Soil Issues for Urban Growers
Patricia Steinhilber
Urban Soils Workshop 2-2-13
Cylburn Arboretum
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What are Soils?
• reactive, dynamic, three-phase ecosystems
composed of solids, liquids and gases
topsoil
several
days after
rainfall or
irrigation
25%
25%
48%
Minerals
Air
Water
Organic Matter
2%
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What Does Soil Provide for
Plants?
• water
• nutrients
• oxygen gas for root and soil creatures
respiration
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Major Sizes Classes of Minerals in Soil
Mineral
Class
Size of Mineral Particles
Feel of
Particles
sands
0.05 – 2 millimeters (mm)
gritty
silts
0.002 – 0.05 millimeters
(mm)
smooth (like
clays
cornstarch or
talcum powder)
less than 0.002 millimeters sticky when
(mm)
wet, very hard
when dry
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The Other Soil Solid
Material: Organic Matter
Humus
75%
10%
Biomass
15%
Residues &
By-Products
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Residues and By-products:
What They Are
• dead stuff
- crop residues, dead roots and bodies of soil creatures
• by-products
- materials that plant roots and soil creatures release or
exude into the soil
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Residues and By-products:
What They Do
• fuel and nutrients for soil organisms
- energy and nutrient source for most of the soil
creatures
• formation and maintenance of soil aggregates
(structure or architecture)
- sticky and gummy by-products of residue
decomposition hold soil particles together in
clumps or aggregates
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Humus
• relatively stable end product of residue
decomposition
• composes the majority of organic matter
• resists further decomposition (1% per year)
• it is not a good nutrient or energy source for soil
creatures
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Humus: What It Does
• very small in particle size & high surface area
• charged sites at many locations on the
surface
• effective at holding water and nutrients
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The Other Half of Soil:
Soil Pores
• soil water
- adequate (but not too much) quantity
- adequate supply of nutrients
- minimize runoff and leaching
• soil air
- source of oxygen for roots and most soil
organisms
- constantly enriched with carbon dioxide from roots
and soil organisms
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The Interplay of Air and Water:
Soil Aeration
• The exchange of O2 and CO2 between the
soil pores and the ambient atmosphere
Hillel
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Balance Between Water
and Air
• macropores (large pores)
- drain quickly after rain or irrigation
- allow rapid infiltration of rainfall and replenishment
of oxygen in the root zone
• mesopores (medium-sized pores)
- “storage pores”
- hold water in form most plants can use
• micropores (very small pores)
- water is held too tightly to be use to most plants
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Biological Classification
of Soil Water
• excess or gravitational water
- water that drains from soil 1-3 days after a rainfall
or irrigation (in macropores)
• available
- water that is in a form crop plants can use (in
mespores)
• unavailable
- water that is held to tightly by the soil to be usable
by most crop plants (in micropores)
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Value of Soil Tests
• best pre-plant indicator we have of potential
nutritional products
• mine them for information!
• excellent investment
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Soil Tests and Their
Interpretation
Soil Test Category Interpretation
very low, low
medium
sufficient, optimum or
high
high, very high or
excessive
nutrient will limiting plant
growth
nutrient may not be limiting;
addition of nutrient advisable
nutrient supply is OK; no
more is needed
nutrient supply is more than
adequate; no more is needed
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Soil test results
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Soil or Manufactured Growth
Medium (MGM)?
• soil – amended in situ soil with lime, organic
matter and nutrient sources
- amended with large/massive amounts of organic
matter
• MGM – imported and mixed together soil
material and compost
- fill soil, manure, compost
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Issues with MGM
•
poor structure, compaction, poor aeration
- low porosity
- excessive micropore space
- inadequate supply of oxygen for plant roots
•
runoff control
- pathways – due to foot traffic
- textural discontinuity between growing beds and
underlying soil
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What Soil Management Issues
Do you Face?
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