Updated: February 16, 2023 Published: February 16, 2023
I knew as soon as I saw last week’s column in the paper that I would be getting questions on whether it is OK to reuse soil, be it seed-starting soil or potting soil. Sure enough!
The short answer is that it is perfectly acceptable to reuse soil if the plants previously grown in it were healthy. We do it all the time in our household. You just have to take steps to ensure the soil has enough nutrients to support a new plant or plants.
If you flip through the internet researching this question, you will find plenty who think you need new soil, which is just wrong. You’ll also find those who justify their answer as a way to save their readers a few bucks. Of course, there is nothing wrong with saving money, but there is so much more to it. In fact, I actually prefer to use soil that has already supported plants.
There are several reasons I like used soil. First of all, the previous plants released carbon substances into the soil in order to activate and maintain the soil food web. This is a good thing. The more carbon you have the better it is for attracting the diverse set of microbes which will feed and protect plants grown in their soils. They produce nutrients in plant-useable form right at the root zone and they release metabolites that keep bad guys in check, break up phosphate molecules and more.
Secondly, if previous plants were harvested properly, the used soil will have good structure, meaning it has nice aggregates and will drain well and has lots of pore spaces to hold air and microbes avoiding their predators. “Proper harvesting” entails simply cutting the plant at the soil level rather than yanking it out of the soil which, naturally, disturbs things.
And, the roots of new plants will follow the tracks left by the old roots. The soil around them harbors those carbon exudates. They also serve as organic matter ready for decay by microbes.
There are some concerns with using old soil. The first is that if harbors some kind of disease, it will pass that on to the new plants. Obviously, if the plants previously grown did not do well or actually showed signs of a pathogen, then that soil should be tossed on a compost pile to be “corrected” by the composting process.
ADVERTIsem*nT
A second obvious problem is lack of nutrients because previous plants used them. This problem is easily overcome by several methods. First, use a fast-acting, liquid organic fertilizer before you plant new stuff in old soil. If you can apply it a couple of weeks before planting, so much the better. Keep the soil moist and activate the microbes in it. And/ or use a fertilizer that is full of active microbes like Down To Earth’s Bio-Live. Remember, whatever you use, it has to be organic
Of course, you can also add a bit of fresh organics to previously used soil. Compost is great as it has the full complement of necessary microbes to support your plants. You don’t need much — 1/4 inch applied on the surface of the existing soil.
New soil or old soil, it is time to start preparing for starting seeds, tubers and bulbs. Build those pots and containers, collect those seeds and get those lights up if you don’t have them going for your houseplants already.
Jeff’s Alaska Garden Calendar
Spring Conference: The Annual Spring Garden Conference will be back in person this year, as well as offering virtual opportunities! All speakers on March 8 will be virtual via Zoom. Sessions on Friday, March 10 and Saturday, March 11 will be held in person on the Alaska Pacific University campus.
Alaska Botanical Garden: Do check www.alaskabg.org as there are lots and lots of things going on which you won’t want to miss.
Forcing bulbs: Time to get yours out into the light.
