Each species of plant has some unique requirements, though all plants share the basic needs.
Nitrogen, Phosphorus and Potassium are the primary elements required by all plants. Certainly, there are many more such as Sulfur, Calcium, Boron to name only a few. These other elements are typically regarded as micro-nutrients, as the plant requires them only in very small quantities.
There are three ways to obtain the necessary elements. Fertilizers, Nutrients and Organically.
Fertilizers are what most are exposed to first. They are pretty common and easily found.
Nutrients are relatively new and supply the elements to the plant in a very different way.
Organically is what typically happens in nature… Decay, bacteria and water eroding elements from the earth, bringing all that together at the root-zone for the plant.
We are not going to talk a lot about Organic growing here, as there are plentiful sources of information. It is as simple as putting a seed in the ground and coming back a few months later.
This is the method that fed the Earth for a very long time. It is by far the simplest method, though, unless the plants are native to the area and soil conditions, it is not the method that will produce the high yields expected by growers these days.
Amending an outdoor garden usually only employs manuer and/or lime, depending on the physical size of the plot being adjusted.
Fertilizers, whether they are Organic or chemical do not feed the plant directly. They feed the bacteria growing in the soil which in turn, feed the plants. This suggests that soil composition is critical for the species of plant. Different soil compositions will support different bacterial cultures.
Some plants are slow growing and have low requirements. These plants can thrive in an abundance of conditions. Rapid availability of the primary elements would not be required. So, unless the soil is completely used up, (or otherwise contaminated) a slow growing plant should do fine.
Some plants grow so fast that the bacteria in the soil has a difficult time keeping up. In the past, when the soils were much more fertile than now, a fast growing plant could do well providing the crops were rotated so the soil could recover. Now, to have a successful crop of a fast growing plant, one is pushed into supplementing the available nutrients. When growing in containers, this problem becomes even more important.
Nutrients, on the other hand, eliminate the problem of fast growing plants by supplying the elements to the plant in a form the plant can use directly. No bacterial action or chemical breakdown to delay the uptake. This also means that soil is not required. In fact, although one can use nutrients in soils, it is much better suited for application in an inert medium like sand, gravel, rockwool and such. Even plain water with no medium will generate healthy and productive plants.
This method is not without it own unique issues. Water quality, cleanliness and reliable equipment are vital to maintain a healthy nutrient reservoir and root-zone.
Because of the direct nature of the nutrient feeding, the water chemistry is limited to a narrow range. Water with excess calcium, magnesium, iron and the like will cause some problems with the stability and availability of the carefully balanced element makeup of the nutrients. It is possible to get nutrients that take poor water quality into account, but these are usually very complex and require some chemistry knowledge.
Each method has requirements that need to be met to maintain the quality of the growing area. With organics, crop rotation and leaving it fallow for a time are strategies used to keep the plants healthy.
When employing fertilizers, especially in containers, there is a problem of salts buildup that must be addressed by flushing clean water through the soil periodically. Containers of sufficient size to support the size of bacterial colony to supply the nitrogen needs of the plants must also be considered.
Systems that use nutrients are sensitive to pH swings, water quality issues, root fungus and light damage. The systems that use these are typically designed to reduce or eliminate these issues.