Physical Properties of Soil Chapter 4 Physical Properties of Soil

Objectives After completing this topic, you should be able to: Describe the concept of soil texture and its importance Identify the texture of a sample of soil Describe soil permeability and related properties Describe structure and its formation and importance Explain other physical properties Discuss soil compaction and tilth

Physical Properties of Soil Soil characteristics: grower can see or feel Neither chemical nor biological, but both affect them Greatly affect how soils are used to grow plants or other activities

Soil Texture Most fundamental soil property Determined by the proportion of soil particles Sand (large) Silt (medium) Clay (small) The size of soil particles, in turn, affects such soil traits as water-holding capacity and aeration

Effects of Particle Size Soil particle size affects two important features: Specific surface area Soil pores: number and size Specific surface area is defined as the amount of surface area exposed by all the particles in a certain weight of soil.  The smaller the soil particles, the greater the specific surface area Soil surface area is important because reactions occur on the surface of soil particles, and because water is held as a film around soil particles Thus the smaller the particles in a soil, the more water and nutrients the soil can retain

Soil Pores Soil pore number and size depends on particle size Macropores (aeration pores): large Micropores: small (A) small particles create many small pores. (B) Pores are larger but fewer in number between large particles. Micropores usually hold water, macropores air. Sometimes the larger micropores are distinguished as mesopores, medium-sized pores that hold readily plant-available water

Soil Separates Soil scientists divide mineral particles into size groups called soil separates This consists of three broad classes Sand (divided into four subcategories) Silt Clay These three together make up the fine earth fraction of soil used to determine texture Larger particles, such as gravel, are considered to be coarse fragments, and are not considered in texture

The USDA system of soil separates The USDA system of soil separates. The comparison shows the difference by setting a very coarse sand grain equal to 3 feet in diameter. Engineers use a different system Separate Diameter (mm) Comparison Feel Very coarse sand 2.00–1.00 36″ Grains easily seen, sharp, gritty Coarse sand 1.00–0.50 18″ Medium sand 0.50–0.25 9″ Fine sand 0.25–0.10 4½″ Gritty, each grain barely visible Very fine sand 0.10–0.05 1¾″ Silt 0.05–0.002 7/16″ Grains invisible to eye, silky to touch Clay <0.002 1/32″ Sticky when wet, dry pellets hard, harsh

Comparing the size of soil separates Comparing the size of soil separates. On this scale, very coarse sand would be 3 feet across

Sand Characteristics Largest of the soil separates Composed mainly of weathered grains of quartz or other minerals Particles range in size Enough sand in a soil creates large pores, so sand improves water infiltration (rate at which water enters the soil) and aeration Large amounts of sand lower the ability of the soil to retain water and nutrients

Silt Characteristics Medium-sized soil separate Silt particles are silky or powdery to the touch, like talc Best ability to hold large amounts of water in a form plants can use Erodes readily in moving water and wind

Clay Characteristics Smallest of the soil separates Consists of tiny, sheet-like crystals Results from chemical reactions between weathered minerals to form tiny particles of new minerals USDA size classification for stones in the soil. Class Diameter Range (mm) Diameter Range (in.) Gravel 2–75 1/12–3 Cobbles 75–250 3–10 Stones 250–600 10–24 Boulders >600 >24

Textural Classification Soil usually consists of more than one soil separate All three separates are found in most soils Actual percentages are called soil texture 12 textural classes are shown in the soil triangle Another important textural name is loam, a soil in which sand, silt, and clay contribute equally to the soil’s properties. Determining soil texture Amount of sand, silt, and clay in a soil can be measured by mechanical analysis Mechanical analysis is based on the fact that the larger a soil particle, the faster it sinks in water E.g. it takes only 45 seconds for very fine sand to settle through 4 inches of water, but it takes about 8 hours for large clay particles

Question Identify a soil that is 10% cay, 10% silt and 80% sand

Soil Triangle Each side of the triangle is a soil separate Soil Triangle Each side of the triangle is a soil separate. The numbers are percentage of soil particles of that type.

Soil triangle redrawn to show fine-, medium-, and coarse-textured soils.

Characteristics of Textural Classes Soils can generally be classified as fine, medium, or coarse Indicative of a number of soil properties Infiltration –water entering the soil Percolation –water draining through soil Water-holding capacity Fine soils retain plant nutrients better than coarse soils. This is true partly because the rapid percolation of water through coarse soil leaches out nutrients. Also, clay particles have the best ability to retain nutrient chemicals.

Modifying soil texture Impractical, except in small areas e.g. golf greens or potting soils Very large quantities of sand are needed to loosen clay soils—enough that sand grains touch and there isn’t enough clay to fill all the gaps.

