Download - Common Soil Parameters
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
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
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 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 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 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
Finger assessment of soil texture for mineral soils
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
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 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 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 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
a) 40 x 2.04 g = 81.6 gb) 40 x 0.40 g = 16 gc) 81.6-16 = 65.6 gd) Silt 66.8%, clay 16.3%, sand 16.9%e) silt loam
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
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-, SO4
2- and HCO3-).
Soluble fertiliser may also contribute K+, NH4+ and
NO3-.
High EC is undesirable for most plants.
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
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 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
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
Organic content
determined by oxidative digestion (dichromate)
measured by: back titration of excess dichromate colorimetry of green product
Chemical Attributes
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 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 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) predictor of salinity measure Ca, Mg & Na
in mmole/L use eqn or nomogram
2
])Mg[]Ca([
]Na[SAR
22
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?
5.9
223
15
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 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 residues left in soils from pesticides or their by-
products
analysed by GC-MS for sensitivity and ease of identification