Greenhouse Media
-soil
-peat moss
-etc
Media: terminology is strictly horticultural (eg loam has 2 different definitions, depending on whether you are a horticulturalist or a soils expert)
Properties of a good medium
1. Provides good support for the plant
2. Reservoir for minerals and water for plant growth
3. has good pore space--good aeration so plant roots can get oxygen
4. holds up well under heavy watering (ie does not collapse--eg vermiculite collapses over time)
5. Is pasteurizable; free of pathogens, insects, etc.
6. Possibly reusable crop after crop (depending on what you are growing)
7. The recipe is simple--able to be duplicated crop after crop: ie standardize a quality medium
What to use: Soil of Soilless mix?
-Common recipe for soil mixes:
· 1/3 topsoil
· 1/3 peat
· 1/3 mineral amendment (ie sand)
· fertilizer
· lime (if needed for pH control)
Advantages & disadvantages to soilless & soil based mixes
1. Plant nutrition
a) Soil mixes provide more plant nutrients than soilless (especially micronutrients)
b) Micronutrient (B, S, Mo, Cu, Cd) deficiencies are rare in soil mixes.
c) When steam pasteurized, some soil mixes release toxic levels of nutrients like manganese.
d) Macronutrient deficiencies may occur in soilless mixes using undecomposed organic materials (eg wood chips, peat) because microorganisms may tie them up (N, P, K, Ca, Mg)
e) Lower initial nutrient levels in soilless mixes
f) Soilless mixes require a greater dependence on liquid fertilizer feeding
g) In some parts of the country it is difficult to get large quantities of topsoil to mix soil based mixes
h) Difficult to have quality control of soils unless they come from the same source
i) Soil require steam pasteurization or fumigation
j) Soils are heavier and more difficult to handle
k) Choice may affect the post-production life of a product
· Life of plants outside greenhouse (ie after sold to consumer)
· Soilless mixes dry quickly (consumer may not want to water twice a day!)
· How many consumers can provide continuous liquid feed of fertilizers (macronutrients & micronutrients) for soilless mixes?
How to choose components of mix
Choice of additives -- most important factors
1. must be free of toxic substances
2. Availability (rice hulls in Alaska? I think not!)
3. Cost (if your costs are too high the consumer will go elsewhere)
4. Local experience in using it (can you get help & advice?)
OR: use premix provided by another.
Some common components of greenhouse mixes
1. Peat
a) Most widely used material in the world
b) Usually deficient in major plant nutrients
c) Provides root stabilization and a medium which holds nutrients
d) Not uniform in composition or consistency (products differ in species composition--see below--and degree of plant decomposition)
· Because of these differences peats are classified into:
A) Sphagnum moss peat (peat moss): 75% sphagnum moss (oven dried weight)
B) Hypnum moss peat: 50% hypnum moss (this decomposes more rapidly than sphagnum peat; unsed mostly in the northern US (Michigan, Minnesota, etc)
C) Reed-sedge peat: 33% sedge, reedgrass, cotton grass, cattails, etc. This peat is less acid than other peats, usually pH 7 & up. It also decomposes rapidly (=bad!), ahs a very fine particle size and is not really suitable for greenhouse mixes, though it is ok for outdoor mulch.
D) Peat humus = humus peat = muck: total fiber content of dry peat is less than 1/3. This muck is more highly decomposed and has few individual plant parts distinguishable. Very fine granular particle size. In greenhouse mixes it does not improve aeration or drainage, but it is ok for lawns. It can be used if mixed with wood chips or other chunky medium to improve the drainage.
The choice of peat will determine
1. Water holding capacity of the medium, as well as aeration
2. Nutrient availability (added nutrients can be tied up in some peats)
3. Activity of certain growth regulators
Best peats
· Spagnum peat moss with a particle size up to 3/8 inch and fiber size 1/16 to 1/4 inch in diameter.
Here in Fairbanks, Lemeta Peat:
· Sphagnum and sedge peat
· Uneven decomposition
· Anaerobic and cold decomposition
· Ok for hydroponic tomato growth
· Fair for container nursery
· Ok for ground beds
Other components for soil mixes (mulches incorporated into the media)
· Shredded bark
· Wood chips
· Sawdust
-cheap if local
-higher N fertilizers needed (to feed microorganisms and plants)
-Caution! Some produce toxic substances like resins, turpentine, tannins. If they are composted for a year this will often remove the toxins. Beware! These toxins are often released with steam pasteurization.
Toxic woods:
· Most toxic:
o California Incense cedar
o Walnut
o Sitka spruce
o Doug fir
· Moderately toxic:
o Japanese larch
o European larch
· Mildly toxic:
o Redwood
o Cedar
o Ponderosa pine
o Tulip tree
o White pine
o Scotch pine
Other additives
· Corn cobs - mulch for greenhouse roses, chopped; nursery media
· Peanut hulls - mulch bed crops; often mixed with peat in azalea media. Be careful to pasteurize; if often carries nematodes.
