In order to succeed as an organic grower, of greenhouse tomatoes
or anything else, you first have to be a good grower. In other words, many
of the necessary skills and techniques are neither organic nor conventional.
These include: building the greenhouse (structure, heating, cooling, irrigation);
managing the environment (light, temperature, soil moisture, CO2); plant
management (variety selection, transplant production, grafting, pruning,
pollination); and marketing.
In a few of these areas some small changes may come with organic
production, such as:
- the greenhouse structure cannot include pressure-treated wood where
it can come in contact with crop roots;
- there are relatively few organic fertilizers that can be dissolved
and applied through a drip irrigation system;
- use of organic seeds is required, unless you can document that
the variety you want is not “commercially available”; and
- marketing organic products requires labeling that conforms to
The areas where organic production differs significantly from conventional
are: type of growing media; sources of plant nutrients; and pest management.
The difference has to do with what kinds of media, fertilizers, and pesticides
are allowed on organic farms.
The national organic standards (www.ams.usda.gov/nop)
do not allow the use of any ‘prohibited materials,’ which includes most
synthetic fertilizers and pesticides. Exceptions to this prohibition are
on a national list of ‘approved synthetics’ that includes materials like
insecticidal soap and sulfur. There are also some products on the national
list of ‘prohibited natural’ materials, such as arsenic and nicotine.
All the materials in any product to be used on an organic farm must
be allowable under the national standards, including so-called ‘inert’
ingredients. As a result, some allowable materials, such as rotentone and
B.t. for potato beetle control, are not currently available in an approved
organic formulation. The OMRI (Organic Materials Review Institute) seal
on product container signifies it is OK to use in organic production (see: www.omri.org).
However, many brands of fertilizers, pesticides and other products may
be allowable but have not been reviewed by OMRI. For these, always check
with your certifying agency to get approval before using them. Use of a
prohibited material may result in decertification.
Organic greenhouse tomatoes are usually produced using ‘solid’ media
rather than hydroponic culture. The system of culture is often in-ground,
but can be in beds, buckets, or bags. The rooting media may contain topsoil,
or it may be soil-less. Compost, peat, perlite, sand and vermiculite are
the other common rooting media ingredients.
Compost also acts as a slow-release source of nutrients. Compost
in the rooting media must be made according to an approved organic process,
or else a delayed harvest requirement of 90-120 days after planting may
Some combination of organic fertilizers is usually added to the rooting
medium to assure the necessary supply of plant nutrients. Testing the nutrient
status of the rooting medium is best done with the saturated media (potting
mix) test rather than a field soil test, since the rooting media usually
very high organic matter.
Commonly used organic fertilizers include:
• calcitic or dolomitic limestone (Ca, Mg)
• greensand, potassium sulfate (K) or sul-po-mag (K, Mg)
• rock phosphate or bone meal (P)
• blood meal, Chilean nitrate (N)
• plant meals such as alfalfa, peanut and/or soy (N, P, K)
Trace elements are usually provided in sufficiency by compost and/or
plant meals, but that can be supplemented using natural materials (volcanic
minerals) and synthetic compounds (chelates, sol-u-bor, etc.) that are
allowed under organic production, but in some cases only if a deficiency
has been demonstrated by soil testing.
Organic (and conventional) disease, insect and weed management starts
with prevention through exclusion, sanitation, and cultural practices that
interrupt pest life cycles. If prevention fails, then biological pest controls
and organic pesticides may be required.
Biological pest controls (predators, parasitoids, etc.) are important
in organic production to avoid pesticide use. To be effective, biological
controls must be introduced when pest populations are still low. Rigorous
scouting is key to identifying pest problems early.
Commercially available biological controls for insects include:
- Aphidoletes, Aphidius, ladybugs (aphids)
- Amblyseius (thrips)
- Encarsia (whiteflies)
- Phytosieulus (spider mites)
Organic pesticides must be used according to the label; common materials
- Azadarachtin = neem (many insects)
- Beauvaria bassiana (many soft-bodied insects)
- B.t. israeliensis (fungus gnat larvae)
- Copper hydroxide (many foliar diseases)
- Insecticidal soap (soft bodied insects)
- Pyrethrins (many insects)
- Spinosad (caterpillars, leafminers,thrips)
- Sulfur (two-spotted spider mite, russet mite, powdery mildew)
- Trichoderma (root and foliar disease suppression)