Chemical fertilizers are synthesized inorganic materials that provide essential nutrients like nitrogen, phosphorus, and potassium but may contain harmful acids. Organic fertilizers rely on microorganisms to release nutrients from natural remains and byproducts but provide nutrients in unequal proportions. While chemical fertilizers equally provide all essential nutrients, organic fertilizers vary in nutrient levels and their slow release does not always meet plant demands. Chemical fertilizers also have higher acid content that can harm soil bacteria, whereas organic fertilizers support bacterial growth. Though organic fertilizers were historically more common, chemical fertilizers saw explosive growth after World War II due to improved productivity but concerns over sustainability have increased organic fertilizer usage.

1. Organic vs chemical
fertilizers
A chemical fertilizer is defined as any inorganic material of wholly or
partially synthetic origin that is added to the soil to sustain plant growth.
Chemical fertilizers are produced synthetically from inorganic materials.
Since they are prepared from inorganic materials artificially, they may have
some harmful acids, which stunt the growth of microorganisms found in the
soil helpful for plant growth naturally. They’re richin the three essential
nutrients needed for plant growth. Some examples of chemical fertilizers
are ammonium sulphate, ammonium phosphate, ammonium
nitrate, urea, ammonium chloride and the like.
Organic fertilizers are substances that are derived from the remains or by
products of organisms. Organic fertilizers depend upon the microorganisms
found in soil to break them down and release the essential nutrients. Organic
nutrients are rich in phosphorous, nitrogen, and potassium, but in unequal
proportions. Examples of organic fertilizers are cottonseed meal, l fish
emulsion, and manure and sewage sludge. There are two types of organic
fertilizers: first is the synthetic type which is organic compound produced
artificially (e.g., Urea, a common organic fertilizer; the other type is natural
organic fertilizers because 100% of the ingredients used to create a typical
natural organic fertilizer come from nature (e.g., fish extract, seaweed and
manure, guano, and compost materials).

2. Distribution of nutrients
Fertilizers are used to provide nutrients to the plants for their good growth.
Soil nutrients’ deficiency is the prevalent problem among home-garden
owners. One of the distinct advantages of chemical fertilizers over organic
fertilizers is that chemical fertilizers are richequally in all three essential
nutrients: nitrogen, phosphorous, and potassium. On the other hand, organic
fertilizers may be richin one of the three nutrients, or may have low levels of
all the three nutrients.
Cost
Organic fertilizers are generally much more expensive than chemical
fertilizers, mostly because chemical fertilizers have more concentrated levels
of nutrients per weight of product than organic fertilizers do. One needs
several pounds of organic fertilizer to provide the same soil nutrient levels
that a single pound of chemical fertilizer provides, and the higher cost of
organic fertilizer is one of the biggest reasons that organic produce is more
expensive than non-organic. (The other big reason being lower organic
yields, on average.) Although it is possible to make a lot of one's own organic
fertilizer as well, once the labor, time, and other resources are accounted for,
homemade organic fertilizer is usually more expensive than store-bought
chemical fertilizer too.
Supply of nutrients
One aspect of the organic fertilizers is their slow-release capability. Slow-
release capability of organic fertilizers has both advantages and
disadvantages: Slow-release means there is less risk of over-fertilization but
sometimes this slow-release of organic fertilizers is not able to fulfill to
needed supply of the nutrients, whenever required. In contrast to organic
fertilizer, chemical fertilizers are always there to provide immediate supply of
nutrients to plants if situation demands.
Acid content
One of the main disadvantages of chemical fertilizers is that, in contrast to
organic fertilizers, several chemical fertilizers have high acid content
like sulfuric acid and hydrochloric acid. This high acid content results in the
destruction of the nitrogen-fixing bacteria, which is helpful in supplying the
nitrogen to a growing plant. In contrast, organic fertilizers support the growth
of nitrogen-fixing bacteria..

3. NPK ratio
Chemical fertilizers always have a high total NPK
(nitrogen:phosphorous:potassium), from 20 to 60 percent or more. The total
NPK for organic fertilizer blends will always be low. Fourteen percent is about
as high as it gets.
History
Natural fertilizers like manure have been in use for centuries as these were
the only form of nutrition that could be provided to crops before the invention
of chemical fertilizers. Chemicals were added to natural fertilizers after the
second world war. Post the war, with advancement of technology there was
an explosive growth in artificial fertilizers due to improved productivity. But of
late, there has been mass awareness of the eco friendliness of the use of
organic fertilizers and many are using those methods again.
Use
There are two ways to measure fertilizer use in a country. One is by nutrient
content — how much nitrogen, phosphate and potash are contained in the
fertilizer applied. IAnother way to measure is in total tonnage — the total tons
it takes to deliver the nutrient content.. The world's largest producers and
users of fertilizers are the United States, China, India, Russia and Brazil.
Some reports suggest the US Fertilizer market to be around $40 Billion of
which organic fertilizers occupy only about $60 Miliion. The rest of it is the
share of the various artificial fertilizers.
References
 Chemical Fertilizer or Organic Fertilizer - EcoChem
 Wikipedia: Fertilizer#Organic fertilizers
 Fertilizer Statistics - The Fertilizer Institute
 Organic fertilizer companies see growing market, but efficacy debated - MarketWatch
Jayadeva de Silva for Humantalents International