There are basically 2 types of fertilizers natural organic fertilizer and chemical or synthetic fertilizer. Organic fertilizers are produced by nature when organic substances such as plant matter, manure and so forth break down they become the food source for new plant life. They gradually accumulate and force nutrients into the soil which improves its vitality and its value to plant life in the process.
Synthetic Chemical Fertilizers are actually harmful to the soil and eco-system over extended periods and there is also recent evidence that indicates some may not actually do what they were intended to. One study conducted by the University of Illinois has demonstrated that synthetic nitrogen reduces soils organic matter content.
"As organic matter dissipates, soils ability to store organic nitrogen declines. A large amount of nitrogen then leaches away, fouling ground water in the form of nitrates, and entering the atmosphere as nitrous oxide (N2O), a greenhouse gas with some 300 times the heat-trapping power of carbon dioxide. In turn, with its ability to store organic nitrogen compromised, only one thing can help heavily fertilized farmland keep cranking out monster yields: more additions of synthetic Nitrogen." So basically once you have used synthetic fertilizer an addiction occurs calling for more and more chemicals to obtain satisfactory results.
Chemical Fertilizers generally do not contain micro nutrients and trace minerals. After only several seasons the soil is depleted from those essential elements. Over extended periods those elements disappear from the produce we harvest further diminishing their quality and nutritional value.
To raise or lower the pH level in the soil either Limestone or Sulfur is utilized. There are other materials that will do the job also, but the aforementioned are the most widely used.
In addition to facilitating an addiction to chemicals and diminishing the quality of the harvest, the use of chemical fertilizers has a number of unwanted effects on the environment.
Groundwater contamination - Nitorgen fertilizer breaks down into water soluble nitrates that saturate the soil. They reach the water tables and remain in groundwater for decades having a negative cumulative effect. Nitrogen groundwater contamination in shore and lakeside areas contributes to marine "dead zones" and fish kills.
When coupled with pesticides chemical fertilizers are proven to have altered immune system and nervous system functions in mammals. Air and water-borne nitrogen can cause respiratory ailments, heart problems, and some cancers.
As organic fertilizers break down, they improve soil structure enhance its water retention ability and its capacity to retain nutrients.
As the plants we are growing mature, they use up the mineral content within the soil. In an uncultivated wild setting as these plants die and decay the minerals are put back into the soil. In a garden or farm setting, or even on our lawns, as we harvest these plants we are taking the mineral elements away from the soil along with the plants we harvest. We are slowly depleting the soil.
To maintain a fertile soil the minerals we are depleting need to be maintained. Small amounts will be replaced by natural processes, but not enough to maintain a viable fertile soil. 13 minerals as well as trace elements are required for adequate plant nutrition.
Adequate plant nutrition facilitates plant growth and health and enhances the plants resistance to insect damage and disease. It is essential in providing a suitable crop.
13 minerals needed for plant nutrition are Nitrogen (N), Phosphorus (P), Potassium (K), Sulfur (S), Calcium (Ca), Magnesium (Mg), Zinc (Zn), Iron (Fe), Copper (Cu), Manganese (Mn), Boron (B), Molybdenum (Mo), Chlorine (Cl).
Although not always as cost effective in the short term when purchased as a packaged product, making your own home made organic fertilizers is a snap.
Compost is one method that helps, but does not 100% cure depleted soils. Compost is decaying organic matter on its way to becoming fertilizer, it provides the essential organic material needed for optimal soil structure. It also adds microbes to the soil, microbial activity is needed to make the mineral nutrients available for uptake by the plants. It does not provide much in the way of minerals however.
Make Your Own Organic Fertilizers
Use finished compost. Unfinished and unaged compost will usually contain harmful bacteria and pathogens and the nutrients are not as readily released in a soluble form.
Fill a suitable container 1/3 full of well rotted compost.
Add boiling water to the compost, leaving a few inches of head space. The boiled water serves to kill off most unwanted pathogenic contaminants.
Let the mixture steep for slightly less than a week, stirring it at least once daily but preferably more.
You can either strain the mixture before using it or pour the liquid into another container accepting that some residue will go along with it. Dilute the mixture at about a 3 to 1 ratio, [3 parts water to 1 part compost tea] and Pour it around the base of your plants.
Remaining residue can either be thrown back into the compost pile, worked into the soil or placed around plants in the same fashion as mulch. Nutrients remaining will seep into the ground adding further benefits to your garden.
Specific elements can be added to the garden on a case by case basis
There is no reliable homemade method to test nitrogen content of soil. You can either have your soil tested or buy a soil testing kit. If you are sure you have a nitrogen deficiency there are a number of ways to add nitrogen.
Chilean Nitrate aka Natural Nitrate is a quick fix in some cases, it is a mined product from a Chilean desert. It is the only known deposit of this mineral salt in the world. It is used as a form of nitrogen for plants in cold soils. It is basically sodium nitrate and its chemical signature is identical to synthetic nitrogen. Even though it is a naturally occurring substance some source claim it is not suitable for organic gardens.
Plant and animal By-Products although slower acting are more suited to organic gardening. Manure from herbivores - grass eating animals contains an abundant amount of nitrogen and organic matter. Used in excess it can easily 'burn' plants.
Fish Emulsion is a faster acting nitrogen source as opposed to manures and can also be used as a foliar spray, directly on the plants instead soil drenches.
Soybean Meal is not a quick fix but releases nitrogen slowly and uniformly. It is more commonly used on lawns but is suitable for vegetable gardens as well.
Alfalfa Meal has been proven to enhance microbial activity. It breaks down quickly but generates heat which can damage root systems so should not be added directly to planting holes. It also adds phosphorous and potassium to the soil.
Legumes and beans are a long term strategy for adding nitrogen to the soil. They draw nitrogen from the atmosphere and fix it into the soil. After harvesting these crops it is best to plow them into the ground or grind them into your compost allowing the absorbed nitrogen to be released into the soil as the plants decay.
Phosphorous is available organically from Bone Meal, rock phosphate.
Potassium sources include kelp meal, organic potash, citrus rinds, wood ash.
Sulfur is frequently used to adjust the soil pH and is available in many sulfates.
Calcium - Eggshells contain a large amount of calcium, as well as a small amount of nitrogen.
Zinc helps plants produce chlorophyll. Manure and or compost is generally sufficient to remedy a zinc deficiency.
Boron can be supplied with phosphate rock or seaweed. Liquid seaweed spray applied as a foliar spray will result in almost immediate results.
Magnesium - Epsom Salts is comprised of magnesium ,sulfur and oxygen and in gardening it is used to correct a magnesium or sulfur deficiency.