Physical Properties of Soils

This lecture deals with physical properties of soils and relates directly to Lecture 2. A significant portion of soil taxonomy deals with soil physical properties. These properties in turn have a significant impact on plant growth and productivity.

II. Learning Objectives
 

	Understand the role of soil texture and structure in crop growth
	Be able to relate texture and structure to soil aeration and water infiltration
	Be aware of influence of soil temperature on seedling development and crop growth and development
	Understand the role of tillage in ameliorating problems caused by poor structure and drainage
	Understand the positive and negative relationships between tillage practices and water and wind erosion

Soil physical properties have a significant influence on crop plant growth and development. Soil texture and structure are the most important of these properties. Soil texture is most important in the areas of water holding, nutrient supply, and on ease of tillage. Soil structure has the greatest influence in the areas of soil aeration, water infiltration, and soil temperature.

The primary method of addressing many of these soil physical properties has been the use of tillage. Tillage can change the soil structure, improve moisture intake and storage, improve aeration and fertility. However, tillage can also impact on soil loss from both soil water and wind erosion.
 

Two basic physical properties of soils are texture and structure

• Texture - various sizes of soil particles
• Structure - arrangement of particles
• Effected by management

• Nutrient supplying ability
• Water relationships
• Aeration
• Root growth and health
• Soil microbial population and vigor

Clays have greater water-holding capacity than silts and water movement is slower

• They become sticky when wet
• Cloddy on drying
• Often called "heavy soils"

Sands are called "light" soils  They do not become sticky and hard to till Nutrient holding ability varies with the textural class

CONCEPT   Textural class (sand, silt or clay) has a lot to do with planning a cropping system.

There are 3 clay minerals found in soils

• Kaolinite
• Illite
• Montmorillonite

Soil Structure

Principle forms generally recognized are:  Platy  Prismatic  Columnar  Blocky  Granular

CONCEPT   Soil structure influences water and air movement in soils.

Soil structure is markedly affected by tillage operations

• Usually done to partly loosen soil
• Make favorable environment for roots
• Can cause deterioration in soil structure
• Usually hastens decomposition of organic matter

• Degree of aggregation and arrangement and stability and aggregates affect:
• Aeration
• Water flow
• Water retention
• Temperature
• Impedance to roots
• We don't have system to value this

• The part of soil volume not occupied by solid particles, but filled with air or water
• It is possible to calculate from the release curve the volume of water drained at different pressures, and thus determine volume of pores

Bulk density  Weight for a given volume of soil  Can use to evaluate structure  More compacted/less pore space  Subsoil layers generally have higher bulk densities/less O. M./are less well aggregated/compacted by heavy equipment

CONCEPT   Compacted soils limit root growth and development.

	Ability to provide air of suitable composition to plant roots and to organisms growing in the soil
	If aeration impeded, soil air higher in CO2, lower in 02
	Diffusion of air through soils is directly dependent on volume of air-filled pores

	Is the rate which rain or irrigation water enters the soil	CONCEPT
	Depends on both surface soil and subsurface horizons	If water runs off and not into soil, water is not available for plants and contributes to erosion.
	Rate is at maximum value soon after water application begins  Then falls to a lower value
	Condition of soil surface has significant influence on infiltration rates of soil
	O. M. and soil texture also have influence
	Success of a cropping system may be greatly influenced by the ability to capture and hold limited rainfall

Affect the biological processes in the soil  Micro biological activity  Seed germination  Root growth

CONCEPT   Temperature influences microorganisms. In hot climates, organic matter consumed at a much faster rate than in cold climates.

