Market Farming

[Alex McGregor <waldenfarm AT sprintmail.com>]

Rose & Fred Lieberman wrote:

> Are soil temps generally warmer than air temps?  In other words, if I chose
> to direct seed larkspur the 2nd week in April when air temps might just well
> be 55, would the soil temps be generally higher or lower?

There is no generality. It all depends. Soil heats up due to radiant energy 
from the sun.
If it's exposed to direct sunlight (no mulch or cover), it will heat up 
faster. Soil
gives off heat during the night, but not as much as it has gained- during 
warm weather,
Thus, soil heats up slowly over time. (Difference between short and long wave 
radiation
if you want to look up the physics of this.)

> My thinking (here in upstate New York) is that soil is darker, it would hold
> on to heat and therefore be warmer; but then I think that the soil is also
> wetter at that time of year and could easily be cooler, or at least not as
> warm as drier dark soil.

Good thinking! What happens is that darker soil heats faster from absorption 
of sun
radiation due to the color- just like dark clothes being hotter in sunlight 
than light
colored- the darker the color, the faster it heats. All soils give off heat 
at the same
rate- color is not a factor here.

One of the things that cools soil is addition of water- through rainfall or 
irrigation.
Water takes more energy to heat than dry soil. So, the wetter the soil the 
longer it
takes to heat. And addition of water cools soil because the water absorbs the 
energy and
it is released as the water turns from liquid to vapor- like sweat 
evaporating from our
skin cools us.

How can we use this, you may ask? Uncovering the soil in the spring will 
cause it to heat
up faster in the spring so we can plant earlier. Or we can mulch or use 
living mulch to
keep the soil from getting too hot for certain crops in the summer. Mulch 
will block the
sun's rays (like standing in the shade) and will also retain moisture. 
Irrigation also
helps keep soil cool in the summer.

I use a long probe thermometer to check soil temps. Long so I only had to buy 
one
thermometer for soil and compost. These read only at the tip, so I put it in 
the soil as
deep as I'll plant seeds to see what the temp is. I check it morning, noon 
and early
afternoon to see what the temperature range is like. If it's too hot, I 
water. Too cold,
I delay direct seeding.

> I need help with my thinking here, please.

You don't need help with your thinking, just some knowledge. Your thinking 
has led you to
some good conclusions. I'll bet you're a successful farmer because of it. 
Where you may
have been short changed, as many of us were, is through an educational system 
which
doesn't connect the theoretical content with how to use this knowledge in our 
everyday
lives.

I wish you perfect soil temperatures for germination!
Alex McGregor
Walden Farm
Walden Ridge, TN