Explain how soil is not frozen after 3 weeks of below freezing weather

[gary350]

TL;DR Summary

Explain how soil is not frozen after 3 weeks of below freezing weather.

It has been below freezing for 3 weeks, 12°F at night and in mid 20s during the day. Yard and garden are frozen 2" deep hard as cement everywhere except 3 row in the garden covered with 1/2" of pine needles. Soil under pine needle is not frozen I can push my finger into the soil 3" deep. How can 1/2" of pine needle keep soil from freezing?

Wood mulch is frozen hard as cement too.

This is winter potatoes. There should be 200 lbs of new potatoes about May 15 in these 2 rows. Potato rows with no pine needles soil is frozen 2" deep.

 

[jack action]

Only water becomes solid at 32°F, the rest of the soil already being solid. The only explanation I can think of is the lack of humidity under the pine needles.

 

[phinds]

If the pine needles were fresh cut then possibly there is a small amount of very slow decay that produces just enough heat to keep things from freezing. Unlikely but possible.

 

[Lnewqban]

The insulation effect is created by the volume of trapped air, as well as the number of little pockets of it.
Regular mulch can't compete with that.

Besides, the leaves themselves, even when dry, may keep some of their anti-freeze properties.
Please, see:
https://www.earthdate.org/node/139

:)

 

[256bits]

The insulation effect is created by the volume of trapped air, as well as the number of little pockets of it.
Regular mulch can't compete with that.

Add to that the fact that direct radiation loss from the soil to the sky has been eliminated.

It is note worthy to mention that in a similar fashion but in the reverse direction of heat flow, straw covered snow has a very difficult time to melt, staying as snow well into the spring, or summer if the cover is thick enough.

Or straw as an insulator for the soil.
These results indicate that just over 3 ft of straw would be enough to keep the ground surface fmm freezing if the minimum mean monthly temperature is -20" F and the ground is at +40' F when the straw is spread.

http://web.mit.edu/parmstr/Public/NRCan/brn119.pdf
for a calculation.

2 inches of pine needles seem to be good for at the very least 3 weeks period, if not longer, of 12 F air temperatures as expressed in the OP.

 

[gary350]

Pine needles fall off the trees to the ground dry out and turn brown color in a few months. I rake up pine needles and put them on the potato rows. It makes sense pine needles allow good air flow so water & moisture evaporates away. No water no ice. Soil is hilled up on potatoes to allow water drainage. Wood chips have no air flow so water stayed and froze. Wood chips also have more of candle wick effect that holds water. I assume pine needles also hold heat even though they are a very thin layer in many places. Pine needles must have the same effect as covering up plants to prevent frost on plant leaves. 3 potato rows with pine needles are not frozen, can't freeze with no water. Soil does not appear to be dry like hot summer but not wet either.

 

[256bits]

Soil is hilled up on potatoes to allow water drainage

I have never heard that as being a reason for hilling potatoes.The land is flat conserving water as it will not drain off.
The hill can have some less moisture than the underlying, but it is still there to freeze.
A better yield is produced by hilling.
1. Weeds are covered up near the plant and die off.
2. Prevention of green potatoes, which are a reject.
3. Less labour needed to separate the good potatoes from the green ones.

A green potato contains solanine, a toxin.
The toxin is produced by exposure to sunlight. Hilling, sometime after the plant has grown a height( a foot as a guide ), puts the roots farther from the surface with less chance of a potato breaking through the surface, being exposed to sunlight and producing solanine. You will have more good potatoes in the harvest.

Pine needles must have the same effect as covering up plants to prevent frost on plant leaves

Same effect, though this time one wants the soil not to freeze.

It makes sense pine needles allow good air flow so water & moisture evaporates away

You do not want active air flow.
You want still, trapped air, which the pine needles do, acting as insulation.

 

[256bits]

Wood chips also have more of candle wick effect that holds water

The candle wick effect could be correct, in which case that is undesirable.
Evaporating moisture has a cooling effect, which is why you sweat.
If your soil is loosing moisture, heat is also being lost driving the temperature down.
You want what moisture is there in the soil, to stay there.

 

[jack action]

These results indicate that just over 3 ft of straw would be enough to keep the ground surface fmm freezing

Knowing that the ground never freezes deeper than 4 ft, that is not an extraordinary finding. Just putting 4 ft of soil on the ground would prevent the ground surface from freezing.

 

[gary350]

I have never heard that as being a reason for hilling potatoes.The land is flat conserving water as it will not drain off.
The hill can have some less moisture than the underlying, but it is still there to freeze.
A better yield is produced by hilling.
1. Weeds are covered up near the plant and die off.
2. Prevention of green potatoes, which are a reject.
3. Less labour needed to separate the good potatoes from the green ones.

A green potato contains solanine, a toxin.
The toxin is produced by exposure to sunlight. Hilling, sometime after the plant has grown a height( a foot as a guide ), puts the roots farther from the surface with less chance of a potato breaking through the surface, being exposed to sunlight and producing solanine. You will have more good potatoes in the harvest.Same effect, though this time one wants the soil not to freeze.You do not want active air flow.
You want still, trapped air, which the pine needles do, acting as insulation.

