Just to share some thots, research and experience, and also to get feedback from other's experience/knowledge.
I've recently taken to dosing my N-limited planted tank with KNO3. The noticable difference, was the plants grew slower... but bushier. It seems that they stopped rushing to reach the surface and grow sideways instead. I guessing its due to the fact that when there's a lack of N in the water and substrate, the next source is air where they can get atmospheric Nitrogen. I've ruled out inhibition of growth by excess N as I only raise my NO3 to 5ppm. In addition, when I was dosing 1ppm per night, the NO3 remained at zero, implying the NO3 was being used. It only started rising when the dosage was raised to 2ppm per night. (Note also, with reference to the following paragraphs, that in addition, KNO3 also dosed K by ~0.8ppm per 1ppm of NO3.)
I've also started dosing my tanks with K2SO4 every week to raise K by 20ppm. Same results... plants stopped rushing for the surface and plants like Hygrophila started putting out horizontal stems at mid-tank rather then at the surface. From what I've read, K plays a key role in many functions of a plant, including photosynthesis, absorption and transportation of nutrients, manufacturing of food (sugars, etc) and enzymes, and cell wall structures. One important point for me was that K makes photosynthesis and food production more efficient and I'm speculating that's the reason my plants stopped rushing for the surface, i.e. light. (One farming article stated that K is also known as the quality nutrient due to its positive effect on attributes of crops like size, shape, colour, taste, shelf life, fibre quality and quality factors)
So far, experienced aquarists have not noticed algae problems associated with excess K, but it can limit the uptake of Magnesium which in turns effects the utilisation of Iron. Unfortunately, there isn't a easy way to test for K. The only indicators are (to my knowledge) Iron defeciency symtoms when there is still iron in your water (excess K) and yellow spots/holes on your leaves (K deficiency). Recommended levels range from 20ppm or less up to 40ppm per week.
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