How is it that you know you have enough? There are several ways to tell. First, you should know from the start what your N levels are from the water report. A heavily planted, extremely fast growing and highly optimized tank can suck up about 6ppm of NO3 per day. On average, you can figure about 2-3ppm per day if your tank is growing well. If you have the light, proper CO2 levels, good trace dosing, and your plants are at a standstill, a lack of N may be a problem. If you have colored plants, their coloration will be affected, and their color is largely dependent upon N. A lack of N will result in the plant not being able to provide enough chlorophyll to plant leaves, resulting in a deeper shade of red/orange/pink/etc. Many plants, such as Ludwigia repens, are mostly "green" plants with very soft hues of color. A lack of nitrogen will provide them with their intense coloration. This is not always a good thing, because it's an indication of N starvation. The use of a plant like this can be very useful to provide a visual indication of nitrogen levels. Unlike test kits, the plants will never lie. Another way would be to calculate your N dosing by using dosing calculators and estimated uptake rates. Begin conservatively, and gradually increase dosing rates to find a point where extra fertilization has no effect, then back off until there is an effect, and gather an average. Week-long increments of a particular (set) dosing rate should be used. I will elaborate on this later in this article.
Phosphorous
Phosphorous (P) has always been commonly thought of as an algae-inducing element and not so much as a helpful nutrient. This is the "old" way of attempting to place blame on the algal woes many folks once had. It's about as true as high light causing algae. P's role in the planted aquarium is an important one, supplying much needed elements in the role of DNA & RNA replication and such things related to growth. P can be made available in the organic and inorganic forms. Some organic forms include fish waste, food, decaying plant matter, bacterial processes, etc. Inorganic forms would include fertilizers such as KH2PO4.
When given other necessities that promote plant growth such as light, CO2, and other nutrients, Phosphorous can limit growth when not in adequate supply. Unlike Nitrogen deficiencies, P deficiency will not bring growth to a screeching halt, but it will hinder growth and a plants ability to uptake other nutrients. When P limiting conditions exist, the conditions become ripe for algae to take over due the slowed uptake of other nutrients--thus decreasing the plants ability to out compete algae for nutrients. In the case of P limitation, algae growth is typically not as severe as Nitrogen limitation and Phosphorous excess. When N & P are in excess and another nutrient(s) becomes limited, massive algae growth is likely to occur. Fortunately, when P is limited in the well-lit, CO2 injected aquarium, Green Spot Algae is a likely warning sign. It does not mean P is zero, but it does mean P is being limited, and plants are not allowed to partake in the luxury uptake (excess uptake allowing for the storage of nutrients within the plant) of nutrients.
Phosphorous can be a tricky nutrient to understand and to test for. When dealing with a such a small range of quantitative values, and a lack of accurate test kits, it's important that the aquarist gain a substantial feel for Nitrogen in order to understand how Phosphorous complements it. In other words, if you know N is at an adequate level and growth slows, P could be limiting. Visually, plants can display signs of N deficiency by turning colors (red, orange, etc.), as P limitation can have a similar effect, but usually not as profound. In conclusion, I would recommend to know the amount of P in your water as found in your local water report, and then use an appropriate calculator to know for sure how much P you are dosing. While planted tanks may differ in their needs, I would recommend a minimum of 0.5ppm to 2.0ppm. While more can be kept, I would advise against it in the minimalist approach this nutrient guide is taking.
Potassium
Potassium's (K) role in aquatic plants is an important one, but is very different than N & P. While N & P play the role of plant "infrastructure," K's role is one of plant "optimization," as it optimizes photosynthesis. It doesn't really do anything fancy, like replicate DNA/RNA or produce enzymes/proteins, but it helps keep them in a constant state of activity. K can be supplied in part by just about all "preferred" fertilizers such as Flourish, TMG, KNO3, and KH2PO4. One of the best facets of K is that it does not cause algae growth. Another is that there is no real minimum or maximum level that must be held constant--a ballpark figure often works well. The worst is that it cannot be tested for (without having a lab handy). However, as I propose in the article, you wouldn't need to test for it anyhow.
It can be difficult to tell if the addition of K in the planted tank actually does anything. However, a limitation can hinder the use of N & P in the plants. While I discourage blindly dumping K into the aquarium, the use of fertilizer calculators can give you a good ballpark figure so as to prevent limitation and excess. A sure sign of limitation are pinholes in plant leaves. This can most commonly be witnessed in Hygrophila species, such as polysperma and corymbosa varieties. It is suggested that enough K can be dosed by using KNO3 and KH2PO4, and I believe in some cases, it is. When KNO3 & KH2PO4 dosing may be low, either due to excessive fish load, high tap water levels, lower lighting levels, etc., the use of K may be mandated. If in doubt, you might wish to dose K for a certain period of time (maybe 2 weeks) to see if you find a benefit. In conclusion, I would propose that you shoot for a minimum of 10ppm, as estimated by a dosing calculator. While larger values will not hurt, excess K will likely have no effect, is wasteful, and goes against this minimalist approach.
Up next will be carbon! This might justify a second article in itself.
Continue to post #21 for carbon!