Soils

This lecture provides a brief discussion of soils. The material here introduces the role of soils in plant growth, soil texture, soil orders, and how soils are named and classified.

II. Learning Objectives
 

	To learn how soil provides physiological needs for plants
	To become familiar with the comprehensive soil classification system
	To understand the relationship between soil texture and productivity
	To become familiar with the soil orders
	To understand how soil characteristics relate to soil names and soil capability groupings

 

Soils supply many of the physiological needs of plants. These include water holding, providing nutrients, support, temperature and aeration. The soil provides the environment for plant root growth and development.

Soils are classified in 10 major soil orders based on morphology and formation. Soil orders are divided into suborders, great groups, subgroup, family, and series. Understanding how to interpret the soil naming system tells a scientist much about the characteristics and capabilities of a soil.

Soils are then placed in soil capability groupings. These groups are determined by the classification system, and provide:

	general usefulness of soils for group production
	a basis for crop production
	a basis for soil change due to normal agriculture practices (liming, fertilization, tillage, etc.)

 

Soil

Provides for many of the physiological needs of the plant.

 

• Water-holding capacity, stores water in soil for plant use.
• Essential nutrients come predominantly from the soil.
• Mechanical support.
• Soil temperature/aeration effect plant growth.

Water and nutrients used by plant come from portion of soil that is in direct contact of roots.

•  Soil as environment for plant roots is of major importance for success of crop.

Comprehensive soil classification system.

• 10 major soil orders.
• Orders based on similar morphology and formation (1) example: soil developed under prairie vegetation.
• Orders divided into suborders by difference in climatic environment.
• Suborders divided into great groups by similarities in profile characteristics.
• Great groups further divided into subgroups.
• Then families and finally series.

• Order - Mollisol
• Suborder - Aquoll
• Great Group - Haplaquoll
• Subgroup - Typic Haplaquoll
• Family - Fine-silty, mixed, non calcareous, mesic
• Series - Drummer

	Family name provides textural, clay mineralogy, pH and climatic information.
	The series name is equivalent to the common name of the soil.
	Texture is one of the more important physical properties of soil.

• Sand greater than 0.05 mm (particles).
• Silt 0.002-0.05 mm in diameter.
• Clay less than 0.002 mm

As particles in the soil become smaller, the ratio of the particles’ surface area to their mass becomes larger.

•  Small particles (clay) have greater water and nutrient-holding capacities.

  Entisols

•  Mineral soils with only the beginning of horizon development.
•  From recently formed or exposed parent material.

Inceptisols

•  Mineral soils more highly developed than entisols.
•  Light colored surface horizons.
•  Not highly developed (could be due to extreme coldness).

Alfisols

•  Mineral soils with surface horizon too thin or too light colored to be Mollic.
•  Substantial clay movement out of the surface into the subsurface horizon.
•  Typical forest soils of central U. S.

Ultisols

•  Mineral soils more highly weathered than Alfisol.
•  Less than 35% base saturation in argillic horizon.
•  Soils typical of southeast U. S. - predominant in subtropical areas of the world.

Spodisols

•  Mineral soils with extreme weathering in a cool moist climate.
•  Subsoil accumulation of O. M. and oxides of aluminum.
•  Soils typical of north central and northeastern U. S.

Oxisols

•  Mineral soils of tropical regions subjected to intense weathering and leaching in hot, wet climates.
•  Occur mainly in tropics.

Aridisols

•  Mineral soil formed in dry climates.
•  Climate has restricted normal soil development.
•  Soils may have accumulations of soluble salts due to restricted leaching.
•  Arid regions, deserts and near deserts.

Vertisols

•  Mineral soils with high content of swelling type clay minerals (montmorillonites).
•  In dry periods these soils develop wide, deep cracks.
•  Form in old marine deposits.

Mollisols

•  Mineral soils with thick, dark colored, not highly leached surface horizon.
•  Soils generally form under prairie vegetation.

Histisols

•  Organic soils, properties due primarily to organic matter, not mineral matter.
•  Wet, boggy or swampy conditions.

	Orders do not fall nicely into a sequence.
	Orders occur in widely different climates and can show major profile differences.

Suborder, great group, and subgroup names can be used to supply useful information about specific soils.

•  Example:

• Bloomfield - Psammentic Hapludalf
• AFF = Aflisol, UD = Humid climate, drainage better than somewhat poorly drained.
• HALP = A weakly developed Alfisol minimal subsoil development.
• PSAMMENTIC = PSAMM stands for a sandy soil, ENTIC for Entisol-like. The whole term implies a weakly developed Entisol-like soil due to resistance of the parent material, sand.

An additional source o valuable information are county soil surveys.

•  A survey consist of:

• Detailed maps of the county
• A discussion of the general nature of county
• How the survey was made
• A general soil map showing locations of important soil associations
• A detailed map of all the soil series in the county
• Formation of classification/formation
• Use and management of soils
 CONCEPT
Soil survey results are available for most every country in the U.S. and are
free - first step in farm planning, management and financial considerations.

	Give soils general usefulness for crop production.
	The soils are grouped according to their restrictions when used to produce crops.
	Allows for soil change due to normal agricultural practices, liming, and fertilization.
	Capability groupings are based on two or more levels:

•  Designated by Roman numerals I-VIII.

• I. Few limitations
• II. Moderate limitations/require moderate conservation methods
• III. Severe limitations/reduce plant choice/very careful management
• IV. Very severe limitations/reduce plant choice/very careful management
• V. Little or no erosion limitation/other limitation impractical to remove - pasture, woodland, wildlife
• VI. Unsuited for cultivation/pasture, woodland, wildlife
• VII. Very severe limitations - unsuited for cultivation/pasture, woodland, wildlife
• VIII Only wildlife, water supply or aesthetic
•  Subdivided into subclasses based on special limitations.
• "C" climate - too cold, too dry.
• "E" erosion.
• "S" shallow, stony, droughty.
• "W" excess water.

Water and nutrient supply capability of the soil is often most severe problem of crop production.

  Can be caused by two things:

 

V. Summary

As you plan a cropping system, the first resource you evaluate is soil. Soil is essential for providing for physiological needs of plants. If the soil order is known, then much can be predicted about the climate, water relations, and fertility of the soil. The limitations of soils in the order predict the crop production problems you might expect to encounter. The more you know about soil, soil classification and soil capability, the easier it is to propose successful crop selection, sequence, and management.

 

IV. Self Assessment
 

	What physiological needs does soil provide for plants?
	Be able to explain a soil order.
	Be able to explain how a soil order can influence crop productivity.
	Understand soil capability groupings including subclasses based on specific limitations.