1. SOILS AND
POTTING MIXES
Green Industry Training
Spring 2013
Dr. Heidi Kratsch

2. Outline
•Soil texture, structure,
chemical properties
•Topsoil qualities
•Organic matter
•Potting media
•Ingredients
•Proportions
•Selection

3. Plant roots need gases
• Oxygen - burns (respires) sugars provided by the
canopy (leaves) for energy
• Release carbon dioxide in the process
• If the two gases cannot freely exchange with the
atmosphere
• respiration shuts down
• roots die off
• Plants can’t get water or nutrients.
• Many root- and wood-rotting organisms (fungi)
thrive in low oxygen soil conditions.

4. Composition of a typical soil
Water
Mineral
fraction
Air
Organic matter

5. Soil texture
The
mineral
particles:
sand, silt,
and clay

6. The effect of particle size
Sand particles Clay particles
Air flow
Water flow

8. Texture effects on soil physical properties
Texture Available water Aeration Drainage Compaction
Sand
Loam
Silt loam
Clay loam
Clay

9. Soil texture and drainage
Coarse Medium Fine
Texture Texture Texture
Silt Loam Clay Loam
Sand
Can’t I just add Sand or Clay
to balance the condition of
the soil?

10. Answer: No…!
•Why? It’s a problem of scale:
• Soil weighs about 90 lbs per cu. ft.
• Soil from a hole 3 ft. in diameter by 2 ft deep is
about 18 cu. ft., and weighs 1600 lbs.
• To change the texture by 10 to 20% would require
160 to 320 lbs of material (sand or clay)
• Requires considerable expense and effort
• Could just be creating cement!

11. WHAT CAN I DO?
Answer: Enhance soil
STRUCTURE

12. Soil structure
• The combination of sand, silt and clay (with
organic matter) into secondary particles called
aggregates
Soil aggregate

13. Structure develops over time

14. Compaction
•Destroys soil structure
•Seals off soil surface
•Stress – perhaps death of plants

15. Impact is worse when soil is wet

16. Effect of compaction on plants

17. Soil layers (horizons)
•In an undisturbed
setting, soils are
allowed to form
naturally.
•Over time, provides
ideal conditions for
plant roots.

18. The reality in urban areas….

19. Excavation and Fill Soils
•Needed to provide proper grade and
surface drainage, but…
•Generally low in organic matter
• Excavated subsoils (basement, grade cut…)
• Often stockpiled for extended periods (much of
the organic matter decomposed in 2 to 6
months)
•Thoroughly disturbed, mixed and broken
up, structure has been reduced, even
eliminated.

20. Effect of construction on soil drainage

21. Excessive drainage problem
•Very sandy soil
•Coarse soils are naturally droughty within
hours after rain
•Add extra organic matter (for retention)
•Precise water management (frequent, low
volume – like drip/trickle systems)

22. Amending soils with organic matter
• Improves drainage and aeration of clay soils
• Improves water-holding capacity of sandy soils
• Reduces compaction
• Provides/retains nutrients
• Locally lowers soil pH
• NOTE: Add no more than 25% by volume
• Higher levels can cause significant soil settling as OM
breaks down

23. Water and Mineral Nutrients

24. Water and mineral nutrition
• Water action helps release minerals into the soil
solution (dissolving, freeze-thaw breakdown—
weathering of rock)
• Water is the medium by which mineral nutrients travel
to, into, and through the roots

25. Soil chemical properties greatly affect the
release of nutrients or the movement of water
• Soil texture
• pH affects mineral form and release
• Accumulation of salts: carbonates, sodium,
chloride and sulfates, etc.) can restrict water and
nutrient uptake, or alter soil structure

26. What is pH?
• pH is measured as the ―activity‖ or concentration of
hydrogen ions (H+) in the solution.
• The higher the concentration of hydrogen ions, the
lower the pH (more acidic).
2 4 6 8 10 12
acidic Neutral alkaline
(7.0)

27. • Why worry about soil pH?
• Affects the dissolution of
soil minerals
• Generally, higher pH =
lower mineral availability

28. CAN’T I JUST
ACIDIFY
MY SOIL?

29. Answer: No
• Why? Another problem of scale:
• Western soils have VERY large reservoirs of pH
buffers in the soil (solid carbonates and other
minerals, ex. ―free lime‖)
• 1% CaCO3 in an acre-foot of soil weighs 40,000 lbs
• Nevada soils frequently contain 20-30%
• All buffering compounds would have to be
dissolved and neutralized before the pH will
drop.

30. Buffering reactions:
CaCO3 + CO2 (in water)  Ca2+ + 2 HCO3
(Calcium Carbonate) (Bicarbonate)
HCO3 + H+ (in water)  CO2 + H2O
(this is just one acid neutralization reaction -- no
change in pH, i.e., no increase in free H+)
Added acid (H+) is consumed until all Carbonates
are dissolved, or other cations leached from the
system (i.e., Total Alkalinity is neutralized).

