It shouldn't be too difficult to figure how much energy is required to raise the temperature of a given volume of soil about 20 degrees. Then there is the heat-loss on top of that (the maintenance, as Ed said). If daytime high averages 60 and nighttime low 40 (as an example), then the average is 50, so if the surface of the soil is being held at 70 then the loss is very large. If the soil is wet as it would be in spring then it's a pretty darn good radiator. The natural solar gain would ameliorate these figures somewhat.

It is indeed a hare-brained scheme. The clear plastic is much more reasonable.

ahh pnbrown :) I've read and appreciated and respected your opinions for awhile, but if 'it shouldn't be too difficult to figure out' the entropy lets hear it? .. why call the guys idea a 'hare-brained' scheme just because your solution is 'more reasonable'.
if you need the math done and can't do it yourself why comment... your first comment.. "Could get expensive is an understatement"

I like people thinking out of the box... and sharing those thoughts.... i'd hate to discourage them from sharing their ideas with us because they think they will get treated that way.
Ed

I do this in my greenhouse. I heat greenhouse with a wood burning stove, so that is my heat source. I put a 5 gal. gas can filled with water on top and bury a garden hose in the soil. using a circulating pump from harbor freight 2 do the circulating. works very well and have tomatoes in april here in mich.

I like the hot water circulation idea. But as thepodpiper explained you would need a circulating pump.

I don't think it is easy at all to figure out how much power would be needed based on analysis and modeling. In the immediate vicinity of the wire, it is a heat conduction problem, and seems not that difficult. But the proximity of the soil surface makes it much more complicated.

At the soil surface, the heat loss would be due to convection and radiation. So heat will flow away from the wire in all directions, but more heat will flow upward toward the surface, because the surface is colder than the other nearby soil. So now we have to model two additional heat transfer mechanisms, and estimate the heat transfer in all different directions. I believe it would be much easier to try it than to calculate it.

To calculate it, if so inclined, you have to know the conductivity and thermal mass of the soil, and the exposure to the sky (angular exposure), the radiation temperature of the sky (depends on cloud cover), etc. Wind speed and heat capacity of the air (varies with humidity and altitude).

Thinking about what thepodpiper said, a wood stove can put out something like 50,000 BTU's per hour. Let's say thepodpiper is running his (or hers?) for what, 4 hours? So 200,000 BTU's. That is 58 kWh. 10 bucks. Per night. Not all of it is going into the soil, either, since it also heats the air in the greenhouse.

But maybe the OP doesn't need that much heat, because he is not in Michigan like thepodpiper. (Is the OP a he or she? I don't care, it is just easier to pick one pronoun and stick with it.)

Too many assumptions and estimates (we don't even know what size thepodpiper's greenhouse is). I still say it is a bad idea (only based on expense and perhaps some danger with a buried wire), but maybe the only way to find out for sure is to try it.

What would happen if you severed the wire with a spade or shovel or whatever? Would the conductor be exposed?

--McKenzie

In any case, the outcome would not be worth the expense if using electricity.

Ed, how is sticking some electric heating elements in the ground thinking outside the box? Like I said, it's comparable to running the heat in a house with everything wide open. I think most would agree that latter would be really foolish.

Isn't the OP talking about an uncovered outdoor piece of ground? He spelled 'outdoors' in all caps. You are suggesting I am innumerate; I suggest the OP is innumerate, and does not grasp how much heat earth can hold nor how rapid heat exchange is between moist earth and cold moving air. I'm not going to do the math for him, I think maybe he can do it himself.

I admitted the idea might be hare-brained...that's the point. I was interested in this type of discussion.

I'll try clear or black plastic, as that seems a reasonable way to do it.

Black only works well if you can get really good contact between the soil and the plastic - any air bubbles and the heat transfer into the soil is greatly slowed. If the soil is dark in any case then I suspect you'll do better with clear - much faster. Be sure the ground is very damp first.

Sometimes I lay black plastic, taking care to try to get it really flat, and then put a cold frame over that to prevent wind from blowing away the warm air that builds just above, and to reduce the night-time loss (that is what really kills on this kind of project). Usually though if the ground is bare I just put the cold frame over. If I am really obsessive and cover the cold frame at night with a blanket or something like that then the warming is much faster. One could think of the clear plastic as being similar to a cold frame, which it is, and there is no reason it couldn't be covered at night as well.

This kind of thing could easily move up germination time 2-3 weeks.

"I like the hot water circulation idea. But as thepodpiper explained you would need a circulating pump."

Not actually. As the water heats up it will rise in the tank and go out the hot water side of the heater, pulling the cooler water in the cold water side.

