Mixing Soil for Organic Transplants

By Rupert Jannasch

Transplant mixtures that consistently produce robust and fast growing plants are an important feature of successful vegetable operations. In organic culture, the prohibition against conventional fertilizers and traditional reliance on compost means soil mixtures are often of varying quality and consistency. Below are a few production guidelines that will help get plants off to a running start. The key is to start early.

What's the rush? It takes time for inert substances like sand and peat moss and biologically active ingredients like compost to blend together and consolidate. Settling time improves soil structure, and nitrogen in the ammonium form will convert to nitrate. Early preparation provides time for soil testing and perhaps adjusting the final nutrient content. Allow at least three months for the mixture to age.

Can potting soil be purchased? Yes, but beware of commercial products with "organic" plastered on the bag. For a certified organic operation, the mixture must be approved by your organic certifier. The product should be weed-free, pathogen-free, and with a moderate nutrient content so as to yield sturdy plants without too much top growth. Naturally, the soil should not contain synthetic fungicides.

Be wary of advertising schemes that use impressive shoot or aboveground growth statistics as a sales pitch. There is no denying that leaves and stems are essential, but sturdy roots are even more important for a plant to thrive in organic conditions.

Is compost necessary? The increasing separation between organic crop and livestock production means that manure-based compost is increasingly scarce. Moreover, compost quality depends on the materials it is made from. The nutrient analysis is never guaranteed. Consequently, many growers are sourcing alternative ingredients such as seaweed extracts, fish emulsion and other concentrated nutrient sources that act as partial substitutes. These are in addition to the more usual supplements such as lime (calcium), bone meal (phosphorus), and possibly blood meal (nitrogen) and greensand (potassium), as well as field or garden soil.

The website at the end of this article describes several soil mixtures made without compost. However, compost-free growing media should not be encouraged in organic systems. Research has shown that compost can act both as a disease suppressant and growth stimulant. Though at times these qualities are overrated, they should not be underestimated. It stands to reason that if a plant is meant to be grown in a biologically active field soil, then it should be reared in similar conditions.

Organic systems achieve stability through biodiversity. Granted, compost could introduce disease to a greenhouse, but, by and large, a well cured soil mixture rich in bacteria, fungi and organic matter will do far more to prevent disease than cause it. Taking shortcuts without compost means chopping off the hand that feeds you. The problem is, in organic farming there are fewer pesticides available for first aid.

Should transplant mixtures be sterilized? With sloppy husbandry, sterilization may be a useful tool, but it runs counter to the purpose of organic. According to master gardener, Eliot Coleman, "…damp-off and similar seedling problems, which are usually blamed on unsterilized soil, are actually a function of cultural mistakes like over-watering, a lack of air movement, not enough sun, over-fertilization, and so forth."

Good compost must be aged. A key feature of good compost is the type of nitrogen it contains. The decomposition process must be advanced to the point that microbes no longer need nitrogen to break down materials such as straw. Secondly, much depends on whether nitrogen is in the ammonium or nitrate form. High ammonium levels can damage foliage and roots and stunt growth. Most vegetable transplants, but not all, do better with the nitrate form. It takes 3 months after the active composting stage (heating) for ammonium to be converted to the more benign nitrate. Add that to the three month settling period for the transplant mixture and it means a bare minimum of 6 months lead time.

Don't forget the roots! The importance of healthy root systems in organic production cannot be emphasized enough. Choose a cucumber plant from a suburban garden center and chances are it looks healthy enough. Peel back the container and the wherewithal to make it grow is probably missing. No roots, just meager, spindly fibres. Often the cause is too much infant formula - excess soluble fertilizers used in plant rearing.

A plant with underdeveloped roots will start poorly in an organic field soil. The margin for error is greater in a conventional system because more fast-acting fertilizers are at the grower's disposal.

Remember the organic rule of thumb, "feed the soil, not the plant." Transplants must be equipped to harvest these nutrients, so a healthy root system is essential.


A very informative publication on potting mixes for certified organic production that details a variety of ingredients including recipes with and without compost is available at the Appropriate Technology Transfer for Rural Areas (ATTRA) website

Rupert Jannasch, M.Sc., P.Ag. is a consultant for the Organic Agriculture Centre of Canada (OACC). Please send comments or questions by phone to 902-893-7256 or by email to oacc@nsac.ca.