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Soil Temperature and Organic Methods of Gardening

Last week the soil temperature in our garden at City College of San Francisco finally reached 60 degrees F, warm enough for reasonably quick germination of summer crops such as beans and squash.

The beans we put in are Purple Podded Bush beans, which are reputed to be able to grow in slightly colder soil than other garden beans. We put them in about 3 weeks ago, when the soil was in the mid fifities. The first leaves started out tinged with yellow around their edges, which looked like they had some kind of disease, but I knew that it was caused by inability to get enough nutrition in the cold soil. This week they are looking almost completely green.

Cold soils are the Achilles heel of organic gardens. My cousin, who raises organic seedlings in the Midwest each spring, in an unheated greenhouse, and then sells them to gardeners, says that they suffer more from chilly weather than they would if they were being grown with synthetic fertilizer. Why? Because organic fertilizer needs some action from soil bacteria to release its nutrients, the simple, water soluble elemental compounds that plants can use for growth.

Synthetics are manufactured to provide the compounds without any bacterial action, but at a cost to the earth. First, they require considerable petroleum energy to create. Second, they are so water soluble that the plants have to catch what nutrients they can while the fertilizers are on their way through the soil into the groundwater. So you have to apply more. Synthetic fertilizers that enter groundwater will pollute it. If they run off they pollute nearby bodies of water. If you can get your organic fertilizer from nearby sources, or make compost from wastes found on site, you can save the petroleum needed to transport your fertilizer. (Think twice before you decide you have to have bat guano from South America.)

So maybe my seedlings get a bit slower start, but I'm sure they catch up, and I feel good knowing that my garden is less of a burden on the earth.


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Hi Tim,

I don'th think that the amount of organic matter in organically amended garden soil is enough to raise its temperature significantly. High nitrogen organic matter (like fresh horse manure or fresh grass clippings) does heat up due to the life activities of bacteria, and when a compost pile is made using a large enough component of such materials, the whole pile heats up, but we don't want to be planting directly in any organic material that is in the "heating up" stage of decomposition. The French intensive market gardeners around Paris used what they called "hot beds," made by burying fresh horse manure deeply in a bed, then adding soil, then planting on top. That warmed the soil. However, they knew not to plant directly in the manure, since the bacteria that do this would be likely to decompose the seedlings as well.

Tim Lester

With organic methids the soil tempurature should be a little warmer as there is more organic matter which stimulates microbes which produce heat.
Kind Regards
Tim Lester

matthew mcgee

i monitor temprature in my front garden as well as humidity and soil temprature in my back yard i have a personal weather station with wind speed and direction temprature humidity and rain an uv index
this info is helpful for maintaining a garden the green thumb
is the word for it nice for my iris mimosa plant (sensitive plant)
oak tree and cattails


i use a soil probe on a 10 ft cord with a wireless transmitter
with a base unit


Hi: I just moved into a house near City College with an old and overgrown garden filled with ornamental shrubs and trees and a number of fruit trees. Can you recommend a source for identifying the fruit trees and flowers? I'm going to get Golden Gate Gardening as well, but I'm a real novice when it comes to gardens. I only know how to weed and turn on a hose and I'm eager to learn how to handle my lovely garden. No one has lived in the house for 7 years, but the previous owner was a rose and fruit fan and obviously took great care with her backyard. Thanks.

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