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June 4, 2015
Wanted: A Substitute for Sodium Nitrate as an Algae Fertilizer
Judi Ramses Nieves (email@example.com): Could someone suggest a substitute for sodium nitrate, which has been classified as “dangerous goods”, takes forever to source and costs a small fortune to ship to the Philippines? We use it as diatom fertilizer in a shrimp hatchery.
Dallas Weaver (firstname.lastname@example.org): The disadvantage of urea is that you can only add small amounts at a time as the culture grows. Bacteria have enzymes that turn urea back into ammonia and high concentrations of ammonia are toxic. That means careful monitoring of concentrations of both urea and ammonia and a lot of small or continuous additions. Nitrate can be added to cultures at high concentrations without toxicity, but it is more expensive than urea and the diatoms have to spend energy reducing NO3 to produce organic N in amino acids.
Brian Boudreau (email@example.com): I agree with Dallas, the disadvantage with urea (or even ammonium nitrate) is greater when you intend to feed shrimp larval stages with algae that might still have relatively high ammonia levels from these two fertilizers.
Dallas Weaver (firstname.lastname@example.org): My noting that you must use small amounts frequently also makes it possible to deplete the culture of ammonia very fast with photosynthesis of the algae. A heavy fast growing algae culture can use up 2+ mg/l ammonia N an hour, so if you are controlling at about 4 mg/l, in a couple of hours without more N feed, the algae can use it all up. Being N deficient, that same algae hitting the shrimp larva tank will consume the ammonia in that tank—but the shrimp larva eating the algae will produce more ammonia.
Daniel Gruenberg (email@example.com): Dallas, I may be wrong but I thought the original question was referring to diatom culture that did not contain shrimp larvae. We add quite a bit of urea to our cultures >5mg/l and the diatoms quickly absorb it, and its levels come down quite quickly. The chemistry of the water has to be correct for this to work, and there should be sufficient light as well. One just has to be sure that the ammonia levels have come down before you use urea in the larval rearing tanks. I also use diatoms in ponds stocked with shrimp, and I have never had a problem with ammonia toxicity when using urea as a nitrogen source. We fertilize only once a week, and we also make a pretty good spike in ammonia. We do this on a sunny day in the morning, and by afternoon the ammonia levels are down below 0.5mg/l.
Dallas Weaver (firstname.lastname@example.org): Daniel, it’s just that algae culture people using nitrate often add 150 mg/l as N. That much urea N could kill everything if the pH was high (low CO2) and the bacteria converted the urea to ammonia. It is hard to convince some clients that it would be more economical to use a chemical feed pump and urea with semi-continuous additions holding a defined level of N in the water and avoid all the alkalinity changes and resulting changes in CO2 caused by the Na part of NaNO3 being added.
Judi Ramses Nieves (email@example.com): Thanks for your kind replies and inputs to my query. Actually, the objective is to produce diatoms for shrimp larvae in a hatchery, starting with small, phyco-lab, pure cultures and progressing to lager volumes till we reach outdoor algal tank volumes. NaNO3 is classified as “dangerous goods” post 9/11. To illustrate how expensive it has become to ship to the Philippines, we bought a kilo of the stuff for $35, and the cost of shipping, along with some other small items, ran up to $1,600. That’s why we would really like to explore other alternatives.
Ali Ghavampour (firstname.lastname@example.org): I don’t know if you ever used ammonium nitrate, but it works better than urea because it is immediately absorbed by phytoplankton. I have used also KNO3, but both were in outdoor, diatom cultures.
Source: The Shrimp List (a mailing list for shrimp farmers). Subject: Substitute for NaNO3 (Sodium Nitrate) Algal Fertilizer. June 2–4, 2015.
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