NPK ratios?

Thu, 31 Jan 2008 02:42:42 PST
Jim, Hannon,

most phosphorus is supplied in nature as animal wastes (guano is high in 
PO4) and will typically not be readable as a major component of most 
soils.  It simply is removed very quickly and does not easily combine 
with other soil elements.  Nitrogen can combine with calcium rather well 
and remain stored. Not that I think that it is overly important in the 
ratio, just that its very nature is quite different than nitrogen or 

As to its importance, it is further down the line of nutrients, but is 
very neccessary for all plant developement.

Jim mentioned that ammonias are not taken up directly by plants, which 
is quite true.  Ammonia (NH4, that is nitrogen with 4 hydrogen atoms 
attached) is first converted to NO2 via bacteria.  This is the first 
step of nitrification.  Plants are still incapable of using this 
compound.  It is further brocken down to NO3 via other bacteria and then 
is available to plants as their main energy source.

The total equation runs something like this:

2NH4 + 2OH + 3O2  >  2H+  + 2NO2-  from which the H (hydrogen) 
evaporates/recombines leaving 2NO2-  + O2  >  2NO3 , with nitrate as the 
end product.  The process is understood, but the actual bacteria are 
still not 100% confirmed.  It may be various types in various 
environments, as this is a fundimental process found in all organic systems.

As to ratios in general, I believe one must consider what they wish to 
achieve and how they plan on doing that.  A plant in a container is not 
really a very natural thing.  The potting mixes, other than providing 
nutrients and a specific type of substrate for the plant, also provide a 
specific type of environment for other life forms.  A classic case is 
with potted orchids.  When they are planted in pine bark they require 3 
times the nitrogen as when they are planted in a non reactive medium.  
The bacteria found in pine bark take up nitrogen to degrade the bark at 
an enourmous rate!  The catch is, when the bark is finally digested by 
the bacteria, the nitrogen is once again available, often burning the 
roots of the plant!  A reason why repotting is an important issue in 
general.  It is not just a case of providing new material for the plant, 
rather a complete exchange to remove dangerous wastes, as well.

As bulbs are largely resting plants, with only a handful being quasi 
evergreen, they are storage oriented.  This would indicate a larger 
requirement of PO4, which, assumedly in nature, is provided by animal 
droppings diluted and transported via the seasonal rains.  Although I am 
not certain, it is possible that these same rains dissolve potassium 
(K+) from the soil.  I take it this reaction will largely hang from the 
soil type.

Something that lit a little light in my head was Jim's mention that 
bacteria are a general enemy of bulbs.  Of course, this is generalized, 
as there are beneficial bacteria involved, it's just dependant on the 
view point!  Could feeding ammonia based fertilizers exacerbate the 
bacterial problems?  Clearly, the plants cannot profit directly from 
ammonias, rather require a converted form (nitrates) to process.  This 
would tend to say that, especially for potted plants in a closed 
environment, that urea is a poor nutrient!  It is ammonia and must first 
be assimilated by bacteria before the plants can profit from it.  In the 
open garden this may not be a problem, but in a pot, it could simply 
feed the bacterias and damage the plant.

Just a few thoughts,

by the way, I always choose a lower nitrogen fertilizer, such as a 
14-9-15 (+2Mg +Tr) time release (8 month), but that is only low in 
relation to the potassium!  I use this with all my potted plants.

Jamie V.


Köln (Cologne)
Zone 8

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