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Thread: Fertiliser Calculations

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    Senior Member ladycrofter's Avatar
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    Fertiliser Calculations

    Can someone explain the maths to me:

    I have spread 600kg 16-16-16 on 10 acres. How many "units" NPK is that? Whay exactly is a unit? Thanks

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    Senior Member davidroberts30's Avatar
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    Re: Fertiliser Calculations

    17-17-17

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    Re: Fertiliser Calculations

    Quote Originally Posted by ladycrofter View Post
    Can someone explain the maths to me:

    I have spread 600kg 16-16-16 on 10 acres. How many "units" NPK is that? Whay exactly is a unit? Thanks
    A unit is 1 cwt/acre multiplied by the % of each ingredient written on the bag.So 1cwt per acre of 16 16 16 fert is 16 units of N,16 units of P and 16 units of K per acre.You have applied 1.2 cwt per acre so that gives 1.2 times 16 units each of NPK .Which is 19.2 units each of N,P and K.
    Last edited by saltoun; 21-05-13 at 01:46 PM.

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    Re: Fertiliser Calculations

    Would it not have been better to work out how much you are giving it before you applied it!?

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    Re: Fertiliser Calculations

    Quote Originally Posted by Chae View Post
    Would it not have been better to work out how much you are giving it before you applied it!?
    now where would the fun in that be?

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    Re: Fertiliser Calculations

    1 unit per acre = 1.23kg/ha
    100 units per acre = 123kg/ha

    I think

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    Re: Fertiliser Calculations

    Quote Originally Posted by miketm150 View Post
    now where would the fun? in that be?
    I suppose it's is so cheap you are aswell just chuck it on and worry later!

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    Senior Member b slicker's Avatar
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    Re: Fertiliser Calculations

    If you apply 1 cwt/acre (50 kg/acre) of a particular fertiliser containing (say)30% Nitrogen, you have applied 30 percentage units (or 30 units for short) of Nitrogen/acre.

    One unit used to weigh 1.12.lb. ( a hundreth of a cwt) But now that the cwt is 50kg, one unit weighs 1.1lb, which is half a kilogram.

    So one unit is equivalent to 0.5 kg. Which means that you have applied 15kg N/acre. Multiply by 2.47 gives 37kg N/hectare.

    Even those of us who can work in nanograms and understand the theory of relativity, apply fertiliser in units/acre, but then have to convert our figure to kg/ha so that we can adjust the fertiliser spreader accordingly. Napoleon's civil servants and Ted Heath have a lot to answer for.

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    Member choochter's Avatar
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    Re: Fertiliser Calculations

    My brain hurts.

    LC, you've put on 1.2 cwt per acre. Giving 24 kg/ha N. Which is not really enough for a cut of hay/haylage. Might be OK for grazing & you can put more on later.
    I put on double that this spring on silage ground and I'm quite frugal with the bagged fert.

  10. #10
    Senior Member ladycrofter's Avatar
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    Re: Fertiliser Calculations

    Quote Originally Posted by Chae View Post
    Would it not have been better to work out how much you are giving it before you applied it!?
    b_slicker thanks, did someone say somewhere that grass takes up 2unitsN/day?

    You are too cheeky there chae, I know what works on it, I just have never got my head around the unit thing. SOil test was low on stuff so changed from 20-10-10.

    Thanks guys, I will have to study this later. choochter we have a massive undercrop of clover and it must make a difference although I am doubtful that it is the incredible 200kgN/acre or whatever was quted elsewhere.

    P.S. for all you lime affectionados, yes, 2T/acre two autumns back based upon soil test.

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    Re: Fertiliser Calculations

    I start my calculations with the anticipated cutting date, this year I went for the 1st wk in June (usually 3rd/4th wk in May), then count the days from the date of spreading, minus 10 days for uptake, then work on 2kg Nitrogen use per Ha.

    agreed with choochters comment re the OP's rate. Doesn't want much more for making hay mind....it could end up been a bit 'firey' with more on!!

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    Re: Fertiliser Calculations

    100kg acre here 25-5-5 gives a decent crop with rain allowing 8-10 weeks growing.

