Many urban soils are disturbed by grading, mixing, compacting, and adding soil imported from other areas. Because of this, urban soils change characteristics abruptly between horizons or layers, which can affect aeration, drainage, water-holding capacity, fertility, and of course root growth and function. The soil is intentionally compacted in order to create a stable building base. Soil engineers call pore space “voids.” Their goal is to reduce the voids in creating a stable surface. The compacted soil area is overbuilt or extended beyond the edge of the structure to provide stability for the structure. Therefore, plantings adjacent to structures will encounter compaction. Also, grading the site results in soil compaction. Ideally, temporary fencing would be used to protect future planting areas from stripping, grading, and compaction. In practice, this rarely occurs.
Soils and water quality are closely linked. The availability of water to plants is decreased in saline soils because of increased osmotic tension. A white crust on the soil surface is usually a mixture of sodium, calcium, and magnesium salts. SAR expresses the accumulation of exchangeable sodium in soils. High SAR can cause soil permeability problems because the sodium can cause dispersion of soil aggregates, which decreases both drainage and soil aeration. Soils high in clay are most susceptible.
Most soil mixes also contain mineral ingredients, also known as “coarse aggregates.” These materials provide structural air spaces to growing media. Sand is used occasionally in some mixes, especially those specialized for cactus. Its general use is limited by its heavy shipping weight. Unless used in larger proportions (>50%), sand can settle and create a perched water table in pots, interfering with water drainage. Perlite is a popular additive to potting mixes because it is inert and light weight. Like sand, it does not hold moisture. Unfortunately, it has a tendency to float to the top of potting media during watering. Vermiculite is another popular additive that improves drainage and holds moisture. It also provides Mg and K, two nutrients needed for plant growth. It does tend to compress in potting media, so mixes containing it should not be “pressed.”
Many calcined clays have properties which make them desirable as potting mix components. Calcined clays are essentially indestructible particles, which provide pore space to a mix due to the large spaces created between particles, and hold water internally within their open-pore particle structure. Most calcined clays have good nutrientretention but addno nutrient value of their own. They have long been used in creating optimal bonsai and orchid mixes because they provide the excellent drainage required by these plants and do not shrink like peat-based mixes do over time. Potting mixes which decompose and shrink once installed in commercial interiorscapes (plants grown indoors) are difficult to manage and often contribute to premature plant replacements.
A potentially more environmentally friendly alternative to Sphagnum peat, coir dust is a byproduct of the coconut husk processing industry and is, therefore, a renewable resource. Also known as coco peat, coir dust has a water-holding capacity and drainage characteristics similar to peat moss. It also contains no disease pathogens or weed seeds, which can be a problem in some organic materials. It is structurally very stable and resists shrinking, which can be a problem when Sphagnum peat is used in containers. It’s also easier to re-wet than Sphagnum peat. It does not provide nutrients, so fertilizer must be provided to plants grown in mixes made with coir dust.
Other additives are sometimes added to a soil mix to customize it to the needs of the plant or grower. Fertilizer almost always needs to be added to soilless mixtures because the ingredients provide no nutrient value of their own. Fertilizers are usually provided in a slow-release form so that nutrients are made gradually available. Limestone is usually added to balance out the acidity of Sphagnum peat moss in many mixes. Some plants and mixes perform better when wetting agents are added. They help the mix hold water for a longer time so that plants can be watered less frequently. Limestone Wetting agents vary in their effectiveness and should be tested prior to large-scale use. Nutrients and wetting agents are added after the mix is steam-pasteurized, so that the additives aren’t destroyed by heat. Sometimes polystyrene pieces are added to mixes to make them lighter during plant transport.
2013 Green Industry Training: Soils and Potting Mixes
SOILS ANDPOTTING MIXESGreen Industry TrainingSpring 2013Dr. Heidi Kratsch
Outline•Soil texture, structure, chemical properties•Topsoil qualities•Organic matter•Potting media•Ingredients•Proportions•Selection
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.
Composition of a typical soil Water Mineral fraction Air Organic matter
Soil texture The mineral particles: sand, silt, and clay
The effect of particle size Sand particles Clay particles Air flow Water flow
Texture effects on soil physical propertiesTexture Available water Aeration Drainage CompactionSandLoamSilt loamClay loamClay
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?
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!
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.
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)
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
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
Soil chemical properties greatly affect therelease 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
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)
• Why worry about soil pH?• Affects the dissolution of soil minerals• Generally, higher pH = lower mineral availability
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.
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).
pH tolerant = iron-efficient plantsIron-inefficient Intermediate Iron-efficientQuaking aspen Red maple AshSugar maple European beech LindenSweetgum Horsechestnut Scotch pineSilver maple Baldcypress GinkgoPin oak Quaking aspen Burr oak
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.
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
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
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!
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
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
―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.
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
Potting mixes1. Anchorage and stability2. Water3. Nutrients4. Aeration
Possible components• Field soil• Sand• Calcined clay• Perlite• Polystyrene• Peat moss• Pine bark• Hardwood bark• Coconut fiber (coir) Usually combine 2 or more ingredients
Why not field soil alone?1. Anchorage and stability2. Water3. Nutrients4. Aeration
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
Properties of soilless potting mixes •Water retention •Aeration •DrainageThe goal is to increase aeration without decreasing waterretention.
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
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
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
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
PeatsLess 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; lowMore water-holding capacitydecomposed
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
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.
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
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)
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.
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
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.
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.
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