Soil scientists classify soil into 13 different orders based on the presence or absence of diagnostic horizons and major differences in soil forming factors or properties. Soil is also further classified based on texture. The texture of a soil is due to the presence of clay, silt and loam.
Sand is predominantly made of quartz; the sand particles have size range in between 0.05 to 2 mm. The particles are large enough to feel the individual grains. Silt particles have size range in between 0.05 to 0.002 mm; the particles are broken down enough and thus individual particles cannot be recognized; it has a texture like a flour. Clay particles are smaller than 2 nm (nano-meter) and clay can be molded into any shape.
Soil is an eco-system that is alive. Not only trees need soil, but soil is home to insects, fungi, bacteria, nematodes and protozoa on which depends the survival of many small mammals, birds and other bigger insects. Thus, if the soil is destroyed, not only an ecosystem that will die but also many birds, mammals and arthropods.
Soil usually refers to the topsoil or the "dirt" under our feet. Resource Conservation Glossary define it as "the unconsolidated mineral and organic material on the immediate surface of the earth that serves as a natural medium for the growth of plants." Simonson in 1957 defined it as "the link between the rock core of the earth and the living things on its surface."
This topsoil or soil has developed over millions and billions of years. It can never be recovered or replaced if destroyed. Soil has formed over millions of years by the action of organisms and climatic forces, modified by the topography, upon the parent material over time. Thus, rocks and boulders are weathered into smaller particles (which will eventually become soil) through physical (wind, air, water) and chemical (due to heat and pressure inside volcanoes, in the deep core of the earth) processes. These weathered particles, organic matters and living organisms are decomposed into inorganic materials like minerals and chemical elements make up the soil. Thus, topsoil sold by garden stores are not soil; they are just some compost. Once the topsoil is destroyed, it is gone for ever because, as you see, to create it, we have to wait at least million years.
Soil is made up of minerals, organic matters, water and dissolved salt and air (nitrogen, oxygen, carbon-dioxide and water vapor). About 50% of soil is made up of solid components (minerals and organic matters); the other 50% is made up of pore-spaces which are actually occupied by either air or water. Thus, for plants, soil acts as bank of nutrients, water reservoir, in which plants anchor their roots; soil also helps in the diffusion of gas (exchange of carbon-dioxide and oxygen between soil and air) and plants can breathe. Soil is also part of the earth's hydrologic cycle. Rainwater percolates through the topsoil into the groundwater reservoir. Trees draw rainwater from soil and gives out water through transpiration into the atmosphere (which again comes down as rain). Water also gets evaporated from soil into atmosphere which will eventually again come down to soil as rainwater.
Now for us gardeners: a good soil is characterized by the soil texture, presence of minerals, organic matters, porosity, fertility, productivity, microbial activity, water retention and water infiltration abilities. Good soil texture allows for the roots of the plants to move easily, spread out and anchor. Microbial activity (which is responsible for decomposing the organic matter into plant nutrients, compost), soil fertility and productivity, presence of minerals and organic matters are responsible for the plants to thrive. Porosity is important for the diffusion of gases to take place through which plants breathe. Plants die or become extremely weak if soil is so compacted or of poor quality that it has no pore spaces and plants cannot breathe. Pore spaces also ensure that roots can move freely and anchor properly. Plants need water and thus soil that can retain water will be good for plants. Sandy soils have large pore spaces and water move freely through it and gets drained. Thus, plants growing on sandy soils will not have any water available. At the same time, water infiltration (that is how fast or how slowly water drains out and spreads out) is also important. If the soil is so heavily compacted that water cannot drain easily, then the roots of the plants will be sitting in water (and we gardeners know what a deadly situation that is for most plants); roots will rot or will be very poorly developed; a healthy plant is one whose roots are spread out and deeply anchored. Thus, water in the soil needs to be filtered out at a proper rate for prevention of root disease and deeper roots to be formed.
Good qualities of soil can be achieved through the addition of compost (organic matter) and humus (organic molecules which cannot be broken down any further). Such addition results in good porosity, water retention and infiltration and microbial activities. The addition also helps in maintaining the 10:1 carbon nitrogen ratio which is optimal for good plant growth. A good soil should have about 50 to 60% carbon, 5% nitrogen, 0.6 - 1.2% phosphorous and 0.5% sulfur. The 10:1 ratio of Carbon to Nitrogen can also be obtained by growing legumes and to some extent young grasses. However, addition of pine needles, leaves, straw, hays, manure, corn-stalks and woody materials disrupts that ratio; for example, addition of leaves results in a ratio of 60--100:1 (that is more carbon is present and less nitrogen). However, as leaves decompose, the ratio slowly becomes 10:1. But before the balanced ratio is achieved, plants have to compete with other soil micro-organisms to harvest the available nitrogen and such competitions are not good for plants.
Those of us who have clayey soil should consider ourselves lucky because clayey soils have high Cation Exchange Capacity, that is the ability of soil to retain nutrients. However, clayey soil have poor water infiltration quality. It tends to retain water and water spreads out through the soil very very slowly. Thus, if you water the soil, that water will be available to the roots down below the soil after much much long time because it takes a LONG time for water to percolate through clayey soil. Thus, there is a high chance that the water will evaporate from the top of the soil before it even gets infiltrated down into the soil. Thus, people with clayey soil should mulch a lot so that the water is not lost through evaporation.
Essential plant nutrients are supplied by soil and soil minerals. Nitrogen, Phophorous and Potassium are the macro-nutrients; calcium, magnesium, sulfur, boron, copper, iron, manganese, zinc are the micro-nutrients. Besides macro-nutrients, plants also need air, water, temperature and light. These and the macro-nutrients are considered the limiting growth factors for plants. If one of the limiting growth factor is deficient, it needs to be fixed for the plant to survive. Deficiency of the micro-nutrients will not affect the plants much.
An optimal balance of nutrients and other factors (as mentioned above) are required for healthy plants. Over-surplus of nutrients is as BAD as deficiency. Over-surplus creates toxicity for plants. So, how do you do know what your soil has? Do Soil-testing. Soil scientists say that it is an absolute must especially if wants to have good production.
In summary, prevent compaction of soil so that pore spaces between soil does not decrease. Small pore spaces result in water retention and limited gas diffusion. Thus, do not walk on the soil on which you are growing your fruits, vegetables and flowers. Avoid soil erosion; topsoil lost is gone forever and it is the soil that is responsible for human and other animals existence. Do soil testing to determine the quality of your soil and take steps accordingly. Do not add fertilizers just because the garden center says so. Add LOTS and LOTS of organic matter into your soil - they are key to everything good for soil.
Our composting places. I also have another small bucket for compost. And, I also compost by digging in all the compostable material into the garden soil directly. Now, this might not be a good ideas as such material takes up nitrogen from soil as they decompose but I am too lazy to do anything.