Chapter 9

Greenhouse Plant Growing Systems

Soil Culture

In soil culture, also known as in-ground culture, crops are raised on level ground as well as in mounded beds (See Figure 9.1). Soil culture is more often used in growing vegetables (tomatoes, lettuce, cucumbers, and peppers) rather than ornamental crops such as herbs, roses, freesia, and foliage plants as well as popular with smaller commercial and organic growers. The soil should be well drained, fertile, have a medium to light texture and be high in organic matter. One of the reasons organic growers prefer soil culture to hydroponics is that soil-building practices are an already familiar concept based on decades of research and experience. The soil in a greenhouse should be worked as deeply as possible to break any existing hardpans or those that may have developed because of repeated traffic over the area. Fumigation or steaming of the soil is a must before each crop or at least once a year. This will help to destroy disease, nematodes and weeds. If soil is steamed, hold it at 180 degrees for 4 hours. Avoid deep tillage after fumigation or steam sterilization. Production techniques used in ground culture are similar to intensive crop production methods used in the field. Rear-tine tillers or mechanical spaders are used in soil preparation, and specialized wheel hoes and hand tools are used for cultivation and weed control.

Greenhouse Soils Amended with Compost or Manure

Field soil in a greenhouse is usually amended with a large volume of organic matter such as well-made compost before growing greenhouse vegetables. Apply the compost or manure before the soil is fumigated so weed and disease pests in the compost and manure are destroyed. If uncomposted manure is used it should be applied at least 4 months prior to harvest to avoid contamination from human pathogens such as E. coli. Greenhouse soils amended with compost or manure may or may not require additional fertilization or liming for good plant and root growth.

National Organic Program Standards for Compost

The National Organic Standards Final Rule (USDA National Organic Program (NOP) states that any plant and animal material can be composted for organic production, provided that it does not contain any synthetic substances is prohibited by the National List or any incidental residues that would lead to contamination. However, recognizing that background levels of pesticides are present in the environment (referred to as unavoidable residual environmental contamination—UREC—in the regulations) and may be present in organic production systems, NOP regulation does not mandate zero tolerance for synthetic pesticide residues in inputs, such as compost.

NOP Standards on Composts Used for Crop Production

Composted plant and animal manures (§205.203(c)(2)) are those that are produced by a process that: (i) established an initial C:N ratio of between 25:1 and 40:1; and (ii) maintained a temperature of 131 to 170 degrees F for 3 days using an in-vessel or static aerated pile system; or (iii) a temperature of between 131 and 170 degrees F (77 °C) for 15 days using a windrow composting system, during which period, the materials must be turned a minimum of five times. Records documenting compost production methods must be maintained for on-farm produced compost as well as for compost obtained from an outside source.

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