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  1. #1
    OllieMartin's Avatar
    OllieMartin is offline Early Fruiter
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    Default Trying to understand NPK ratings (esp. manure)

    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?
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  2. #2
    OllieMartin's Avatar
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    Default

    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
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  3. #3
    Bigmallly's Avatar
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    Default

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

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

  4. #4
    OllieMartin's Avatar
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    Default

    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
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  5. #5
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    Default

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

  6. #6
    OllieMartin's Avatar
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    Quote Originally Posted by taff View Post
    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.
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  7. #7
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    Quote Originally Posted by taff View Post
    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
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  8. #8
    OllieMartin's Avatar
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    Quote Originally Posted by Nicos View Post
    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
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