soil bulk density_ print

8/10/2019 Soil Bulk Density_ Print

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SOIL BULK DENSITY

Soil bulk densityg / cm3) is defned as the mass per unit bulk volume o

soil that has been dried to a constant weight at 105 OC. The volumeincludes all pore space as well as space occupied by soil solids.

Since fne-textured soils generally have more total pore space thancoarse-textured soils, the fner soils also generally have lower buldensities. !ul density values o" fne-textured soils commonly range "rom#.$ to #.3 g / cm3, while those o" sandy soils range "rom about #.3 to #.% g/ cm3. &espite this general di'erence in bul density between sandy andclayey soils, sandy soils are re"erred to as (light and clayey soils as(heavy. This terminology re"ers to relative ease o" tillage, not typicalbul densities.

Soil compaction, due to tra*c "rom machinery or livestoc or due tonatural processes, decreases soil pore space and, there"ore, increasesbul density.Since clay particles are plate-lie, clay soils can be readily compressedand molded. Such compressibility, together with the low bul density o"clay soils, allows "or substantial increases in bul density when clay soilsare compacted.+n contrast, sand grains cannot be molded together. Thus, compaction o"sandy soils with relatively small porosities does not lead to as great o"increase in bul density as occurs when clay soils are compacted.lthough fne-textured soils generally have lower bul densities thancoarse-textured soils, the opposite can be true in compacted soils.

ccumulation o" organic matter in soil lowers bul density in two ways.irst, the particle density o" organic matter is much less than thosemineral particles. Secondly, and more importantly, organic matterpromotes the "ormation and stabiliation o" soil aggregates.&ue to inter aggregate pore space the porosity o" well-aggregated soil isgreater than that o" a poorly aggregated soil.

ccordingly, bul density is lower. +n general, bul density is determinedby soil texture and modifed by soil structure. ithin any textural class acertain range in bul density is expected and whether, within this range,bul density is relatively low or high depends on the degree o" structuraldevelopment. hereas texture is not a'ected by soil management, soilstructure is a "ragile property that can deteriorate with intensivecultivation, exposure to raindrops and machinery tra*c.

!ul density depends on the soil particles densities such as sand, silt, clay

and organic matter and their pacing arrangement.


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ulk density values are re!uired or converting gravimetric soil water

content to volumetric and to calculate soil porosity which is the amount

pore space in the soil.

"esearchers oten need a bulk density value to use in models#

characteri$e feld conditions# or convert to volumetric measurements.Soil bulk density is a basic soil property in%uenced by some soil physical

and chemical properties. ulk density is a dynamic property that varies

with the structural condition o the soil. &his condition can be altered by

cultivation# trampling by animals# agricultural machinery# weather# i.e.

raindrop impact. 'nowledge o soil bulk density is essential or soil

management# and inormation on the soil bulk density o soils is important

in soil compaction and structure degradation as well as in the planning o

modern arming techni!ues. ( both# bulk density and particle density are

known# the total porosity can be calculated by using these values.

Soil bulk density should be used as an indicator o soil !uality parameter.

) unit increases in organic matter and clay content caused a relatively

larger decrease in soil bulk density. ) soil system can be thought as a

network o soil properties.

ulk density is an indicator o soil compaction and soil health. (t a*ects

infltration# rooting depth+restrictions# available water capacity# soil

porosity# plant nutrient availability# and soil microorganism activity# which

in%uence key soil processes and productivity.

ulk density varies rom soil to soil and rom hori$on to hori$on and is

primarily a unction o the amount o pore space in the soil. &he percent

pore space in a soil is a unction only o bulk density since particle density

is a constant or a given hori$on or soil. ,hen bulk density increases

percent pore space decreases. &here is also a change in the distribution o

micro to macro pores with changes in bulk density. (ncreases in bulk

density# which are usually caused by compaction# tend to destroy the

large macro pores in preerence to the smaller micro pores. Since themacro pores are those pores that drain ree o water and hence serve as

the pathways or air and water movement# compaction tends to increase

problems associated with e-cess water perched water tables/ and

associated with poor aeration.

igh bulk density indicates

1. 2oor environment or root growth i.e "esistance to penetration

hence restricts rooting depth# which reduces the uptake o water

and nutrients by plants/


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3. "educed aeration

4. "educed water infltration# thereby increasing runo* and the

ha$ard o water erosion

Factors Afecting Bulk Density

(nherent actors that a*ect bulk density such as soil te-ture cannot be

changed. ulk density is dependent on soil organic matter# soil te-ture#

the density o soil mineral sand# silt# and clay/ and their packing

arrangement.