· Others:
o Bagasse - shredded sugar cane
o Fish meal
o Spent mushroom compost
o Spent hops
o Manure
o Rice hulls
Sand
· Fine: 0.02 to 0.2 mm in size
· Coarse: 0.2 to 2.0 mm
· "Gravel" (only from a horticultural sense) more than 2.0 mm in size
· Most used is fine sand
· Must be free of carbonates
o Huge pH increase if there is carbonates in it.
o Nutritional (micronutrient) disorders due to pH change.
o To test for carbonates, add dilute hydrochloric acid:
· Slight foam, just detecting CO2
· Froths and fizzes wildly = carbonate
o Sand is very heavy --100 pounds per cubic foot
· Good fro adding weight to large plants to prevent container from toppling over
· Can be too heavy, especially if you are handling it a lot
Vermiculite
· Aluminum/Iron/Magnesium silicate mineral
· Mined in the USA and South Africa
· The mineral is heated to 1000° C for 1 minute, which causes it to exfoliate
o Water trapped between plated becomes steam and forces the plates apart
o Volume increase of 15-20 TIMES
· Best size is 1/4 inch diameter (for horticulture)
· USA vermiculite has a pH of 7 to 7.5
· South African vermiculite has a pH of 9.8
· Light weight: only 5 pounds per cubic foot
· 5-8% potassium (by weight)
· 9-12% magnesium (by weight)
· Sterile
· Problem: it will collapse over time (the plates shrink back together)
· Problem: possibility of asbestos contamination (from the mine in Libby Montana, which had the mineral in conjunction with asbestos)
· For more information see The Vermiculite Association
Perlite
· Mineral of silicon and aluminum dioxide
· Volcanic origin
· Huge deposits in US and New Zealand
· Rocks crushed and heated to 1000° C
· Expands like popcorn
· Does not break down like vermiculite
· Light weight (8 pounds per cubic foot)
· May "float" in container & rise to top of soil
· Graded particle sizes (horticultural: 1/8 to 1/4 inch)
· Does not absorb water
· Virtually no nutrients
· Sterile
· PH 7-7.5
· Very dusty
Rock wool
1. Used extensively in Northern Europe
2. Aluminum and silicone mineral
3. Heated to 1500° C, and forms as fibers when cooled
4. Blocks or cubes for propagation
5. No nutrients
Other components used as media
· Coal cinders
· Pumice
· Calcined clay (kitty litter without the perfume!)
Useful standard recipes for mixing
· John Innes Composts
o 1st standardized mixes
o not widely used today
o John Innes Horticultural Institute in the UK
o (NOTE: 'loam' used in his terminology is not the same as the loam referred to in soils classes!)
· U.C. System (University of California)
o Mixes of sand and peat
o Quality often depends on the quality of the sand
§ Round grains, not sharp
· GCRI (Glasshouse Crops Research Institute in the UK)
o mix of sphagnum moss and sand, with fertilizer added
· Cornell Peat-lite mixes
o Most used in US
o Basis for nearly all US mixes
Mixing medium
§ For volumes of less than 6 cubic feet:
o Usually hand mixed
o With a shovel or a trowel
§ For volumes greater than 6 cubic feet
o Motorized mixing
o Cement mixer: about 2 cubic feet per batch
o Cement truck: 6-10 cubic yards
Pasteurization
§ Steam
§ Aerated steam
§ Chemical fumigation
Steam is the most common. Some
advantages:
§ Use as soon as cool
§ Killing temperature is 180° F (80° C) for 30 minutes
§ (includes beneficial organisms)
Aerated steam
§ Same as above, but uses extra air to help reduce the water logging of the soil that occurs when you use steam.
Chemical Fumigation: Not recommended
Problems with pasteurization
1. Heat increases solubility of many compounds:
· Phosphates
· Potash
· Magnesium, Zinc, Iron, Copper and Boron
· This may lead to toxicities in soils
· Especially those already high in soluble salts
2. Ammonia toxicity
· If organic sources of N are used, this may cause ammonia toxicity:
a. Bacteria that convert organic matter to ammonia are not killed at 180° F.
b. These bacteria multiply rapidly into the niches left vacant by organisms killed by sterilization
c. The bacteria that convert ammonia to nitrates and nitrites ARE killed by sterilization above 160° F
d. Ammonia will peak about 20 days after sterilization
e. Re-colonization by the ammonia consuming bacteria will reduce ammonia levels after 20 days
3. Recontamination
· Pasteurization creates a void
· Growth after pasteurization may lead to higher levels of bad organisms
Fertilizers
· Levels tied directly to
1. medium
2. crop
3. pH
4. time of year
5. watering schedules
6. water quality
7. growth regulators
Fertilizers and plant nutrients
o 17 or 18 essential plant nutrients
o 3 not provided by 'normal' fertilizers: carbon (which comes from CO2 in the air), oxygen (which comes from air oxygen, water or CO2)
o 2 others rarely added because they are usually already available: Nickel and chlorine.
o Macronutrients: nitrogen, phosphorous, potassium, calcium, magnesium, sulfur
o Micronutrients: iron, manganese, molybdenum, zinc, boron and copper.
