Assessing soil quality,
soil testing procedures,
soil test interpretation
Weston Miller,
OSU Extension Service
Preview of presentation
• Safety precautions
• General soil quality assessment
• How to take a soil test
• What to test for
• Interpreting results
Safeair.Oregon.gov
Based on soil data collected by DEQ in late February 2016, OHA concludes that the levels
of metals in soil around Bullseye Glass in SE Portland and Uroboros Glass in N Portland are
too low to harm the health of people in the surrounding community.
As a precaution for urban food gardeners in general, State agencies are advising that
gardeners perform soil tests for metals in urban areas regardless of proximity to air
pollution sources.
How to interpret results of soil test
for heavy metals in urban areas
Safety first with metals!
How to minimize exposure to heavy
metals through vegetable gardening
• Research inherent hazard of site
– Old paint on/near buildings, industries like gas stations/plumbing shops,
air pollution from roads/industry, etc.
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Increase soil pH by adding lime to the soil
Add compost and other forms of organic matter
Avoid root crops and greens in suspect areas
Wash produce thoroughly
Take off shoes before entering home
Wash hands after gardening, even if you use gloves
Questions to assess soil quality
1. Does soil have good structure and tilth?
2. Is the soil free of compacted layers?
3. Is the soil easily worked?
4. Is the soil full of living organisms?
5. Are earthworms abundant in the soil?
6. Is plant residue present and decomposing?
7. Do crops/weeds appear healthy and vigorous?
8. Do roots grow well?
9. Does water infiltrate quickly?
10. Is water available for plant growth?
Drainage
• Avoid low areas
• Raised beds
• Organic matter
Look for signs of life
Look for response of
plants to flush of
nutrients in early summer
Soil Texture
• How coarse or fine a mineral soil is.
• The proportion of sand, silt, and clay.
Soil Triangle
Laboratory texture analysis
Soil texture
by feel
Relative Size of Soil Particles
(University of
Nebraska, Lincoln)
General properties of sandy soils
Sandy soils tend to be:
• Low in organic matter content
• Low native fertility
• Low water holding capacity
• Low cation exchange and
buffer capacities
• Rapidly permeability of air
and water
General properties of finer soils
Finer soils tend to be:
• Higher in organic matter
content
• Higher native fertility
• Higher water holding capacity
• Higher cation exchange and
buffer capacities
• Slower permeability of air and
water
• Good up to a point, then…
Ideal soil structure
(CA Fertilizer Association)
OM helps to aggregate particles,
creating pore spaces
OSU soil testing resources
Keyword search “OSU small farms soil test”
• How to take a sample
• List of labs
• How to interpret results
Why do chemcial analysis of soils?
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Establish baseline nutrient status
Determine application rates
Assess pH and need for liming
Measure changes over time
Avoid excess nutrient application
or build up of salt
How to take a soil test
Determine management units
Sample where the crop will be planted
What to test for
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pH (acidity-basicity)
SMP buffer (ease of change of pH)
Organic matter (OM)
Cation Exchange Capacity (CEC)
Phosphorus (P)
Potassium (K)
Calcium (Ca)
Magnesium (Mg)
Sodium (Na)- salts
Texture?
Date
Organic matter
Organic Matter (OM)
• Tests not so accurate, tend to overestimate
• Look for 5-8% OM for veggies
Soil Organic Matter (SOM) fractions
Organic Matter
• Key to long term fertility
• Improves quality of most soils
• Breaks down to form humus (natural glue)
• Strengthens aggregates
• Improves water holding capacity
• Releases plant nutrients
• Provides food for beneficial organisms
Too much of a good thing?
• High Organic Matter (OM) content leads to:
– High rates of mineralization of soil leading to
potential excess available nutrients (P and K)
– Excess nutrients can lead to plant problems
• High P can affect micronutrient and N absorption
– Potential for excess nutrients to leach into
groundwater or surface water as non-point
source of pollution
Cation Exchange Capacity
A soil’s capacity to hold cations is called
its cation exchange capacity or CEC.
CEC is a calculated value
CEC determines
availability of cations
• Higher in clay soils
because negative charge
• Organic matter and
increase pH can increase
effective CEC
pH
pH Scale
pH 6-7 is best for vegetable production
Soil pH determines if lime is needed
Soil pH and nutrient availability
Soil bacteria abundance
Soil pH is Key
120
Relative Yield, %
100
80
Beets
Spinach
60
Rutabaga
40
Turnip
Radish
20
Lettuce
Green bean
0
5.6
5.8
6
6.2
6.4
6.6
6.8
Soil pH
Courtesy John Hart, OSU Crop & Soil Science
Yield, t/a
6
3
0
5.5
6
6.5
Soil pH
Courtesy John Hart, OSU Crop & Soil Science
SMP Buffer pH
SMP Buffer tells how
much lime to apply
To Increase Soil pH
• Lime (Calcium carbonate)
–Per soil test recommendations
–Or 5-10 # / 100 sq. ft.
–Dolomite also has Mg
–Apply in fall, if possible
Phosphorus soil test
Potassium
Calcium
Magnesium
Trace minerals: B and Zn
Consider tissue analysis to test for B if noticing signs of deficiency
(chlorotic young leaves; death of main growing point; stunted growth)
Zn = 6.8ppm, which is high, but Zn rarely
causes toxicity except in saline soils
What about nitrogen?
• Forms available for plant uptake
– Nitrate (N03-)
– Ammonium (NH4+)
• Concentrations depend on bio activity
• Vary widely with temp and moisture
• Difficult to interpret soil test results
Nitrate nitrogen
Residual soil nitrate
General recommendation: Apply 3 # actual N per
100ft2 over course of season in splits with. No more
than ½ in initial application
Review of presentation
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Safety precautions
General soil assessment
How to take a soil test
Safety first!
What to test for
Interpreting results