31. Major pH-related problem: iron chlorosis

32. pH tolerant = iron-efficient plants
Iron-inefficient Intermediate Iron-efficient
Quaking aspen Red maple Ash
Sugar maple European beech Linden
Sweetgum Horsechestnut Scotch pine
Silver maple Baldcypress Ginkgo
Pin oak Quaking aspen Burr oak

33. SOIL SALINITY

34. Soil salinity = soluble salts in soil
• Salts inhibit plant growth through ―chemical
drought‖ (induced water stress), specific ion
toxicity, or soil dispersion (Sodium salts)
• Visual diagnosis: salt crusting/salt burn
• Electrical conductivity (EC) is the measure of soil
salinity.
• EC > 2 deciSiemens/meter can harm plants
• SAR – Sodium Adsorption Ratio
• SAR > 13 is sodic, but soil problems can occur at lower
levels.

35. Salinity and plant adaptation
Soil EC (dS/m)
0 2 4 6 8 10 12 14 16
Berries
Apple
Alfalfa---|
Corn----|
Spinach----------|
Bluegrass
Tall fescue-------------|
Alder-----------|
Cottonwood----|
Barley----------------------------------------------|
Wheatgrass-----------------------------------------------|

36. Sources of salts
• Natural deposits
• Residual salts in new development areas (watch
fill soils)
• Irrigation waters
• natural sources (esp. shallow wells)
• water softeners (high in sodium)
• Deicing salts (road throw and sidewalk runoff)
• Over-application of fertilizers or manure and
compost

37. Other salt problems
• Sodium saturated, or Sodic soils can become
dispersed (involves clay particles).
• Breaks down soil structure
• Seals soils to air and water penetration
• Specific toxicities to specific salt constituents

38. Flocculated, aggregated clay
Dispersed, crusted clay

39. Boron
• Essential mineral (specific ion) – becomes toxic
above 0.5-1.0 ppm
• Occurs in arid, young soils
• Other sources: well water, reclaimed water,
geothermal springs, earthquake faults
• Mobile in soils – moves up with moisture
evaporation from soil
• Boron-laden soils are often salty too!

40. Boron toxicity symptoms in bur oak

41. Leaching salts with water
• Ensure that soil has good internal drainage.
Water must move through the soil to carry salts
out
• Add organic matter
• Deep tillage/ripping (not near established trees)
• Apply water over 1-2 days
• 6 inches of water to cut EC by 50% (in top foot)
• 12 inches of water to cut EC by 80%
• 24 inches of water to cut EC by 90%
• Only effective if water table below 6 to 8 feet

42. Do amendments help?
• Gypsum (calcium sulfate)
• Used along with leaching
• only effective for sodic soils
• Sulfur
• May be effective for sodic soils
• Limited effectiveness for pH on a landscape scale
• Organic matter
• Improves soil structure
• Does not lower salinity
• Does not affect boron levels

43. Step Back – Big Picture Review

44. ―Typical‖ Nevada soils
• Arid/Droughty conditions
• Low precipitation
• Coarse, sandy soils
• High pH (alkaline – 7 to 8+)
• Reduced mineral nutrient release (especially
Iron)
• Higher evaporation levels produce saline or
sodic conditions
• May not be able to ―fix‖ the conditions.

45. IF I CAN’T FIX THE
SOIL, WHAT DO I
DO?
• Choose species adapted
to the conditions at hand
• Prepare soils for best
possible condition

46. Potting mixes
1. Anchorage and
stability
2. Water
3. Nutrients
4. Aeration

47. Possible components
• Field soil
• Sand
• Calcined clay
• Perlite
• Polystyrene
• Peat moss
• Pine bark
• Hardwood bark
• Coconut fiber (coir)
Usually combine 2 or more ingredients

48. Why not field soil alone?
1. Anchorage and stability
2. Water
3. Nutrients
4. Aeration

49. Pots restrict how water drains
Gravity

50. Shift towards soilless potting mixes
•Do not need to be
pasteurized
(sterilized)
•Lighter in weight
(lower shipping
costs)
•Mixes are more
consistent – you
know what to expect

51. Properties of soilless potting mixes
•Water retention
•Aeration
•Drainage
The goal is to increase aeration without decreasing water
retention.

