Growing in 100% Compost?
I'm going to be in Chicago from April to June installing 30 community/school gardens. We are looking into growing mediums and have found that quite a few suppliers are growing in 100% compost. I don't personally have any experience with this. What are the implications? What is the optimal growing environment for vegetable's in Chicago's climate?
There's a wide range of opinions about growing in 100% compost, which is not surprising considering that there's a wide range of "compost" available! Whether this is a good idea depends heavily on the composition and qualities of the compost as well as characteristics of site where the garden will be installed. Unfortunately, there's no standardized label for compost that details sources, texture, pH, micro-nutrients or other properties of a particular compost. To add to the confusion, some folks use the word "compost" in the British tradition, which is as a synonym for "growing medium" that may contain little "compost" as we use the term.
In theory, yes, you can grow in 100% compost, at least if it's the kind of rich, dark, crumbly compost that you might make on your own from a good variety of sources. Much of the commercially available compost tends to be more homogenous, though -- "mushroom compost", "composted manure", "composted bark" and so forth. While these products can serve as excellent soil conditioners or amendments, as stand-alone media they can drain and dry out too quickly or tend toward clumping with little structure to support roots. A common complaint is that in pots or raised beds, significant shrinkage occurs as compost further breaks down and/or washes away.
Presuming you're installing 30 (raised bed) gardens in 30 different places, I think you may encounter a variety of underlying site conditions. (We Cook County Extension Master Gardeners do quite a bit of the same, so we can attest to that variety!) If that's the case, and you intend to use the same media for all gardens, I think you might consider using a mix of compost and soil-based media that will likely do a better job of even moisture retention and yield a more forgiving, "lower maintenance" garden than most commercial compost-only media would provide. The University of Maryland extension has a short fact sheet on vegetable garden media that you may find useful in considering materials for a mix:
To the extent that these are educational gardens, it might be interesting to have the students conduct experiments in comparing their crops' progress in different soil mixes if there will be multiple beds at a site or a bed can be segmented to hold different media. (Research suggests that compost-only media tend to yield somewhat smaller plants than soil-compost mixes, but pertains mostly to greenhouse grown ornamental plants, not vegetable yields.)
Chicago's climate can be erratic -- we come to our average daily highs and lows through swings in temperature more than steady conditions. Similarly, we can have long dry, or wet, spells, so it's important that a bed be able to retain moisture in the former yet drain well in the latter. Many locations have a fairly heavy clay-based soil, so in the ground we're more likely to be working in compost to promote a more aerated structure and drainage than to help sandy soils retain water. When building raised beds to avoid clay we need to ensure they can drain well, lest our raised bed become a soggy pot at its bottom. That having been said, a bed that fails to hold moisture can become a high-maintenance, water-guzzling nuisance when our summer days turn hot and gusty.
For what it's worth, the National Weather Service's Climate Prediction Center notes high probabilities of Chicago's entire 2013 growing season being above average in temperature, with a wetter-than-typical April-June and a normal-precipitation summer. You may wish to bring along a large umbrella when you come!