Who can explain NPK please?

[Bigmallly]

I think it all depends what you are growing Martin. For leafy plants you are looking at a higher Nitrogen content. For roots it will be the Phosphorous content you will be more interested in & for fruit/flowers it's the Potassium content.

[Nicos]

My thoughts are that every plant will require differing amounts of each.
Presumably 100s of years ago they just grew in the wild ( a version of it would) where it was best suited.

Thing with GYO is that we are mixing all sorts of plants together...hence another bonus for plant rotation ( not just because of pests)...at least it gets a bit easier to remember things like you want to encourage leaves and not flowers on lettuce etc.


It all gets very confusing doesn't it?

As every one has different soil types, for the amateur gardener,it really does come down trial and error (with a basic knowledge) for your particular plot!

[Penellype]

I have no idea why they calculate the N:P:K ratio the way they do, but to do so by weight would not give an accurate comparison in any case. Nitrogen is a much lighter element than either phosphorus or potassium, and plants could not make use of any of them in their elemental form. Nitrogen is a very stable gas, forming about 78% of earth's atmosphere, but inaccessible in this form because it is so stable. Phosphorus is a toxic, unstable solid which does not exist naturally, and will ignite spontaneously on exposure to air. Potassium is a reactive metal which burns on contact with water. Fertilizers therefore contain compounds of these elements in the form of ammonia, nitrates, phosphates and oxides which are able to be absorbed by plants.

That still doesn't explain why they calculate it as they do though...

Much better to think in terms of nitrogen for leaves, phosphate for roots and potassium for flowers and fruit. I like that

[Snadger]

Does anyone here know in what proportion plants use N, P and K? I wouldn't know what an ideally balanced fertilizer would contain, but surely plants use more N than they do the other two?

My take on it is:-

Nitrogen and Potassium are the biggies and plants utilise roughly equal ammounts of each for there respective purposes.

Most soils have ample phosphates to last for many years as its not so easily leached as the other two,but it can be helpful in small amounts for root formation.

Nitrogen is mainly used by the plant through the spring and summer (leaf and stem growth) and Potassium is used mainly through the Autumn and winter (Flowering fruiting and general hardening of plant)
There is also an overlap where the plant will utilises both N & K in varying amounts.

[Kirk]

Probably more the 100% wrong but K (Potassium) has an atomic weight of 39, so taking 39gm as the start. To get the same (theoretical) number of Potassium atoms you need 31gm, Nitrogen would be 14gm.

And that is if it were the pure element, throw in a compound mix and it starts getting complex.

For chemistry to get equal "amounts" as in same number of atoms you use "moles" not grams etc. A mole is related to the atomic weight of the element. As in 1 mole of K has the same number of K atoms present as 1 mole of P, but they are different weights in gms.

Never have understood the difference between 5:5:5 and 1:1:1

[Bigmallly]

I think the higher the number the greater the %...........so although 5:5:5 has the same ratio as 1:1:1, one has 5% of each nutrient whilst the other only has 1% of each.

[Martin H]

Probably more the 100% wrong but K (Potassium) has an atomic weight of 39, so taking 39gm as the start. To get the same (theoretical) number of Potassium atoms you need 31gm, Nitrogen would be 14gm.

And that is if it were the pure element, throw in a compound mix and it starts getting complex.

So if the potassium in the fertilizer is potassium chloride, KCl, then chlorine has an atomic weight of 35 so the molecule that gives you an atom of potassium must have a weight of 39 + 35 = 74.

Similarly if the nitrogen in the fertilizer comes from sodium nitrate, NaNO3, then to get one atom of nitrogen you would need 23 + 14 + 48 = 85. So to get equal numbers of potassium and nitrogen atoms in your fertilizer you would need to mix these two chemicals in the ratio of 74 : 85 by weight.

For example.

But all this is a bit beside the point. Before we can design the right fertilizer from first principles, we would need to know
- what proportion of these elements our plants actually need for optimum growth; and
- what quantities of these elements are already available to our plants from the soil.

We can obviously answer the second question by growing hydroponically: there are no nutrients available except the ones we put in ourselves. But I suspect that the first one can only be answered by trial and error; I wouldn't be surprised if it depends on what species of plant we are growing and also at what stage of development the plant is at.

It all seems too complicated so I think I'll just stick a handful of blood fish and bone in and let the plants pick out what they want to use.