Nitrogen (N), phosphorus (P), potassium (K) are elements. As such, there is no difference between those elements from an organic nutrient and from a chemical nutrient. Elemental N, for example, is the exact same regardless of the source.
For this article, let’s keep looking at N: the most important of the above elements. Unfortunately, plants can’t absorb pure elemental N directly and there isn’t a way to feed plants a pile of single N atoms.
There is plenty of nitrogen gas (N2) in air, but plants can’t split the two atoms apart. What garden plants most often use as their source of N is nitrate (NO3)—which is an N atom connected with three oxygen (O) atoms—because this compound is easy for plants to separate. Woody plants like trees can also use ammonium (NH4) as a source of N.
Animal waste and plant material that has fallen to the ground are two sources of N that occur naturally in untended wilderness. To emulate this, manufactured organic nutrients tend to be made from naturally occurring materials with minimal processing.
One advantage to this is that the materials (i.e. leaves, lawn clippings, livestock manure etc.) can often be collected cheaply, 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 assorted other plant material winds up on the ground when 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 natural setting plants with nutrition.
As these things decompose (or compost), bacteria and fungi convert them into ammonia (NH3) and ammonium. Another organic source of ammonia is the waste products of animals, which contain N in the form of urea (CO(NH2)2). The urea is converted to ammonia by bacteria using the enzyme ureasec.
Regardless of the source, if the ammonia is exposed to acidic conditions (pH less than 7) it picks up another hydrogen (H) atom and converts to ammonium—this part of why pH can have an effect on plant growth; if the pH is too high, this conversion is inhibited. Beneficial bacteria then convert the ammonium to nitrate, which can then be used by the garden plants.
I like to compare organic nutrients to eating oatmeal for breakfast, as they tend to be bulky and release their nutrients over a long period of 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.
Also, with the exception of high-ammonia “hot” manures, organic nutrients are less prone to overfeeding; compost, worm casting and fish excrement can be used in almost unlimited quantities without causing nute burn—which is wrinkled-paper-like damage on the leaves of the plant from using too much fertilizer.
However, since organic nutrients tend to be less processed, they can to be more prone to clogging hydroponic systems that rely on sprayers and pumps. The NPK values for organic nutrients are also usually lower than for chemical-based solutions because the percentage of nutrient to total mass tends to be lower.
And since they are closer to a natural state, the NPK values for organic products tend to be less exact than with chemical-based fertilizers, which can be made to exact recipes.
However, there is more than one way to make ammonia. It can be manufactured chemically from nitrogen gas by applying heat, pressure and an iron catalyst. Ammonium sulfate and ammonium nitrate are other manufactured forms of N.
All of these chemical nutrients allow some (or all) of the steps needed to create nitrate to be skipped because they start the N further along the path and closer to the finished nitrate. However, they also do not last as long as organic compounds before giving up the N it contains.
Chemical nutrients are more like having an energy drink for breakfast. They are shortcuts to the process and release their nutrients quickly, then need a top up to avoid a “crash.” However, chemical nutrients can allow for a greater level of control as to how much and when the N becomes available to the plants since they are shortcuts to the natural process. This can allow for a higher nutrient level and 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—one which can result in nute burn or overloading and damaging natural systems with the runoff.
For example, adding a chemical nitrate allows for the entire nitrate-creation process to be skipped and an immediate supply of N to the plants, but—because chemical nitrates are water soluble—what isn’t taken up by the plant will quickly wash downstream (unless recirculated).
Overdosing plants with chemicals can also imbalance a natural system to the point that it becomes inhospitable to beneficial bacteria and fungi. Depending on the exact chemical used, there may also be “leftover” residue that can build up in the system over time. This is where the practice of watering heavily without nutrients for a time (flushing), which helps wash away any leftover chemical buildup, comes from.
So, 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 and they are both tools that can be used successfully when used correctly. The primary differences are in how many shortcuts are taken and what remains afterwards.
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.
After all, sometimes a big hearty high fiber breakfast is what is what a person needs to start the day and sometimes all one needs is a good, strong cup of coffee.
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.