When it hits the soil surface, tiny air pockets within it warms and expands. When they expand, some of the air gets pushed out of the soil. Once the soil cools again, air is drawn back inside. The cycle plays out daily and seasonally.
Soil air content is a very important factor in plant health. This is because oxygen drives cellular respiration in plants. Respiration is where plants turn energy made through photosynthesis, and stored in its body as carbohydrates, into energy for growth.
Even the roots respire and without oxygen, they start to produce less energy and eventually die. Without oxygen, roots cannot take up nutrients from the soil either.
Moderate sunlight can be beneficial for soil because its warmth encourages beneficial soil microorganisms to be more active.
However, too much sunlight on uncovered soil will make it dry out faster, kill microorganisms, and remove essential air pockets.
Darker rich in organic content (like compost) warm absorb much more warmth from the sun than the lighter, sandier soil you might use with desert succulents.
Remember that you should always store bags of potting mixture in a cool, dry, and sheltered place. If its exposed to direct sunlight, the bag can heat up and encourage pathogens like Legionella bacteria to grow.
Plants with large foliage create a sun-screen that prevents light from hitting the soil directly. This helps keep soil at a consistent temperature, meaning that it is much better at retaining its air, water, and oxygen content.
Farmers replicate this effect by mulching their fields. You might have seen rows of vegetable crops with a layer of straw covering bare soil inbetween the plants. This is to prevent temperature fluctuations related to sunlight. The straw (mulch) protects the soil from cooling in the winter and warming up too much in the summer.
Well-aerated soils make for a healthy plant. However, the presence of many air pockets can present problems in places where there are many environmental changes. More oxygenated soils are much quicker to heat up on warm days and cool down on cold days.
These temperature swings can make it difficult for plants. Low soil temperature results in reduced tissue nutrient concentrations while overheating can lead to fewer air pockets and therefore reduced plant respiration.
Ideally, you want your soil to be at a consistent, moderate temperature - not too warm, not too cold.
One of the best ways to achieve this is by using soils that are both well aerated andstocked with beneficial microbes.
A soil that supports a dense community of microorganisms are more resistant to overheating and they are much able to keep the soil temperature at a steady level, despite environmental changes. In the colder seasons where there is less sunlight, microbial activity keeps the soil at a more constant, warmer temperature.
To cultivate a soil rich in beneficial micro-organisms, you can add in organic biostimulants when you prepare your potting mix.
Apart from regulating soil temperature, soils with diverse microbial communities will increase your plant's tolerance to stresses and pathogens.
In conclusion, we can say that soil does not needsunlight to support plant growth. However, sunlight can affect soil properties in ways that make it easier or more difficult for plant roots to respire and take up nutrients. The best way of helping your plant along is to prepare your potting mixes to be well-aerated and full of diverse micro-organisms.