@ vertach

You have to isolate the water heater from the city water . The use it as a hydronic heating boiler, with a closed circuit system, at LOW pressure. Need some implementation.
You can further lower the temperature as needed.

Yes seysonn, it will be a closed system.

Hopefully the sun will provide most of the heat.

In my younger day's I built several closed systems to supplement greenhouse and dwelling heat. I moved and left those behind.........

I did manage to get my old broken bones working, long enough, 2 years ago, to install one on my detached garage.

Thermo-siphons are extremely clever. They are most viable in lower latitudes especially outside of freezing zones so that pipes don't have to be insulated and/or don;t have to add antifreeze to the tank water (antifreeze causes a loss in efficiency).

Again, though heating ground requires relatively a lot of energy. Of course it stands to reason that the amount of solar heat that would be gained by a tank painted black and then piped into the ground would be less than the amount that would gained simply by letting that solar incidence hit the ground instead of the tank. IOW, a much larger solar collection area vs the heated area is required. My guess is larger by an order of magnitude in the case of a thermo-siphon (passive) for the time of year we are talking about (before the spring solstice, right?) I tried this once with an evacuated-tube collector and a small 12v pump connected to a pv-panel to pump the water through garden hose buried in a bed (so an active system). The pump turned out to be too weak to move the water through the hose - small-diameter hose has a great deal of friction depending on the length. So the water wasn't moving fast enough to keep the collector from over-heating and blowing the hose connections (water was boiling inside the pipe).

With a thermo-siphon as well pipe diameter is critical because the friction will easily overcome the tendency of the hot water to flow, preventing the siphon and possibly damaging the collector.

hey pn :) i'm suggesting nothing of the sort... wait, whats 5 + 5 ?
its not a simple calculation,... and to presume that the cost doesn't equal the benefit for anyone by calling it hare-brained is wrong IMO. based on your zone we probably live in pretty close proximity... so you know what a few week on either end would do.

hairmetals idea has merit, if you want to open your doors and windows to heat your house... that's your call.

if your in MA you know people who run the kind of wire I was talking about to prevent ice dams... I have them on a couple rental propertys... the power usage doesn't seem to bankrupt all the people using them... oh and by the way... they are outdoors.

furthermore, solar power is getting cheaper and cheaper. I could see a self sufficient system with a little up front investment to accomplish what he's trying to.

happy holidays all,
Ed

Wouldn't be hard to figure out the heat-cost of the plan. The issue is the rate of heat loss. The question is how many watts per unit area are required to keep that unit area at the desired temperature. Bury a short cable with a given wattage, looping it evenly into a square area. That's your unit area. Put a thermometer in the middle, halfway between the cable runs. What temp does it stabilize at? You can raise that equilibrium temperature by looping the cable tighter, but then the area becomes smaller.

Should be pretty straightforward that way to figure out how much cable and how many watts you'd need to properly heat a bed. If you're trying to raise the temp 1 degree above ambient, that's probably easy. If you're trying to raise it 10 degrees above ambient, I suspect that's gonna require a lot of watts.

Look at it this way. Averaged over a day and night, sunlight from a clear and unobscured sky will put a few hundred watts per square meter into your soil. So if you want to add that much heat electrically, you're talking of order a dollar per day per square meter. If you want to do it for a few weeks, and for a reasonably sized bed, you're talking quite a few bucks.

'sunlight from a clear and unobscured sky will put a few hundred watts per square meter into your soil'

again I love the 'wouldn't be hard to figure' and 'should be pretty straighforward' talk... but this is so much better... I laughed pretty hard at this 1.

It's called the solar constant. Look it up. But that wasn't about how it wouldn't be hard to figure. It was just a comparison of electrical heating with solar heating. I proposed a pretty simple experiment -- short heating cable and thermometer. But I'm glad you had a nice laugh.

One way to find out if it works or not and how, it just do it. This is not something to be analyzed by numerical calculation. I think.

Another method is just to utilize the ground heat/temperature. To do this you just prevent the heat loss from the ground. Natural ground temperature is about 60F (+/- ?). So insulate the top (By straw, leaves) , cover it by black plastic( except a tiny opening for the seed) . This is what I am going to do next early spring . So in reality , you are not warming up the soil but preventing it from cooling/heat loss.

