called gravitational water.

 

GRAVITATIONAL WATER

Gravitational water also moves finer grained particles down through the soil.

 

This removal of soil components from the topsoil is called eluviation.

 

The particles then are deposited at a lower depth through illuviation.

 

ELUVIATION AND ILLUVIATION

By these processes, the upper soil becomes coarser (eluviation) over time and less able to hold water.

 

In some cases, the subsoil gets very dense (illuviation) and a clay hardpan is formed.

 

LEACHING

Gravitational water affects the chemical composition of a soil moving the soluble chemicals (nutrients) deeper into the soil profile. This depletion is called leaching.

 

Leaching, eluviation, and illuviation are the causes of the characteristic stratification of soils.

 

 

CAPILLARY WATER

Some soil water is held to the surface of the individual soil particles by surface tension and this is called capillary water.

Capillary water can move in any direction in the soil profile from wetter to dry areas.

•      Storage during dry periods

Capillary water also moves chemicals in solution. Lime deposits can form and produce a cement-like layer called caliche.

 

SOIL AIR

A large part of soil (up to 50%) is made up of the voids between soil particles. When not filled with water, these spaces are filled with air.

 

Soil air supplies the oxygen and carbon dioxide necessary for plants.

•      Worms

ORGANIC MATTER

Soils also contain organic matter.

The decayed remains of plant and animal material, partially transformed by bacterial action, is called humus.

Humus is important: 

•      Catalyst in the extraction of nutrients

•      Improves soil structure

•      Holds water

•      Source of food microscopic organisms

 

CHARACTERISTICS OF SOIL

The characteristics of soil include:

•      Color

•      Amount of Humus

•      Texture

•      Structure

•      Acidity/Alkalinity

 

COLOR

The color of soil is indicative of the type of minerals in the soil and the amount of humus.

 

•      Red and Yellow

•      Light Gray

•      Black

TEXTURE

Soil texture varies according to the size of the particles- clay – silt – sand - gravel.

 

Texture is important in the ability of soil to hold water and air.

 

Loam soils are those with textures that are a mixture of silt, clay and sand which are ideal for agriculture.

 

 

STRUCTURE

Structure and texture influence porosity and permeability.

 

Porosity is a measure of how much space in a soil that can contain water.

 

Permeability is a measure of the rate at which fluids pass through the soil.

 

 

ACIDITY AND ALKALINITY

The acidity of alkalinity of a soil is measured on a scale of 0 to 14, called the pH scale.

This is a measure of the concentration of highly reactive hydrogen ions present in the soil moisture.

A little acidity is good because the nutrients can be dissolved and are available to the plant.

Too acidic and all the nutrients get leached out

 

FACTORS AFFECTING SOIL FORMATION

There are several factors that affect the development of soils including:

•      Climate

•      Parent Material

•      Organic Activity

•      Topography

•      Time

 

CLIMATE

Climate is usually the dominant factor in soil formation.

•      Precipitation

•      Temperature

•      Humus

 

PARENT MATERIAL

Soil is derived form the weathered fragments of rock called parent material.

•      Residual Parent Material

•      Transported Parent Material

 

ORGANIC ACTIVITY

Plants and animals affect soil formation.

 

Tree roots and earthworms make the soil looser, and more porous.

 

Vegetative cover.

•      ­ Plant Life  ­ Available Humus

•      ­ Humus   ­Soil Fertility

 

 

 

 

 

NUTRIENT CYCLES

Nutrient cycles are important to plant growth.

 

•      Mid-Latitude Climates

•      Tropical Climates

•      Dry Climates

TIME

The younger the soil, the more the soil will be influenced by the parent material.

 

The older the soil than climate will have more to do with the soil’s characteristics.

 

Generally, it takes hundreds to thousands of years for a soil to reach maturity.

TIME

Transported parent materials have not had time to develop into leached out soils.

•      River Sediments (Nile)

•      Glaciated Outwash

•      Wind Blown Material (Loess)

•      Volcanic Activity

 

Some of the best soil for agriculture is on these surfaces.

 

SOIL PROFILE DEVELOPMENT

Over time, soils develop vertical, distinct layering called horizons.

These layers or horizons are distinguished by their different physical and chemical properties.

 

SOIL FORMING REGIMES

At the broadest scale of generalization, there are three primary soil-forming regimes that relate to climatic differences including:

 

•      Laterization

•      Podzolization

•      Calcification.

 

LATERIZATION

Laterization is a soil-forming process that occurs in humid, tropical and subtropical climates.

•      Little to no humus

•      A horizon leached out except Fe and Al.

•      Topsoil is reddish, coarse and porous.

•      The B horizon has a heavy concentration of illuviated materials

PODZOLIZATION

Podzolization – occurs in its purest form in the high middle latitudes – moist, cool summers – long, severe winters.

•      Associated with coniferous forests.

•      Abundant Humus

•      Acidic (Iron and Aluminum Leached out)

•      Pine Barrens (NJ)

 

CALCIFICATION

Calcification occurs in dry climates where evaportranspiration exceeds precipitation.

•      Alkali Dusts (Caliche)

•      Grasslands (Abundant Humus)

•      Deserts (Very little humus)

•      Not Leached Out