Ideal soil has a fairly fixed composition. A good soil for landscaping is made up of almost 25 percent water and 25 percent air. Minerals, organic matter, and animal life make up the rest. Air and water are essential resources in the soil for growing healthy plants. You can’t do much about the mineral content but you can make a big difference by your own behavior. You can add the organic matter and you can add the microorganisms. The microbes eat the organic matter so they can make the spaces for the air and the water. The organic matter is the kick-start for making a lousy soil into a good soil.
For the Techies - Content of Good Soil
A healthy, well-structured soil should have the following components. These estimates are for 100 cubic feet of soil.
Minerals. 3,900 pounds; ideally about 45% of the volume
Organic Matter. 250 pounds, about 5% of the volume
Live Organisms. 4 to 6 pounds
Water. 844 pounds, about 20 to 25% of the volume
Air. 20 to 25 %by volume
The gross weight for this soil is 5,000 pounds.
Critters in the Soil
There is more life concentrated in the three inches below the soil surface (assuming a "healthy" soil) than anywhere in the world above the soil! The macroorganisms (bigger guys) like earthworms, springtails and mites move through the air spaces in soil while the microorganisms (very little guys) like bacteria, nematodes, and fungi live in the water film.
These microbes perform an enormous amount of work for you and your landscape. They break down minerals from the rock particles to make them available to plants. Along with the macroorganisms, these little guys recycle through decomposition enormous amounts of organic matter, again producing valuable nutrients for plants. Soil microlife also prevent lots of problems by attacking potentially problematic fungal spores and dangerous bacteria and viruses.
The more active the microbiotic life in your soil, the more likely that you will be able to reduce your use of fertilizers and pesticides, saving you time and money. There will then be reduced risk of problems from inadvertent excessive use of fertilizers and pesticides. If you do have to resort to use of pesticides, a healthy population of soil microorganisms will help break down those chemicals more quickly before they cause long term problems.
The bigger creatures in the soil, the macroorganisms, make important contributions as well. Creatures like nematodes that are less than a millimeter in length and the comparatively giant earthworms that are 3 to 5 inches long have jobs to do. Also included in this crew are a number of different insects including millipedes, sowbugs, springtails, ants and mites just to name a few. Under favorable conditions, there can be almost 2 million macroorganisms in 1000 square feet of soil around plant roots in your yard. Virtually all these insects will be killed by an application of any broad spectrum insecticide on your lawn.
Lots of earthworms are desirable because they aerate the soil, improve soil drainage, aid decomposition of organic matter, and provide fertilizer for plants. Don't worry if you have not seen an earthworm in years. As soon as you aerate and add some organic material and make sure the pH of the soil is adjusted, you'll start seeing earthworms. Earthworm egg casings can lay dormant as deep as 12 inches in the soil for as long as years. All they need is some encouragement to hatch and thrive. They are everywhere, everywhere!!! However, one application of a granular quick-acting nitrogen fertilizer over your lawn will chase these good guys away for at least a month., sometimes as many as four.
As for the other macropals, they contribute to healthy soil too. The mites and springtails promote decomposition of organic material, producing nutrients for plants. The tiny nematodes produce nutrients for plants, control pathogens in soil, and even control pest insects in larval and egg stage.
Organic Material Is Food
Organic materials provide much of the source matter for this microscopic operating system. The higher the percentage of organic matter you have in your soil (more than 4% is desireable), the more complete will be the mix of nutrients these microscopic creatures are able to produce for your plants. This is one reason why the addition of organic matter to soil is so beneficial to the lawn, trees, shrubs, and beds. Without a proper environment to host such beneficial microbes, plants do not thrive, because reduced microbial populations produce fewer nutrients for the plants, shrubs and trees.
Soil has a structure
IdeaI soil has lots of empty space, or empty microscopic compartments, created by that healthy microbe population. The more easily water carrying oxygen soaks into the soil, the better off the turfgrass, the trees, shrubs and plants are. It is the existence of those spaces that make it possible for the air and water to move through and feed the microbes and roots. Microbes make those spaces if they have the organic matter they need to fuel these activities. If there is little organic matter, then the small population of microbes can continue to make spaces in the soil.
If there is no space, because the soil has been tamped down and compressed from some kind of weight, that is what we called “compacted” soil. If plants appear to be stunted, the cause is almost always hard, or compacted soil. Compaction is caused by pressure from equipment, frequent foot traffic, or even from the weight of a single footstep. Remember, we walk over every square foot of our lawn 25 to 30 times a year just to keep it mowed. Compressed soil interferes with the drainage of water away from the soil surface to a depth where it is available to plant roots. The resulting shortage of water and oxygen also means that organic matter cannot decompose properly. No matter how much organic material is piled on the surface, it will not benefit the plant if the top foot or more of soil is compacted. As if that were not harmful enough, compaction discourages the good work done by earthworms.
Soil offers nutrition
Plants do not get their food directly from the soil. They make their own food (protein and carbohydrate) using air, water, sunlight, and certain critical ingredients from the soil such as nitrogen, phosphorus, and many other compounds and elements that are called plant nutrients. There are several essential nutrients: nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur are needed in large quantities. Minor ones such as cobalt or sodium are required in much smaller doses.
All these nutrients are important; however, what is equally important is that they are available to the plant in certain balanced amounts--not too much and not too little. What this means is that we need to figure out how to create soil conditions in which soil microorganisms will release the balance of nutrients that each plant requires. If you add organic material to your soil each year and you take steps to reduce and minimize compaction, the system of microbes will usually handle the nutritional task with no more assistance needed from you.
Explanation for the Techies - In a fertile, well-kept soil containing at least 5 percent organic matter and having a near neutral pH (6.2 to 6.5), natural biological processes can provide as much as 3 pounds of actual nitrogen for every 1,000 square feet of yard and garden area each season. This is equivalent to a 50 pound bag of 6-10-4 commercial fertilizer. About half of this annual nitrogen supply comes from decomposition of organic matter, such as manure, plant residues, or compost. The other half is captured from the air by nitrogen-fixing organisms.
The following files deal with fixing and managing soil in the home landscape:
Fix Soil Under Turfgrass
Soil Under Trees & Shrubs
Soil in Flower Beds