In response to many questions about dosing I've decided to explain my views on why we do what we do. I need a reference to send people to rather than repeating the same thing over and over. If anyone notices errors or problems please let me know since I will refer to this frequently.
It seems that most people learn "how" to dose fertilizers without understanding why those steps work. Learning the "how to method" first causes a lot of confusion when we finally ask the question, WHY?
We need to supply all the necessary nutrients to the plants in adequate quantities. Quantities that are high enough so the plant growth is never limited by the amount of any one nutrient. This idea is based on Liebig's law of the minimum
Look at the barrel in this image. The water height of this barrel is limited to the lowest slat. Imagine each slat as a single nutrient and the barrel as the plant; the water height representing growth rate. Now the maximum growth rate will be defined by the lowest slat, or nutrient. So the growth rate of the plant would follow the nutrient in least supply.
So what would be the best option to get maximum growth rate from our plants? The answer seems obvious. Supply nutrients above the top of the "barrel". This would represent non-limiting nutrients.
Now we know the concept behind non-limiting nutrients. Let’s talk about the steps to obtain that condition. Again I'm going to use science. Hoagland's solution is nutrient rich water for plants to grow in. Sound familiar? Here’s what Wikipedia says about Hoagland’s Solution...
The Hoagland solution is a hydroponic nutrient solution that was developed by Hoagland and Arnon in 1933 and is one of the most popular solution compositions for growing plants (in the scientific world at least). The Hoagland solution provides every nutrient necessary for plant growth and is appropriate for the growth of a large variety of plant species. The solution described by Hoagland in 1933 has been modified several times (mainly to add iron chelates and the like), but the original concentrations for each element are shown below.
• N 210 ppm
• K 235 ppm
• Ca 200 ppm
• P 31 ppm
• S 64 ppm
• Mg 48 ppm
• B 0.5 ppm
• Fe 1 to 5 ppm
• Mn 0.5 ppm
• Zn 0.05 ppm
• Cu 0.02 ppm
• Mo 0.01 ppm
The Hoagland solution has a lot of N and K so it is very well suited for the development of large plants like Tomato and Bell Pepper. However, the solution is very good for the growth of plants with lower nutrient demands such as lettuce and aquatic plants with the further dilution of the preparation to 1/4 or 1/5. Hoagland solution must be made from 7stock.
Wikipedia, Hoagland's solution
Notice the nutrients listed as ppm (parts per million) above. The amounts in that list are far higher than we want in a planted tank. Remember, we have fish, shrimp, bacteria and an entire eco system in our tanks to worry about. In addition, the ratios are different for our purposes. So where do we turn now? We know we want to supply those 12 nutrients at various levels but what levels? Well, a hobbyist named Tom Barr thought the same thing. He’s determined the appropriate range for each nutrient through years of testing. The method of supplying non-limiting nutrients to our plants is known as the EI method or Estimative Index
. I prefer the term EI concept because “method” implies a one size fits all mentality. This couldn't be further from the truth.
The ranges for EI are listed below.
CO2 range 25-35ppm
NO3 range 5-30ppm (KNO3)
K+ range 10-30ppm (K2SO4 or GH booster)
PO4 range 1.0-3.0 ppm (KH2PO4)
Fe 0.2-0.5ppm or higher (?) (Plantex CSM +B)
GH range 3 degrees ~ 50ppm or higher (GH Booster)
I know what you may be thinking. Where are all the other nutrients? And what does GH have to do with nutrients?
The "other" nutrients are supplied through one dry fertilizer called the trace mix. It's a combination of all the other nutrients mixed at the proper ratios, the most popular blend being Plantex CSM +B. The GH is listed as a nutrient because GH is a measurement of calcium plus magnesium; more on this below.
So from those 12 nutrients in Hoagland's solution we only have to deal with six. Nitrogen, Phosphorus, potassium, iron, GH and traces (the other nutrients).
We can dose nutrients using any frequency we like, daily, weekly or monthly as long as we stay within range. There are advantages to dosing smaller amounts over time. We get into a daily routine when we do something over and over. This helps us to remember to dose. It also prevents large swings which can impact some inhabitants. Most hobbyist with CO2 injected tanks dose every other day; macros one day micros the next. This can prevent nutrients in the trace mix from binding to macro nutrients, iron and phosphate in particular. However, low tech tanks can easily withstand monthly dosing with many other variations in between. Remember, EI is a concept not a step by step method.
Before we get into how to calculate doses let’s talk about water changes.
Clearly if we dose nutrients “above the barrel” there is a point at which some of them will become toxic. To prevent this we do regular water changes. The frequency of water changes is no different than the dosing schedule we choose. We can do water changes 3 times per week or once per year. It all depends on the system we happen to be running. The common theme being that we keep nutrients in a specific range. Balancing water changes and dosing schedules can easily be visualized using wets calculator
As you can see, with a little tinkering on a calculator we can tailor any nutrient level we want. This is the easiest way to balance water changes and nutrients. Simply keep nutrients within the given range.