That's a good idea, seysonn, except it might work better to use transparent plastic if what you're trying to do is heat the soil. Black plastic will absorb the sunlight, and just heat the plastic, which will cool rapidly, since it is exposed to the outside air, especially if there is an air gap between the plastic and soil. With transparent plastic, you heat up the (well, not quite black) ground underneath, and the plastic sheet insulates it from the cold air. The disadvantage of clear plastic is that weeds will grow under it. There is a good comparison of the soil-heating effects of colored and clear plastic at the attached link.

Here is a link that might be useful: plastic mulches

not just me dan,,, lots of laughs for sure... I just hope that mind isn't doing any real engineering.

C'mon, Ed, you aren't seriously comparing a use with high utility (preventing ice-dams on roofs) with one with low utility (or maybe zero utility), are you? Plus, as you know, to cause melting and thus slippage between ice and a slanted roof surface does not require anything like the energy that would be required to maintain cubic feet of soil (how much roof, how much soil, we none of us even know what we are talking about at this point) at significantly above the temperature of air above and even the ground below and beside.

And why the displeasure at my pointing out that the OP correctly characterized his own idea at the outset?

Yes, the math involved in trying to exactly quantify these kinds of things would be complex, probably most of us wouldn't know how to calculate it, but we can easily know that it doesn't make sense. OTOH, blasting around in a powerboat or muscle-car doesn't makes sense either...

Ed, your responses are unconstructive. If you want to have a discussion, then let's do so. That's what we're here for. BTW, we're not talking "math", we're talking basic thermal physics or engineering. If you're scared of that, it's forgivable, I guess.

As seysonn and I pointed out, it's not hard to get a practical answer to the problem if you do it in a very limited area. But the cooling rate of the soil (which is how fast you have to pump energy into it) and the resulting equilibrium temperature are going to depend on a lot of things. Not just air temperature, but also on the wind speed. Probably soil moisture as well. Wet soil will be a much poorer insulator than dry soil and, for that matter, will take more energy to heat. So even the practical answer one gets this way will have a lot of caveats.

Dan, thanks a bunch.
That link has a lot of good reading material, regarding plastic mulch.
So, now I know that for the purpose of soil heating I should use clear plastic. It works, in a way, like a greenhouse. Or we may say it works on GREENHOUSE EFFECT.

Seysonn, let us know how it works. Yes, that's one reason why we don't paint greenhouses black! I've never used either kind of plastic mulch, but it would be interesting to find out. If I interpret the PSU link properly, the problem with clear mulch is that weeds can grow underneath it. Now, I don't see a big problem with that, in that once the crop starts to sprout, and you remove the plastic, you just have a bit of weeding to do.

I tried this last year. And it did okay for starting the season early for my tomatoes. But not melons or corn.

I have 40' raised beds and I ran the cables down those. Then I planted and covered the row with plastic over hoops.

First night I discovered that the breaker was tripped, so I had to run extention cords to different breakers for each cable.

It did raise my electric bill considerably, and the corn didn't grown one inch faster or taller than the corn planted out 2 weeks later. My melons were a disaster! They just can't take any cold, air or soil.

This did work with the tomatoes tho, I used them in conjunction with wall-o-waters. My tomatoes grew strong and healthy right from transplant, even tho some of my night time temps were still dipping into the 30's. Still, I didn't pick my first ripe tomato until late July...so it didn't help in that regard.

All in all, it was a waste of time, money and effort for me.

Interesting. How did the soil temp compare to the air temp? And it would also be interesting to know how the soil temp outside your hoops compared to the air temp.

Dan ,
I will definitely do it. I have already covered my raised beds with clear plastic(since October). Because it is rainy here from November til April . And the sun angle is way down. So actually I am prepared.

We can come up with all kinds of Hi-Tech soil heating methods. But in my opinion it is not economical. You have to find a way to use natural alternative free energy which in this case are SOLAR and GEOTHERMAL.

Seysonn, I have to wonder about the soil temp. It would be really interesting to compare the temperature of your bed, after it had been covered for several months, with that of nearby uncovered soil. Say, at the same depth. That would be a quick way of deciding if that cover helped.

I guess there are actually two effects going on here. One is the solar heating of the covered bed, and the other is the self-generated heat by the composting that is probably still going on in the bed. I have to assume that the same process that makes a big compost pile steam up is happening in the soil, especially if you've dug in some green stuff. Covering the bed holds in both kinds of heat. Maybe we could consider the latter to be geothermal, no?

Of course, what that would mean is that digging some green stuff into a cold bed could warm it up after a while Hmmm.

"I have to assume that the same process that makes a big compost pile steam up is happening in the soil, especially if you've dug in some green stuff"

Not IME. The rule of thumb for a noted effect from thermophyllic microbes is close to a cubic yard; clearly that sort of profile is not going to happen with tilling in or mixing in green matter. The warming will be primarily from solar and to be notably ahead of the ambient soil requires insulating in some way to prevent loss.

