SOIL - CHAPTER 2

Soil Physical properties

are often the most important properties for evaluating the suitability of soil for a particular purpose

Soil color

Property that will give important information about the soil characteristic; Reflects an integration of chemical, biological and physical transformations and translocation that occurred within the soil

Organic matter

Organic coatings tend to darken and mask the brighter colors of the mineral.

Water Content

Soils are generally darker when wet than when dry; Influence of oxygen level, and there by the rate of OM accumulation which darken the soil

Presence and oxidation

states of Fe and Mn oxide

Manganese oxide

black

Glauconite

green

Calcite

whitish color

Soluble salts

whitish color

Soil Color Determination

It is determined by comparing the color of the soil to the chips in Munsell Color Chart

The Munsell Soil Color Chart

is one of the most influential color-modeling systems devised by Albert Henry Munsell, an American artist.

Munsell system categories

hue, value and Chroma

Hue

the specific color

Value

the lightness and darkness of color

Chroma

the light intensity

Brown to Black (Surface Horizon)

Accumulation of OM

Black (Subsoil)

Accumulation of Mn; Parent material e.g Basalt

Yellow to reddish

Ferric; Well aerated soils

Gray, bluish-green

Ferrous; Poorly drained soils

White to gray

Accumulation of salts; Parent material: marl, quartz

Is soil color a good indicator of soil fertility?

Soil color has little direct effect on plant growth but is an indicator of properties

Black

High in organic matter but drainage may be poor

Bright red and yellow

well drained with oxidized iron

Gray colors

poorly drained reduced iron.

As indicator of soil fertility or productivity

darker soils are generally fertile or productive due to the amount of OM present and leaching of nutrients present;

As indicator of parent material

in young soils color is indicator of parent material; dark colored minerals give rise to dark soils; light colored minerals give rise to light colored soils.

As indicator of drainage

mottling is an indication of intermittent reduction and oxidation of minerals in the soil due to water.

As indicator of climatic condition

in mature soil, color is an indicator of the climate in which they developed; warm climate brings about red soils; light color is a result of leaching of iron

Soil Texture

Single most important physical property of the soil

Soil Texture

The percent of sand, silt and clay in a soil sample

Soil Texture

The texture of the soil cannot be altered. A sandy soil remains sandy and a clay soil remains clayey.

Soil Texture

This is the most stable property of the soil.

Sand

2.-0.05 / 2-0.02 /Coarse, gritty, mostly primary minerals (quartz and feldspar), cubic to spherical in shape

Silt

0.05-0.002/ 0.02-0.002 /Smooth, powdery, mostly primary mineral (quartz, feldspar), cubic to spherical in shape

Clay

Coarse fragment

2mm / Gravels, cobbles and boulders /Not considered as part of fine earth fraction

Sand

Sand

Coarse texture

Sand

Visible without microscope

Sand

Rounded and angular in shape

Sand

Some ________ in soil will be brown, yellow or red because of Fe and Al oxide coating

Sand

Feel gritty

Sand

Considered non- cohesive- does no stick together in a mass unless it is very wet

Sand

Low specific surface area

Sand

Has less nutrients for plants than smaller particles

Sand

Voids between ________ particles promote free drainage and entry of air

Sand

Holds little water and prone to drought

Silt

Silt

Not visible without microscope

Silt

Quartz often dominant mineral in silt since other mineral have weathered away

Silt

Floury feel

Silt

Wet silt does not exhibit stickiness and plasticity

Silt

Creamy slick and slippery feel

Silt

Smaller size allows rapid weathering of non-quartz mineral

Smaller particles

retains more water for plants and have slower drainage than sand

Silt

Hold more plant nutrients than sand

Clay

< 0.002 mm

Clay

Fine texture

Clay

Flat plates or tiny flakes

Clay

Small clay particles are colloids

Clay

If suspended in water will not settle

Clay

Large surface area

Clay

Wet clay is very sticky and is plastic or it can be molded readily into a shape/rod

Clay

Easily formed into long ribbons

Clay

Shrink swell- none to considerable depending on the kind of clay

Clay

Pore space are very small and convoluted so movement of water and air are slow

Clay

Water holding capacity; high capacity to adsorb water

Clay

Soil strength –Shrink/swell; affect building, roads and walls

Clay

Chemical adsorption is large

Loam

is the ideal soil texture for crop production where there is equal proportion of sand, silt, and clay.

SANDY SOILS

Lower total porosity

SANDY SOILS

More macro-pores

SANDY SOILS

Low water holding capacity

SANDY SOILS

Very good aeration

SANDY SOILS

Easy to till (light soil)

SANDY SOILS

Non-sticky and non-plastic

SANDY SOILS

Low nutrient holding capacity

SANDY SOILS

Less fertile

CLAYEY SOILS

Higher total porosity

CLAYEY SOILS

More micro-pores

CLAYEY SOILS

High water holding capacity

CLAYEY SOILS

Poor aeration

CLAYEY SOILS

Difficult to till (heavysoil)

CLAYEY SOILS

Very sticky and plastic

CLAYEY SOILS

High nutrient holding capacity

CLAYEY SOILS

More fertile

How to determine Soil Texture

Roll method. Feel method, Hydrometer method, Pipette method

Hydrometer and pipette method

uses the stokes law. It states that the speed or velocity with which particles settle out of a liquid medium is dependent on a constant factor (K) and the radius of the particles or the bigger the particle, the faster it will fall out of suspension

Nutrient supplying capacity of the soil

coarse textured soils have low supplying capacity while fine textured soils have high supplying capacity.

Water holding capacity

coarse textured soils have low water holding capacity while fine textured soils have high water holding capacity.

Infiltration and percolation

______________ rates are rapid in coarse textured soils and slow to very slow in fine textured soils.

Soil tilth

coarse textured soils are easily tilled while fine textured soils are difficult to till.

Soil aeration and drainage

coarse textured soils are well aerated and well drained while fine textured soil are poorly aerated and poorly drained.

Water flow

under saturated conditions, coarse textured soils have higher capacity to conduct water flow but below saturation point fine textured soils have higher water conductivity.

SOIL STRUCTURE

Defined by the way individual particles of sand, silt, and clay are assembled. Single particles when assembled appear as larger particles. These are called peds or aggregates.

Soil Aggregates

a group of soil particles that bind to each other more strongly than to adjacent particles; The space between the aggregates provide pore space for retention and exchange of air and water

Humus

also binds soil particles together

Aggregate Stability

Refer to the ability of the soil aggregates to resist disruption when outside forces (usually associated with water) are applied

Aggregation affects

Erosion; Movement of water; Plant root growth

Importance of Aggregate Stability

Desirable aggregates are stable against rainfall and water movement