For new systems water changes of 2-3 times per week for the first 1-2 months can prevent many problems such as substrate leeching or absorption, removing nitrogen cycle wastes, and other problems we may face with a new setup. Many of these can lead to algae outbreaks and inhabitant stresses. All of which can be easily alleviated by frequent water changes and healthy plant growth.
To explain how we dose each nutrient let me use nitrogen as an example.
Nitrogen, represented by NO3 (nitrate) above. This is the result of nitrifying bacteria. This means we can have nitrogen without dosing. The amounts we have are based on the amount of organic matter that is processed by the bacteria. We can also get nitrate through our tap water. Many water supplies have nitrate levels from agricultural runoff. Since there are other sources for nitrogen we need to test for that first before we calculate a dose. In the list above the range for NO3 is 5-30ppm. If we will never go below 5ppm even after a 50% water change we may not need to dose nitrogen. If we will go below we should calculate how much is needed to keep nitrogen in that 5-30ppm range.
Let’s assume we have 0 nitrates and we want to calculate for 10ppm. Let’s further assume we want to dose every other day. If we want 10ppm then three doses per week would be…oh let’s just round it up say 4ppm. Notice we don’t have to be exact since the range is 5-30ppm! Always round up. We can taper off dosing later until we see changes in plant health. This nutrient level is called the critical point; the point at which nutrients become limiting. Just like everything else, this point is tank specific.
Now we have to figure out how much of something we need to add to raise NO3 to range. We can use a list of fertilizers to raise nitrogen. The most common one used is KNO3 or potassium nitrate. Below is a list of the common fertilizers used to raise the other nutrients within range.
NO3 (nitrogen) – KNO3 potassium nitrate.
K+ (potassium) – K2SO4 potassium sulfate. Found in many GH boosters as well.
PO4 (phosphorus) – KH2PO4 mono potassium phosphate
Iron (Fe) – Ferrous gluconate, EDTA iron chelate and DTPA iron chelate.
GH – Magnesium MgSO4.7H2O (Epsom salt) OR GH booster
Calcium CaSO4 (calcium sulfate) OR GH booster
Making these calculations couldn't be simpler. We simply use a nutrient calculator
. There are many options on the calculator listed. Suffice it to say most people can figure out how to use the calculator rather than explain its use. This is how we decide how much of a fertilizer to add to raise that nutrient within range. The calculator can calculate for dry dosing or solutions, solutions being more appropriate for smaller tanks.
One point I would like to make in regards to the calculator output. Many fertilizers supply more than one nutrient as seen below. We need to keep a "running total" to account for this.
Notice how the KNO3 above also adds 2ppm of potassium?
We make these calculations for each nutrient in the list.
Three glaring issues still remain that we need to talk about; GH, iron and traces.
Gh, often noted as dGH (degrees of general hardness), is a measurement of the concentration of divalent metal ions such as calcium and magnesium per volume of water. Notice, other things can influence this measurement. However, calcium and magnesium are the two most valuable players here. Generally a GH of 3-5 degrees is considered adequate for plant growth. Sometimes the ratio of calcium to magnesium can be skewed to one side leaving the other out of range. So if we have access to a water report we can make an informed decision on whether we should dose additionally Ca or Mg based on total GH. This usually isn’t necessary. If we wanted to be safe we could add enough GH booster to raise the GH 1-2 degrees. This won’t have any impact on plant growth. It can however impact very sensitive fish and invertebrates.
Iron and trace mix are interrelated because most trace mixes contain iron. Many hobbyists have realized that higher iron levels are beneficial to multiple plants. Yet adding more of “everything else” is not needed. So a basic dose of trace mix is usually calculated to obtain an iron concentration of about .2ppm. That leaves iron at the minimum level while supplying sufficient levels of “everything else”. If we want to add additional iron we can use various forms of it.
Iron comes in many forms. The two basic types are non chelated or chelated. Essentially, chelated is a fancy name for time released. We can use iron without chelates which will last a short time, hours. This would be analogous to an injection from your doctor. It won’t last long but it works fast. We can also use iron that has been chelated with various chemicals. The time it lasts and the availability to your plants depends on the chemical used. The two most popular are EDTA and DTPA. EDTA can last up to 24 hours depending on the PH of your water. EDTA is the form of iron used in Plantex. DTPA can last up to 48 hours and is even more dependent on your PH. The higher the PH the more need for a stronger chelate such as DTPA.
So now we know why we need to dose, what we need to dose, how much we need to dose and more importantly, how to do it. Many debates are forged arguing levels, which fertilizer works better, or many other factors. The root issue always remains the same. Supplying non-limiting nutrients is the most efficient method to obtain maximal growth rate.