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OllieMartin · #1 (**permalink**) 14-05-2010, 12:53 PM

I'm trying to get my head around NPK ratings (yes ,for the website!), and it generally makes sense, but I've come across a discrepancy regarding their use.

Having read the Wikipedia article on it:

Quote:

Originally Posted by Wikipedia

...an 18−51−20 fertilizer contains by weight:

18% elemental (N)
22% elemental (P), and
16% elemental (K)

So because N is the only factor that doesn't need converting (as it is not in an oxide form unlike the other two), this example suggests that the fertilizer contains 18% N and the other elements are converted from their oxide states into an elemental form.
Q1: So when buying fertilizer, do we have to mentally convert the P and K content?
(Apparently UK fertilizers can show the elemental ratio, but in brackets afterwards)

Also, having browsed the internet for nutritional values of various manures I have found that sheep manure is reported as both 18-11-26 and 0.7-0.3-0.9.
So if nitrogen is the % by weight I would think that 0.7% N is more likely than a massive 18% N..
Q2: Which do I believe?
And
Q3: What's the difference in the way they're reporting these figures?

OllieMartin · #2 (**permalink**) 14-05-2010, 01:05 PM

Hmmm, just realised that 0.7-0.3-0.9 is approximately equal to 18-11-26 as a ratio, but how do we know if a bottle of liquid fertilizer is made entirely out of N, P and K, or if it only has trace amounts? Surely a ratio is just proportionate to each other unless it contained a "all other materials" comparison

Bigmallly · #3 (**permalink**) 14-05-2010, 02:28 PM

I lost the will to live around 2pm Ollie but I hope this helps.

What is NPK? – Understanding Fertiliser Labels | FluidsensorOnline.com

OllieMartin · #4 (**permalink**) 14-05-2010, 02:42 PM

Thanks it did help a lot.
So the NPK 18-11-26 is probably just a ratio and doesn't reflect the % mass of a sample of that manure, where as the 0.7-0.3-0.9 probably does.

Cheers, hope the will to live comes back before an opportunity to end it all arrised

taff · #5 (**permalink**) 14-05-2010, 03:28 PM

you could always not buy it, and use comfrey, nettles and manure instead?

OllieMartin · #6 (**permalink**) 14-05-2010, 03:32 PM

Quote:

Originally Posted by taff

you could always not buy it, and use comfrey, nettles and manure instead?

I'm a shoe-string gardener, I don't buy things I can pinch off my friend who keeps horses for free

Besides, I'm collating the infromation to try and make a definitive list of Soil Conditioners, their uses and nutrient contents on the website.

Nicos · #7 (**permalink**) 14-05-2010, 03:42 PM

Quote:

Originally Posted by taff

you could always not buy it, and use comfrey, nettles and manure instead?

I think it's a bit of research for the excellent site he's creating

I've never truely understood it to be honest apart from where each bit is aiming for in the plant!

Here's how I remember it if it's any help Ollie...

http://www.growfruitandveg.co.uk/gra...p-k_14880.html

OllieMartin · #8 (**permalink**) 14-05-2010, 04:02 PM

Quote:

Originally Posted by Nicos

I think it's a bit of research for the excellent site he's creating

I've never truely understood it to be honest apart from where each bit is aiming for in the plant!

Here's how I remember it if it's any help Ollie...

http://www.growfruitandveg.co.uk/gra...p-k_14880.html

Thanks, I already had a read of that one

tbh it looks like the NPK of manures varies wildly depending on how old the animal is, what sex it is, what it's reared for, what it's fed on, what's mixed with the manure (bedding) and how they run the tests on the ingredients. So I'm just going to have to go for ball-park figures, it's that or show a minimum and maximum for each which would end up looking something like: 0.7-18-0.3-11-.09-26 and would be as much use as a wet fart in a windy teacup...er...yea

Two_Sheds · #9 (**permalink**) 14-05-2010, 06:36 PM

Quote:

Originally Posted by OllieMartin

tbh it looks like the NPK of manures varies wildly depending on how old the animal is, what sex it is, what it's reared for, what it's fed on, what's mixed with the manure

Yep!
I think of manure as more of a soil improver than a fertiliser (feed). For those without horses, or money, there is comfrey (or borage): more nutritious than farmyard manure

Snadger · #10 (**permalink**) 14-05-2010, 07:58 PM

Quote:

Originally Posted by OllieMartin

Thanks it did help a lot.
So the NPK 18-11-26 is probably just a ratio and doesn't reflect the % mass of a sample of that manure, where as the 0.7-0.3-0.9 probably does.

Cheers, hope the will to live comes back before an opportunity to end it all arrised

It is a ratio

..............Carlsberg don't make manure, but if they did.........

OllieMartin · #11 (**permalink**) 14-05-2010, 08:21 PM

Quote:

Originally Posted by Snadger

It is a ratio

..............Carlsberg don't make manure, but if they did.........

