Rejuvenate used container soil

July 21, 2009 4:15 AM

Every year on my porch in Boston, Ma. I grow a few different vegetables in large planters. This year one is pumpkins. After several years of growing crops in the same soil, how can I rejuvenate the soil to make it better for the next crop?
posted by boby to home & garden (11 comments total)

Improving the soil generally means adding organic matter / compost. You may also want to add fertilizer.
posted by jon1270 at 4:37 AM on July 21

Get a big bag of leaf compost and a big bag of cow manure, add them both to your current soil. Mix all the way down to the bottom of the planter - you may need to dump some onto a tarp or empty plastic bag if you're spilling. Water in some compost tea, and water with compost tea every couple of weeks during growing season.
I don't recommend commercial fertilizer for planters growing food unless you want to replace the soil completely every couple of years.
Yay you for doing this! Everyone can grow some of their food, it doesn't have to be a giant raised bed in the back yard.
posted by pomegranate at 4:53 AM on July 21

You could always plant legumes, many of which fix nitrogen through this awesome symbiosis with bacteria in their root nodules. This is what as known as crop rotation.
posted by sciencegeek at 4:54 AM on July 21 [2 favorites]

Bone meal, blood meal and wood ash.

Put a smallish handful of each in the hole when you plant.
posted by Stewriffic at 5:41 AM on July 21

I'm not an experienced gardener, but i've read it's good to alternate nitrogen fixers with plants that aren't, season over season. A soil tester may also help you learn.
posted by maniabug at 5:47 AM on July 21

The usual crop-rotation thing for fixing nitrogen is to plant legumes.
posted by polyglot at 5:49 AM on July 21

When you prep the containers, put lots of leaves and twigs in the bottom. I use brush from the yard. It provides drainage, and creates compost over time. Also, add soil amendments, as above. Clover is nitrogen fixing; once your plants are well started, you could add some clover seed. I find that ground cover in my large potted plants helps keep weeds down. Pumpkins - great idea.
posted by theora55 at 6:46 AM on July 21

Heat the soil.

Small amounts of soil may be heated in various ways to get rid of the unwanted pests. Clean the soil of physical debris. Water if necessary, the soil should be very moist but not dripping wet. Next step is to apply heat.

The soil temperature must reach between 180 and 200 degrees and stay there for awhile; below that pathogens may not be killed, at higher temperatures, organic elements may begin to break down and toxins form. The soil must be cool before using.

You can use the microwave oven to sterilize soil. Fill clean, quart-size plastic containers with lids with the potting mix. For microwaves with temperature probes, punch two holes in the lid, insert the probe in one hole to the middle of the soil, the other hole will allow steam to escape. Program the oven to heat until the soil reaches 180 - 200 degrees F and hold it there for twenty minutes. Remove and allow the soil to thoroughly cool before using or storing. A large microwave may hold up to 7 quart containers.

Another way of using the microwave to sterilize soils is to put about 2 pounds of moist soil in a thick, plastic bag; leave the top open and place in the center of the microwave. Treat for 2 to 5 minutes on full power, check the temperature in the middle of the soil with a thermometer, when it reaches 180 - 200 degrees, close the bag carefully and put in a cooler to hold the heat in the soil. Allow to cool as before.

Pressure cookers, steamers and solarization are also acceptable methods for heating soil.

If the planters are moveable, prepare the soil as for planting, add a little organic matter and make sure it mosit. The move the planters to a bright sunny spot. Soil solarization uses radiant heat from the sun to heat the soil. It is a common practice in veggie gardens and annual beds; in fact this is the right time of the year to use solarization in gardens; June, July, and August have the hot sun and proper sun angle needed for good solarization.

Use a thermometer to make sure the temperature reached the right level. Leave the cover on the planters for 4 to 6 weeks, this length of time is necessary to heat the soil to a depth of 6 to 8 inches.

If you are not planting again until spring, leave the planters covered until then. If you will use them for fall planting, remove the cover a couple of weeks before installing the plants to allow the soil to cool.
posted by yarddoccarol at 7:02 AM on July 21

I would highly suggest checking out the container forum over at gardenweb, especially the posts by tapla, aka Al.

some especially good threads:
Tapla's 5-1-1 Container Mix in More Detail
A Soil Discussion
Container soils and water in containers (long post)

One of the common thmes you'll find is that soil compaction and proper aeration are your biggest concerns in containers, rather than nutrient balance and disease (i.e. things solved with crop rotation and compost).
posted by aquafiend at 9:06 AM on July 21

If it were me, I'd add a bunch of good compost in the fall, and then plant a nitrogen-fixing cover crop as sciencegeek suggests, then let it over-winter. Since you're in Boston, you might want to mulch the whole thing with straw or hay. (Or look for a cover crop that can survive Boston winters. I assume there are some out there.)
posted by mudpuppie at 9:17 AM on July 21

Thanks for all the advice. I think I'll start a small compost pile and add it
to the container soil in the fall. Will try planting clover for the nitrogen addition and weed cover also.
posted by boby at 8:30 AM on July 22