The essential difference between organic fertilisers and inorganic fertilisers is that organic fertilisers are ‘natural’. They are derived from plants or animals. Basically anything that has lived! Almost all inorganic fertilisers are manufactured or known as synthetic fertilisers.
Our earth has an abundant supply of nutrients in the soil - largely from the inorganic - but natural - sources of rocks. The largest source of Nitrogen is actually in the air we breathe. Some plants have adapted themselves to make good use of this free Nitrogen.
The term 'natural' in reality does not mean instant health, or even healthier insofar as feeding vegetables are concerned. (I await the irate emails!). There have been enough cases and worries about the use of bonemeal as a 'natural' or 'organic' fertiliser. So much so, that it now has to be sterilised by steaming before it can be sold and used as a garden fertiliser. The bones of camels and whatever were at one time used.
For practical purposes, fertilisers can be divided into bulky manures and concentrated fertilisers. The bulky manures are all organic types - such as farmyard manure and compost. They are essential for keeping the soil in good heart, and they also release plant nutrients slowly.
Most of the concentrated fertilisers are inorganic salts such as ammonium and potassium nitrate. Superphosphate, or compound mixtures such as Growmore. They are used in much smaller quantities, release the nutrients more rapidly, but do little to improve the soil structure.
It is interesting to note that all three macro elements occur 'naturally' from mineral sources - as does the other important element of Lime (Calcium) and all of the micro-nutrients which are necessary for plant growth.
Nitrogen is the single most important plant food. It is sold in various forms, and is 'applied' organically in various forms. It should be understood when talking of organic or synthetic fertilisers - lets call them feeds; for that is what they are!
Vegetables and other plants, can only use Nitrogen from the soil (there are other sources) in the form of the chemicals Nitrate ions or Ammonium ions. All living animals need Nitrogen also, and the only way they can obtain Nitrogen is by the eating of plant materials - somewhere in their food chain. Hence most carnivores kill and eat ' grazing' animals - not other meat eaters.
The provision of many organic manures, actually reduces the amount of Nitrogen in the soil in the short term. This is because for it to be 'converted' into nitrogen, microbes in the soil have to take Nitrogen OUT of the soil in order to do their job! This is especially the case with organic materials containing high cellulose content - wood chippings, sawdust, straw and the like.
The nutrients that plants can absorb for food, are essentially chemicals. Plants cannot 'eat' raw bonemeal nor rotted farmyard manure - fortunately! Whatever is put on or in the soil as a potential plant food, firstly has to undergo state change within the soil, to end up as soluble compounds that vegetables can absorb as the nutrients. For the purpose of this article, we discuss only the Macro Elements, or Macro Nutrients N P K. There are other nutrients that plants need, though not in such great amounts as these three.
Whether the source is organic or synthetic, it has to be converted into the chemicals that the plant can absorb - through various means. You can now decide if you trust the well-packaged and researched boxes of fertiliser and bottles of liquid fertiliser, or your variable compost heap. A simplification, and an explanation to simply put both sides of the argument. Yes, I have come down rather on the side of synthetic fertilisers, but simply to redress the balance a little, and give food for thought - excuse the pun!
However, Bonemeal and Blood Fish and Bone are not 'bulky' manures. They are fully organic fertilisers. Together with this, there are several liquid feeds which are fully organic - such as seaweed extract.
So, the main criteria for classifying them is
Organic fertilisers - manures - from something that has lived. | Inorganic fertilisers - are normally manufactured, or at least processed.
For those who espouse the organic only code as a way of life, study how the 'organic' fertiliser Fish Blood and Bone is sourced and manufactured. The blood element in the main comes from the world's slaughter houses. The Bone from the Wholesale butchers who prepare the meat for sale, and the Fish from the innards of 'prepared' fish, and other 'fish' resources that happen to get tangled in trawler nets.