From Chapter 8 of "Healthy Garden Healthy You" by Milo Shammas
To the naked eye, soils look pretty much the same except sandy desert soils or tropical rain forest soils. Yet a gardener needs to know the soil's unseen composition. Correct analysis of the soil, and correcting any defects you find, is fundamental to your success in growing, whether you have a fruit and vegetable garden or plants of any other type. The kind and quality of your soil determines which plants you can grow.
Soil consists of mineral particles, organic matter, air and water of different kinds and in a variety of proportions. The nature of your soil depends on the nature of the underlying rock. If you live in a river valley, particles there have been ground down to form silt or clay. In other areas that have only a thin covering of soil, the land may well be rocky or sandy. These underlying conditions also determine how acid or alkaline the soil is. The acid/alkaline balance (also called "pH") is another important factor in determining what you can grow in your soil.
You can improve your soil structure with a little work. We know that roughly 90 percent of soil consists of non-living solid mineral material. Of that 90 percent, half is air space known as pores. Organic matter makes up the other 10 percent of substance in soil. Some of the living matter will move freely, grow and expand, such as organisms and roots. However, the majority of the visible biomass will be plant roots. Even though living matter makes up 10 percent of soil mass, it is responsible for far more than 10 percent of plant health. Nature can cope well with poor soil conditions, ensuring that only suitable plants will survive to propagate themselves. But the gardener wants a far wider scope. To grow a larger range of plants than nature will allow in your locale, you must improve the soil in various ways.
As an aside, hunter-gatherers lived off what they found and had to adapt to it. What we might call "agricultural man" or "gardening man" takes what he finds and adapts it to fit his needs and desires. Both the organic gardener and the corporate farmer take steps to modify what they find in the ground. Two key differences between them are how far they go to modify or interfere with the natural processes in their environment and the consequences they are willing to accept.
You will find a spectrum of different textures in natural soil. Depending on the type of soil you have, adding compost and/or organic fertilizer may not be enough to get a beautiful tilth. You may need to add other substances such as grit to get the desired texture. Experiment to find the best way to achieve a super fertile soil.
Plants will not thrive unless they have enough oxygen for their roots. Heavy clay soil has such small particles that very little air penetrates it. With clay soils the gardener must create a more porous texture in the soil, normally by adding lots of organic matter and possibly some grit. Another way to improve heavy clay soil is to add gypsum. If the soil is waterlogged and fails to drain well, you may also consider some form of artificial drainage.
Light sandy soil, by contrast, drains too freely and retains very little moisture. In periods of drought, plants in sandy soil will suffer and possibly die. Again, the answer is to add plenty of bulky organic matter to help bind the soil particles together and hold onto precious water.
To learn what your soil consists of, take a lump in your hand and crumble it between your fingers. If the soil is very sandy, you will hear the sound of grains rubbing together and feel them between your fingers. A less sandy soil, often found in areas surrounding a riverbed, is silt, which has a soapy feel. Clay is very heavy and sticky, with a glaze on its surface that causes it almost to shine. You will find one of these basic soil types in your garden.
To succeed at gardening you must create a loamy soil, a rich mixture of soil that feels light and friable to the touch and has a pleasant earthy smell. Loamy soil encourages earthworm activity and is well aerated. Because it is neither too dense nor too crumbly, this soil type holds moisture to the right degree with plenty of oxygen for microbes, mycorrhizae and, most importantly, healthy plants.
Clay has the finest particles of minerals and the least amount of air in its structure. Unless you work a lot of grit and organic matter into it, it will be hard to grow a good range of plants.
Silt lies somewhere between sand and clay in particle size. Provided it contains lots of organic matter, silt makes good garden soil. Silt has a silky feel and is often found in river valleys.
Sand has the coarsest mineral particles and, although it is well aerated, water runs through it very easily. Sand needs lots of organic matter added to bind the particles and improve its moisture retaining properties.
Pores are spaces between soil particles where water and air can flow. In the best conditions, about half the volume of soil consists of pores. The water and air will fill these gaps and attach to the particles. Pore space is crucial to good tilth. Without them, any type of fertilizer will not be able to get into the soil to feed the plants or even the microbes. Also, pores enable moisture to make its way up from beneath the roots through capillary action.
Loam is what all gardeners aim for. This soil structure has a good combination of organic matter with the basic mineral particles, whether sand, silt or clay. The structure is achieved by generous and regular applications of composted material, which helps create valuable pore space for roots to travel through and accumulate the necessary nutrients for their growth.
How well a soil holds together can be influenced by the action of microbes that digest organic materials in the soil. Their action helps stabilize the soil by physically binding soil particles together. Microbes also release a by-product called glomalin that acts like a glue to help bind mineral particles and organic materials together. This greatly contributes to building better soil structure by creating semi-stable soil aggregates.
Plant health largely depends on the amount of water and oxygen that can enter the soil. Tightly compacted soils do not let plants breathe and allow minimal amounts of water to penetrate deep into the root zone, inhibiting root growth. To get a friable soil with good tilth, two things that will always help are amending it with organic matter and cultivating the soil as often as possible.
Milo Lou Shammas
Founder and Formulator