Hi all,
It could be that you were adding too much plant food and there was enough in the water to sustain the plants for the last week........ You can check your tap water for phosphates, mine is really high.
That would be my suggestion as well, you always get a bit of a lag between fertiliser addition and growth. Also the plants responses to each different nutrient, whether it can store them in the tissues and then transport them to areas of growth etc, are different.
It is the production line analogy, instead of the speed of the line being set by the slowest process, plant growth is limited by one factor, either the amount of a nutrient ion in solution (usually K+ or NO3-), concentration of dissolved CO2 or light (photosynthetically active radiation - PAR) .
Although we think of N P K (Nitrogen (N), Phosphorus (P) & Potassium (K)) as macro-nutrients, plants actually need about x10 more N and K than they do P. In the UK if you use tap water you will definitely have more phosphorus in your water than the plants will ever need because the water companies add orthophosphate (PO4) to control heavy metal solubility.
This was the idea behind EI fertiliser dosing ("Estimative Index"), CO2 and nutrients are never limiting, growth is limited by light availability. Quote from its originator, Tom Barr.
The idea behind EI is simply introducing an excess amount of nutrients within an aquarium, throughout the week. This excess of nutrients floods the water column and feeds the plants. This is an estimative method; measuring specific nutrient uptake rates is not necessary and no test kits are involved. EI provides a surplus of nutrients that helps to prevents plant deficiencies, and allows plant growth to out compete algae growth.
Therefore more light means you need to add more CO2 and fertilisers to keep things in balance. I don't like EI, because you are dealing with a system where you have a lot of potential for things to go wrong. I think of it like juggling, in this case you have 3 balls (light, nutrients, CO2) and the more of each you have, the higher the balls are thrown. If your "balls" aren't thrown the same height (CO2 isn't distributed through out the tank, one nutrient is limiting plant growth) algae and deficiencies symptoms appear. Basically the plant is dining every day at Heston Blumenthal's restaurant and if it has to go to KFC it isn't happy
This is why I developed the low tech "duckweed index", in this case CO2 is atmospheric (so no added CO2) and the light is irrelevant, as nutrients are always limiting, you can tell when a nutrient (usually either N or K) become severely limiting, as the "Duckweed" (can be
Salvinia or Amazon Frogbit) becomes yellow and growth slows to a stop.
In the restaurant analogy, the plant is scavenging from the bins at the organic, vegetarian restaurant and just managing to keep alive, with the occasional trip to Nando's when death is imminent. This doesn't produce lush growth of EI, but it does produce a tough resilient plant and less input of fertilisers, no CO2 etc. means a "lower juggle", and less to go wrong in the tank. I want stability much more than I want rapid plant growth.
cheers Darrel
