The parts of the soil that are produced by the erosion of bedrock give the soil its basic characteristics such as its texture and pH but the gardener cannot easily change these properties.
However, gardeners can influence the amount of air, water and organic matter that go to form the soil's structure. If water and air filled pores are too small then the soil becomes waterlogged in winter and solid in summer. If the pores are too big then water drains away too quickly and the soil dries. Dry soil is damaged by wind erosion of small, light particles.
As roots need oxygen to respire and produce energy to grow, waterlogging will be detrimental to root extension and may even result in root death. Water is a vital constituent for photosynthesis and needs to be provided in sufficient quantities.
The rapid drainage of water from the soil will cause leaching that the production of nutrients from decomposition cannot keep pace with. It may also be difficult for the addition of man made fertilisers to replace lost nutrients. Small, light soil particles can relatively easily be removed in the mass flow of water. These are the clay and organic matter particles that have charged surfaces and contribute to the retention of nutrients in the soil.
Surface runoff, particularly from hard surfaces, will also exacerbate the loss of clay and organic matter from the top soil. Where I have trapped surface run off, and along the path where puddles form, you can see a fine layer of soil particles covering the surface when water has evaporated.
To prevent this from happening we need to regulate the size and shape of the soil water filled and air filled pores. A more varied range of sizes means that both water and air can be trapped effectively providing these essential resources for soil life.
We need to stick the soil particles together. Soil glue is produced from a large number of different sources. Decomposition of vegetable and animal organic matter produces a sticky gel, which is quite difficult to remove from the hands and leaves a stain for several washings.
In order to obtain sustenance from organic matter both bacteria and fungi secrete enzymes into the soil matrix. Not only are these enzymes sticky themselves but they form sticky substances when they break down organic matter. These enzymes can stick to both clay and organic matter and continue to function for some time. Both bacteria and fungi also secrete sticky mucilage as a protection from predators.
The larger soil fauna, like nematodes, earthworms, slugs and snails secrete sticky mucus which is quite difficult to wash off the hands. I know cus I've been slimed. I always find it remarkable that, even though these animals produce a sticky mucus, they are remarkably clean animals.
The excreta of most animals contain sticky mucus type compounds and these are left on the surface of the soil if not within it.
Plant roots secrete sticky gels and organic compounds. This can be seen to most dramatic effect under tussocks of grass. There always seems to be a beautifully structured, friable top soil enmeshed within the adventitious roots. This soil structure can be quite different from the surrounding soil and be full of worms and other small animals.
After the complete decomposition process, the final product is humus. This is made up of organic compounds that are very difficult for the bacteria and fungi to break down. It is sticky and forms a thin layer over soil particles.
All of these sticky compounds will aid the production of a wide range of different sized pores by sticking soil particles together. With this variety of different sized pores both water and air can be retained in the soil. Small and light particles can be trapped and the mass flow of water slowed. Reducing the mass flow allows the water to soak throughout the soil profile; nutrients to be taken up by plant roots rather than leached and allows nutrient production by decomposition to keep up with nutrient loss.
Compost and manure contains lots of these sticky compounds together with the organisms that produce them. Why would you not add them to the soil?
Man made chemical fertilisers do not contain these sticky compounds or the soil organisms that make most of them, so they do not help to produce a good soil structure.