Soil Testing
•Backyard
• Nutrients
• Contaminants
•Biological
Backyard Soil Tests
•Texture
•Structure
• Color
Texture:
Proportions of Sand, Silt, and Clay
Sand .05-2 mm
Silt .002-.05 mm
Clay <.002 mm
Coarse Fragments >2 mm
Soil Structure
Aggregation of Sand, Silt, and Clay Particles
Structure affects:
•Macroporosity
•Infiltration
•Aeration
Formation of Soil Structure
• Growth of roots and movement of organisms
create pores and aggregates
• Soil organisms break down organic residues,
producing glues that stabilize aggregates
• Fungi provide structural support to aggregates
• Physical, chemical processes also involved
Soil Color
Gray, mottled colors
indicate seasonal high
water table.
Major Soil Types
of the Puget
Sound Area
Glacial Till Soil
Ablation till
• Not compacted
• Permeable to water and
roots
Basal till
• Compact and cemented
• Barrier to water and
roots
What Is A Soil Nutrient Test?
• A chemical evaluation of the nutrient
supplying capability of the soil.
• Soil test results are calibrated with crop
response research to develop fertilizer
recommendations.
Soil Nutrient Testing
Soil Test vs. Crop Response
Interpreting Soil Tests
• Nutrient status
Low: fertilizer response likely
Medium: sometimes a fertilizer response
High: fertilizer response unlikely
• Fertilizer recommendation
Reference: EC 1478. Soil Test Interpretation Guide
Soil Nutrient Test
• Basic garden test: P, K, Ca, Mg, B,
pH, lime requirement
• Options: Micronutrients (Zn, Mn, Cu,
Fe), N, S, CEC, base saturation, organic
matter, salts
When to Sample?
• Standard tests can be taken at any
time before fertilization.
• It is best to be consistent from year to
year.
How Often to Sample
• Sample each unit every:
• 3 to 5 years for yards and
gardens.
• 1 to 3 years for commercial
production.
Taking a Sample
• Sample defined area
• Take 10 or more
subsamples (0-8” deep)
• Dry and mix well
• Subsample
mixture and
fill sample bag
Choosing a Lab
• Does the lab routinely do agricultural and
garden tests?
• Do they use OSU or WSU test methods?
• Do they give fertilizer recommendations?
• What information do they need?
• How to send sample?
• Cost?
• Turn-around time?
• What does report look like?
What About Soil pH?
• Indicates relative acidity or alkalinity
• pH 7 = neutral; less than 7 = acid; more than 7 = alkaline or basic
• Logarithmic scale
Why is pH Important?
• Nutrient availability
• Availability of toxic metals
• Microbiological activity
Desirable pH Ranges
Vegetables 6 to 7.5
Pastures 5.5 to 8
Acid loving plants 4.5 to 5.5
Contaminant Sampling
• Usually not needed
• You may want to test for lead and/or
arsenic if you have good reason to
suspect contamination:
Plume of Asarco smelter (lead, arsenic)
Growing food around foundation of old
house (lead)
Biological Sampling
• Population or activity
of soil organisms
• Not enough research
has been done to
recommend these
tests