2 primary methods of adding fertilizer:
o Pre-plant (to medium)
o Post-plant (as liquid feed)
Pre-plant
o Ca, Mg
o Normally added to adjust pH
o Optimum pH 6-6.8
§ Ericaceous species need pH of 5-6.5
§ Venus fly traps need 4-5
§ If the pH is too low
· Toxicities of Zn, Cu, Mg, Fe
· Deficiencies of Ca, Mn
· Loss of phosphorous due to leaching (ie becomes too mobile in the soil)
§ If the pH is too high
· Deficiencies of micronutrients Fe, Mn, Zn, Cu, B
Adding Ca & Mg
o When you want to raise pH
o Add dolomitic limestone for both Ca and Mg 'fertilization'
§ 0-10 pound per cubic yard, for both soil and soilless cultures.
§ 10 pounds per cubic yard for soilless
o Add calcitic limestone for just Ca 'fertilization'
o
WARNING:
§ Speed and level of pH rise with limestone is directly related to particle size:
· <100 mesh (0.01") is too small
o dissolves too fast
o high and fast pH rise
· >20 mesh (0.05") is of no value
o dissolves too slowly
o If you don't want to raise the pH
o Add gypsum (Ca sulfate) for calcium
§ 0.5 pound per cubic yard for soil and soilless mixtures
§ may also use old sheetrock (which is made from gypsum)
o Add Epsom salts for magnesium (magnesium sulfate)
§ Up to 1 pound per cubic yard
Phosphorous
Both pre and post planting fertilizer
Pre-plant
o triple super phosphate (calcium phosphate) (0-45-0 is the fertilizer grade)
o
Phosphorous is held by clay particles in the soil
o soil mixes: one dose as pre-plant is sufficient
o soilless mixes: phosphorous often leaches out; may need pre and post-plant fertilization (within 1 year)
Sulfur
o mostly as pre-plant fertilizer
o gypsum is often used (calcium sulfate)
Micronutrients
--3 methods
1) Pre-lant; incorporate into mix
o added all at once to medium
o micronutrients imbedded into clay particles, pulverized glass, slow release particles or drying oils (eg Ozmocoat bubbles)
o nutrients last up to one year
o fritted trace elements
o glass with the trace elements embedded in it and then crushed. The trace elements then leach out of the glass slowly
2) post plant single dose as liquid: lasts up to 4 months
3) constant feed with other nutrients
o minute doses; micronutrient must be soluble
Nitrogen, P & K
o Nearly always added post-plant
o May be added as slow release
o Eg Osmocote
§ Fertilizer impregnated linseed oil balls
§ Only for long term crops
§ Does not work in cold soils (released too slowly)
Complete fertilizers
· Nitrogen, phosphate, potash.
· Eg 17-17-17 = % by weight of the 3 elements
· 20-10-10S = sulfur added
Major considerations:
1. Frequency of application
· weekly
· continuous feed = fertigation
2. Amount of N
· 500-300 ppm continuous
· 200-600 ppm weekly
3. Types of N fertilizer
· Urea and ammonia
o Slower availability
o Dependent on breakdown by microorganisms
o PH dependent (lower pH reduces rate of breakdown)
o Temperature dependent: lower temperatures slows microbial activity
o Best for ericaceous crops.
· Nitrate - readily available, not pH dependent
4. Acidifying effect
· + cations (ammonium--NH4+) taken up by plants, which release H+ ions
o substrate becomes acidic
· Negative anions (NO2---nitrate, SO4---sulfate)
o When taken up by plants a hydroxide ion (OH-) is released in trade
§ OH- combines with H+ to yield water
§ Thus H+ taken out of system and pH rises
Potential acidity-measure of the acidifying effect of a fertilizer
Nutrient monitoring
· Need to know nutrient deficiencies and toxicities
· This is NOT EASY!
o Small changes in plant tissues are hard to observe
o May be masked by multiple deficiencies and toxicities
o May be masked by insect and disease damage
o Different crops may have different manifestations
o Nutrient interactions change symptoms
Trouble shooting
· Soil testing (get lab that does soilless mixes, if that is what you deal with)
· Tissue analysis (useful only if tissue research is complete, so you know what indicates deficiency or toxicity
Handouts for today:
1. List of soil mixes & predominate contents
2. Number of pots by size filled with one cubic yard of medium
3. Standardized growing media, recipes
4. Calculation of liquid fertilizer rates
5. Temperature necessary to kill pathogens and other organisms harmful to plants
6. Calculation of fertilizers, from Greenhouse Operation and Management, by Nelson.
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