The three primary nutrients or “Macronutrients” that most plants need are Nitrogen (N), Phosphorus (P), and Potassium (K). They are the common ingredients in most fertilizers and can be purchased in dry or liquid form. Commercial fertilizers will provide the detailed Guaranteed Analysis label of all the nutrients present in the fertilizer. On the label, there will be three numbers separated by dashes that will indicate the percentage (by weight) of nutrients in each package.
So a 50 pound bag of 5-10-5 contains:
5% nitrogen or 2.5 pounds per bag (.05 x 50lbs = 2.5lbs)
10% Phosphorous or 5 pounds per bag (.1 x 50lbs = 5.0lbs)
5% Potassium/Potash or 2.5 pounds per bag (.05 x 50lbs = 2.5lbs)
The remaining 40 lbs of the bags weight is filler (and sometime intermediate and micro-nutrients as well) to help you spread the fertilizer at an even rate over a given area.