	Soil temperature may vary appreciably with depth below the surface
	Surface temperatures may undergo a wide fluctuation
	Dark colored soils absorb a higher proportion of the radiant energy
	Angle of incidence makes difference
	Fluctuations in soil temperature are influenced by both heat capacity and thermal conductivity of the soil  Thermal conductivity of water higher than soil and air  Rate of heat flow increases with increasing moisture content
	If surface layer is wet, soil warms slower than dry soil because heat is conducted to lower layers and it takes more heat to change the temperature	CONCEPT   Cool wet soil delay planting. It can also be critical in areas with short frost free periods.
	Variation in temperature much greater in bare soil compared to one with vegetable cover  Bare soil reaches high temp. Greater than air temp.  At night, temperature falls below air temperature
	Shading effect beneficial in avoiding excessive soil temperatures  Inter seeding crop avoids excessive soil temperature in tropical area  Seed new crop in crop residue  Both methods conserve soil moisture and reduce maximum soil temperatures
	Germination of seeds affected more by soil temperature than any other factor except moisture  In temperate climates - temperature too low for germination  In tropical climates - may be too high
	Roots are more susceptible to injury by high temperatures

Soils are tilled for three general reasons  Change soil structure  Manage crop residues  Control weeds

CONCEPT   Tillage has both positive and negative influences on crop and soil management.

• Improve moisture intake
• Improve moisture storage
• Improve moisture transmission

• Light
• Moisture
• Nutrients

Tendency is to reduce number and intensity of tillage operations

• No-till
• Zero-till
• Conservation tillage
• savings in cost to operator
• savings in time
• Minimum tillage
• Wheel track planting
• Plow plant

	In 1875, it took 150 man hours of labor to produce 270 kg of corn. By 1975, it took 4 man hours.
	Tillage can be done closer to optimum time  Most midwestern farmers can plant their entire acreage in one week

Undesirable tilth usually leads to soil erosion   Tillage when too dry or too wet can result in excessive cloddiness

CONCEPT   Erosion is a major environmental problem in the area of tillage.

• Universal soil loss equation computes average annual soil loss as product of two quantitative factors
• ability of soil to erode
• rainfall erosion ability
• Univ. Soil loss equation developed for cropland east of Rocky Mts. In U. S.
• Estimates soil lost quite well in that area
• Tropical regions differ from eastern U. S. in most factors included in equation
• There are five primary means to reduce erosion
• vegetation
• plant residues
• improved tillage methods
• residual effects of crops in rotation
• mechanical supporting practices
• A combination of crop canopy and plant residues can provide year round protection of soil surface from raindrop impact
• With multiple cropping, soil may have adequate protection without following practices which leave abundant plant residues
• No-till systems utilize plant residues quite fully
• can reduce soil loss by 95%
• however, cannot be used on all soils (delay in soil warming in poorly drained soils)
• weed control and fertilizer placement difficult
• Conservation tillage result in less soil loss from water erosion than from moldboard plowing and secondary tillage

• Land is most vulnerable to wind erosion when vegetative cover is sparse and soil surface dry
• Adequate cover of soil surface with a growing crop or with crop residues affords good protection from wind erosion
• Second crop may be grown primarily for protection from wind erosion.

 

As long as the majority of our food crops are grown in the soil, a basic understanding of soils will continue to be essential to crop production. This section discussed two of the most important physical properties of soils, that of texture and structure. Soils that are too sandy (course textured) leach quickly and do not hold water well. This can influence both fertility and the environment, as well as limit the plant's ability to obtain water for turgor, photosynthesis, and nutrient uptake. Soil structure has a primary impact on soil porosity and aeration. If water does not move through soils, air is excluded and plant roots die. If water does not enter the soil (infiltration) and instead runs off, the water is lost to the plant and erosion often occurs. A secondary property, but none-the-less important, is soil color. This effects temperature, how slowly or quickly a soil dries, and has a direct influence on seeding date and germination. Finally, mankind uses tillage to effect the soil physical properties. This can be both beneficial and harmful (water and wind erosion potential of tilled land).
 

 

	Know the difference between soil texture and structure.
	Be able to explain how texture and structure influence crop plant growth.
	Know the relationship between pore space and plant root growth and development.
	How does soil temperature effect crop plant growth?
	What is tillage and how does it influence soil physical properties?
	What is tillage and how does it influence soil physical properties?