Saturday, March 7, 2009

Mixed media

The definition of the term growing medium, as outlined by the Growing Media Association (I'm not kidding, it exists) is "material used in a container to grow a plant."  In other words, potting soil.  In the past, I just bought the cheapest bag of whatever at the hardware store.  But this year, as I wait impatiently for my seed order to make its way though my mail slot, I thought I'd do a little research to ensure that my organic seeds had a comparable home.

As The National Gardening Company plainly states, "A good seed starting mix holds moisture, drains well, and is fine textured.  Fair enough.  But what makes a growing medium organic?  Some companies sell organic potting mixes, or else you can make your own.  The organization ATTRA has published a detailed guide of what is allowed in organic potting soil.  These agents include soil, sand, compost, spaghum peat moss or other forms of peat, coir or coir dust (a byproduct of the coconut fiber industry), newspaper, alfalfa, sawdust, clay, kenaf, perlite, vermiculite, limestone, and alternative fertilizers.

What is not considered organic is any mix containing wetting agents.  An explanation in the Global Garden online magazine reveals that "soil wetters are essentially the same as detergents."  Part of a class of chemicals known as surfectants, these wetting agents cannot be biodegradable to do their job effectively.  Nevertheless, some soils are manufactured with them because organic mixes can be hydrophobic.  Perhaps you've had this experience before; you pour water on your seedlings but it doesn't get absorbed.  Wetting agents can reduce the surface tension of the water to so it is easier for it to enter the pores of the organic matter instead of sliding off of it.

A few commenters to a previous post have suggested soaking the soil before planting the seeds.  It was not clear what kind of mix they were using, but if I find a good organic product I'll heed their advice.  Even though my MSG plot and my gardens at home are not located near a stream or pond I would prefer to not to use any agent that poses a risk to aquatic organisms or potentially my plants as well.   I checked two reliable local gardening centers; one has not opened for the season yet while the other only carried a couple of mixes that contained wetting agents.  I was instructed to check back in a few weeks, if I can wait that long.


Dan said...

Interesting post, I have never thought of wetting agents as being bad for the environment. I guess one thing to consider is the amount would be very minimal from a small cell pack of veggies. It would also be unavoidable if any plants were purchase from the nursery. Maybe I should start purchasing organic potting soil.

I use pro-mix potting soil by the way, I have found it to be the best quality you can buy. They also have an organic line but I have not seen it around much. I have also thought of mixing by own and storing in a large bin.

One other thing to consider from an environmental point of view is almost all soil-less mix is made of peat. It is harvested from bogs and is really hard on the environment.

Bryan Bunch said...

My local garden store did not have any seed starting soil without wetting agents, but I was too impatient to start tomatoes and basil that I bought some anyway. It was a small bag!

Sally said...

I think both you make a good point about how little the proportion of wetting agents is to the soil in general. And it's usually the last ingredient listed in those mixes. I'll probably look around a little more for organic this weekend, but if I have to resort to the usual stuff I won't beat myself up about it.