The rule of thumb you quote is about getting a compost pile steaming. I'm not saying your garden bed will start steaming. I'm just saying that the composting process in the bed releases heat by microbial breakdown of organic material. It WILL produce heat, though certainly not a lot of it. If you put enough insulation on the surface, it has to get warmer than the air, for just this reason.

Using microbial processes to generate heat has been done for thousands of years, and you don't need steaming compost piles to do it.

Yeah, I guess there would be some. As you say, with enough insulation at night and on overcast days it could accumulate. It would be very tiresome to have to move that insulation twice a day to take advantage of solar gain.

IME, significantly beating the natural course of warming is a lot of effort in some way or another. To me the best use of the effort and expense is to sculpt areas to be permanent, solar bunkers with glazing, essentially. Even without glazing the effect can be notable - a bed at the base of a south-facing masonry wall with fill behind it, for example.

The idea of a south-facing masonry wall is a good one. Of course, that's like a permanent Wall-O-Water. Actually, speaking of steaming compost heaps, it might be clever to run some water plumbing under one, and circulate that water in an early-season bed. Run tubes instead of wires. The energy costs would be minimal.

This thread has turned out to have some legs.

I know I have read about using horse manure to heat beds. If I remember correctly, you put a few inches of horse manure covered by a few inches of soil. Something like that. Horse manure can give off a lot of heat for a long time. That seems like the simplest embodiment of that basic idea.

For passive systems, I have this idea. In my latitude (37 north) the winter sun doesn't get very high. Also, the biggest heat loss I have occurs on clear nights via radiation transfer to the night sky. So If I built a slanted roof over my beds, blocking the northern sky, it would greatly reduce night cooling without blocking the sun. Thermal ballast would also help moderate the temperature swings. It could maybe be an A-frame type of thing with the northern wall opaque, and the other walls clear. I think it would make a big difference.

--McKenzie

McKenzie, I've made made "hotbeds" from manure numerous times - in fact, the only reason I stopped is because I can only get ahold of stable manure that is based on pine shavings, and in my light soil the long-term effect is not good.

What I used to do is buy the cheapest hay bales I could find - spoiled hay is ideal - and make a big box out of the bales. Two bales high is best but sometimes I just did one bale high. First I remove most of the garden soil from the where it will be located; fill the box within a few inches with fresh stable manure; put the soil back on top; pop a cold frame over the whole box.

I would do the in late february or early march. With the super insulation of the hay that puppy will heat up wicked, in fact it is tough to control oneself from planting right away. often I couldn't stop myself and i'd sow radish and lettuce the day I built the bed - the radishes will grow incredibly large cotyledons, I guess because of the very warm soil. They wouldn't make true leaves until the first flush of heat backed off. Lettuce will germinate and then just sit there until it cools off some, so best to wait 4-5 days.

It works like anything but it's a job. If I could get stable manure made from straw I would do one every year.

I'm a little surprised to hear about that hot-bedding. Putting a lot of non-composted material into a bed can really rob nitrogen, but I guess if it is buried deeply enough, that won't be a serious issue, because the composting happens slowly, and ammonia can't easily escape. Who would have guessed that an added benefit of in-situ composting (which I do routinely with fall leaves) is bed warming?

But if you went to the trouble to plumb the hot pile, and move the heat to the bed, as I suggested, you'd have a lot more temperature control.

Plumbing a hot-pile is a neat idea for lots of purposes, but there is nothing simple about it. We had a thread about it at the soil forum.

Making a hotbed is laborious but very simple in comparison. Simplicity has big advantages.

This morning I had the idea of finding somebody who would let me buy straw to bed their horse in exchange for reserving the manure for me. It would be well worth the cost of the bedding.

Hey, Daninthedirt, from what I understand, these hotbeds have been used for a long time. So I am inclined to think it must work OK. Like you say, the manure is buried under soil. Also, the straw would be soiled horse bedding. So there should be a fair amount of readily available nitrogen in the straw as well as the manure.

--McKenzie

I don't think I said that it wouldn't work. I just said I was surprised to hear about it. The point is that composting competes with plants for soil nitrogen. I'm thinking that this doesn't happen in a hotbed because the stuff that is being composted is at depth. That works. The recipes for hotbeds I've seen start with a deep excavation. Half of that is filled with the manure/straw, and the top is filled with good soil, amended by fully composted material. So what you're actually growing in isn't really what is making the heat.

