Trying to understand NPK ratings (esp. manure)

[Pies]

Bit more info

Understanding Fertilisers from the National Vegetable Society

[OllieMartin]

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]

Bit more info

Understanding Fertilisers from the National Vegetable Society

Thanks for that, swiped some more info

[Nicos]

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

db stands for 'dangling bond'

....whatever those are!!!!

[OllieMartin]

db stands for 'dangling bond'

....whatever those are!!!!

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]

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]

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]

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]

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.