Common soil parameters
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Common Soil Parameters. What are the common parameters of soil that we could test?. The 3 soil attributes. Physical attributes Texture, structure, etc Biological attributes Organic content Chemical attributes pH, conductivity, cations, anions etc. Physical Properties. Colour.

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Common Soil Parameters

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Common soil parameters

Common Soil Parameters

What are the common parameters of soil that we could test?


The 3 soil attributes

The 3 soil attributes

  • Physical attributes

    • Texture, structure, etc

  • Biological attributes

    • Organic content

  • Chemical attributes

    • pH, conductivity, cations, anions etc


Physical properties

Physical Properties


Colour

Colour

  • described by using a Munsell Colour Chart

  • describe colour in terms of Hue, Value, and Chroma.

  • A typical designation is 10YR 5/2. This is the symbol of a greyish brown colour with 10YR hue, 5 value, and 2 chroma

  • The symbol components are always arranged in this sequence.


Soil colour determination

Soil Colour Determination

  • Hue - colour of pigment that must be mixed with black and white (or the proper shade of grey) to produce the colour to be matched

  • Soils range in hue from red (R) through yellow-red (YR) to yellow (Y) with some spots of green-yellow (GY) or even green (G).

  • 0-10 prefixes divide the space between the absolute hues

  • eg 7.5YR

  • value - the square root of the percentage of light reflected

  • soils range from 2 to 8


Soil colour determination1

Soil Colour Determination

Chroma

  • the amount of pigment that must be mixed with the proper value of grey to produce the particular colour

  • pure grey colours have 0 chroma

  • increasing brightness is indicated by chroma’s up to about 8 in soils


Soil texture

Soil Texture

  • Soil Texture is the most commonly used descriptor of a soil.

  • Texture is a property of the fine earth (<2mm) fraction that depends on the particle‑size distribution.

  • Particle size in this fraction varies from 2mm diameter down to less than 0.1mm

  • The distribution of particles over this size range influences many important soil properties such as ease of cultivation and water‑holding characteristics.


Soil texture1

Soil Texture

  • Soil is allocated to a textural class, depending on its content of sand‑, silt‑ and clay sized particles.

  • In the field, determined subjectively from the feel of a moist soil molded between the fingers and thumb

  • This is because the particle‑size distribution influences the mechanical properties of the material.


Class exercise 1

Class Exercise 1

Finger assessment of soil texture for mineral soils


Soil structure

Soil Structure

  • Described as weak, moderate, or strong depending on how distinct the peds? are.

  • Fine, medium, or coarse depending on the size of the peds.

  • Shape of the peds - platy, granular, blocky, columnar, or prismatic,


Settling rate lab method

Settling rate lab. method

for the silt‑size particles plus some of the coarse clay,

  • rate of settling is proportional to the square of the diameter of the particles

    • v = 6000d2

    • v is the settling velocity in cm/minute

    • d the particle diameter in mm

  • two sizes needed for determining soil texture; limits between:

    • sand and silt (0.05 mm) and

    • silt and clay (0.002 mm)


Example 8 1

Example 8.1

  • Calculate how far a 0.05 mm diameter particle would travel in one minute.

  • v = 6000 x 0.052

  • = 15 cm/min

  • in 1 minute, particles of this size would have travelled 15 cm


Exercise 8 2

Exercise 8.2

  • Would larger particles than 0.05 mm travel more or less than 15 cm in this time?

  • More

  • What does this mean about the sand fraction?

  • It would sink faster

  • Calculate how long a 0.002 mm diameter particle would take to travel 1 cm.

  • v = 0.024 cm/min

  • 1 cm in 41.7 minutes


Applying this in the lab

Applying this in the lab

  • a uniform suspension of soil in water is allowed to settle for one minute

  • a sample is then withdrawn from a depth of 15 cm

  • will contain no particles larger than 0.05 mm diameter

  • larger particles will have settled beyond that depth even if they started at the surface

  • the sample will contain silt and clay only

  • any particles smaller that had sunk from this level will be replaced from above

  • repeat at a depth appropriate for 0.002 cm

  • eg 3 cm after 2 hours


Exercise 8 3 answers

Exercise 8.3 answers

  • 40 x 2.04 g = 81.6 g

  • 40 x 0.40 g = 16 g

  • 81.6-16 = 65.6 g

  • Silt 66.8%, clay 16.3%, sand 16.9%

  • silt loam


Soil water

Soil Water

  • The amount of water present in the soil at any one time is most commonly expressed as a percentage of the oven‑dry weight of the soil

  • Percentage is commonly determined for the field capacity of the soil and for air‑dry soil.