Except if it is ONLY a ratio then that creates the following problem:

Fertiliser 1 has an NPK of 1-1-1
Fertiliser 2 has an NPK of 5-4-4

You'd assume that Fertiliser 2 is better/stronger/has more nutrients in it, but:
If it's JUST a ratio, then who says the 5-4-4 doesn't make up 1% of the fertiliser and the rest is just water, and Fertiliser 1 is 33% N, 33% P, 33% K and 1% water making it actually stronger.

There surely has to be some indicator to suggest how much (by mass or volume) that N, P & K actually take up.

D'ya get meh?

Snadger · #12 (**permalink**) 15-05-2010, 06:47 AM

Quote:

Originally Posted by OllieMartin

Except if it is ONLY a ratio then that creates the following problem:

Fertiliser 1 has an NPK of 1-1-1
Fertiliser 2 has an NPK of 5-4-4

You'd assume that Fertiliser 2 is better/stronger/has more nutrients in it, but:
If it's JUST a ratio, then who says the 5-4-4 doesn't make up 1% of the fertiliser and the rest is just water, and Fertiliser 1 is 33% N, 33% P, 33% K and 1% water making it actually stronger.

There surely has to be some indicator to suggest how much (by mass or volume) that N, P & K actually take up.

D'ya get meh?

Erm........I think so?

I see where you're coming from but lets get this out of the way first. The 0.7-0.3-0.9 you quoted earlier will still be a ratio of 7-3-9 in my opinion.
As far as I know, most fertilisers have 'carriers' which are inert materials the nutrients are impregnated into. The ratio will be as stipulated but the weight of each element, in a granular or powder fertiliser should also be stipulated on the packaging. This means that if you do a soil analysis and the results indicate you nead 'X' ammount of a single nutrient applied over a season, you should be able to calculate dozage per application to give you the required ammount annually?
I aint no mathematician, but thats the way I see it anyway!

Bigmallly · #13 (**permalink**) 15-05-2010, 08:18 AM

Quote:

Originally Posted by OllieMartin

something like: 0.7-18-0.3-11-.09-26 and would be as much use as a wet fart in a windy teacup...er...yea

If all else fails, they look like good lottery numbers.............

OllieMartin · #14 (**permalink**) 15-05-2010, 08:34 AM

Quote:

Originally Posted by Snadger

Erm........I think so?

I see where you're coming from but lets get this out of the way first. The 0.7-0.3-0.9 you quoted earlier will still be a ratio of 7-3-9 in my opinion.
As far as I know, most fertilisers have 'carriers' which are inert materials the nutrients are impregnated into. The ratio will be as stipulated but the weight of each element, in a granular or powder fertiliser should also be stipulated on the packaging. This means that if you do a soil analysis and the results indicate you nead 'X' ammount of a single nutrient applied over a season, you should be able to calculate dozage per application to give you the required ammount annually?
I aint no mathematician, but thats the way I see it anyway!

Okay that does explain it for labelled fertilisers (thank you).
But how do we know how good a manure is, even if we have a vague idea of its NPK?
I agree, I thought 0.7-0.3-0.9 = 7-3-9!

I did find this comprehensive list:
Ohio Livestock Manure Management Guide, Bulletin 604-06
But it still seems they've done the maths wrong (although more likely I have)
E.g. They say a "lactating cow producing 90lb milk a day":
Produces 153lb faeces containing just over 1lb of Nitrogen.
They also say that is "5.4% N db" (whatever "db" is)
But I calculate that as 0.65% N

Still not totally happy

Snadger · #15 (**permalink**) 15-05-2010, 09:38 PM

How about this then?

If a 100-pound bag of fertilizer has an N-P-K ratio of 5-7-4, it contains 5 pounds of nitrate, 7 pounds of phosphate (which contains phosphorous), 4 pounds of potash (which contains potassium) and 84 pounds of filler.

As far as I know, cow manure, which is mainly used as a soil improver rather than a fertiliser is roughly 1-1-1 so a 100-pound bag would add 97 pounds of bulk and 3 pounds of fertiliser.

So methinks the ratio AND percentages ARE linked.

I must admit, being the saddo that I am, I find this quite interesting!

Pies · #16 (**permalink**) 15-05-2010, 09:54 PM

Bit more info

Understanding Fertilisers from the National Vegetable Society

OllieMartin · #17 (**permalink**) 15-05-2010, 09:56 PM

Quote:

Originally Posted by Snadger

How about this then?

If a 100-pound bag of fertilizer has an N-P-K ratio of 5-7-4, it contains 5 pounds of nitrate, 7 pounds of phosphate (which contains phosphorous), 4 pounds of potash (which contains potassium) and 84 pounds of filler.

As far as I know, cow manure, which is mainly used as a soil improver rather than a fertiliser is roughly 1-1-1 so a 100-pound bag would add 97 pounds of bulk and 3 pounds of fertiliser.

So methinks the ratio AND percentages ARE linked.

