Cooperative Extension
MG Manual Home

  MG Manual Reference
Ch. 2, pp. 22 - 23

[Fertilizers: fertilizers | analysis | types | organic | applying | application | improving | compost ]

Soil Fertility Report
There are 18 elements essential to plant growth. Nitrogen, phosphorous, potassium, calcium, magnesium, and sulfur are considered fertilizer macronutrients because plants require them in the largest quantity for maximum growth.Nitrogen, phosphprous and potassium are the primary nutrients, which are most likely to be present in inadequate amounts. Calcium, magnesium, and sulfur are secondary macronutrients and usually are either present in sufficient quantities or are added coincidentally with other materials (e.g., fungicides, irrigation water). The other 12 nutrients, called micronutrients, are just as important but necessary in smaller amounts. If plants lack any of these elements, they exhibit signs of nutrient deficiency. Some of these symptoms are given in the discussion of nutrients in Chapter 1 on Botany.


The analysis prouded on a fertilizer package refers to the amount of an element present in a formulation based on percentage of weight. All fertilizers are labeled with three numbers, giving the percentage by weight of nitrogen (N), phosphate (P2O5), and potash (K2O) respectively. Often, to simplify matters, these numbers are said to represent nitrogen, phosphorus, and potassium, or N-P-K. We should remember that it is not N-P-K, but N, P2O5, and K2O. Moat fertilizerscontain no P2O5, and K2O, but phosphorus and potassium are expressed as the amount of fertilizer would contain if they were in P2O5 equivalent, and K2O form equivalent. (These forms of expression are leftovers from the early days of chemistry, when all elements were expressed as oxides.) For example, if we have a 100 pound bag of fertilizer labeled 10-10-10, it contains 10 pounds of N, 10 pounds of P2O5, and 10 pounds of K2O. To convert the P2O5 content to actual phosphorous content, multiply the given weight by 0.43. To convert K2O content to potassium content, multiply the given weight by 0.83. The rest of the fertilizer's weight is filler.
Filler is important so that we can evenly spread the fertilizer and avoid burning plants with too much fertilizer.
For many years, there has been a model label law which some states have adopted for the classification of fertilizers. The law also establishes minimum levels of nutrients allowable and provides specific labeling requirements. To date, model label legislation has not met with total acceptance, so there are still differences from state to state as to what constitutes a fertilizer and the type of information on labels. Even so, the information contained on fertilizer labels has been well standardized, and the consumer is protected by state laws requiring manufacturers to guarantee the claimed nutrients.
The law requires that the manufacturer guarantees accuracy of what is claimed on the label. In some cases, a fertilizer will contain secondary nutrients or micronutrients not listed on the label because the manufacturer does not want to guarantee their exact amounts. The gardener/consumer is assured that nutrients listed on the label are actually contained in the fertilizer. On fertilizer labels, the initials W.I.N. and W.S.N. stand for Water Insoluble Nitrogen and Water Soluble Nitrogen, respectively. The water soluble nitrogen (W.S.N.) dissolves readily and is usually in very simple form, such as ammoniacal nitrogen (ammonia) or nitrate nitrogen. Nitrogen which will not dissolve readily may exist in other forms in the fertilizer. These are usually organic forms of nitrogen (with the exception of urea) that must be broken down into simpler forms before it can be used by plants.
Water insoluble nitrogen (W.I.N.) is referred to as a slow-release nitrogen source and delivers nitrogen at different rates according to the amount and kind of material in its composition.
The best fertilizer to use depends on many factors, such as the nutrients needed, soil structure, soil chemistry, and method of applying the fertilizer.

Next Next
Search Index Comment

This site was developed for the Arizona Cooperative Extension, College of Agriculture, The University of Arizona.
© 1998 The University of Arizona. All contents copyrighted. All rights reserved.