soils, fertilizers and potting mixes

Green Industry Training

Spring 2015

Dr. Heidi Kratsch

SOILS, FERTILIZERS AND POTTING MIXES

Outline•Soil texture, structure, chemical properties

•Topsoil qualities•Organic matter•Fertilizers•Potting media•Ingredients•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

Air

Mineral fraction

Organic matter

Soil texture

The mineral particles:sand, silt,and clay

The effect of particle sizeSand particles Clay particles

Water flow

Air flow

Texture effects on soil physical properties

Texture Available water Aeration Drainage Compaction

Sand

Loam

Silt loam

Clay loam

Clay

Soil texture and drainage

SandSilt Loam Clay Loam

Coarse Texture

Medium Texture

Fine Texture

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!

WHAT CAN I DO?Answer: Enhance soil STRUCTURE

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

Soil aggregate

Structure develops over time

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

Impact is worse when soil is wet

Effect of compaction on plants

Soil layers (horizons)•In an undisturbed setting, soils are allowed to form naturally.

•Over time, provides ideal conditions for plant roots.

The reality in urban areas….

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.

Effect of construction on soil drainage

Excessive drainage problem•Very sandy soil•Coarse soils are naturally droughty within hours after rain.

•Add extra organic matter (but not to tree planting holes)

•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 Nutrients

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 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

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

Neutral (7.0)

acidic alkaline

•Why worry about soil pH?

•Affects the dissolution of soil minerals

•Generally, higher pH = lower mineral availability

CAN’T I JUST ACIDIFY MY SOIL?

•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.

Answer: No

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).

Major pH-related problem: iron chlorosis

pH tolerant = iron-efficient plants

Iron-inefficient Intermediate Iron-efficient

Quaking aspen Red maple AshSugar maple European beech LindenSweetgum Horsechestnut Scotch pineSilver maple Baldcypress GinkgoPin oak Quaking aspen Burr oak

Step Back – Big Picture Review

“Typical” Nevada soils

•Arid/Droughty conditions• Low precipitation• Coarse, sandy soils

•High pH (alkaline – 7 to 8+)• Reduced mineral nutrient release (especially Iron)

•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

Soil Organic Matter• Originates from living organisms,

consisting mostly of carbon and nitrogen.

• Includes living organisms (bacteria, fungi, earthworms) and decaying plant matter.

• Soil organisms use decaying plant matter as a food source.

Humus• An organic component of soil, formed by the

decomposition of leaves and other plant material by soil microorganisms.

• “Stable” vs. “active” (compost)

• Stable humus does not add nutrients, but it does bind and store nutrients.

• Presence of stable humus can prevent leaching of nutrients from the soil.

Cation exchange capacity (CEC)

The degree to which cations can be held by soil particles and exchanged with soil

water.

Cation Anion positively charged negatively charged

ex. Mg2+ ex. SO42-

Positive ions attract negative ions.

Essential Nutrients

Chemical elements involved in the metabolism of the plant or necessary for the plant to complete its life cycle

Fertilizers are mineral salts.

Essential Nutrients (Elements)Macronutrients:• Nitrogen (N)• Phosphorus (P)• Potassium (K)• Sulfur (S)• Calcium (Ca)• Magnesium (Mg)

Micronutrients:•Boron (B)•Chloride (Cl)•Copper (Cu)•Iron (Fe)•Manganese (Mn)•Molybdenum (Mo)•Zinc (Zn)•Nickel (Ni)

Complete Fertilizer

32% Nitrogen

10% soluble Potassium (K2O)

10% availablePhosphorus (P2O5)

FertilizersInorganic• Release elements quickly in water

• Excess can “burn” plants• Urea is treated as inorganic because of “quick release” of N

• Solubility not affected by temperature

• May leach from soil

Organic •Release inorganic ions slowly

Examples:•Urea formaldehyde• Isobutylidene diurea (IBDU)

•Manures•Sewage sludge•Blood•Bone meal

Slow-release fertilizers

•Release nutrients over an extended period

•Higher cost•Reduce leaching and burn problems

•Release rate may be affected by soil moisture and temperature

Slow-release: urea aldehydes•Urea formaldehyde

• 36-38% nitrogen

• Slowly released• Relies on microbial breakdown

• IBDU – isobutylidene diurea• Slowly released • Not dependent on microbial activity.

Slow-release: sulfur-coated•Prills of various fertilizers (urea, triple superphosphate, potassium sulfate, potassium chloride)

•Coated with sulfur and wax-like sealant

•Not dependent on microbial activity

Sulfur-coated urea

Water-Insoluble Nitrogen (WIN)

GUARANTEED MINIMUM ANALYSIS

Total Nitrogen (N) 12.0 %

Water Insoluble Nitrogen (N) 10.8%

Iron (Fe) 0.2%

Organic Matter 80.0%

Look for WIN that is at least 50% of total Nitrogen.

Forms of Nitrogen•Ammonium (NH4+)• Potential toxicity• Acidifying• Should be no more than 40% of total N for container plants

•Urea – broken down to ammonium

•Nitrate – less chance of toxicity but greater chance of leaching

Ammonium toxicity symptoms

Turfgrass FertilizationConsiderations:•Minerals required for growth•Natural soil fertility•Fertilizer selection•Turfgrass species, desired

quality and use•Application schedule.

pH adjustment for turfgrass?•Use of regular sulfur applications can deteriorate soil structure and cause build-up of soluble salts.

