Soils are classified in 2 ways: by their texture and structure. Texture refers to the size of the mineral particles derived from decomposed rock. Structure refers to the aggregation of the decomposed rock, in other words, how it's all held together—the crumbs of dirt.
This decomposed rock is all held together by organic matter.
Fine-textured soils are either clay or silt, both of which have tiny particles. By virtue of their small size, they pack together tightly, leaving small pore spaces that easily absorb and retain water with little room left for air.
Soils with flatter clay particles pack solid, resulting in slow water penetration. With coarse-textured sandy soils, water penetrates immediately and drains quickly as well. Because of the speed with which it drains, it doesn't leave many dissolved nutrients behind and they are instead pulled away.
Few soils can be classified as just fine or coarse-texture, but are made up of a variety. The best soils are good combinations of these fine and coarse textures. This is called loam.
Loam is a mix of pore sizes. It allows for penetration of water, but also allows it to drain, but not completely. A loam mix will hold onto the moisture and retain some of the dissolved nutrients. This is excellent for plant roots to thrive.
The natural activity of decomposing organisms helps cement soil particles together into crumbs that characterize the soil's structure. This decomposing helps create a soil with both small pores to hold water and larger pores to allow air to penetrate.
A good healthy soil, with a good structure that is capable of nurturing healthy plants may contain as much as 50% organic matter.
Soil problems arise when this ideal balance becomes out-of-balance. This can happen for various reasons. Most of which involve the loss of microbe activity.
Too much water (places that regularly have standing water) kills off the micro-organisms that live in the ground. The decomposition process stops. The pores of the soil collapse and in short order, the soil becomes compacted, meaning the soil's pores have closed up.
Too much traffic (foot traffic for example) compacts the soil so that it squeezes the soil's particles together. This lose of pore size reduces the soil's ability to drain properly, hold on to moisture, and to allow air to circulate within the soil's structure.
Contamination also can kill off the micro-organisms. A chemical spill or the dumping of other contaminants into the soil can kill off the microbes having the same results almost as the too much traffic already mentioned.
Healthy plants grow in healthy soils. Plant problems (disease, weeds, insect infestations) can often can be attributed to poor soils. Most healthy plants can fend for themselves in the world if given a fighting chance by providing a healthy environment for them to grow. Take away that healthy environment and they become weakened and these opportunistic pests can over take them.
Therefore you want to make sure your soil is always improving. You want to make it a haven for microbes. Microbe activity is the key indicator.
The best way to keep the microbes happy is to give them plenty to eat in the form of organic matter. You obviously don't want to drown them out, or kill them with chemical contaminations.
If standing water is a problem, take steps to improve drainage. Don't block natural drainage pathways by creating a landscape that traps water in localized spots. The landscape should have natural paths for water to follow without pooling. Water always moves downhill—just don't block that movement by putting in a raised flower bed across its natural path.
Next, avoid using insecticides in the lawn and landscape. Many insecticides kill everything it comes in contact with. Be absolutely certain of what you are doing before spraying any insecticide as the results can be far reaching and quite devastating to the health of your entire soil causing many more problems for you than a few insects could ever do.
Incorporating organic amendments to the soil is really a good thing. Whenever you can, add organic matter. For your lawn, this could be leaving the lawn clippings on the lawn instead of bagging them. For the garden, add compost to the soil in the fall or early spring.
Healthy soils require less fertilization and less watering. Healthy soils mean you'll have healthier plants, better looking plants and microbes with big smiles.