Onions are the first seeds that we start for the year, and earlier this week we finished sowing about 5,000 onion seeds in flats indoors. In late March and early April, we’ll transplant the best of the seedlings into the ground, one by one. If all goes well, the first of the sweet onions will show up on the market stand in late June. Storage onions will be available sometime thereafter, and we’ll still be serving some of the storage onions on our table at this time next year.
We’re undertaking an experiment with our seed starting approach this year in an effort to move towards more sustainable methods. The approach that we’ve used for the past few years has been to grow transplants in a commercial, certified organic potting mix in reusable plastic flats under grow lights in the house. We’ve known all along that this has been an interim method, and within the next few years we hope to be growing our transplants in a passive solar greenhouse with a custom potting soil mix composed mostly, if not entirely, of on-farm ingredients. The passive solar greenhouse isn’t in the picture for this year, but testing some alternative potting mix options certainly is.
Moving away from commercial potting mixes is desirable for several reasons: 1) They violate the ethic of on-farm closed loops. 2) They’re expensive. 3) They may be sold out or otherwise unavailable when needed. 4) Quality may be inconsistent. 5) Many of the components are not as sustainable, local, and/or ethical as we’d like. For example, peat moss is a major component of most potting mixes, but it is non-renewable in the long run. Vermiculite, another common component of potting mixes, is a mined mineral that is often geologically associated with asbestos. Furthermore, these and other ingredients must be shipped a long distance if we are to use them.
So, to wean ourselves off of commercial potting mixes, we started with some research regarding the potting mixes that other people use. This publication from ATTRA was especially useful. The appendix has a list of recipes for potting mixes based on a wide range of ingredients. Joanna spent a couple of hours compiling these in a spreadsheet and doing unit conversions to get an overall picture of the breakdown of various components (organic matter, mineral matter, fertility amendments) by percent. Using this information as a guide, we came up with nine initial test mixes that currently reside in the flats with the onion seeds. They range from fully off-farm to fully on-farm. It would be nice to do have full analyses from a soil testing lab for each mix, but that’s beyond the means of the farm budget, so we’ll let the onions tell us what works and what doesn’t.
Watching how this experiment plays out will be interesting and informative. Some initial results are already coming in: As expected, the water retention for the on-farm mix isn’t as good as the commercial mix. But it remains to be seen whether that helps or hurts in the long run, given that damping off tends to strike when conditions are too consistently wet. One certainty is that the on-farm mix will require more regular attentiveness from us to keep the plants from drying out too much. A couple of months from now, hopefully will have a good population of healthy onion seedlings and a better sense of potting mix performance to guide our future management.