When I lived in Michigan we planted potatoes day of last frost June 6 growing season was barely long enough to harvest potatoes in Sept. Potatoes were easy to grow lay them on the surface then cover with 3" of soil.

When I lived in Illinois potatoes were easy to grow plant day of last frost summer seldom got hotter than 90°F harvest new potatoes 4 months later.

When I lived in Phoenix AZ plant potatoes Oct 15 then harvest new potatoes end of Feb 15.

TN is a very hard place to grow potatoes spring rain every day March to June we had 47" of rain last spring. Potatoes rot in a swamp soil surface needs to be 2" above water level, lay seed potatoes on the surface then cover with 6" of soil. When saved potato eyes start to grow Oct start planting them as eyes appear on seed potatoes. Try to have all seed potatoes planted by Dec 1st. 4 month crop takes 6 to 7 months to grow in cold weather. There will be no above ground plants they freeze off. May 15 still no plants but it is time to dig up new potatoes.

I learned green potatoes are not caused from visible sun light. If I cover potatoes with 1" of soil they still turn green sun radiation goes 1" deep into the soil and still makes green potatoes. New potatoes need to be covered with more than 1" of soil to prevent green potatoes. My experiments show if 4 potatoes are covered with different amounts of soil, ¼", ½". ¾", 1", each potato is very noticeable progressive greener color. Different soils are made up of different materials, AZ soil is basically powdered rock mountain rivers flooded desert from Phoenix to Tucson for millions of years with snow melt water every spring.

 

[256bits]

Knowing that the ground never freezes deeper than 4 ft, that is not an extraordinary finding. Just putting 4 ft of soil on the ground would prevent the ground surface from freezing.

Certainly in some places for bare ground that would be true, such as Quebec City and Montreal.
In others such as Saskatoon or Yellowknife the frost penetration is greater.

It depends upon the degree-days below zero, and upon snow cover.
snow cover provides another layer of insulation for the soil, and the frost penetration is reduced.
Frost depth penetration of 4 feet under bare ground ( or asphalt ) translates to something like 2 feet under snow covered ground ( depending of course upon amount of snow cover ).

Further north areas have permafrost where the ground can actually be frozen to depth all year round. The concern in these areas is thaw penetration into the frozen ground.

 

[anorlunda]

Good points @256bits . I just looked up how deep can it be. I was surprised to learn "miles"

https://en.wikipedia.org/wiki/Permafrost
Permafrost does not have to be the first layer that is on the ground. It can be from an inch to several miles deep under the Earth's surface.

 

[jack action]

I don't think 3 ft of straw will affect 'several miles' of permafrost!

 

[jrmichler]

Back to the OP... We can look at this as a 1D heat transfer problem. Soil temperature is constant at 30 feet down and below. Between the surface and 30 feet down, it varies seasonally, with additional daily variation in the top few inches. Good search terms for more detail are soil temperature vs depth. The pine needles are an insulation layer, or more correctly, a layer with lower thermal conductivity. A simple, possibly oversimplified, diagram:

The temperature 30 feet down is a constant 40 degrees F in my Northern Wisconsin USA climate. The temperature between that level and the surface will be warmer than that at the end of summer. Since the temperature gradient is warmer toward the surface, there will be heat flow upward. The rate of heat flow is a function of the temperature gradient in degrees F per foot, and the thermal conductivity of the soil.

Next, an insulation layer (pine needles) is added. The temperature drops. Now there is heat loss upward through the insulation layer, and the rate of heat loss is a function of the temperature gradient in degrees F per foot, and the thermal conductivity of the pine needle layer.

Next, the effect of thermal mass needs to be added to the above model. The thermal mass of soil is a function of the specific heat of dry soil, the density of the dry soil, and the moisture content of that soil. It will typically vary with depth.

At some point, the heat loss upward through the pine needles overcomes the heat flow upward through the soil, and the soil layer underneath the pine needles freezes.

A simple experiment would take two or more low cost digital inside/outside thermometers. Bury one probe under the pine needles, and the second probe a few inches down, then watch them for a few days or weeks.

 

[256bits]

We can look at this as a 1D heat transfer problem

Pretty much as what I as treating the problem, but since you mentioned it, one should be careful with edge effects around the perimeter of the insulating material. If the area is large enough as one consideration, edge effects can be neglected. Though if you are wrong in the assumptions, your potatoes can end up becoming mush, not from a frost layer moving top down, but inwards from the sides, freezing the soil as well as your potatoes.

 

[gary350]

Pretty much as what I as treating the problem, but since you mentioned it, one should be careful with edge effects around the perimeter of the insulating material. If the area is large enough as one consideration, edge effects can be neglected. Though if you are wrong in the assumptions, your potatoes can end up becoming mush, not from a frost layer moving top down, but inwards from the sides, freezing the soil as well as your potatoes.

It has been many years sense we had continuous cold below freezing weather here every day for 3 weeks. It is common to have 15° after dark then warm up to 40°F every day all winter. Weather cycles are up and down weekly sometimes it warms up to 50° like yesterday. 43 years of planting potatoes none have frozen that were covered with 6" of soil. Last year 47" of rain 1/2 the crop rotted in the swamp but harvest was still 80 lbs of new potatoes.