Fertilizers do not make soil fertile.

What is soil fertility? Many people get confused about this. Fertilizers do not make soil fertile. Fertilizers are like vitamin supplements. You can't make a sick, diseased, malnourished person healthy with vitamins. Fertility is the soils ability to hold the nutrients in fertilizers when applied. When thinking of irrigation we use the image of a sponge. The bigger the sponge, the more water it can hold. That's obvious. The sponge image is good for fertility too. Think of it this way.

Soil particles are like tiny magnets with a positive charge. Nitrogen atoms have a negative charge. Some soil particles have a strong attraction to nutrient molecules like nitrogen and potassium. Others have weaker attraction so they cannot hold on to the molecule for very long. If the root does not take the molecule in fast enough then water will carry the molecule down through the soil profile into the water supply.

The soil particles with the bigger charge + grabs the nitrogen molecule N faster and holds it stronger.

It's like a sponge with tiny little holes will hold more water when you lift it out of the bucket. A sponge that has big holes will let the water flow down into the bucket faster. Finer soil particles like clay will hold water and nutrients longer than courser soils like sand.

In these illustrations water droplets carry nitrogen atoms down through the soil profile. The larger gaps between the soil particles allow the water to flow past the particles and not release the nitrogen atoms N

The smaller soil particles have smaller spaces between them and together have a stronger magnetic charge. They enable the root to take the nitrogen N up to the plant leaves.

Most of the time the location of the soil in the environment is the main factor to soil fertility. We have all heard the term "fertile bottom land". This refers to the process by which erosion by water transports soil, minerals, and other matter from high mountains to low valleys where it collects and is deposited. These areas are good for farming because the soils are fertile. They can hold molecules for a long time. The particles are smaller and have large amounts of minerals washed down from the rocks of the mountains. These minerals add magnetic charge to the soils. In fact, they can hold natural sources of nutrients that have arrived over time so no additional nutrients are needed. This is true in the middle of the United States. The fertility of that land is legendary. Soils in Florida are almost all sand. Sandy soil particles are bigger and have weak magnetic attractions so water and fertilizers will move quickly through the soil profile.

So how do you find out what kind of soil you have in your landscape?

One way is to take soil samples and send it to your local agricultural extension agent. This is a group of people that work with local universities to provide information and services to the public about agricultural issues. They will produce a detailed report about your soils fertility and nutrient levels.

http://en.wikipedia.org/wiki/Soils can provide more detailed information about soils.

Soils lacking in fertility can be improved.

For this discussion we will focus on how to improve soils that are lacking in nutrient holding capacity, and organic matter. It's hard to improve the existing soils in your yard without replacement or major tilling and disruption. But once you get the PH balanced and the moisture levels at optimum levels you can encourage the growth and development of your soils natural microbes, fungi, and microscopic worms. These organisms are the building blocks of healthy soils.

There are many sources to buy organic fertilizers. The trick is to grow your soils own microbial populations. These organisms fall into two basic categories,

Bacteria - Processing, or fixation, is necessary to convert gaseous nitrogen into forms usable by living organisms. Some fixation occurs in lightning strikes, but most fixation is done by free-living or symbiotic bacteria. (From Wikipedia.)

Fungi. - A mycorrhiza (Greek for fungus roots coined by Frank, 1885[1]; typically seen in the plural forms mycorrhizae or mycorrhizas) is a symbiotic (occasionally weakly pathogenic) association between a fungus and the roots of a plant.[2] In a mycorrhizal association the fungus may colonize the roots of a host plant either intracellularly or extracellularly. It is an important part of soil life. (From Wikipedia)

These two organisms are what will help you apply less artificial fertilizers because they will produce natural nutrients from the air. As stated above it's called nitrogen fixation (wikipedia link). Certain bacteria can take nitrogen from the air that is not available to plants, and change its from so it can be taken up from the soil in plant roots. Certain fungi attach to plant roots and create a symbiotic relationship where the fungi help in the uptake of nutrients from the soil and receive a little food from the plant to survive. And as another bonus, these same microbial colonies help break down the old dead and dying grass leaves called thatch. This is significant because faster decomposition of thatch also increases the ability to leave clippings on the ground naturally. The process of getting nitrogen from the air down to plant roots in an available form is complex and fascinating.