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Heat Capacity: Its Use in Gardening


Have you ever gone to the beach on a hot summers day, got out of your car and stepped onto flaming hot beach sand? Perhaps from this experience you expected the water to be just as warm, but were surprised to find the water is ice cold? Well, heat capacity can explain why this happens very well.

Heat capacity is the amount of energy needed to raise 1 gram of a substance by 1 °C. The heat capacity of quartz sand (beach sand) is 830 cal/g/°C while liquid water has a heat capacity of 4182 cal/g/°C. This simply means that 1 gram of beach sand requires 830 calories of energy to heat up by 1 °C, but water needs five times more energy than sand to heat up the same mass (1 gram).

Heat capacity, however, is not just limited to the beach, but you can find its use everywhere, specifically in the garden. With heat capacity you can determine the medium that should be placed around your plants to protect the plant from excessive heat and extreme cold. In this post I will explore four specific mediums: mulch, wet soil, grass and water.

Most gardening blogs and YouTube channels advise amateur gardeners to remove the grass from their garden and plant in the now exposed soil. Or they advise the future gardener that they should invest in raised garden beds filled with wood and soil. Perhaps they even advise the new gardeners to place mulch around their plants to protect the plant from the sun (and even pests), but why do these content creators give out these specific advice? Is it something they truly believe in, is it something they have heard elsewhere or is it specifically something they have found works for them?

Dry soil has a heat capacity of 800-1480 cal/g/°C (depending on the moisture). Let’s assume in your garden your soil is exposed and your plants are not so congested (therefore the soil has direct access to sunlight). On a hot day your soil’s heat capacity will fluctuate around 800-1480 cal/g/°C, depending on how wet the soil is. As the day proceeds, not only is all the soil surrounding your plant heating up quickly (due to the low heat capacity), but the water in the soil will eventually start evaporating out from the soil. This leads to the soil’s heat capacity decreasing, making it even easier for the soil to heat up. This can all culminate to the soil getting so hot that it ends up cooking your plants, killing them off. This is the reason why most content creators advise their followers to protect their soil. The most typical solution… mulch (wood).

Wood has a heat capacity of around 1300-2400 cal/g/°C, comfortably ahead of exposed soil. This range exists because the heat capacity is dependent on the wood material and also the water-content of the mulch. Therefore, wetting your mulch on a hot day regularly would protect your soil from heating up too quickly and will keep your plant from cooking. The type of wood you use can also have a positive affect, for example, pockwood timber has a much higher heat capacity than red pine timber. This makes using mulch in summer a perfectly good suggestion for gardeners.

But why not just grow your plant directly among grass? Surely the grass will ensure the surrounding soil (around the plant) does not heat up too quickly (since the grass blocks the soil and even uses the sunlight to grow) and could even potentially protect your plant from pests. Well… most gardeners believe that doing this will make the plant and grass compete for resources (nutrients and water), affecting both the growth of the plant and the grass. This is probably a valid reason in certain cases, but if you think about it, each grass plant that grows is competing with thousands of other grass plants growing around it. Yet, well-kept lawn looks great and does not look like its lacking any resources. Therefore, growing plants among grass, especially plants with deep roots, would probably be a good idea, however is not advised if your grass is already looking sickly.

But what about placing some sort of barrier around your plant and the grass? That way the grass roots do not interfere with the plant roots and you are not facing resource competition. This should be a perfectly acceptable solution for those who cannot afford to keep their lawn in great condition.

Finally, what if we try a pretty insane idea? What about growing your plant in an environment where it is surrounded by liquid water? The water would not be making direct contact with the soil. Since water’s heat capacity is so high, on a hot day it would keep your plant thoroughly cool, not just only because of low heat transfer, but the evaporated water will moisten your plant throughout the day. The water environment would also ensure most pests don’t get access to your plant. Naturally, the water solution would be a terrible idea in winter, especially areas where the temperature dips below 0 °C. This is simply because at low temperatures, the water would be cooling down the plant more rapidly (compared to just soil) and you could risk your plant dying from frost.

In the next few weeks I will be starting my experiment – growing the same plant type in four different environments. One where the soil is constantly kept wet, another where the soil is covered with mulch, another where the plant is growing among grass and lastly, a plant is surrounded by water. I will be investigating if there is any change in the plant’s growth, appearance and how it is affected by pests, if at all. Once my experiment has ended, I will post an update on this website and you can see if heat capacity truly is useful in gardening and if my hypotheses were correct.

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