Optimising Plant Growth and Minimising Algae
Author: Jennifer Hamlin
Photo credit: Aakash Sarin
The main nutrients for plants are Carbon, Nitrate, Potassium, Phosphate, Iron and Magnesium.
Those are usually known as macronutrients. The micronutrients are all of the other trace minerals just like those found in your own multivitamin tablet.
Some key points about plant growth.
1. When plants are growing optimally, they use up nutrients in the water column.
2. The more light they have, the more plants will seek nutrients.
3. Without nutrients the plant growth will be limited.
Light is a key factor to growth demands. If the lights are on too long, the plants will use up the available nutrients and leave behind whatever they can't use. Plants have a limited ability to adapt quickly to changes in nutrient and light levels. Plants require enzymes like rubisco to produce energy for growth and when conditions change the plants' enzyme levels also change, but it takes time. In other words, if you suddenly add a lot more light and a lot more nutrients, the plants will not be able to use it until they adapt their enzyme levels to the new conditions.
Algae is not fussy about nutrients and will use whatever is available; it can also adapt much more quickly than plants so it can easily take over when conditions are right. if plants are growing well, they will strip the nutrients from the water column faster than algae can and this will prevent algae from getting a foothold.
There are certain things that can cause alage. One of the biggest culprits is fluctuating carbon (CO2) levels. More than other nutrients, carbon can be challenging to add to the tank and this can result in fluctuations. Like any nutrient, fluctuating carbon levels will prevent the plants from growing well and algae are right there ready to take up the opportunity.
Another main cause of algae are spikes in other nutrients that are not as quickly utilised by plants, such as ammonia. Ammonia is produced by fish respiration, waste products and decaying organic material like dead plants and plants. Even a cycled tank can have a temporary ammonia spike.
Algae can quickly be controlled by adjusting the nutrients you are dosing. This works surprisingly well. For instance, if you are getting black beard algae, increase the carbon/CO2; if you are getting green spot algae, increase the phosphate levels.
Estimative Index dosing is a method of fertilising developed by plant guru Tom Barr. It is based on the principle of ensuring that nutrients are always available to the plants so that they can grow optimally when the lights are on. This means that nutrients are not the limiting factor for growth.
A typical target range for EI dosing in your tank is:
CO2 25-35 ppm
Nitrate (NO3) 10-30 ppm
Potassium (K+) 10-30 ppm
Phosphate (PO4) 1.0-2.0 ppm
Iron (Fe) 0.2-0.5ppm or higher
GH range 17-40 ppm or higher
Notice that carbon is a vital nutrient. This is provided in the form of liquid carbon or CO2.
The ranges above can be achieved in a low or high tech tank, the difference being the speed at which the plants will use up the nutrients (and thus the frequency that you need to dose the tank). Either way, aim to add micronutrients at the same frequency that you add the macronutrients above, they will not be used up as fast as macronutients but you don't want any deficiencies so you need to keep them up as well.
The difference between high and low tech setups is just the speed of plant growth (amount of plant biomass that is produced). The only reason to have high light and high nutrients is to have fast growing plants. Fast growing plants are like teenage boys - they need a lot of food.
So, to make a long story short:
1. make slow adjustments when you are changing nutrients, CO2 and light in the tank
2. make sure your lighting level matches the amount of nutrients in the water
3. add micronutrients and macronutrients (including carbon) as often as needed to match your lighting levels
Each tank is different in terms of the lighting intensity, photoperiod, filtration, stocking levels and plant biomass so dose rates will vary slightly to achieve the target ranges above.
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