The Mystery of NPK
As promised, I’m starting this thread to compliment Mike’s excellent ‘The mystery of pH’ thread. I’ll keep it as simple as possible, otherwise it with develop into a chemistry class.
Basically NPK is: N=nitrogen P=phosphorous and K=potash, the three main food elements necessary for plant life. Nitrogen is required for foliage growth, phosphorous for root growth and potash for growth of flowers and fruit.
These elements can be applied in chemical or organic form…..the latter being preferred by most gardeners and the former mainly used by commercial growers.
Although there are several types in there chemical form, the most common available to us gardeners being, Sulphate of Ammonia = N (nitrogen) Superphosphates = P (phosphorus) and Sulphate of Potash = K (potash)
For an example, blood, fish & bonemeal (a favorite amongst gardeners) offers these properties.
Dried blood is a slaughter house by-product and is an excellent source of quickly available organic nitrogen, when used as a top dressing or watered in. Dried blood completely soluble and can be mixed with water and used as a liquid fertilizer.
Fish meal is a great natural fertilizer, high in phosphorous and high in organic nitrogen. Fish meal is quick acting, offering a sustained supply of nutrients.
Bone meal is used as a long-lasting source of phosphorous as well as low levels of nitrogen, potassium and calcium. The extremely slow availability of nutrients from bone meal makes it a very safe fertilizer, especially when planting of potting very young or new plants.
Posts
The way I think of this is -
P - For the roots
N - For the leaves
K - For the flowers/fruit
simples.
Thanks David
In the sticks near Peterborough
I'll let you off then Mike
(I was only joking anyway 
). Hope you feel better tomorrow 
"Although there are several types in there chemical form"......I wouldn't believe I penned that...so sorry!
Thanks for this thread - always up for learning the science in bite size chunks !
A question - what does seaweed extract do on the NPK front? And things like tomato feed are high in K - but do they have any of the N or P too?
Chicky - Yes there are the other two elements in tomato feed. The analysis of say Tomorite (you need to read the labels) is 4-3-8.... the figure 8 indicating the potash content.
As far as I know, seaweed contains nitrogen (N) & trace elements, but negligible amounts of the other two.....I would need to research it to be absolutely sure.
It seems liquid seaweed has an NPK rating of 0-0-1, so I was right. That doesn't mean that it's anything other than an excellent organic feed.