Many vegetable crops are established from greenhouse-grown transplants. The advantages of transplants include better emergence, uniformity, healthy plants, and earliness. Since there are only few commercial suppliers of organic transplants, most organic growers produce their own transplants or buy them locally. Production of transplants in small cells (plastic trays) in peat-based media is the most common and widely practiced method.
Organic transplant production, however, needs additional attention as commercial growing media/potting mix often contains synthetic ingredients prohibited for use under the National Organic Program. Even though many organic potting mixes are available, most organic growers formulate their own potting mixes.
Specifically, the growing media should be uniform and supply the needed nutrients to sustain seedling growth until field establishment. The nutrient level in the growing mix can be managed by adding compost, an excellent organic amendment. Growers need to remember to make sure that the compost they use is ready for transplant production.
The compost used to formulate potting mix should be mature and stable. The maturity of the compost determines its fitness, in terms of its chemical and biological activity for particular end uses. The stability of the compost is its resistance to further rapid decomposition. Immature compost could have high soluble salt concentration, accumulation of harmful organic acids, poor water holding capacity, and could harbor plant pathogens that could infect growing transplants.
A practical check to determine the end of active composting is to thoroughly wet a small amount of compost and store it in a sealed plastic bag at room temperature. If the compost does not emit a foul smell after a week in the bag, it can be considered stable and is ready to be stored/cured.
Most small-scale organic growers are involved in on-site compost production, predominantly dairy-based compost. Some growers efficiently manage their composting operations and finish the composting process a year in advance. This does not always ensure good quality compost just because it had a curing time of a year.
Quality of compost is highly dependent on storage conditions. If the compost is stored outside or stored as a big pile, situations that promote anaerobic conditions could reduce the quality and biological parameters of the finished compost. Compost stored outside is exposed to rain, snow, and other conditions that can lower its overall quality.
Soluble Salt And pH
The two most important chemical characteristics of compost that growers should test before using their compost are soluble salt concentration and pH. Higher salt concentration causes increased salinity, which directly affects seedling germination and development.
As a general rule, a compost with salinity of less than 6 deciSiemens per meter (dS/m) will cause no damage. When using compost with higher salinity, make sure it is mixed in a relatively lower proportion with other ingredients in the potting mix. A normal range of salinity for good quality potting mix amended with compost for vegetable seed germination and transplant growth is 1 to 2 dS/m. Salinity levels above 2.0 and 2.5 dS/m are detrimental for seed germination and transplant growth, respectively.
The pH range of the compost should be between 6 and 7, which could vary based on feedstock, processing, and handling of the composting process. If the compost has a low pH, growers could correct it by adding the required quantity of liming material (lime). The amount of lime to add for pH adjustment depends on the buffering capacity of the compost, which is generally high.
To bring about one unit change in pH for a highly buffered medium, approximately 5 pounds of finely ground lime may be required per cubic yard of medium. To calculate the exact lime requirement, growers should add different quantities of lime to small batches of compost and then incubate them for three to four weeks. Based on pH measurements at the end of incubation and depending on the desired pH range, the amount of lime should be determined and applied to the complete lot of compost.
Planning ahead is critical in this case as it takes three to four weeks to determine the correct amount of lime required. Similar methodology could be used to rectify high pH issues using elemental sulfur. Growers should be careful while using elemental sulfur as it can drastically reduce the pH, if applied at high rates.
Count On Quality
The best way to ensure the quality of compost would be to have a complete nutrient analysis done through a plant/soil testing laboratory. If the compost-based potting mix will be used for growing small-seeded crops, an herbicide residue analysis is also recommended, but this should not be a major problem if the material used was from a certified organic facility. If the compost is from an offsite source, growers should know the source of compost, as composts widely differ in nutrient concentrations, depending on the source of origin.
It is not easy for the grower to test the biological characteristics of the compost. One step that could be adopted is to grow few transplants in the compost-based potting mix very early in the season and monitor for any disease development. This is easier said than done, owing to time constraints and busy schedules.
On a broader note, one practical approach would be to ensure that the compost produced either on-site or bought from a composting enterprise is of the highest quality. Even though compost is a good nutrient source, it alone cannot provide a sufficient amount of nutrients until the transplants reach the finishing stage.
Adding a large quantity of compost in the potting mix is not a wise option, as it would lead to increased salinity, adversely affecting seed germination and seedling growth. For sustained and continuing transplant growth, growers should supplement their compost-amended media with a standardized commercial organic fertilizer.