A fertilizer is said to be a complete or mixed fertilizer when it contains nitrogen, phosphorus and potassium (the primary nutrients). Examples of commonly used complete fertilizers are 6-12-12, 10-10-10, 15-15-15 and 20-10-10. An incomplete fertilizer will be missing one or more of the major components. Examples of incomplete fertilizers are: 34-0-0 (ammonium nitrate), 46-0-0 (urea), 18-46-0 (diammonium phosphate), 0-46-0 (triple super phosphate) and 0-0-60 (muriate of potash).
Incomplete fertilizers are blended to make complete fertilizers. As an example, if 100 pounds of 46-0-0 (urea) were combined with 100 pounds of 0-46-0 (concentrated super phosphate) and 100 pounds of 0-0-60 (muriate of potash), a fertilizer grade of 15-15-20 would result. When these quantities are combined, each quantity is diluted by the other two materials by one-third, provided each fertilizer material contributed equal weight to the blend.
The fertilizer ratio indicates the proportion of N, P2O5 and K2O containec the fertilizer. The specific fertilizer ratio you will need depends on the soil nutrien level. For example, a 1-1-1 ratio (10-10-10 or 15-15-15) is widely used at planting time when both phosphorus and potassium are in short supply (soil tests low medium). When soils test high or very high in phosphorus and potassium, ratio (34-0-0 or 46-0-0) may be a more appropriate choice for use at or near planting.
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