What Soil Temperature is Too Hot?

adamsn06December 31, 2009

I'm planning on growing tomatoes in beds with clear plastic this year and wondered at what point should I put a cooling mulch on top of the plastic. I understand that the soil needs to be 55-60 degrees at the low end -- what is the high end?

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I read somewhere that tomato blossoms begin failing to set at 86 degrees and upwards for most varieties, but I'm not sure if thats soil temp, evening air temp or average air temp. I know some hybrid varieties such as Heatwave will set fruit at higher temps than others, but in my experience these determinate varieties give far less overall production than most indeterminate varieties. Your question is a good one and I hope someone can give us a more specific answer. Have a happy New year.

    Bookmark   December 31, 2009 at 11:02AM
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Thanks for the feedback hardclay and happy new year to you ... absent any other information I think I'll cover the clear plastic with grass clippings when I regularly see 2" soil temps above 80F ... but I've got to think there has been research on what temperature is optimal and what is too high for tomato roots.

    Bookmark   January 1, 2010 at 10:53AM
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Generally between 26°C and 30°C (~80°F - 86°F) is the high end. It's a range because higher soil temperatures have been found to have a more negative effect on more mature plants.

One research paper in 1971 found that the combination of 30°C root temp and 25°C air temp was the point of being detrimental to plants. A cooler root zone (mulched) would be able to withstand higher air temps. Research in field plants in the south found that planting the root ball deeper helped fruit set in high air temps.

On the other end, Lingle and Davis (1959) observed, and later papers (Cornillion, 1974; Bugbee and White, 1984) confirmed, that root temperatures around 15°C (~60°F) or lower severely restricted growth.

    Bookmark   January 1, 2010 at 12:09PM
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There is a moisture interaction too. Higher levels of moisture in the soil maintain cooler temperatures because of better heat flow into the subsoil. If the soil dries out, the plant goes into moisture stress and the soil temp tends to rise at the same time for a double whammy on the plant. This is a huge part of the reason for the mulch. It helps with moisture regulation in the soil.


    Bookmark   January 1, 2010 at 2:42PM
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