Greenhouse Fertilizers

1
Greenhouse Fertilizers

• By
• Jay Holcomb

2
Essential Nutrients

• Nitrogen
• Phosphorus
• Potassium
• Calcium
• Magnesium
• Sulfur

• Iron
• Manganese
• Copper
• Zinc
• Boron
• Molybdenum

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Fertilization Programs

• Pre-plant applications--fertilizer added to the
  media before the crop is planted.
• Post-plant applications--fertilizer added to
  either the crop or media during the production
  cycle.
• Remedial applications--fertilizer added to the
  crop in response to a diagnosed problem.

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Pre-Plant Fertilization

• pH adjustment (calcium and magnesium)
• pH adjustment (sulfur)
• Micronutrients application
• Nitrogen and potassium application
• Phosphorus
• Slow-release fertilizer

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pH (calcium and magnesium)

• Generally pH is low
• Low supply of Ca and Mg
• Ca and Mg do not move rapidly in soil
• Applied as ground limestone (dolomitic)

• To raise the pH use limestone at up to 10 lbs per
  cu yd
• Calcitic lime to increase Ca
• Dolomitic lime for Ca and Mg
• Gypsum to add Ca, but not raise pH

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pH (sulfur)

• If pH is too high
• If sulfur content is too low.

• To lower pH use finely ground sulfur at 1/4 lb/cu
  yd or aluminum sulfate at 1.5 lbs/ cu yd or iron
  sulfate at 1.5 lbs.cu yd
• These should create about a 1 unit change in pH

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Micronutrients

• Low levels of Fe, Mn, Cu, Zn, B, and Mo
• Soilless mixes often have low micronutirents

• Add micronutrient fertilizer such as Esmigram at
  3-6 lbs/cu yd, Micromax at 1-1.5 lbs/cu yd,
  Pro-Max at 1-1.5 lbs/cu yd

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Nitrogen and Potassium

• If N is low in the media
• If K is low in the media
• Most soilless mixes are low in N and K

• Use calcium nitrate at 1 lb/cu yd
• Use potassium nitrate at 1 lb/cu yd
• This is a starter fertilizer that should last 1
  or 2 weeks

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Phosphorus

• If phosphorus is low in the media.
• For soil-based media use preplant phosphorus

• To add P use superphosphate (0-45-0) at 1.5 lb/cu
  yd

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Slow-release Fertilizer

• Use when nutrients are low and little capability
  to add fertilizer during the crop
• Use when concerned about nutrient leaching

• Use Osmocote 14-14-14 at 14 lbs/cu yd
• Use Nutricote at
• Do not steam media after slow-release fertilizer
  addition

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Post-plant Fertilization

• Liquid fertilization using a complete fertilizer
  is most common
• Liquid fertilization to correct a deficiency
• Dry manual application

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Liquid Fertilization

• Fertilize at each irrigation with a dilute
  concentration of nutrients in the irrigation
  water.
• Rate of N from 50 ppm to about 300 ppm
• Ratio of nutrients is important
• Amount of micronutrients matters
• Fertilizer must be completely soluble

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Liquid Fertilization--Frequency

• If fertilize at each irrigation, use dilute
  solution of no more than 200 ppm N
• Constant fertilization provides more uniform
  nutritional environment
• If fertilize weekly, use up to 600 ppm N
• The decision of constant or weekly may be a labor
  or equipment decision

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Liquid Fertilization--Balance

• Ratio of N-P-K is very important
• High N causes soft, vigorous, vegetative growth
• Generally the K rate is similar to the N rate
  except for a few crops
• For P soil based mixes that have preplant P will
  need little if any soluble P
• P rate for soilless mixes needs to be about 1/2
  the N rate

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Liquid Fertilization--Nitrogen

• Ratio of ammonium to nitrate-- 60 of N should be
  in nitrate form
• Ammonium causes soft, vigorous growth--use for
  young plants and in spring and summer
• Nitrate causes harder, slower growth--use for
  maturing plants and in fall and winter

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Liquid Fertilization--Ammonium

• Nitrifying bacteria convert NH4 to NO3 at pH of
  6-7
• Ammonium can become toxic during cool weather
• Ammonium fertilizer will acidify the media

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Select a fertilizer-Solubility

• Fertilizer must be completely soluble in water.
• Do not mix P, Ca and SO4 in the same concentrate
  tank
• Some fertilizers prilled with clay which will not
  dissolve

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Select a Fertilizer--Reaction

• Will pH of the rise or fall based on the
  fertilizer program?
• Read the label
• Ammonium tends to drive down pH

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Select a Fertilizer--N Form

• No more than 40 of N should be in ammonium form
  unless weather warm
• Read label
• Ammonium can become toxic

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Dry Fertilization

• Top dressing pots is labor intensive.
• It may be good when injection equipment is not
  available
• It is best to use a slow-release fertilizer

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Types of Slow-Release Fertilizers

• Limited Solubility
• Low Solubility, Biodegradable
• Membrane regulated

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Slow-Release--Limited Solubility

• These materials dissolve very slowly
• Dissolution rate is affected by particle size and
  hardness
• Primarily a physical breaking down
• Lime and MagAmp would be examples

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Slow-Release--Biological

• These products are slowly soluble, but biological
  activity is necessary to release nutrients
• Primary example is ureaform

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Slow-Release-Membrane Regulated

• Membrane covers a water soluble fertilizer
  granule
• Plastic-coated and sulfur-coated
• Membrane affects absorption of water and release
  of fertilizer solution.

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Osmocote Fertilization

• Topdressed vs incorporated
• Linear release rate for the stated time
• Release rate higher at higher temperatures.
• Broken granules release nutrients more quickly

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Nutrient Concentration in Leachate

• Irrigation Water
• Liquid Fertilizer
• Runoff (constant fert.)
• Runoff (constant fert plus Osmocote
• Runoff (Osmocote)

• 2.1 ppm NO3
• 174 ppm NO3
• 204 ppm NO3
• 213 ppm NO3
• 2.2 ppm NO3