Soil Physical Properties Used to Assess Soil Quality Field Exercise

Two Ways to Assess Soil Quality Measurements over time. Comparing management systems. Examples: Side-by-side comparisons of management systems. Measurements in the same field over time. Problem areas versus non-problem areas. Compare measured values to a reference soil condition or to the natural ecosystem.

Soil quality is assessed by measuring a number of soil properties to evaluate the soil’s ability to perform basic functions. Physical Property: Bulk Density Porosity Macroporosity Microporosity Air-Filled Porosity Hydraulic Conductivity Field Capacity Plant Available Water Aggregate Stability Infiltration Penetration Resistance Condition: Compaction Aeration Percolation Infiltration Drainage Aggregation Waterlogging Erodibility Crusting Root Growth Plant Growth

Bulk Density The ratio of oven-dried soil (mass) to its bulk volume (g/cm3). Range: 1.0 to 1.7 g/cm3. Used to convert soil water content in percent by weight to percent by volume. Used to calculate porosity. Calculation: BD = Oven-Dry Soil Weight/Core Sample Volume Indicator of: Compaction, aeration, root growth, microbial activity, infiltration, and drainage. 

Porosity That portion (%) of the soil not occupied by solid material. The ratio of the soil pore volume to its bulk volume. Calculation: Porosity (%) = 100 – 100(BD/PD) where, PD = Particle density. Indicator: Compaction, aeration, aggregation, root growth, microbial activity, water movement and storage.

Macroporosity That portion (%) of the soil occupied by pores of large size. Because of their large size, they are not able to retain water against gravity by capillary action. The ratio of large pore volume to the soil bulk volume. Calculation: % Water by volume at saturation - % Water by volume at field capacity.   Indicator: Compaction, aeration, aggregation, root growth, microbial activity, water movement and storage.

Microporosity That portion (%) of the soil occupied by pores of small size –also called capillary pores. Because of their small size, they are able to retain water against gravity by capillary action. The ratio of small pore volume to the soil bulk volume. Calculation: % Total Porosity - % Macroporosity. Indicator: Compaction, aeration, aggregation, root growth, microbial activity, water movement and storage.

Air-Filled Porosity That portion (%) of the soil occupied by air-filled pores. The ratio of air-filled pore volume to the soil bulk volume. Calculation: % Water by Volume at Saturation - % Water by Volume at Sampling. Indicator: Aeration, excessively wet conditions, microbial activity, drainage.

Saturated Hydraulic Conductivity Indicator of the soil ability to conduct water (in/hr) in saturated conditions. The ratio of the flow density or “flux” per unit hydraulic gradient. Estimation: Soil Water Characteristics Calculator (http://www.pedosphere.com/resources/texture/triangle_us.cfm Indicator: Water movement, compaction, aggregation.

Field Capacity The water content (in/ft) of soil after free drainage from a saturated condition. The amount of water (in/ft) that a soil is able to retain after free drainage. Calculation: (% Water Content by Volume)/100 x (12in/ft). Indicator: Water retention, plant growth, rooting, microbial activity, compaction, aggregation.

Permanent Wilting Point The water content (in/ft) below which plants are generally unable to extract water from the soil. Calculation: (% Water Content by Volume)/100 x (12in/ft). Estimation: Soil Water Characteristics Calculator (http://www.pedosphere.com/resources/texture/triangle_us.cfm).

Plant Available Water Holding Capacity The quantity of water (in/ft) that a plant is able to extract from a soil at field capacity. Calculation: Field Capacity (in/ft) – Permanent Wilting Point (in/ft). Indicator: Available water retention, plant growth, rooting, microbial activity, compaction, aggregation.

Water Stable Aggregates (Aggregate Stability) Measures the amount of stable aggregates (%) against flowing water. Calculation: % Water Stable Aggregates = 100 [(Weight of Stable Aggregates)/(Weight of Aggregate Sample Used)]. Indicator: Soil erodibility.

Infiltration The entry of water into the soil (in/hr). The height (in) of water entering the soil surface per unit time (hr). Calculation: Water Height (in)/Time (hr). Indicator: Water runoff, erosion.

Penetration Resistance (Soil Strength) Soil resistance (kg/cm2) to penetration by a metal rod. Calculation: Cone Index = force applied (kg)/cone tip basal area (cm2). Indicator: Compaction, root proliferation, infiltration, drainage, microbial activity.

