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.

USDA-NRCS Field Staff Soil Quality Workshop

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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.

Increasing soil organic matter improves these soil properties.

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

Ways to Increase SOM?

Soil Quality

Physical Chemical Biological

Soil Organic Matter

Minimizing Tillage

Applying Soil Amendments

Managing Crop Residues

No Tillage and Cover Cropping Improve Soil Quality

in North Carolina, USA

Charles W. RaczkowskiSchool of Agric. & Env. SciencesNorth Carolina A&T State University

Soil Quality, Soil-Water Relations, and Soil Erosion:

A “hands-on” Demonstration Used for Training of Field Personnel

Chris Lawrence, Agronomist, USDA-NRCS, Virginia