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Measuring Soil Physical Properties to Assess
Soil Quality
Charles W. RaczkowskiNorth Carolina A&T State University Presented at the Soil Quality Workshop held in Nong Lam UniversityJune 2007
Soil Quality
“The capacity of a soil to function within ecosystem boundaries to sustain biological productivity, maintain environmental quality, and promote plant, animal and human health.”
Assessing Soil Quality
“A quantitative assessment of soil quality is invaluable in determining the sustainability of land management systems”
Soil Quality Assessment
• Choose indicators of soil quality based on the multiple functions of soil that maintain productivity and environmental health.
• Must include soil physical, chemical, and biological properties.
• Give importance to those soil functions that need to be improved.
Characteristics of U.S. Piedmont Soils
• Weak structure
• Crust prone
• Low infiltration & high runoff rates
• Low nutrient & water retention
• Highly Erodible
Choosing Soil Quality Indicators
Condition Indicator PropertiesWeak soil structure Aggregate stability, slaking,
qualitative assessment
Crust prone soil Infiltration, aggregate stability
Low infiltration & high runoff rates
Infiltration
Low nutrient & water retention
CEC, organic matter content, water holding capacity
High erodibility Aggregate stability
A “minimum set” of physical, chemical and biological properties is chosen to assess the overall function of soil.
Soil Properties
Physical Chemical Biological
Bulk Density Soil pH Soil Respiration
Infiltration Soil Nitrate Earthworms
Aggregate Stability
Electrical Conductivity
Soil Slaking
The USDA-NRCS Soil Quality Test Kit
Collectively, indicators: are designed as a
screening tool to provide immediate results for comparing management systems
can monitor changes in soil quality over time
can diagnose possible health problems due to land use and management.
The USDA-NRCS Soil Quality Test Kit
SWTK Characteristics: Tests can be easily
conducted on the farm by field personnel or by landowners.
The kit allows field personnel to be an active participant with the landowner in the assessment of soil quality.
The assessment provides the opportunity to discuss management options.
Property Standards
Unlike air quality and water quality assessments, standards do not exist for most soil property indices of soil quality.
Two Ways to Assess Soil Quality
1. Measurements over time.2. Comparisons.
Examples:1. Measurements in the same field over time.2. Problem areas versus non-problem areas.3. Compare management systems.
Soil Physical Properties Included in the SWTK
Bulk Density Aggregate Stability Slaking Infiltration Morphological
observations Porosity Pore-Size Distribution Soil Strength Water Retention
Bulk Density
• The ratio of oven-dried soil (mass) to its bulk volume (g/cm3).
• Range: 1.00 to 1.80 g/cm3.
• Calculation: BD = Oven-Dry Soil Weight
Core Sample Volume
• Indicator of: Compaction, Pore Space
• Related to: Water Dynamics, Root Growth
Aggregate Stability (AS)
• Measures the amount (%) of stable aggregates against flowing water.
• Calculation:
AS = 100 [(Weight of Stable
Aggregates) ÷ (Weight of Aggregate Sample Used)]
• Indicator: Soil erodibility, soil aggregation (structure).
Infiltration
• The entry of water into the soil (cm/hr).
• The height (cm) of water entering the soil surface per unit time (hr).
• Calculation:
Ir = (WV ÷ CA) ÷ Tir
where, Ir = Infiltration rate (cm/hr)WV = Volume of water (cm3) infiltrating in time Tir (hr).CA = Cylinder area (cm2)
Indicator: Water runoff, erosion, surface crusting.
Soil aggregates form around organic residues
Processes that disrupt aggregates increase loss of SOM
AS
Ir
BD
AS
Ir
BD
16 % clay 39 % 49%
More OM is needed to stabilize fine textured soils
Adapted from Russell (1973)
16 % clay
39 % clay
49 % clay
Effect of long term sod on bulk density
http://soil.scijournals.org/cgi/content-nw/full/65/3/834/FIG3
25 yrs of CT corn
25 yrs of CT corn
20 yrs of bluegrass, then 5 yrs CT corn
20 yrs of bluegrass, then 5 yrs CT corn
After adding waterAfter adding water
25 yrs of 25 yrs of conventional conventional corncorn
25 yrs of 25 yrs of conventional conventional corncorn
20 yrs of bluegrass, then 5 yrs conventional corn
20 yrs of bluegrass, then 5 yrs conventional corn
“Slaking” after adding water
No Tillage and Cover Cropping Improve Soil Quality
in North Carolina, USA
Charles W. RaczkowskiSchool of Agric. & Env. SciencesNorth Carolina A&T State University
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