an intentionally bioactive living matrix

by Joey Klein and Karl Hammer

Vermont Compost Company

Vermont Compost Company has been making compost and blending potting soils since 1993.  As participants in the growth of this business, and as an organic vegetable growers with  greenhouses, we are enthusiastic users of compost based potting mixes.  Karl originally developed potting soils for organic growers the early 80’s in Vershire, VT.

We were dissatisfied with the blends available that were made for use with chemical fertilizers.  The happy use of compost-based blends requires some changes in attitude and practice.  For growers who have been using potting media that are blends of peat, vermiculite, and perlite with a chemical wetting agent and  nutrient charge, there may be a learning curve to the proper management of plants in compost-containing media. For growers seeking to run their greenhouse within the criterion of certified organic production, there is no better choice than potting soils containing substantial amounts of appropriate compost

The compost in the potting media provides many benefits.  It emulates the growing conditions  to be found in a compost-enriched soil, where there will be a large, diverse, and vibrant population of microbes.  This microbial community is a proven suppressant of soil borne disease organisms.    The microbial interaction with the roots of the seedling results in a plant/microbe mediated release of nutrient ions from the humus, allowing the mix to meet the changing needs of the growing plant. The compost serves the role of organic matter in the soil, and as it is broken down its nutrients are slowly released, providing a long-term nutrient supply. The compost also provides sites for the retention of soluble nutrients from other ingredients. 

  Leaching of soluble nutrients is minimized. (For an excellent discussion of these issues, see Organic Soil Fertility Management, by Steve Gillman, Chelsea Green, 2002.)

There are some management issues in the use of compost based potting media.  The most important is maintaining adequate soil temperature, as you are working with a living system that will slow and stall if it is not warm.  Providing heat to the soil directly is ideal.  One strategy is to grow in soil blocks on a heated concrete slab.  Another is to have the forced hot air of the heating system delivered under the benches, where it will have to rise through the media.

A common approach is to install a hot water tank in your green house and water with tempered water.  If the water is 60 degrees when it goes into the potting media, it will tend to retain that heat.  If you water with cold well water of 40 to 45 degrees, it will take a lot of sunshine and forced hot air just to get back to the root zone goal of 60 degrees.   A hot water tank and plumbing to mix the water to 60 degrees (lukewarm) will more than pay for itself in saved heating fuel, and give visible results in rapid plant growth.

Compost and peat are both very water retentive.  Most mixes for organic growers contain no wetting agents.  This means that the media must be gradually wetted and mixed, usually by hand in small batches, until it is uniformly moist but not wringing wet

The higher the percentage of compost in the mix, the less watering the plants will need. 

Crushed granite, sharp sand, perlite, and vermiculite may be added to these mixes help with drainage and porosity.  In general, any potting soil with a large percentage of compost will require much less watering.  For growers just switching over from other mixes, this will be an adjustment.  They will most likely prefer a mix with more perlite and more rapid drainage.  Let the surface appear dry before watering.  Ideally, the media surface should be dry overnight. Even better is to invest in an “ebb and flow” bottom watering system.  Algae thrives on cool and damp soils, so the best control is to avoid these conditions

Growers producing food crops in the greenhouse need to be cautious in keeping a balance between soil temperature and the available light.  In low light, short photo periods, typical of late winter in , and in cold soil where there is soluble nitrate, plants can tissue accumulate large and potentially dangerous amounts of nitrate in the leaves. Of particular concern are spinach and mache.  Well made organic mixes tend to avoid this problem by minimizing nitrate availability. Cell, pot, or block size is crucial to the successful use of compost-based potting media.

With compost-based potting soils, the grower must provide adequate soil volume to meet the need of the growing plant, while balancing this with the need to minimize the heated area of the greenhouse.  Conscientious potting-on as the plant develops, and prompt setting out as the seedling reaches the correct stage to deal with the outdoors best achieve this.  Small-celled plug trays are possible with blends with a higher percentage of perlite, but the smallest cells are to be avoided.

Brenda Hedges of Greystone Gardens (Waterbury, VT), a plug producer, lists 144 cell trays as her smallest and sells many 98’s, 72’s and 36’s of plants to pot on to her organic greenhouse customers.

It is best not to try to meet the needs of a rapidly growing plant with any sort of soluble fertilizer.  Many that are approved for organic growers fail to live up to their stated analysis.  The smell of freshly applied fish emulsion in a retail greenhouse can be very off putting.  Much better is to move the plants to larger pots with a fresh charge of compost-based media.  For Julie Rubaud of Red Wagon Plants in Shelburne, VT, this means, choosing the 12 inch pot over the 8 or 10 inch for her premium hanging baskets.  She more than gains back the extra cost of the pot and the media in the selling price.

For organic vegetable growers needing a high volume of seedlings to transplant, the plastic-free option has its merits.  The Dutch have led the way in this technology, moving beyond the hand or stand up block maker to the block making machine.  Soil blocks require a media that will hang together well, containing no perlite at all.  Soil blocks provide an excellent environment for plant roots, with air on all sides and good drainage.

Sandy and Paul Arnold of Pleasant Valley Farm (Argyle, NY) even take trays of soil blocks to their farmers’ market and tear off plants as they sell.  Their customers appreciate the vigor of the plants and the lack of plastic trash.  The Dutch blocking machines are very quick, seeding and blocking automatically. 

Pete Seely of Springdale Farm (Plymouth, WI) has pioneered in the import of these machines and can answer all questions about them

Please do not tinker with adding additional fertilizers to a mix without the ability to test for electrical conductivity. Higher conductivity in potting soils will suppress germination of many plant varieties.  Some sensitive varieties want almost no nutrients in their germinating mix.  Conductivity meters are available from many catalogs, and are an essential investment if you are blending or adjusting your own potting media

An organic beginning for plants that will receive organic care gives them benefit of adaptation to the environment in which they will finish their growth.  This is a visible boon both to the farmer setting a crop to grow to maturity, and to a greenhouse grower selling plants to organic gardening customers.  Remember, under organic management we are not seeking to provide the soluble nutrients directly to the plant roots.  Rather, we should strive to create the appropriate conditions for the plants to communicate with their microbial communities via sugar/enzyme root exudates.  The plants’ microbial herds, given appropriate and sufficient humus in the media from which to work, will make available the appropriate nutrients for the plant’s stage of growth.  Organic growers need to leave behind the paradigm of direct feeding of the plants, and instead work to create the appropriate environmental conditions for the plants to husband their own bio-collaborative community, to make the right nutrients, (and other things i.e. communities of disease suppression), available at the right time.  Careful watering and soil temperature management are both crucial to your success in working with compost-based potting medias.