Class 3:

Soil Sampling and Testing

Chris Thoreau

What is Soil Testing?

What are We Testing For?

Soil Sampling Methodologies

Soil Testing

Interpreting Soil Test Results

What is Soil Testing?

Soil testing is the use of various laboratory

methods to discover chemical and/or biological

qualities of a soil sample as part of an overall soil

management plan

Soil testing can also be used to discover the

presence of contaminants such as heavy metals or

pollutants

Testing for chemical properties is the most common type of soil testing and includes:

• pH

• Organic Matter %

• Nutrients N, P, K, Ca, Mg, Fe, Cu, S, Mn, Zn, B

• C:N Ratio

• Electrical Conductivity (EC)

• Cation Exchange Capacity

• Base Saturation

• Particle size distribution (texture)

Soil testing involves three basic stages:

1. Sampling

2. Testing

3. Interpreting

Which, ideally, lead to actions to improve

our soil quality.

Soil Sampling:

• The process of collecting soil to be submitted for

testing. The collected soil is called a “sample”

• Soil must be collected carefully to ensure a

representative sample is submitted for testing

Collecting Samples – 2 basic methods

1. Composite Samples: • Several samples are taken from a field and then

mixed together to get a representative sample

Samples must be taken randomly or in a set pattern to

ensure a statistically representative sample is

collected as we are collecting a small amount of soil

to represent a large area

Collecting Samples

Random Sampling Random locations

Easy way to get a

representative sample

Must be truly random to

be accurate!

Zig-zag motion can be

suitable

Collecting Samples

Topography Sampling

• Variation of random

sampling method

• Addresses variation in

field topography or

field features

Collecting Samples

2. Variation Sampling: • Samples are taken from a field in a fixed pattern

and each sample is kept separate

• Good for identifying variations in a field

Collecting Samples

Grid Sampling

• Good for uniform field(s)

• Does not take into

consideration topography

• Can be used for variation

sampling

Variation in Fields – e.g., OM and pH

Collecting Samples – Tools:

• Clean trowel and shovel; Oakfield probe

• Clean bucket

• Plastic bag

Plus, if doing variation sampling:

• Individual Bags

• Labels

• Field Map

Collecting Samples – in the field:

After establishing sampling points… 1. Remove vegetation from soil surface

2. Using shovel, dig 20-30 cm deep hole in a

small area (20 cm diameter hole)

3. Mix soil in hole well

4. Using trowel, scoop up a standard, level

sample and place in bucket

Where don’t we collect samples from?

• Anomalous/non-representative locations

• Around trees

• Field edges

• Near manure or compost piles

• Around standing water

Once all samples have been taken…

1. Collect about 1 litre of soil from your

mixture of samples

2. Place it in a bag and label it • Collection date

• Name of field collected from

• Contact info for lab

When is a good time to be sampling?

• Close to time when soil will be “used” or Spring

• Or, end of season to prepare for following year

• Test for Nitrogen: mid- or late-season when soil

is warmer

• When soil not too wet or dry

• Ideally test in the conditions in which plants will

be growing

Once soil sample has been collected it

needs to go to a lab…

Where do we send our samples? • Always to the same lab

Request list of methods use

Methods are all standardized

• If new lab:

Confirm methods used and compare results to

previous test

Once you have test results you need to

know what to do with them!

pH - Measure of soil acidity • Or, the amount of H+ ions in soil

• Must measure pH in solution and residual

Scale is from 0-14 • 0 = acidic; 14 = alkaline/basic

pH in coastal soils is usually acidic Soil Acidity is neutralized through

additions of Lime (CaCO3) Amount of lime required depends on soil

texture and OM content

Chemistry of raising pH

pH Buffering

Adsorption Affinity:

Al3+ > Ca2+ = Mg2+ > K+ (H+) = NH4+ > Na+

Basic Liming Recommendations

• Contingent on measured pH, desired pH, clay

content, OM %

Table 3. Lime Application Rates to Raise Soil pH to Approximately 7.0

Existing Soil pH

Lime Application Rate

(pounds per 1,000 square feet)

Sandy Loamy Clayey

5.5 to 6.0

5.0 to 5.5

3.4 to 5.0

3.5 to 4.5

20

30

40

50

25

40

55

70

35

50

80

80

• Lime application rates shown in this table are for dolomite, ground, and

pelletized limestone and assume a soil organic matter level of approximately 2%

or less. On soils with 4 to 5% organic matter, increase limestone application rates

by 20%.

• Individual applications to turf should not exceed 50 pounds of limestone per

1,000 square feet.

• Avoid the use of hydrated or burned lime because it is hazardous to both

humans and turf (can seriously burn skin and leaves). If hydrated lime is used,

crease application rates in the above table by 50% and apply no more than 10

pounds of hydrated or burned line per 1000 square feet of turf.

Organic Matter %

• % is by soil weight

• Ideal levels = 12-20% or more

• If tests show amounts below this level – add

more organic matter!

C:N Ratio

• Should be 15:1 to 24:1

• If higher than 24:1, be careful to not add woody

or high carbon materials to soil

Electrical Conductivity (EC)

• Measure of salt content in soil

• Tests should return levels below 4 mmhos/cm

• Rare to have high salts in soil around here

• Use gypsum to remove sodium in soil

• Flushing water with soil important

Cation Exchange Capacity

Measure of soil’s ability to hold and

release positively charged particles • Ca2+, Mg2+, K+, Na+, H+, Al+

• Higher CEC value is better

• >15 meq/100g soil or 20 cmol/kg soil

• OM and clay both have high CEC

• CEC Animation

Base Saturation

• Fraction of cation exchange sites occupied by

base cations (Mg2+, Ca2+, Na+, K+)

Usually expressed as a percentage

• Base Saturation increases with soil pH

Base Saturation

• Ideal proportions of Base cations

60-75% Calcium

10-15% Magnesium

3 – 5 % Potassium

<3% Sodium

<12% H+

Nutrient Levels Nitrogen

• Leaf and Stem growth; photosynthesis; proteins

• Total N % = .5 -.7%; ppm = 20ppm

Phosphorus • ADP and ATP synthesis; photosynthesis; N-fixation; roots

• 125 – 300 ppm

Potassium

• Enzyme activator; metabolism; stomata regulation

• 150 – 250 ppm

Nutrient Levels Calcium

• Important for cell growth and structure

• 1500 – 3000 ppm

Magnesium • Essential element of chlorophyll; P carrier; enzyme activator

• 14% of Calcium levels - 200 – 400 ppm

Copper • Seed formation; chlorophyll formation; enzyme activity

• 5 – 10 ppm

Nutrient Levels Zinc

• Component of many proteins; toxic at high levels

• 50 – 100 ppm

Iron • Important for e- transport: photosynthesis; toxic at high levels

• 150 – 250 ppm

Manganese • Assimilation of CO2 in photosynthesis; e- transport in PS

• 75 – 125 ppm

Nutrient Levels Boron

• Cell wall strength; cell division; too much is toxic

• 2 – 4 ppm

Sulfate Sulfur • N-fixation; chlorophyll formation; seed production

• 100 – 150 ppm

Ideals Nutrient Ratios

Calcium: Magnesium = 10:1 • 7:1 in sandy soils

Phosphorus:Potassium = 2:1

Potassium:Sulfate-sulfur = 1:1

Interpreting Soil Test Results

Soil Sampling Fact Sheet

Gaia College Base Saturation

Soil Test Interpretations 1

Soil Test Interpretations 2

Soil Testing for Organic Gardeners