I must admit, being the saddo that I am, I find this quite interesting!

Me Too!

From what I read, the N ratio SHOULD be a % of the total mass. The P-K ratios aren't exact elemental percentages as they represent P2O5 and K2O, but multiplying them by 0.43 and 0.83 respectively gives the elemental figures (this is sometimes shown in brackets on fertilisers).

So long as the person demonstrating the NPK of a manure knows that the the N-P2O5-K2O ratio should be an exact percentage of the mass, then that's fine.
But when I discovered figures for sheep manure that I mentioned before I had to assume one of them was wrong as I seriously doubt any manure could have 18% Nitrogen by mass!

OllieMartin · #18 (**permalink**) 15-05-2010, 10:08 PM

Quote:

Originally Posted by Pies

Bit more info

Understanding Fertilisers from the National Vegetable Society

Thanks for that, swiped some more info

Nicos · #19 (**permalink**) 16-05-2010, 03:42 AM

Quote:

Originally Posted by OllieMartin

They also say that is "5.4% N db" (whatever "db" is)

db stands for 'dangling bond'

....whatever those are!!!!

OllieMartin · #20 (**permalink**) 16-05-2010, 08:04 AM

Quote:

Originally Posted by Nicos

db stands for 'dangling bond'

....whatever those are!!!!

Quote:

Originally Posted by Wikipedia

When a metal is deposited on silicon, these dangling bonds give rise to interface states within the energy band gap of silicon.

...umm...really?

Snadger · #21 (**permalink**) 16-05-2010, 12:48 PM

Quote:

Originally Posted by OllieMartin

Me Too!

From what I read, the N ratio SHOULD be a % of the total mass. The P-K ratios aren't exact elemental percentages as they represent P2O5 and K2O, but multiplying them by 0.43 and 0.83 respectively gives the elemental figures (this is sometimes shown in brackets on fertilisers).

So long as the person demonstrating the NPK of a manure knows that the the N-P2O5-K2O ratio should be an exact percentage of the mass, then that's fine.
But when I discovered figures for sheep manure that I mentioned before I had to assume one of them was wrong as I seriously doubt any manure could have 18% Nitrogen by mass!

I think it depends on the nitro base used i.e urea,ammonium nitrate, ammonium sulphate etc etc. All have differing percentages of N so by weight will give different feed rates. There is also useable Nitro and nitro which is inaccessible to the plant..........just to complicate the issue!

ohbeary · #22 (**permalink**) 19-05-2010, 10:11 PM

NPK, yeah OK, if you want to use chemicals to grow your food (yes I know what it means and how it works

), I prefer to rotate crops, compost anything vegetable in origin and use any manure that comes my way for free, pig manure for sandy soil, horse poo for clay soils and cow muck for general use, the bacteria in the soil will release the nutrients, there is a lot of free cardboard out there for digging in to bean trenches, moisture retention, carpet tubes make great 4-5" pots, line your heap with cardboard, it keeps it together insulates and rots down to make more compost, lets just keep it natural!( urine really does activate compost, "ammoniacal nitrogen"!!)

Snadger · #23 (**permalink**) 20-05-2010, 06:15 AM

Quote:

Originally Posted by ohbeary

NPK, yeah OK, if you want to use chemicals to grow your food (yes I know what it means and how it works

), I prefer to rotate crops, compost anything vegetable in origin and use any manure that comes my way for free, pig manure for sandy soil, horse poo for clay soils and cow muck for general use, the bacteria in the soil will release the nutrients, there is a lot of free cardboard out there for digging in to bean trenches, moisture retention, carpet tubes make great 4-5" pots, line your heap with cardboard, it keeps it together insulates and rots down to make more compost, lets just keep it natural!( urine really does activate compost, "ammoniacal nitrogen"!!)

I do keep it natural (have a bucket of pee ready now for the compost bin!) but its nice to know the science behind the good practice!

OllieMartin · #24 (**permalink**) 20-05-2010, 08:06 AM

Quote:

Originally Posted by ohbeary

NPK, yeah OK, if you want to use chemicals to grow your food (yes I know what it means and how it works

), I prefer to rotate crops, compost anything vegetable in origin and use any manure that comes my way for free, pig manure for sandy soil, horse poo for clay soils and cow muck for general use, the bacteria in the soil will release the nutrients, there is a lot of free cardboard out there for digging in to bean trenches, moisture retention, carpet tubes make great 4-5" pots, line your heap with cardboard, it keeps it together insulates and rots down to make more compost, lets just keep it natural!( urine really does activate compost, "ammoniacal nitrogen"!!)

As I pointed out in the OP title, I'm trying to get my head around NPKs especially manures. I rarely use inorganic fertilisers, only really tomato feed. Otherwise it's all manure, compost and mushroom compost.
Just been finding it difficult to establish a good 'typical' NPK for some of the organic soil conditioners because they vary so much depending on what goes into them.

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