Topic: n .p.k

[5rod]

hi all,
I understand that the N is nitrogen, P is phosphorous and K is potash.I also understand the balance like 20-20-20 is a balanced feed and 20-10-20 is unbalanced feed, and understand why they are.
What I do not understand and would like to know if a whole is 100% then what is the break down of the rest ?, or are these figures not percentages.what are they?.
                      ::) ::)

[saddad]

Predominantly it is a ratio if the numbers are equal then the plants are getting the same amount of each. Growmore is 7:7:7 I think. Some plants prefer unbalanced eg cabbages prefer more Nitrogen. I think it isn't particularly related to the bulk 100% as most of the elements are provided in compounds which break down to release the element. As Nitrogen is a gas it is tricky to provide it as it floats off unless it is tied to another element and that makes up the rest of the weight...  :-\

[chriscross1966]

Also, phosphorous is supplied as phosphate, meaning that it is attached to three oxygen atoms and another atome for every atome of phosphorous..... plus they don't like using potassium nitrate cos it's explosive. THe problem with nitrogen isn't that it's a gas, it's the triple bond the gas has between the two atoms of nitrogen in the molecule. It takes a lot of energy or some clever fiddling with reactions to split that bond to get the nitrogen into an accessible state. The bacteria that legumes host are very good at the fiddling, the other main source of natural nitrate apart from animal wastes which isn't a primary source, just a concentration of what plants have already done, is lightning.... it just rips apart the nitrogen (and everything else) and some of it recombines with oxygen....

chrisc

[daveyboi]

I think it is actually a percentage of the amount of each element in the whole of the fertiliser. The balance is the the other chemical components and any filler used. For example a liquid feed will contain water which is why they tend to be lower ratios than dry fertilisers.

[Tee Gee]

This link might help;

http://www.thegardenersalmanac.co.uk/Data/Fertiliser/Fertilisers.htm

[Sparkly]

This may help too :)

http://www.granvilleparkallotments.org.uk/guide-to/fertilisers.html

[Vinlander]

I think it is actually a percentage of the amount of each element in the whole of the fertiliser. The balance is the the other chemical components and any filler used. For example a liquid feed will contain water which is why they tend to be lower ratios than dry fertilisers.

Correct - but the missing % on the packet aren't necessarily fillers - for example Ammonium Sulphate is a nitrogen fertiliser purely because of the Ammonia (or rather ammonium ion with one extra Hydrogen).

The Sulphate part is much more than half the weight of the molecule and since it is normally sold in a crystalline form there will also be at least as much weight of "water of crystallisation" strongly attached to each molecule...

So though it is regarded as one of the more concentrated chemical fertilisers it is nowhere near 100%.

The sulphate isn't a fertiliser at all - though it will leave the soil more acid when the ammonium is stolen by living things - which can be an advantage or a nuisance.

Actually I'm not entirely sure whether the hydrogen in ammonium is counted as 'filler' - I don't think so - I seem to remember that either ammonia or urea (slightly more nitrogen/kg) is the reference unit for nitrogen fertiliser.

[daveyboi]

It is all very interesting however maybe this will explain it better and I quote from a wikipedia article.....


Macronutrient fertilizers are labeled with an NPK analysis

An example of labeling for the fertilizer potash is composed of 1:1 potassium to carbonate by volume, or 47:53 by weight (owing to differences in molecular weight between the potassium and carbonate). Traditional analysis of 100g of KCl would yield 60g of K2O. The percentage yield of K2O from the original 100g of fertilizer is the number shown on the label. A potash fertilizer would thus be labeled 0-0-60, not 0-0-52.