52. Coarse mineral components
•Perlite
• Volcanic origin
• Low bulk density
• Good drainage and aeration
• Low CEC and water-holding
•Vermiculite pH 7.5
• Heat-expanded mica
• Low bulk density
• Use coarse grades for best
aeration and drainage
• High CEC and water-holding
pH 7.5 (U.S.), 9.0 (African)
Vermiculite

53. Sand
• Coarse concrete-grade
(washed)
• High bulk density
• Excellent drainage and
aeration
• Increases water-holding
when mixed with bark
• Decreases water-
holding when mixed
with field soil
• Low CEC

54. Calcined Clays
• Good water- and nutrient-holding capacity
• Excellent drainage qualities
• Provides Coarse Texture and Aggregated
Structure
• Little influence on pH of a mix
• Bulk density 30 to 40 lbs/ft 3

55. Bulk Density
•How heavy per unit Bulk density at
CC
volume Material (lbs/ft3)
Field soil 106
•Acceptable range: Sand 107
3
40 to 60 lb/ft Sphagnum peat 54
•Too heavy: not Coir (coconut
fiber)
46
economical to ship Vermiculite 46
•Too light: pots with Pine bark 51
Perlite 32
plants topple
Rock wool 54
CC = Container Capacity

56. Peats
Less decomposed • Sphagnum moss - a moss that grows in
acid bogs in North America, Canada, and
northern Europe
• Sphagnum peat moss - the partially
decomposed remains of Sphagnum moss
• Peat moss (or moss peat) – partially
decomposed Sphagnum or hypnum
• Reed-sedge peat – reeds, sedges,
marsh grasses and cattails (variable in color
and other properties)
• Peat humus – highly decomposed; low
More water-holding capacity
decomposed

57. Sphagnum moss Sphagnum moss peat – pH 3.0 to 4.0
Hypnum moss peat– pH 5.2 to 5.5
Reed-sedge peat – pH 4.0 to 7.5

58. Peat-based mixes
•Common formulations:
• Sphagnum peat moss / vermiculite (1 : 1)
• Sphagnum peat moss / perlite (1 : 1)
•Excellent water- and nutrient-holding, good
drainage.
•Very difficult to re-wet if allowed to dry out.
•Must be careful not to over-fertilize and
water enough to leach out excess nutrients.
•Breaks down over time.

59. Coir (coconut) fiber – alternative to peat?
• Made from coconut
husks.
• High water-holding
capacity
• Excellent drainage
• Absence of weeds and
pathogens
• Decomposes slowly.
• Easier to re-wet than
Sphagnum peat.
• Some sources high in ―Coco Peat‖
salts. pH 4.9 to 6.8

60. Bark-based products
• Cheaper than
Sphagnum peat
• pH 4.5, increases
over time
• Excellent aeration and
wettability
• Poor water-holding
• Often mixed with sand
and vermiculite or
peat moss (3 bark : 1
pH of softwoods 3.0 to 4.0 sand : 1 vermiculite or
pH of hardwoods 6.0 to 7.0 peat moss)

61. Pasteurization
• Eliminates disease
organisms, insects,
nematodes, weeds.
• Steam: 160F for 30 min
• Soil-based substrates
must be pasteurized.
• Soilless does not need
it unless reused.
• Does not protect
against future
infestation.

62. Other Pre-plant Additives
• Dolomitic limestone
• Correct the pH or acidity of a
mix
• Phosphate
• Superphosphate (0-45-0)
• Nitrogen and
potassium
• Enough to last 2 weeks
• Micronutrient mix
• Enough to last the growing
season
• Wetting agent
• Gel granules help media hold Hydrogel crystals used as a
water longer wetting agent

63. Organic mixes
• OMRI –
• Organic Materials Review
Institute
• Assures products are
consistent with the
requirements of the National
Organic Standard.
• Challenge is not finding
ingredients but in getting
consistency.
• May not use wetting agents
in certified organic
products.

64. Compost and manure
rules:
Compost 1. Pile must be 131-170F for
• Rarely used alone as a 3 days (closed system) to
potting ingredient (20 to 15 days (open system).
30% is common). 2. Must be turned at least 5
• Has been shown to times.
suppress plant diseases.
3. Measure respiration (CO2
• During composting process:
release, O2 uptake or
1. First phase, most materials
easily degraded (104-122F) temperature).
2. Second phase, cellulose and
pathogens (and some
beneficials) degraded (122-
149F)
3. Third phase, humus content
increases, along with some
beneficials.

65. Summary – container substrates
• Stable product that will not shrink in volume during
plant production / shelf time.
• Bulk density low enough for shipping and handling
but high enough to prevent toppling of plants.
• At least 10 to 20% air by volume at CC (container
capacity) in a 6.5-inch pot
• High cation exchange capacity (CEC) for nutrient-
holding.
• pH of 6.2 to 6.8 (soil-based) or 5.4 to 6.5 (soilless)
– crop dependent

66. Questions?
Contact:
Heidi Kratsch
University of Nevada Cooperative
Extension
Phone: 775-336-0251
Email: KratschH@unce.unr.edu