This material originally appeared in the Spring 1992 Small Farmer’s Journal

Copyright 2010 by the authors, Anne and Eric Nordell

We have hesitated to go public with our potting mix, not because the formula is top secret, but because our greenhouse experience is limited in years and scale. Nevertheless, we would like to offer what we have learned in hopes of showing that something as seemingly insignificant as putting together a potting mix can be integrated into a systems approach to farming.

Like many growers, we raise our own plants so we can select varieties most suitable to our growing conditions and markets. Just as important is timeliness, that is, having starts ready in due season.

Along similar lines, we mix our own potting soil to match growing conditions in the fields. Plants started in a live medium seem to take off quickly when planted out in biologically active soils. To put it the other way around, we have tried to bring the concept of “feed the soil, not the plant” from the field into the greenhouse.

A more site specific reason has been to raise seedlings hardy enough for our unirrigated planting system. A compost-based potting mix helps us to grow the stocky plants with extensive root systems necessary to weather adverse conditions in the field.

It was not until we began to fine-tune manure management in the barn that we arrived at a way of making compost suitable for potting soil. So maybe a little compost history would be appropriate here.

Midwinter of ’89 we roughed together a chicken coop next to one of the composting pigpens in the barn (for details on this animal-powered system, see “Work Hogs and Horse Manure” in the Spring 1991 SFJ). We then filled the coop two and a half feet deep with the partially composted horse manure accumulated in the pigpen. The home laying flock took a genuine interest in this rich, rotted material, scratching and pecking away. Surprisingly enough, feed consumption dropped while egg production rose, the eggs developed the flavorful taste and firm texture associated with free range, and the hens seemed to appreciate warm ground under-claw during the winter months. At this point in the decomposition process the compost also seemed to prefer the lighter touch of the birds compared to the packing power of heavy hogs.

We encouraged the flock of 20 to shred and mix the compost by scattering shelled corn and sprouted oats. We did find it necessary to turn the materials by hand as the hens could only scratch six to eight inches deep. But digging a trench through the compost in the 9×9 foot coop was just a 5-10 minute job twice a week, easily absorbed into the routine of barn chores. Besides being a cheap way to make feed, this extra effort incorporated the chickens’ droppings into the compost, keeping the coop sweet smelling and moist.

According to the books, chicken manure is rich in calcium and phosphorus, two nutrients poorly represented in other manures. So apparently the hens helped to balance out the fertility supplied by the horses and the hogs. In addition, we provided the laying flock with a box of wood ashes tacked waist high in one corner of the coop. Not only did the dust-bath help to keep lice at bay, but when the birds flew out of the ash box they dusted the compost with a fine coat of readily available calcium, potash and trace minerals.

In two to three months the cooped up composts was already fine in texture and rich in color. At this point we layered in some soil, no more than one-half inch for every four inches of compost. Compared to trials without using soil, this addition seemed to help ripen the compost in the direction of humus, to keep the compost (and later the potting mix) more uniformly moist, and to re-inoculate the pile with beneficial bacteria possibly destroyed during the first heat cycles. (We like to use soil from a freshly plowed field of rye and hairy vetch where the good tilth and earthworm activity indicate an active and healthy soil life.) For the final round of heating and then curing, we shoveled the compost, now thoroughly mixed with soil by the hens, into a wooden bin built next to the coop in an unused corner of the barn.

Three advantages to making compost in the barn soon became clear: Under roof, there was no chance of losing nutrients due to leaching; weed seed could not blow in and infest the pile; and the moderated temperatures in the barn allowed for more biologically active months of the year. In fact, by the next winter the compost was mellow enough for use in a germinating mix.

More importantly to us, the resulting compost needed no further amending with fertilizers for strong, balanced plant growth. Because the nutrients had been completely digested and incorporated in this live composting process, we did not have to worry about uneven growth due to poor mixing or unexpected chemical/biological reactions as can sometimes happen when adding raw or soluble elements to a potting mix just before use.

We must admit that we may use larger cell sizes than would be affordable in a commercial nursery in order to provide enduring fertility from compost alone. For example, we grow lettuce in 72-cell pro-trays, and pot our peppers and tomatoes into 24’s. But we see the large plug size as an advantage, rather than a liability, when setting out plants in the field without irrigation. That is, we have been willing to sacrifice greenhouse space in an effort to drought-proof the farm. Despite four months of record hot, dry weather last year, we planted out over 800 lettuce starts every week from April through August. Although not every head reached market size, we lost virtually no plants either.

We have also learned there is a lot more to growing healthy plants than henpecked compost. First of all, finding the right blend of compost and peat moss to insure good drainage, moisture and aeration has been an ongoing trial-and-error process. We have settled on two basic mixes that so far have fared well under most conditions.

For a germinating mix, we combine two parts compost (sifted through a 3/8-inch screen), four parts peat moss, one part each of perlite and vermiculite. We also use this “lite” mix for lettuce seedlings to encourage root growth and discourage the root and stem rots that sometimes appear when the mix is made much heavier. On the other hand, we boost the percentage of sifted compost to six parts for potting on heavy feeders like tomatoes, peppers and coles.

Sunlight and heat seem to be critical ingredients for healthy seedling growth as well. In the cloudy, cool spring of ’90 we found removing the second layer of plastic glazing covering the greenhouse made all the difference in controlling damping off and spindly growth. From our limited experience, sacrificing insulation for light seems wise.

Besides, cool air temperatures seem to benefit most plants as long as the roots and soil are kept warm. The opposite, cold soil and warm air, we found, just about guaranteed weak, spindly growth. To get warmth where it was most needed, we buried a flue in a growing bench filled with gravel and stone. Burning a short, hot fire in the barrel stove attached to the flue each evening warmed the stones in the bench, which in turn, released gentle heat under the flats all night long. (We suspend foam insulation board over tomatoes, basil and peppers to hold heat around these sensitive plants on nights when the mercury threatens to drop below twenty.)

Not being engineers, this flue-in-a-bench design may not be the most efficient setup in the world, but does provide us with several advantages. Bottom heat creates the proper soil temperatures for a compost-based potting soil to come alive and release nutrients to the plants. Root growth is visibly more extensive and covered with fungal fuzz in warm soil as well. Because we fire the stove wide open, we can burn scrap lumber and soft woods cleanly. And we don’t have to get up in the middle of the night to stoke the stove as might be necessary if used in the conventional way for space heating.

If we have learned nothing else from mixing our own, it is that potting soils work best when integrated into the whole farm. For example, we needed a potting mix that would produce plants fit for our growing conditions and planting system. Compost suitable for such a potting mix came out of developing a conscientious manure management system in the barn. And we learned that a live potting soil is only as good as the greenhouse system.