What is N-P-K ?

Nitrogen-N, Phosphorous and Potash (phosphate)-P, Potassium-K, if a plant is deficient in any of these, they will eventually die. See the table below which shows several organic products and their NPK ratings: (numbers are approximate and may vary depending on brands)

Nitrogen-N
(Nitrogen deficiency on maple)
Nitrogen is the first letter N on a bag of fertilizer. Nitrogen is required for vigorous green tissue growth. This is what makes plants green. Yellowing leaves on plants is an indication of nitrogen deficiency, plants will eventually die.

Phosphorous & Potash (phosphate)-P
(Phosphorous deficiency on sugar beet)
Phosphate is the 2nd letter on a bag of fertilizer. Plants need this to stimulate good strong roots. It also aids in the production of fruits and flowers. Symptoms include stunted growth and extremely slow growth rates.

Potassium-K
(Potassium deficiency on tomato plant)
Potassium is the 3rd letter on a bag of fertilizer. It helps promote less water loss in plants and helps thier roots take more water into the plant. It also aids in making plants more disease and insect resistant. Symptoms include dry leathery leaves and brown curly edges.