[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Nitrogen sources

On Thu, 26 Aug 1999, Andy Moore wrote:
> Would someone mind explaining, in simple terms, the potential problems with
> not having enough Nitrogen in a planted tank ?? I have on occasion had very,
> very low PO4 & NO3 levels - am I correct in thinking that the plants would
> not have much of a chance to grow in this environment ?

Nitrogen is one of the most common elements in plants.  It has to be there
because it's an essential part of the biochemicals that make the plant
tick.  Phosphorus is not as common in plants, but it's just as essential.  
In a setting that is devoid of either plant-available nitrogen or
plant-available phosphorus a plant will fail to grow and eventually die.

Nitrogen is the biggest part of air and a common gas dissolved in water,
but that nitrogen isn't available to plants.  Nitrogen in air is in a
molecule (N2) that is so stubborn that only a few specialized organisms
can break the molecule up and use the nitrogen in it.  Plants must have
nitrogen that's in the form of ammonia, ammonium or nitrate.  I suspect -
but don't actually know - that plants can use nitrite as well.

When nitrogen is in short supply the plants may first show it with slowed
growth and yellowing.  Nitrogen is mobile in plants.  That means that when
it gets to be in too short supply the plants can take nitrogen from older
leaves and use it for new growth.  So the plants' second response to a
nitrogen shortage is to drop some of their older leaves.  In some plants
this is a very distinctive process, with the leaves dying back from the
tip to the base of the leaf along a V-shaped line centered at the central

The nitrogen supply in an aquarium doesn't have to be nitrate and it
doesn't have to be at any kind of measurable level in the water.  The
nitrogen source may be present only in the substrate and not at all
measureable in the water.  As a result you may not be able to tell if your
tank is nitrogen deficient just by testing nitrate in the water.

Phosphorus in water is found in phosphates and organic molecules, but
plants and algae can only use the phosphates.  Phosphates, through a
number of routes, tend to end up in the substrate rather than in the
water.  The phosphorus content in water is often low enough to restrict
the growth of algaes because they can only get nutrients from the water.  

There is usually an ample supply of phosphorus in fish food and that
supply seems to get converted into phosphate rather easily.  So we
generally worry more about an oversupply of phosphorus than we do about a
shortage of phosphorus.

The processes that tend to put phosphates in the substrate must not work
very well in some aquariums because people often report high phosphate
levels.  Phosphates may exceed 1 mg/l in aquariums; that much phosphorus
in nature would probably cause serious ecological problems.  There is an
old rule of thumb that water flowing to a lake or pond needs to carry less
than 0.050 mg/l total phosphorus (phosphates and organic phosphorus) to
avoid problems.

Low phosphate readings in water probably aren't a problem for plants.  
Rooted plants are able to get phosphate from the substrate and non-rooted
plants (and algae) probably are adapted to use phosphate at relatively low

Hope that helps,

Roger Miller