Manure and straw have a lot of nitrogen, but it certainly isn't "readily available" to plants. You need to convert it to inorganics like ammonium and nitrates to do that, and that's what composting does. That's why you don't grow things in fresh manure and straw.

Just so, a hotbed sequesters the heat-generating material fairly deep (I was putting about 6 inches of soil on top). By the time plant roots get well into it it has cooled off and broken down some. Basically it gives about a 2 month head-start over planting outdoors. Following crops for the next year or two benefit a lot, of course, so it does double-duty. It is because the wood-shavings interfere with the quality of the long-term effect that I stopped making them.

OTOH, perhaps mixing in some char and slow-release minerals while building the bed could ameliorate the negative effect of wood-shavings.

That's very true. A big advantage to hot-bedding is soil fertility the next year. But hot bedding apparently requires a LOT of digging. You're not going to be doing that with a tiller. Get out your backhoe.

As long as we are brainstorming--part of the problem is finding a way to transfer solar heat deeper into the ground faster and storing it during the cold night period- using the ground as storage to try to get a sort of thermal flywheel effect going to get through the cold night. My idea âÂÂonly half hatchedâ is this 1. Hammer sections of copper, aluminum, steel or whatever conductive pipe you have into the ground-exposed part painted black. 2. cover with a thick layer of your favorite mulch. 3. install a clear plastic cover over the pipes you drove into the ground----Variables to be considered are many--- a few are a) the spacing of pipes. b) the ratio of buried to above-ground c) what thickness of mulch is the most beneficial d) would 2 layers of plastic with an airspace between work better e) instead of pipes how about a L shaped piece of metal like an unassembled length of ductwork 8"x12" purchased from Menards or Lowes. They come in 2 L shaped pieces. Bury the long 12" side. Paint the exposed part black and bend it to an angle it to face the sun, f) would insulation buried alongside your test planting area improve heat retention. g) bury 8â side and angle 12â side to sun. The duct comes in many sizes 12âÂÂx 12â ��"4âÂÂx 16â etc.
Benefits: cheap-easy to build and store -----towards spring and planting season, the benefits of the warmer ground and more direct sunlight might combine with this system to gain us a week or two.
Any other insights as to other variables and/or âÂÂconsiderationsâ before I give this a try in the spring would be appreciated.

"But hot bedding apparently requires a LOT of digging."

The way I did it, yes, although I reduced it a lot by using the hay bales, so all I dug up was the six inches of topsoil. If one has loose soil on hand one could just put the hay bales right on the ground, fill with manure, shovel on the loose growing soil, and pop on the cold-frame.

I think you can manage it without a backhoe :)

Well, it seems to me that the trouble with plumbing your bed with heat-conducting metal is that what conducts heat IN efficiently will conduct heat OUT efficiently. The advantage of plumbing with fluid to move heat is that you can turn it off when it's colder outside than inside the bed. But moving fluids around in a bed is a big deal.

I think the idea of hot-bedding is a good one (although it may be physically challenging), and that in the context of this thread, the right solution is to dig stuff in, rather than plug stuff in.

daninthedirt, sorry if I misinterpreted your position.

Also, I am sure you are right. Since there is good soil on top, it doesn't matter if the composting is making nitrogen temporarily unavailable down below the soil layer.

But the point I was trying to make was that the nitrogen in urine is readily available to plants, and I believe that is correct, even if it is not really important to the main discussion here.

I am enjoying this discussion quite a bit.

Best regards,
McKenzie

mckenziek, that's a good point about urine in the manure. So although the manure, as it composts, will use available nitrogen, it comes with quite a bit of its own in the first place. Interestingly, I hear the best nitrogen source from animal waste is actually straw bedding from stalled animals.

I never really thought of doing buried composting, as composting requires aeration, and stuff buried a foot or so underground isn't going to get a lot of aeration. But I guess it just happens more slowly down there.

" the right solution is to dig stuff in, rather than plug stuff in."

Not sure what this means. "stuff" means fresh manure, or aged stuff?

A heavy layer of fresh manure then about a foot of soil on top of the manure. But then you just might need to create hothouse covers if the air is still too cool.

The presumption here is that we're trying to produce heat. composted/humified stuff won't produce heat, will it? I think that's what "aged" manure is. So go fresh stuff.

Yes, a cover is needed for night and cold days.

Hi All, I did some research on combining row covers and hydronic heating systems at the Student Organic Farm at Clemson University. We've also been doing research on heating the water with hot compost piles and storing heat in insulated ponds. You can learn more about our work in the book "The Bio-Integrated Farm."