)s a rule o thumb# most rocks have a density o 3.5 g+cm4 so ideally# asilt loam soil has 506 pore space and a bulk density o 1.44 g+cm4.7enerally# loose# well8aggregated# porous soils and those rich in organicmatter have lower bulk density. Sandy soils have relatively high bulkdensity since total pore space in sands is less than silt or clay soils. ulkdensity typically increases with soil depth since subsurace layers aremore compacted and have less organic matter# less aggregation# and lessroot penetration compared to surace layers# thereore contain less porespace.)vailable water capacity 9igure 3/ is a*ected by soil te-ture# presence

and abundance o rock ragments# soil depth and restrictive layers.

Bulk Density Management

ulk density can be changed by management practices that a*ect soil

cover# organic matter# soil structure# compaction# and porosity.

:-cessive tillage destroys soil organic matter and weakens the natural

stability o soil aggregates making them susceptible to erosion caused by

water and wind. ,hen eroded soil particles fll pore space# porosity is

reduced and bulk density increases. &illage and e!uipment travel results

in compacted soil layers with increased bulk density# most notably a ;plow

pans available water capacity is also a*ected by organic matter andcompaction. Organic matter increases a soil>s ability to hold water# bothdirectly and indirectly. Compaction increases bulk density and reducestotal pore volume# conse!uently reducing available water holdingcapacity.


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&he ollowing measures increase organic matter# and reduce compaction#which improve bulk density and porosity

i/ 2ractices that increase organic matter such as continuous no8till#cover crops# solid manure or compost application# diverserotations with high residue crops and perennial legumes or grass

used in rotation?ii/ @inimi$e soil disturbance and avoid operating e!uipment when

soils are wet?iii/ Ase designated roads or rows or e!uipment?iv/ "educe the number o trips across a feld?

v/ Subsoil to disrupt e-isting compacted layers? andvi/ Ase multi8crop systems involving plants with di*erent rooting

depths to help break up compacted soil layers.

Determination of Bulk Density

&o obtain core samples is relatively simple i no stones are present#however# it is more diBcult to obtain good !uality cores. &he core sampleris pushed or driven into the soil to the desired depth and then removed.@any samplers are available which are provided with a metal casing tohold the core and permit easy removal and handling o


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the sample during weighing# wetting and drying. ( the soil sampler isassumed to be ull its volume may be used as the volume o soil. ( thesampler is not ull volume may be used as the volume o soil. ( thesampler is not ull an independent measurement must be made o thevolume o soil.

Equipment needed Cylindrical core sampler# 'nie or spatula alance sensitivity 0.01g Oven to dry the core samples at 105oC 2lastic bag 8 large enough to hold the sample ,heighing tin

Laboratory Procedure

i/ Determine the weight o empty containercylindrical tin/ii/ &he sample in a container is then placed in oven at a

temperature 105oC to dry or 3Ehrs. Depending on the si$e o thecontainer more drying hours can be applicable.

iii/ "emove the sample rom an oven cool and measure theweight o dried soil sample F container.

iv/ Determine the volume o cylinder core# cm4

Particle DensitySoil particle density g + cm4/ is mass o soil solids oven8dry/ per unit

volume o soil solids.

2article density depends on the densities o the various constituent solids

and their relative abundance. &he particle density o most mineral soils

lies between 3.5 and 3.G g + cm4. &he range is airly narrow because

common soil minerals di*er little in density. 2article density is a relativelyconstant parameter and is sometimes assumed to be 3.5 g+cm4. (n

contrast# organic soils have lower particle densities since the density o

organic matter is much less than that o mineral particles. (t is easy to

measure the mass o a small sample o soil but not so easy to accurately

measure the volume o soil solids that make up this mass. rie%y# the

volume o a known mass o soil solids is determined by indirectly

measuring the volume o water displaced by the soil solids. &he mass o

water displaced is actually measured# then the corresponding volume

ound rom the known density o water.


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Procedures for Particle Density

raduated !ylinder "et#od$

1. 2repare an air dried soil sample.3. )dd G0 ml o water to a 100 ml graduated cylinder.

4. Asing a mortar and pestle# grind about 40 grams o soil to destroyaggregates. 7rinding is not re!uired or sand.

E. ,eigh the sample and record the weight.

5. Asing a unnel# pour the weighed sample into the cylindercontaining the measured amount o water.

. Shake or stir to release any entrapped air bubbles.

G. "ead and record the new water level.

H. &he change in water level is the volume o water displaced by thesoil# and thereore the volume o the soil particles.

2orosity

hen soil is saturated ,all air in the soil is displaced by water. Thus, thevolume o" water is an indication o" the total porosity o" the soil 0i.e., thevolume o" pores).Soil porosityis the percentage o a soil that is pore space or voids. &he

average soil has a porosity o about 506# and the pores are flled with air

or water depending on the moisture content. Sands have larger pores#

but less total pore space than clays.Soil particles have a more or less fxed density o" 1.2 g cm3. ( both bulk

density and particle density are known# the total porosity can be

calculated Soil porosity can be calculated as "ollows4.


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soil bulk density_ print

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