You may recall from a previous post that we have installed a hoop house/cold frame in The Learning Garden. As part of this project we are following the air and soil temperatures. Those temperatures are summarized in the graph below. We compare the soil temperature inside the hoop house (orange) to a similar bed without a hoop house (green). The hoop house does consistently increase the soil temperature by about 5 degrees. The fluctuations are largely due to the amount of sunlight on the particular day that the readings were taken. The high temperatures during the day are also dependent on the amount of sunlight. On bright sunny days the high temperatures approach 80 degrees. Venting keeps the heat from building up to excessive levels. The low air temperatures probably are similar to the outside air temperature. The hoop house if just too small to hold heat through the night. On April 7 radish and lettuce seeds had germinated.
The other temperature study that is going on is black vs clear plastic mulch laid directly on the ground. It is very clear that clear plastic, the green line, heats the soil faster. The temperature difference is 10 degrees. The soil temperature is similar to the hoop house temperature. It was a bit of a surprise that black plastic did not affect soil temperature compared to no covering. Although the sun does heat the plastic, that heat is not transferred to the soil. We will be continuing this study through the summer.
On March 21st a number of Outagamie County Master Gardeners installed a hoop house on one of the raised beds in a space we call The Learning Garden. Hoop houses and cold frames are intended to create a warmer micro environment. To track the benefit there is a thermometer inside the hoop house that records the high and low temperatures over a 24 hour period. The 3 days following the installation the air temperature ranged from 23 to 77 degrees. Soil temperature had increased from 37 to 57 degrees. The soil in the other raised bed, which is not covered, increased from 36 to 41 degrees. These sunny days were followed by a cold cloud weather pattern. Soil temperature in the uncovered bed dropped to 30 degrees while inside the hoop house the temperature had dropped to 39 degrees. Air temperatures were 34 and 46 degrees. After one week, it does look like the hoop house is increasing soil temperature faster than without. Not surprisingly, sunny days show the biggest benefit.
On the 24th and second experiment was started on the large bed. 4’ X 4’ sheets of black and clear plastic were laid directly on the soil. Soil temperature under each of these sheets is being measured and compared to an uncovered section. The weather on the days after the sheets were laid down was cold and cloudy. The soil temperature actually dropped to the low 30’s. A couple days of sun increased the soil under the clear plastic to increase to 54 degrees while the uncovered and black sections remained a 32 degrees. Black plastic does not allow light to penetrate. The plastic is warmed, but that heat is not being transferred to the soil. Clear plastic allows the light to penetrate and warm the soil. The plastic then traps the heat. We will be continuing this experiment trough the growing season. For more information see this very good article from Penn State. There are related articles in the bibliography, including one on tomatoes and red mulch, that are interesting. http://extension.psu.edu/plants/plasticulture/technologies/plastic-mulches
Written by Tom W. Posted by Rachel
Soil Temperature Experiment in the Learning Garden