Fertilizers

Gardeners are often confused by terminology as it applies to commercial fertilizers. Such terms as mixed fertilizer, balanced or unbalanced, complete or incomplete have little or no meaning to many gardeners. Then when it comes to numbers on the fertilizer bag, such as 8-8-8 or 10-6-4, they are even more confused as to the meaning of these figures. We will try to explain to you what these terms and figures mean as they apply to fertilizer.

A fertilizer is a material which contains one (or sometimes two or three) of the major plant food nutrients--nitrogen, phosphorus, and potassium.

When two or more fertilizer materials are mixed, the product is called a "mixed fertilizer." If the mixed fertilizer contains all three of the major plant nutrients (nitrogen, phosphorus, potassium) it is called a "complete fertilizer". When only one or two of the nutrients are present it is referred to as an "incomplete fertilizer."

A "balanced" fertilizer is one containing the three major nutrients in equal amounts. An 8-8-8 fertilizer is a "complete-balanced" fertilizer since it contains all three major nutrient elements and all three are present in equal amounts.

A "complete-unbalanced" fertilizer is one that contains all three major nutrients but each is not present in equal amounts. A 10-6-4 fertilizer is an example of a "complete-unbalanced" fertilizer.

Now, to get down to the specifics of what the numbers on the bag mean. The term 8-8-8 is the analysis or grade designation of a commonly available complete and balanced fertilizer. The grade or analysis of a fertilizer gives the percentage by weight of each of the three fertilizer nutrients. The percentages are expressed in terms of total nitrogen, available phosphoric acid and water-soluble potash. Thus, an 8-8-8 fertilizer contains 8 percent total nitrogen, 8 percent phosphoric acid and 8 percent water-soluble potash. The nutrients are always listed in this order on the label attached to the fertilizer.

A 100-pound bag of 8-8-8 obviously contains 8 pounds of nitrogen, 8 pounds of phosphoric acid and 8 pounds of potash, or a total of 24 pounds of plant nutrients. The remaining 76 pounds consist of a filler or a carrier of some kind. These fillers may or may not have some value as a source of plant nutrients. Some fertilizer manufacturers include micronutrients in the filler. The micronutrients iron, manganese, zinc, copper, chlorine, molybdenum and boron are required in very minute quantities and less often than the nitrogen, phosphorus and potassium.

Two questions may come to mind. These are: Should you use a complete-balanced fertilizer or a complete-unbalanced fertilizer? How much should you use?

Generally, a complete-balanced fertilizer should be used. Without a soil test or years of observation and experience you have no way of knowing which of the three major nutrient elements may or may not be needed. Therefore, a complete fertilizer is recommended to take care of any possible deficiency of the three major nutrient elements.

As to how much to use, we do not know. Again, without a soil test, definite recommendations as to amounts to apply can not be given. However, general recommendations as to type and amount of fertilizer to apply are available from your county agent's office.