Soil Density and Permeability Particle density (PD) This is the density of solid particles only Average 2.65 grams per cubic centimeter PD varies according to the type of minerals in the parent material and the amount of organic matter in the soil. Mineral Density (grams/cm3) Density (lbs/ft3) Water 1.0 62.5 Quartz 2.65 166 Feldspars 2.5–2.7 156–169 Micas 2.7–3.0 169–188

BD = Weight dry soil/Volume dry soil = g/cm3 Bulk density Actual density of a soil is less than the PD  It is the mass of a volume of undisturbed oven-dry soil. To measure BD, a core of soil of known volume is carefully removed from the field. The soil core is then dried in an oven at 105°C until it reaches a constant weight The example that follows is for a core of 500 cubic centimeters (cm3) that weighs 650 grams (g): BD = Weight dry soil/Volume dry soil = g/cm3 Determine the bulk density of the soil given in the above example Determine the volume of the particles in the soil given in the above example (Hint: use the average particle density of soils to solve this problem)

Questions A oven dry soil weighs 800 grams and has a bulk density of 1.3 g/cm3 What is the volume of the undisturbed soil? What is the volume of the soil particles?

Porosity = wet weight (g) − dry weight (g) x 100 soil volume (cm3) Soil porosity Measure of the soil volume that holds air and water  usually expressed as a percentage Thus, a soil with a 50 percent porosity is half solid particles and half pore space. Porosity can be measured by placing an oven-dry soil core in a pan of water until all of the empty pore space is filled with water. The water volume, which fills the pore space, divided by the total core volume, is porosity. Example: The soil core used as an example before had a volume of 500 cubic centimeters and weighed 650 grams when dry. When wet, the same core weighed 900 grams. Porosity is thus calculated as follows: Porosity = wet weight (g) − dry weight (g) x 100 soil volume (cm3)

Porosity = 900 − 650 × 100 = 50% 500  Porosity can also be calculated from BD and PD. If there were no pore space, then BD would be the same as PD The ratio BD/PD would be equal to 1 The more the pore space, the smaller the BD and smaller the ratio BD/PD The ratio BD/PD is simply the percentage of the soil that is solid matter If one subtracts that percentage from 100 percent, the difference is the percentage of pore space  The following equation can be used to calculate porosity: Porosity = 100% − BD × 100 PD  If we substitute the values for the BD calculated earlier:

Porosity = 100% − 1.3  × 100 = 50% 2.65 The porosity of sand (about 30 percent) is lower than that of clay (about 50 percent)

Permeability Ease with which air, water, and roots move through the soil In highly permeable soil, water infiltrates soil rapidly, and aeration keeps roots well supplied with oxygen. Permeability depends partially on the number of soil pores, but it depends more on the size and continuity of the pores.  Permeability is simply a descriptive term with no numerical value; it cannot be measured directly Hydraulic conductivity is a measure of the rate of water movement through a soil. Coarse soils might have conductivities of 1 and 1.5 inches per hour or more, while fine-textured soils might measure a hundredth of that

Soil Structure Structure refers to the way soil particles clump together into large units These large units are called soil aggregates Peds: naturally occurring aggregates Clods: clumps of soil caused by tillage Well-aggregated soils contain large, continuous pores that promote good air and water movement and that provide easy pathways for root growth. Classification traits Type: shape Class: ped size Grade: ped distinctiveness and strength

Types of soil structure

Structureless Soil Single-grain Massive soils Sandy soils and soils without structure Massive soils Fine textured soils that lack structure or function as a soil mass Lack permeability

Types of Soil Structure Granular structure Commonly found in A horizons Platy structure Usually found in E horizons Blocky structure Typical of many B horizons Prismatic structures Tend to occupy lower B and C horizons

Formation of Soil Structure Two-step process Soil creates a loose ped and the second cements it Weak aggregates are cemented to make them distinct and strong Structure is not a permanent soil feature Can be degraded by mismanagement

Soil Consistence Behavior of soil when pressure is applied Relates to the degree that soil particles stick to one another or other objects Soils types and characteristics Wet soil: stickiness and plasticity Moist soil: loose, friable, and firm Dry soil: determined by trying to crush an air-dried mass of soil in the hand

Soil Tilth Physical condition of tilled soil Tillage Suggests how easy the soil is to till, a good seedbed can be made, how easily seedlings can come up, and the ease of root growth Tillage Improves soil tilth for a time, improving soil air–water relations for new seedling

Compaction Results when pressure is applied to the soil surface Primarily alters soil traits related to pores and soil strength Can create: Reduced porosity and permeability Reduced air exchange and potassium uptake Decreased infiltration rates Increased erosion and evolution of nitrous oxide Reduced percolation and oxygen availability

Physical Property Management Considerations Avoid aggregate destruction Avoid puddling and clods Minimize surface crusting Improve tilth

Soil Channels and Pans Channels Any hardened layer of soil is a pan Large, continuous pores extending from the surface and leading deeper into the soil Any hardened layer of soil is a pan Claypans Fragipans Plinthite Caliche Duripans

Soil Temperature Critical for plant growth and development Important factors Sunlight and air energy inputs Absorption and conductance of heat Loss of heat at the surface Temperature effects Affects seed germination, root growth, as well as water and nutrient availability and biological activity

Soil Temperature (cont’d.) Managing soil temperature Natural conditions Watering and soil manipulations Mulching Frost heaving Inorganic materials and plastic sheeting Fire and soil temperature Soil temperature affects soil and plants in natural ecosystems Fire is present or necessary in many natural ecosystems

Soil Color Indicates: Color as a guide to soil use Nutrient composition Organic matter Drainage in soils Color as a guide to soil use Used to classify soils according to color

Summary This chapter reviewed several topics Texture and textural classes Permeability Consistence Tilth Compaction Physical property management Temperature and color