  • These two percentages are used as limits to classify the water present in a soil after a rain as gravitational, capillary, and hygroscopic


Soil conductivity

Soil Conductivity

  • Determined quantitatively by a conductivity meter.

  • Standard water:soil mix

  • An important parameter when assessing salinity.

  • Used to estimate the concentration of soluble salts in the soil (Na+, Mg2+ and Ca2+ , Cl-, SO42- and HCO3-).

  • Soluble fertiliser may also contribute K+, NH4+ and NO3-.

  • High EC is undesirable for most plants.


Stone content

Stone Content

  • affect soil fertility by taking up space

  • reduce ability of a given volume of soil to hold water and nutrients

  • a hindrance to cultivation

  • measured by eye in the field

  • by separation and weighing in the lab


Soil ph

Soil pH

  • Tested with a calibrated pH meter.

  • Soil solutions are well buffered – do you remember what this term means?

  • By using a standard, valid comparisons between soils can be made (absolute values are difficult to interpret)


Soil ph1

Soil pH

  • Measurements can be made in the surface layer of a moist soil provided sufficient water is present to make liquid contact between the electrodes.

  • Under some circumstances where buffering capacity is not adequate, soils maybe suspended in 0.1M KCl for pH determination

  • There are many ways by which the pH of a soil can be measured – refer to handbook


Calcium carbonate content field method

Calcium Carbonate Content - Field Method

  • Field estimate of CaCO3 content is based on the reaction of soil with dilute acid giving both visible and audible effects.

  • Method is only approximate and not sensitive to differences in CaCO3 contents above 10%.


Biological attributes

Biological Attributes


Organic content

Organic content

  • determined by oxidative digestion (dichromate)

  • measured by:

    • back titration of excess dichromate

    • colorimetry of green product


Chemical attributes

Chemical Attributes


Nitrogen

Nitrogen

  • wet ashing with conc. sulfuric

  • formation of ammonium ions

  • converted to ammonia (NaOH)

  • steam distillation

  • collection in boric acid

  • titration with std HCl

  • known as the Kjeldahl method – standard for N

  • doesn’t pick up NO2/NO3

  • need conversion by initial reduction


Phosphorus

Phosphorus

  • availability vs total a major issue for P

  • various extraction solution to assess availability, eg Olsen 0.5 NaHCO3

  • analysis by:

    • ICP

    • XRF

    • colorimetry


Micronutrients

Micronutrients

  • can exist in soils in:

    • water‑soluble

    • exchangeable

    • adsorbed

    • complexed

    • secondary clay minerals

    • insoluble oxides

    • primary minerals

  • extraction with chelating DTPA and ICP analysis


Sodium absorption ratio sar

Sodium absorption ratio (SAR)

  • predictor of salinity

  • measure Ca, Mg & Na in mmole/L

  • use eqn or nomogram


Exercise 8 4

Exercise 8.4

  • A soil is tested for leachable Na, Ca and Mg, and the results are (in mmole/L: 15, 3 and 2 respectively. What is the SAR?


Cation exchange capacity cec

Cation exchange capacity (CEC)

  • relies in the removal of ions with a concentrated solution of an ionic substance intended to drive off the adsorbed ions

  • techniques used to analyse the released ions include:

    • titration with EDTA – Ca & Mg

    • flame AAS – Na, K

    • ICP emission - all

    • Kjeldahl N analysis – all adsorbed ions are replaced by NH4, which are then released by excess K; the ammonium is then analysed

  • some methods use approximations and correction factors to achieve quick result


Ph buffering capacity

pH buffering capacity

  • adding known amounts of acid (as HCl) or alkali (as NaOH or lime) to soils

  • allowing a equilibrium period before measurement of pH

  • graph of amount added (per kg of soil) vs pH plotted

  • buffer capacity is the slope of the graph

  • quoted as an amount of acid or alkali (typically millimoles H+ or g CaCO3) per kg of soil per pH unit.


Pesticides

Pesticides

  • residues left in soils from pesticides or their by-products

  • analysed by GC-MS for sensitivity and ease of identification


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