    Re-seeds and well managed long terms leys do seem to pay. 600kg on each got me 110 rounds off 8 acres and 80 odd off 6. Burnt out PRG performing very poorly this year even with a good dose at 120/kg.

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    Re: Fertiliser Calculations

    The Scottish Agricultural College used to produce a small leaflet about fertilising grass. Don't know if they still do it. But by the time I've worked out units/acre and worked out how to set up the fertiliser spreader, it is easier to put it on a low setting and drive round and round (or at least up, down, and crossways) until it has all gone! (But I'd never admit to doing that on here.).

  14. #14

    Re: Fertiliser Calculations

    Thought I was the only one to do that!

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    Re: Fertiliser Calculations

    Definitely not.Was doing it this morning because the holes in the bottom of the spreader where the fert drops out of must have shrunk since yesterday.Same setting,same speed,same fert,half the rate.Infuriating.

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    Member choochter's Avatar
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    Re: Fertiliser Calculations

    Quote Originally Posted by Giles1 View Post
    Definitely not.Was doing it this morning because the holes in the bottom of the spreader where the fert drops out of must have shrunk since yesterday.Same setting,same speed,same fert,half the rate.Infuriating.
    If you've got an old crock of a spreader like I have, are you sure the ram + opening mechanism was working properly? Mine wasn't but a good sqooshing of WD40 helped.
    My spreader and I, after a five year dialogue, have finally agreed a setting and speed to spread 2 cwts/acre.

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    Re: Fertiliser Calculations

    I have tried and tried and tried to get my head around the metric system when it comes to fertiliser, but all my efforts have come to nothing, not least because you might as well be speaking arabic to a farmer than talking in kg/ha.

    It is far more straight forward to work in the old school way. A hundredweight is 50 kilos or a 'bag'. The numbers on the 'bag' correspond to how many units you will get by putting on a 'bag'. Each bag or multiple thereof is just added up. IE in applying two bags of AN you will get 2 x 34.5 units = 69 units.

  18. #18
    Senior Member ladycrofter's Avatar
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    Re: Fertiliser Calculations

    My brain is going to explode.

    600kg 16-16-16 on say 10 acres more or less.

    60kg/acre divided by 50kg = 1.2units/acre of fert.

    1/3 of that = 1.2 x .3 of N = .36units/acre????

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    Re: Fertiliser Calculations

    Quote Originally Posted by Uwork4menow View Post
    I have tried and tried and tried to get my head around the metric system when it comes to fertiliser, but all my efforts have come to nothing, not least because you might as well be speaking arabic to a farmer than talking in kg/ha.

    It is far more straight forward to work in the old school way. A hundredweight is 50 kilos or a 'bag'. The numbers on the 'bag' correspond to how many units you will get by putting on a 'bag'. Each bag or multiple thereof is just added up. IE in applying two bags of AN you will get 2 x 34.5 units = 69 units.
    For every 100kg/ha of product you apply it gets 'the number on the bag' of actual N P or K. A lot feckin easier than working out bags to the acre these days. for example today I was applying 200kg/ha of Lithan 34.5% this equals 69kg/ha of actual N.

    Bags (and units) would have been easier when filling the spreader or drill with wee bags I admit.

    Problems arise when people mix the two systems like kg per acre!!

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    Re: Fertiliser Calculations

    Quote Originally Posted by SPACE COWBOY View Post
    Thought I was the only one to do that!
    can always put more on is my theory but u can't take it off

  21. #21
    Dave Applesquasher
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    Re: Fertiliser Calculations

    Quote Originally Posted by ladycrofter View Post
    My brain is going to explode.

    600kg 16-16-16 on say 10 acres more or less.

    60kg/acre divided by 50kg = 1.2units/acre of fert.No, 1.2 cwt/acre (cwt = hundredweight)
    Your fert had 16 units of N, 16 of P and 16 of K per cwt
    So 1.2 x 16 = 19.2 units per acre


    1/3 of that = 1.2 x .3 of N = .36units/acre????