Measurement of Soil Physical Properties for Soil Quality Assessment Parameter Equation Result 1 % Organic Matter 2 Soil Structure 3 Soil Color 4 % Sand 5 % Silt 6 % Clay 7 Textural Class

Soil Water Content at Sampling Parameter Equation Result 8 Soil Moisture Sample Weight, g 9 Hydroscout Reading 10 Mass Wetness, g #9 / (#8 - #9)

Collecting an Undisturbed Soil Core

Data From Undisturbed Core Sample Parameter Equation Result 11 Ring Weight, g 12 Ring Diameter, cm 13 Ring Height, cm 14 Ring Volume, cm3 15 Weight Rubber Band + Dry Cloth, g 16 Weight Rubber Band + Cloth + Metal Core + Field Moist Soil, g 17 Weight Rubber Band + Cloth + Metal Core + Saturated Soil, g

Data From Undisturbed Core Sample Parameter Equation Result 18 Weight Rubber Band + Cloth + + Metal Core + Drained Soil, g 19 Weight Rubber Band + Cloth After Drainage, g 20 Weight Rubber Band + Cloth Saturated, g 21 Core Sample Field Moist Weight, g 22 Core Sample Dry Weight, g 23 Bulk Density, g/cm3

Data From Undisturbed Core Sample Parameter Equation Result 24 Weight of Water at Sampling, g 25 % Water at Sampling 26 Weight of Water at Saturation, g 27 % Water at Saturation 28 Weight of Water at Field Capacity, g 29 % Water at Field Capacity

Particle Density Determination Parameter Equation Result 30 Weight of Moist Soil, g 31 Weight of Dry Soil, g 32 Weight of Flask Full of Water, g 33 Weight of Flask Full of Water + Soil, g 34 Particle Density, g/cm3

Calculation of Soil Physical Parameters Equation Result 35 % Porosity 36 % Macropores 37 % Micropores 38 % Air-Filled Porosity 39 Hydraulic Conductivity, in/hr 40 % Water PWP 41 % PAW

Calculation of Soil Physical Parameters Equation Result 42 Depth of Water at Saturation, in/ft 43 Depth of Water at Field Capacity, in/ft 44 Depth of Water at Sampling, in/ft 45 Depth of Water at PWP, in/ft 46 Depth of PAW, in/ft

Aggregate Stability Determination Parameter Equation Result 47 Volume of Initial Aggregates + Gravel, cc 48 Volume of Stable Aggregates + Gravel, cc 49 Volume of Gravel, cc 50 Volume of Initial Aggregates, cc 51 Volume of Stable Aggregates, cc 52 % Water Stable Aggregates

Infiltration Parameter Equation Result 53 Infiltration Time, minutes 54 Infiltration, in/hr Soil Respiration 55 Soil Temp., oF 56 Soil Temp., oC 57 Draeger Tube Reading 58 Respiration, lbs CO2-C/acre/day 59 Penetration Resistance, kg/cm2

Property Conventional Tillage Strip Textural Class Sandy loam Sandy clay loam % Sand, Silt, Clay 60 – 22 - 18 56 – 20 - 24 Soil Structure Fine Moderate Blocky Organic Matter, % 1.7 2.5 Soil Color 7.5 YR 4/6 7.5YR 3/4 Bulk Density, g/cm3 1.32 1.47 Total Porosity, % 47.4 41.4 Macroporosity, % 16.2 13.8 Microporosity, % 31.2 27.6

Property Conventional Tillage Strip Air-Filled Porosity, % 26.8 9.2 Hydraulic Conductivity, in/hr 2.05 1.20 Field Capacity, in/ft 3.1 3.5 Permanent Wilting Point 1.5 1.9 Plant Available Water, in/ft 1.6 Water Stable Aggregates, % 64.0 62.7 Infiltration, in/hr Penetration Resistance, kg/cm2

Question 1: Based on the soil water content obtained from the core sample, determine the depth of water (inches) needed so that the upper foot of soil reaches field capacity. Conventional Tillage: Water Content at Sampling= 20.6% or 2.47 in/ft Field Capacity = 3.1 in/ft Answer: 3.1 – 2.47 = 0.63 in/ft Strip Tillage: Water Content at Sampling= 32.2% or 3.86 in/ft Field Capacity = 3.5 in/ft Answer: 0 in/ft

Question 2: Calculate the soil water content (in/ft) at 50% plant available water depletion. Next, calculate how much water (inches) needs to be added to the upper foot of soil to reach field capacity. Conventional Tillage: Soil Water Content @ 50% Depletion = PAW/2 + PWP = (1.6/2) + 1.5 = 2.3 in/ft (19.1 %) To reach FC add the amount depleted = 1.6/2 = 0.8 inches

Question 3: Use the soil water characteristic calculator to estimate the soil properties listed below. Compare the estimations with the results obtained from your samples. Conventional Tillage Strip Tillage Actual Estimated Bulk Density, g/cm3 1.32 1.18 1.47 1.15 FC, in/ft 3.1 3.0 3.5 3.4 PAW 1.6 1.5

Question 4: Using the soil water characteristic calculator determine the effect of a two-fold increase in organic matter in each soil environment. Any improvements? Conventional Tillage Strip Tillage Actual Two-fold OM Bulk Density, g/cm3 1.32 1.18 1.47 1.12 FC, in/ft 3.1 3.5 3.9 PAW 1.6 1.8 1.5 1.9