Organic fertilizers are basically fertilizers made out of something that was once alive. They are decomposed living matter and will decompose more and release their nutrients in the soil. Non-organic fertilizers, those that are man made are chemical compounds are carried into the soil by salt. When the salt dissolves in water, the nutrients are made available to the plant. Chemical fertilizers, as well as most organic fertilizers are rated by their N-P-K rates. That is how much N=Nitrogen, P=Phosphorus and K=Potassium is contained in the bag. Home compost from vegetative matter usually has fairly low rates of all of these elements, which is why you can add a lot more of it to your soil than you do any other fertilizer. In general, home compost is usually considered more of a “soil amendment” than a fertilizer when applied in a small amount. Remember, any bag of fertilizer you buy will tell you how much you need to spread over a specific area to get the proper NPK value listed on the label. Nitrogen helps plant growth – plants and soil need just enough and not too much. Phosphorus provides the means for growth and flower bloom and potassium helps plants make fruit and also to stave off disease.
Fish Fertilizer: is dry, and this is my favorite additive to dig into the soil along with a lot of home compost. Each type of fish fertilizer has varying degrees of each element, however in general it has 5-7 parts of nitrogen, and slightly higher rates of phosphorus and potassium. It is considered a gentle, complete fertilizer and is great for vegetables.
Liquid Fish Emulsion: is diluted with water and is either poured directly in the soil or can be sprayed onto the plant. It is also considered a safe and gentle (although slightly smelly!!) fertilizer that has higher rates of nitrogen than it does phosphorus and potassium – it is an excellent fertilizer for perennials and also good for regular fertilization of vegetables.
Manures: are great sources of nitrogen and are also “compost” – meaning they add bulk and break down into the soil providing both essential nutrients and amending the soil. Manure is generally fairly high in nitrogen, especially bird manures and must be applied with a spare hand. Store purchased, bagged manures are generally fairly limited in nutrients. For example, bagged cow manure purchased from a store is usually .2 .2 .2 – that is less than one measurement of each element – similar to home compost. Composted chicken manure has a ratio more in the range 4-5 rates of nitrogen, 2-3 phosphorus of and 3-4 of potassium. Manures are best when composted which makes them release their nutrients more slowly and gently and are dug into the soil.
Seed Meals: are available in bulk and are partly nutritious and also partly good for amending soils. They tend to be quite high in nitrogen and break down very quickly so they can be dug sparingly into the soil for a fast feed. Some plants that love nitrogen such as tomatoes, brugmansia and roses seem to prefer alfalfa meal. Depending on where you live, you may find cottonseed meal, alfalfa meal, soymeal, corn gluten meal and more. Seed meals generally are 5-7 measures of nitrogen, a much smaller ratio of phosphorus in 1-3 parts and a slightly higher amount of potassium maybe 3-4 parts.
Some types of organic fertilizers are considered to contain single elements or mostly single elements. These are particularly effective if you have had a soil test and it shows a deficiency of a particular element.
Bone Meal and soft rock phosphate: are excellent sources of phosphorus and also micronutriennts (which are not measured in the NPK amounts but often appear as percentages on the back label – and each has a different set of micronutrients to offer.). If you want bloom, phosphorus is what you need!
Blood Meal and nitrate of soda: is very high in nitrogen – well above 10 parts. It needs to be spread VERY sparingly, but if you have nitrogen robbed soil, this might be what you need.
Greensand and wood ashes, or sulfate of potassium: contain potassium. These are rarely added to soils, and most specifically when a soil test calls for them because they contain high rates of potassium usually between 5-10 parts.
I recommend you get a soil test for your garden, Most garden centers will have soil test kits, however your local Extension office will probably be able to provide you with a much more accurate test provided you follow directions carefully. If you seek a detailed soil analysis, there are a few labs which can provide you with the services you need (in the USA) The National Sustainable Agriculture Service has a list. Before you add any type of fertilizer to your soil, you should be aware of how much you need, and carefully calculate the rate for which to spread it according to the label on the package. After using organic products once or twice in the garden, you will begin to get the hang of measurements – which makes the job so much easier when it comes soil preparation time. I have only ever used organic fertilizers and compost – because I find it makes my job easier. I can apply both at the same time, and know that I am not only feeding my plant, but also feeding the soil.
Mother Earth News: Making your own organic fertilizer
Dave's Garden: Composting video
Dave's Garden: Organic Fertilizers Measures Table
Gardening For Dummies: Fertilizing Your Home Garden
Cornell University: Nitrogen as Fertilizer study