By Robert Steinkamp
Reading fertilizer labels would be a good cure for insomnia. Most of you only dig into a fertilizer label only when the need arises and specific information is needed. Let us review what is on the label, why it is there, and the
potential usefulness of the information.
Most fertilizer bags are covered with a lot of printed information. Soluble fertilizer bags show suggested application rates, and instructions for making concentrates for various injector ratios and other directions for use. Controlled release fertilizer (CRF) bags show incorporation and top-dress application rates, and information on release characteristics. Every bag of fertilizer also shows a statement of guaranteed analysis and the ingredients used to make for formulation (“Derived from” statement), required by state law. The guaranteed analysis is the official label.
Fertilizers are identified by a 3 number designation termed the “grade” 20-10-20, for example. The first number, 20, indicates the amount of nitrogen contained in the fertilizer as a percentage by weight. On the label, it is shown as “Total Nitrogen (N)”. 100 pounds of this fertilizer contains 20 pounds of elemental nitrogen (N).
The second number, 10 in our example, is used to show the amount of phosphorus contained in the fertilizer. On the label, the terms “Available Phosphoric Acid (P2O5)”, Available Phosphate (P2O5)” and Available Phosphorus (P2O5)” can be used interchangeably. The amount of phosphorus is expressed as percent by weight P2O5 (phosphate). 100 pounds of 20-10-20 would contain 10 pounds of P2O5.
The third number, 20 in our example, indicates the amount of potassium in the fertilizer. On the label, it can be shown as “Soluble Potassium (K2O)” or “Soluble Potash (K2O)” expressed as percent by weight K2O (potash). 100 pounds of 20-10-20 would contain 20 pounds of K2O.
This is where the confusion can start. It is easy to understand that 100 pounds of 20-10-20 contains 20 pounds of elemental nitrogen (N). The 100 pounds also contains 10 pounds of phosphate (P2O5). This is not the same as elemental phosphorus (P) that we are accustomed to seeing in soil test results. Because phosphate (P2O5) is heavier than phosphorus (P), a factor must be used to convert to the more familiar P (phosphorus):
-phosphate (P2O5) multiplied by 0.43 equals P. 100 pounds of 20-10-20 contains only 4.3 pounds of P.
Potash (K2O) is not the same as the more familiar K (potassium) and also requires conversion. To convert potash to K:
-potash (K20) multiplied by 0.83 equals K. 100 pounds of 20-10-20 contains 16.6 pounds of K.
So, 100 pounds of 20-10-20 contains 20 pounds of N, 4.3 pounds of P and 16.6 pounds of K. The actual N-P-K analysis is 20-4.3-16.6. This conversion is necessary for any calculations involving parts per million.
After the 3 number analysis, the label goes into more detail. The total amount of N is broken out into the various sources. The percent by weight of N derived from nitrate is shown as “Nitrate Nitrogen” and that from ammonium as “Ammonium Nitrogen”.
Some fertilizers, like 20-20-20, contain urea as a nitrogen source. The percent by weight of urea derived N would be shown as “Urea Nitrogen” or “Water Soluble Nitrogen” on the label.
The term “Water Insoluble Nitrogen” is seen on some granular top-dress type fertilizer labels. This indicates the percent of urea derived N from urea formaldehyde (UF), methylene urea (MU) or isobutylidenediurea (IBDU), which are urea-containing uncoated slow release fertilizers.
The breakout of Total Nitrogen (N) into the various N sources allows the ratio of nitrate N to ammonium and urea N to be calculated. This N breakout is important because you can change the N source ratio as a crop toning strategy. A common strategy is to increase the ammioniacal (ammonium and urea content) when soft, lush growth is desired. However, one should be cautious about too much ammonical nitrogen. A common recommendation is that no more than 40% of the N source be ammoniacal N when a fertilizer is used regularly. This is especially true in northern climates or during cooler times of the year. Some growers switch away from ammonium and urea containing fertilizers during the winter to ensure problems do not arise with toxicities as well as help maintain toned plant growth. Also, changing the ratio of nitrate N to ammonium and urea N influences the way the fertilizer affects the pH of the growing mix and so can be used in pH management.
On some formulations, the secondary nutrients magnesium (Mg), calcium (Ca) and sulfur (S) are guaranteed on the label in elemental form as percent by weight. It is important to note that not all fertilizers contain the secondary nutrients. Your water quality should be used to determine if the water soluble fertilizer you choose needs these nutrients. Just because they are called “secondary” doesn’t mean they are any less important, it just means they may not be needed in as great of quantity and may be readily supplied by your water source.
Fertilizers can contain trace elements not shown on the label. For an element to appear on the label, it must meet minimum concentration requirements. Fertilizers are regulated at the state level, and the various state labeling laws are similar but not identical. Some of the micro nutrient levels in soluble fertilizers are too low to meet the minimum required for guarantee so they cannot be listed on the label. This is why you will often see lawn fertilizers listed with just the N,P, & K concentrations. However, exceptions are made for soluble fertilizer products where the label shows the phrase “For Continuous Liquid Feed Programs”, and micro nutrients are listed.
As with the primary and secondary nutrients, the micro nutrients are listed in elemental form as a percentage by weight. A fertilizer might contain 0.10% iron, meaning that 100 pounds of fertilizer would contain 1.6 ounces of iron. Some fertilizer formulations contain higher micro nutrient levels than others. Of particular importance is the boron level because plants have a narrow tolerance range for that element. Because some irrigation water sources contain high boron levels, some growers must pay close attention to the amount of boron contained in the fertilizer.
Next on the label is the “Derived from” statement, listing the fertilizer material sources of the nutrients guaranteed on the label. In short, these are the ingredients that make the fertilizer.
Another useful statement is listed only on soluble fertilizer labels- the potential acidity or potential basicity of the material expressed in pounds of calcium carbonate (limestone) per ton. This is very important because it indicates how use of the fertilizer affects the pH of the growing mix.
For example, 20-20-20 has a potential acidity of 597 pounds of calcium carbonate per ton. This means that 597 pounds of calcium carbonate would be needed to neutralize the acidity generated by 1 ton of this fertilizer. In contrast, 20-10-20 has a lower potential acidity per ton, 422 pounds of calcium carbonate. While both fertilizers would have an acidifying effect on the growing mix, 20-10-20 would have less because its potential acidity is less.
Some fertilizers like 15-0-15 have potential basicity, meaning use of the fertilizer can cause the growing mix pH to go up. The potential basicity of 15-0-15 is 418 pounds of calcium carbonate per ton. This means that the use of 1 ton of this fertilizer would have the same pH raising effect as the application of 418 pounds of limestone. The fertilizer’s effect on growing mix pH is a very important factor in nutritional management.
These cryptic fertilizer labels provide much useful information. The 3-number analysis can be used for parts per million calculations. The nitrogen source breakout can be used for seasonal fertility adjustments or for managing crop toning. The presence or absence of calcium, magnesium and sulfur can be determined to match your water quality And, for most fertilizers, the concentration of micro-nutrients are listed, and with soluble fertilizer, the potential effect on growing mix pH can be estimated.