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    Re: Fertiliser Calculations

    Quote Originally Posted by Giles1 View Post
    Definitely not.Was doing it this morning because the holes in the bottom of the spreader where the fert drops out of must have shrunk since yesterday.Same setting,same speed,same fert,half the rate.Infuriating.
    Did you leave any in to get damp overnight?

  23. #23
    Dave Applesquasher
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    Re: Fertiliser Calculations

    You guys should think yourselves lucky with rams and things on your spreaders.
    Yesterday and today I spread a full 600kg bag using nothing more than a bucket and cup.
    I'm sure you are more experienced with mountains than me but they were damned steep fields I was on.
    My calf muscles feel like someone put them on upside down and my left arm is definitely longer than it was!

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    Re: Fertiliser Calculations

    Quote Originally Posted by charlieboy View Post
    Did you leave any in to get damp overnight?
    No,cleaned and dieseled after every days use.Think it may be atmospherics and perhaps different batches of the "same" fert.Happened before.Spreader mechanically sound.And no its not because 600kg bags contain more than the old 500kg ones.....................

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    Senior Member Cowabunga's Avatar
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    Re: Fertiliser Calculations

    Quote Originally Posted by saltoun View Post
    A unit is 1 cwt/acre multiplied by the % of each ingredient written on the bag.So 1cwt per acre of 16 16 16 fert is 16 units of N,16 units of P and 16 units of K per acre.You have applied 1.2 cwt per acre so that gives 1.2 times 16 units each of NPK .Which is 19.2 units each of N,P and K.
    Yes, 19.2 units of each. Which is next door to bugger-all really.
    The Duck 2015

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    Re: Fertiliser Calculations

    Quote Originally Posted by kennyo View Post
    For every 100kg/ha of product you apply it gets 'the number on the bag' of actual N P or K. A lot feckin easier than working out bags to the acre these days. for example today I was applying 200kg/ha of Lithan 34.5% this equals 69kg/ha of actual N.Bags (and units) would have been easier when filling the spreader or drill with wee bags I admit.Problems arise when people mix the two systems like kg per acre!!
    I agree, but the thing is, everyone, young or old, who I talk to still talks in bags/acre or units.What you can't do is have a conversation in kg/ha and units. Everyone understands bags/acre or units. Its far more idiot proof I think. I let the farmer wear his brain out when it comes to spreading though I still remember the x by 0.8 or 1.25 bit!!!

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    Senior Member Cowabunga's Avatar
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    Re: Fertiliser Calculations

    Quote Originally Posted by Uwork4menow View Post
    I have tried and tried and tried to get my head around the metric system when it comes to fertiliser, but all my efforts have come to nothing, not least because you might as well be speaking arabic to a farmer than talking in kg/ha.

    It is far more straight forward to work in the old school way. A hundredweight is 50 kilos or a 'bag'. The numbers on the 'bag' correspond to how many units you will get by putting on a 'bag'. Each bag or multiple thereof is just added up. IE in applying two bags of AN you will get 2 x 34.5 units = 69 units.

    Yes, that's the way it works.

    I work everything out in units per acre but because my spreader insists on an input of kgs/ha the cwts/acre [50kg bags per acre] figure I get [worked out from the desired units/acre] is multiplied by 125.

    So 2cwts/acre is 2*125=250kgs/ha
    The Duck 2015

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    Re: Fertiliser Calculations

    Yes surely you want three 'bags' an acre to be doing some good. I've never seen fert in cwt's but still think well I need 4-5 bags an acre total for my wheat then multiply by 125 to satisfy my spreader control box each pass.

  29. #29
    Dave Applesquasher
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    Re: Fertiliser Calculations

    Talking in "bags to the acre" is even worse.
    It's OK as long as you know you are talking about 1cwt bags but if you don't you are in for an environmental disaster.
    My son is 24, I don't think he has ever seen a 1cwt bag. Tell him to put 3 bags to the acre and see what happens.

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    Re: Fertiliser Calculations

    Quote Originally Posted by ladycrofter View Post
    P.S. for all you lime affectionados, yes, 2T/acre two autumns back based upon soil test.
    Would be more benficial to know the current ph though, might not mean a thing. I stuck 10t/ac on limex 45 on some peat 2011, not sure it would start with a 6 though now

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