Plants need nitrogen (N), phosphorus (P), potassium (K), and other nutrients for healthy growth. These are elements, and as such, there is no difference between the nitrogen (N) from an organic nutrient, or a synthetic nutrient. Elemental nitrogen is the same exact thing, regardless of the source.
The most important (and most interesting) of these is nitrogen (N). Unfortunately, plants can’t absorb pure elemental nitrogen (N) directly. There isn’t a way to feed plants a pile of single nitrogen (N) atoms. There is plenty of nitrogen gas (N2) in air, but plants can’t split the two nitrogen atoms apart, they are bound too tightly together, and so, nitrogen gas (N2) isn’t a good nitrogen source for plants.
What garden plants most often use to allow them to take up nitrogen (N) is a form known as nitrate (NO3), which is a nitrogen (N) atom connected with three oxygen (O) atoms. Nitrate (NO3) is easy for the plants to separate the nitrogen (N) from the oxygen (O), and therefore, makes for a good source of nitrogen (N) (woody plants like trees can also use ammonium (NH4)).
Plant material that has fallen to the ground, and animals leaving waste material behind are two sources of nitrogen (N) that are naturally occurring in untended wilderness. To emulate this, we get organic nutrients from naturally occurring materials with minimal processing. One advantage to this is that the materials can often be collected cheaply (i.e. leaves, lawn clippings, livestock manure, etc.), and require little processing before use, often just maturing or composting. Compost (3-1-2) is very similar to what happens in nature when leaves, and other plant material fall to the ground, and nobody is around to rake it up.
Blood meal (12-0-0) and alfalfa meal (2-1-2) are two other organic fertilizers that are based on things found to supply plants in a natural setting with nutrition. It is as these things decompose (or compost) that bacteria and fungi convert them into ammonia (NH3), and ammonium (NH4), which break down further into nitrites, and finally nitrates.
Another organic source of ammonia is the waste products of animals, which contain nitrogen in the form of urea (NH2)2(CO). The urea is converted to ammonia (NH3) by bacteria using the enzyme ureasec. This process takes time with spread out availability, because the bacteria generate the ammonia as they get to it.
I like to compare organic nutrients to eating oatmeal for breakfast, they’re bulky, and release their nutrients over time. Some forms of organic fertilizers can continue to release nutrients for more than one season, improving the general long-term health of the soil. Because the percentage of nutrient to total mass is usually lower, the NPK values for organic nutrients are also generally lower than with chemical-based solutions. Because they are closer to a natural state, the NPK values for organic products will also be less exact than chemical-based fertilizers, which allow you to make to exact recipes. This is why organic nutrients are less prone to overfeeding, the exception being high ammonia ‘hot’ manures. You can use compost, worm casting, and fish excrement in almost unlimited quantities without causing ‘nute burn.’ Since organic nutrients are less processed, they are also more prone to clogging hydroponic systems that rely on sprayers and pumps.
However, there is more than one way to make ammonia (NH3). It can also be a manufactured chemical made from nitrogen gas (N2) by applying heat, pressure, and an iron catalyst. Ammonium sulfate ((NH4)2SO4), and ammonium nitrate (NH4)(NO3) are other manufactured forms of nitrogen that allow for later parts of the process to be skipped over. Any of these allow for a short cut in the process, and makes the nitrogen available a lot faster, but does not last as long before giving up the nitrogen it contains.
Chemical nutrients are more like having an energy drink for breakfast, they release their nutrients quickly, and then you need more to avoid a ‘crash.’ Since chemical nutrients are shortcuts to the natural process, they can allow for a greater level of control of how much, and when the nitrogen becomes available to the plants. This can allow for a higher nutrient level, and resulting increase in performance than is possible with organic nutrients. With this level of control comes responsibility however, as introducing an overabundance becomes a much more likely temptation, which can result in ‘nute burn,’ or overloading and damaging natural systems with the runoff. Adding a chemical nitrate (NO3) for example, allows for skipping the entire nitrate (NO3) creation process, and immediately supplies nitrogen (N) to the plants, but it is also very water-soluble, and what isn’t taken up by the plant will quickly wash downstream (unless recirculated).
Overdosing plants with chemicals can imbalance a natural system to the point that it becomes inhospitable to the beneficial bacteria and fungi normally responsible for the process. The ability to better fine tune the available nutrients also allows for ease in imbalance creation, and smaller margin for error. Because chemical fertilizers are shortcuts to the process, using them to treat nutrient deficiencies will tend to give faster results than an organic solution, which is better suited for long-term release. Depending on the exact chemical used, there may also be “leftover” residue after plants take up the ammonia or nitrate they need, which can build up in the system over time. This is where the practice of watering heavily without nutrients for a time (flushing) comes from in hydroponics, to help wash away any leftover chemical residue buildup.
Regardless of the source, in acidic conditions (pH less than 7) the ammonia (NH3) picks up another hydrogen (H) atom, and converts to ammonium (NH4). This is part of why pH can have an effect on plant growth, if the pH is too high, this inhibits conversion. Beneficial bacteria then convert the ammonium (NH4) to nitrate (NO3) which can then be used by the garden plants. Nitrogen from organic sources follows a path of several steps to become the nitrate (NO3) that plants need. Chemical nutrients allow skipping some (or all) of these conversion steps, which starts the nitrogen (N) further along the path, and closer to the finished nitrate (NO3).
Phosphorus is available naturally from organic composts, rock phosphate, or bone meal – or it can come from chemicals, such as ammoniated superphosphate (5-50-0) or ammonium phosphate (18-46-0). Overuse of phosphorus is one of the sources of environmental pollution.
Potassium is also obtainable from organic sources like compost (3-1-2), kelp (1-0-4), or greensand (0-0-3), or from a chemical, such as potassium nitrate (13-0-44).
The differences between chemical and organic nutrition are not as absolute as they are often portrayed. They both use the same process to supply the same elements to the plants. The primary differences are in how many shortcuts they offer, and what remains afterwards. They are both tools you can use successfully when done correctly. Although purists on both sides may strongly disagree, I believe there is little reason not to make use of the benefits of both in moderation. Plants awaiting organic nutrients to become available may benefit from a little chemical boost to tide them over, and long-lasting organic materials can help create a buffer for fast-acting chemical nutrient gardens.
Sometimes a big, hearty, high-fiber breakfast is what a person needs to start the day, and sometimes you just need a good strong cup of coffee to get your eyes to open. As always, understanding why you are adding something to your garden, and how it works, goes a long way toward picking the one that’s right for you.