•Hard water used for irrigation can negative the acidifying effects.

•Acidifying fertilizers are a better option for western soils• Offset alkalinity of irrigation water• Temporarily low soil pH at time of fertilization.

N is most important for turfgrass fertility•Elicits the strongest growth response•Enhances green color•Absorbed primarily in NO3− form •Can be translocated to leaf tissue within 24 hours.

Analysis of quick-release N fertilizersN carrier Analysis Burn

potentialSoil reaction

Ammonium nitrate

33-0-0 High Acidic

Potassium nitrate

13-0-44 High Basic

Ammonium sulfate

21-0-0 High Acidic

Urea 45-0-0 High Slightly acidic

Monoammonium phosphate

11-50-0 Moderate Slightly acidic

Diammonium phosphate

20-50-0 Moderate Basic

Analysis of slow-release N fertilizers

N carrier Analysis Burn potential

Activity at low

temperatures

IBDU 31-0-0 Moderately low

Moderate

Sulfur-coated Urea (SCU)

22 to 38-0-0 Low Moderate

Resin-coated urea

24 to 35-0-0 Low Moderate

Urea formaldehyde

36 to 38-0-0 Low Very low

Manures Variable Very low Very low

Activated sewage sludge

4 to 6-4-0 Very low Very low

Turfgrass N requirements by speciesGrass species Lbs. N per 1,000 sq. ft. per

yearCreeping bentgrass 3 to 8 Kentucky bluegrass 2 to 4Perennial ryegrass 2 to 4Red fescue 1 to 3Chewings fescue 1 to 3Tall fescue 1 to 2Dwarf fescue 1 to 2

No more than 1 lb. N in any one application.

Phosphorus requirements of turf •Greatest response to phosphorus seen with turfgrass seedlings.

•Deficiencies rarely observed in established turf• Exceptions include low soil P levels or pH above 7.8

•Applications should be based on soil tests.•High soil P levels increase potential for annual bluegrass (weed) infestation.

Starter fertilizers•Apply before turf establishment

•Incorporate to a depth of 2 to 4 inches. •After overseeding•After aeration

Potassium requirements of turf•K involved stress resistance, wear tolerance, disease resistance

•Factors that affect requirements:• Clipping removal, irrigation, soil texture

•Application should be based on soil tests.

“Winterizers”•Only good for warm-season grasses.

•Cool-season grasses need nitrogen in the fall.

•When? -mean daily temperature for three or more consecutive days is below 50 degrees F.

Turf Fertilization Schedule

Maintenance level

SpringApril/May

SummerJune July

FallSept Nov

Total

Pounds of N per 1,000 square feet

Low 1 - - 1 - 2

Medium 1 - - 1 1 3

High 1 0.5 0.5 1 1 4

Fertilization may be reduced by as much as one half if clippings are consistently recycled

back into the lawn.

Iron deficiency •Most common micronutrient deficiency for turfgrass

• Intervienal chlorosis of leaf blades and thinning of turf

•More serious problem when pH above 7.5 or high soil phosphorus.

•Spray every two weeks with 1 to 2 ounces ferrous sulfate per 1,000 sq. ft. until corrected.

Fertilizer calculations•You have a 50-lb bag of 26-5-10 fertilizer that you want to apply to a lawn at a rate of 1.0 lb nitrogen per 1000 sq ft. How much of the 26-5-10 fertilizer will you need to apply per 1000 sq ft?

• Ignore the weight of the fertilizer bag and divide the amount of nitrogen desired (1.0 lb nitrogen per 1000 sq ft) by the percentage of nitrogen in the bag (26%). 26% = 0.26.

• (1.0 lb nitrogen per 1000 sq ft) ÷ 0.26 = 3.8 lb of a 26-5-10 fertilizer is needed to supply 1.0 lb nitrogen per 1000 sq. ft.

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

Pots restrict how water drains

Gravity

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•Drainage

The goal is to increase aeration without decreasing water retention.

•Perlite• Volcanic origin

• Low bulk density

• Good drainage and aeration

• Low CEC and water-holding

•Vermiculite• Heat-expanded mica

• Low bulk density

• Use coarse grades for best aeration and drainage

• High CEC and water-holding

Coarse mineral components

Vermiculite

pH 7.5

pH 7.5 (U.S.), 9.0 (African)

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/ft3

Bulk Density•How heavy per unit volume

•Acceptable range: 40 to 60 lb/ft3

•Too heavy: not economical to ship

•Too light: pots with plants topple

Material

Bulk density at CC

(lbs/ft3)

Field soil 106

Sand 107

Sphagnum peat 54

Coir (coconut fiber)

46

Vermiculite 46

Pine bark 51

Perlite 32

Rock wool 54

CC = Container Capacity

Peats•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 water-holding capacity

Less decomposed

More decomposed

Sphagnum moss Sphagnum moss peat – pH 3.0 to 4.0Hypnum 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.

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 sand : 1 vermiculite or peat moss)

pH of softwoods 3.0 to 4.0pH of hardwoods 6.0 to 7.0

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.

•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

water longer

Other Pre-plant Additives

Hydrogel crystals used as a 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.

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

Questions?Contact:

Heidi KratschUniversity of Nevada Cooperative

ExtensionPhone: 775-336-0251

Email: KratschH@unce.unr.edu