What Do NPK Values Refer To?
Most home gardeners prefer to buy their plant nutrients as pre-mixed products - the so-called complete fertilizers (often erroneously called balanced fertilizers).
Many years ago, before we knew as much as we do today about plant nutrition, three elements were said to be essential. Although we still know too little about nutrition, we do at least realize that these "Big Three" - nitrogen, phosphorus and potash - by no means supply all elements vital to growth.
Nevertheless, any mixed fertilizer containing these three elements may legally be labeled "complete".
According to law, at least in most states, such complete fertilizers must contain at least twenty units of plant food. Figures that state how many units are contained in a product must appear on the bag or package, with nitrogen (N), phosphorus (P) and potash (K) appearing in that order. It is still possible however to find the figures for the last two elements listed in reverse order.
Learn More About Soil Fertilizers & Additives:
Fertilizer Formula - feeding rates of various fertilizer formula.
Organic Nitrogen And Soil - organic forms for getting more nitrogen into your soil.
Phosphorus And Soil - phosphorus is vital to your soil of course. But bone meal is so overrated.
Fertilizer Costs - calculating the REAL cost of fertilizer.
Chemical Nitrogen In Soil - understanding nitrogen in soil, and how it is used to make fertilizer products.
Potassium And Soil (Potash Fertilizer) - potash is the common term for the fertilizer forms of potassium.
Fertilizer Burn - how to avoid burning your plants/lawn with fertilizer.