Sun Gro’er Blog

What’s On The Fertilizer Label

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 How to read fertilizer labels
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.
Primary Nutrients
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.
 Secondary Nutrients
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.
 Micro Nutrients
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.

Fertilizer Changes Growing Mix pH

By Robert Steinkamp

CarnationsSummary: Limestone is added to growing media to adjust the pH to a desirable level, however other factors influence medium pH while a plant is growing—namely water quality and fertilizer usage. When considering a fertilizer’s influence on pH, a given material’s acid or basic reaction must be known. With water soluble fertilizer, it is shown on the bag. With controlled release fertilizer it can be assessed by the ingredients.
Compared to pH management, fertility management seems easy: you either fertilize or you don’t. With commercial water soluble fertilizers, the water turns a reassuring blue color. With controlled release fertilizers you can actually see and squeeze the prills (firm prills contain fertilizer while soft ones are empty). Growing mix pH, however, cannot be visually judged, even by the crudest methods, until plant growth is impacted. And no grower wants to be alerted to pH problems by plant symptoms. To monitor pH, the mix must be tested.  To manage pH, the 4 factors influencing pH must be understood. These are the: 1. mix, 2. water, 3. fertilizer and 4. crop.
Today, we will focus on these four factors and how fertilizer influences growing mix pH.
The pH is just a measure of the number of hydrogen ions in the mix, measured on a scale of 1 to 14.  If there are lots of hydrogen ions, the pH will be low, and the mix will be acidic. Few hydrogen ions result in a higher, more basic pH. When the pH is 7, the mix is neutral, neither acidic nor basic. Mix pH is important because it impacts the availability of the various ions of fertilizer nutrients.
When making a growing mix, limestone must be added to adjust the starting pH to a desired point—typically 6.0.  Usually, the lime used is crushed dolomite, a mixture of calcium and magnesium carbonate. When the mix is first watered, the dolomite starts to dissolve.  The carbonate fraction of the dolomite reacts with the water, tying up hydrogen ions and reducing their number, resulting in higher pH.  The calcium and magnesium contained in the lime do not tie up hydrogen ions but do compete for exchange sites in the mix. They do not directly influence the pH.
This initial pH adjustment consumes some of the dolomite. At pH 6.0, there are some, but not a lot, of hydrogen ions in the mix.  Because there are few hydrogen ions to tie up, the remaining dolomite is held in reserve.
Even though the pH was adjusted when the mix was made, growing mixes have only a limited ability to resist pH changes during crop production.  The three factors that can move the pH away from the starting point are the type of fertilizer used, the properties of the irrigation water and the crop being grown.
Fertilizers can be classified by their effects on growing mix pH.  Acidic or acid-forming fertilizers lower mix pH.  Basic fertilizers can cause the mix pH to increase.  A third class, neutral or non-acid forming fertilizers have no effect on pH.
Fertilizers can be mixtures of many chemical compounds. With most commonly used greenhouse fertilizers, nitrogen sources are the primary causes of acidity or basicity.  A 20-20-20 mix, for example, is a mixture of potassium nitrate, ammonium phosphate and urea. Each of the three ingredients contains a different nitrogen source. The three sources used are nitrate nitrogen, ammonium nitrogen and urea nitrogen; nitrate nitrogen has a basic affect, raising pH, while ammonium and urea nitrogen are acid forming, lowering pH.
A fertilizer’s effect on pH depends on the ratio of nitrate nitrogen to ammonium and/or urea nitrogen: the more nitrate, the more basic, and the more ammonium and/or urea, the more acidic.  This information is prominently shown on the fertilizer label. The material’s percent nitrogen will be broken out into percent nitrate, ammonium and/or urea.  For any given fertilizer, these percentages can be used to predict its effect on mix pH.
With soluble fertilizers, it is easy to determine the potential effect on mix pH.  Each bag label shows a statement of potential acidity of basicity in terms of pounds of limestone (calcium carbonate) per ton of fertilizer.  For acid-forming fertilizers, the statement indicates the poundage of limestone required to neutralize the acidity produced by 1 ton of that fertilizer, and for basic fertilizers, the poundage of limestone needed to equal the acid neutralizing power of 1 ton of the fertilizer is indicated.  The poundage numbers have no direct meaning other than for comparing one fertilizer to another. Listed below are some common soluble fertilizers along with the potential acidity or potential basicity (from most acid to most basic):
24-8-16                 potential acidity 667 pounds calcium carbonate per ton
20-10-20            potential acidity 392 pounds calcium carbonate per ton
15-16-17            potential acidity 215 pounds calcium carbonate per ton
15-5-15                 potential basicity 141 pounds calcium carbonate per ton
15-0-15                 potential basicity 296 pounds calcium carbonate per ton
By examining and comparing statements, a fertilizer’s potential to change mix pH can be judged. The greater the potential acidity number, the more acid-forming the fertilizer. Conversely, the greater the potential basicity number, the more basic the fertilizer.
Controlled release fertilizer (CRF) labels do not show potential acidity/basicity statements, however, most of the commonly used coated greenhouse type CRFs contain both nitrate and ammonium in a ratio of about 1:1, making them acidifying. Those that contain urea are somewhat more acid-forming. Higher application rates generate more acidity, and soil incorporation will affect pH faster and to a greater extent than top-dressing. The acidic effects of CRF are much weaker than those of an acidic soluble fertilizer.
In a new situation, predicting the degree to which the mix pH might change can be complex. Type of fertilizer, concentration, frequency of application, degree of leaching and time all influence a fertilizer’s effect on mix pH. Also of huge importance, though not discussed in this article, is the effect of the irrigation water’s alkalinity.  Even the plant species being grown influences the growing mix pH. For example, geraniums are acid-producing plants. Just consider the pH management problems experienced by geranium stock plant growers!
Growing mix pH is not a constant. Even though the pH is set when the mix is made, the mix has a limited ability to resist a pH change during crop production. After the initial watering-in, the mix pH is subject to change by the fertilizer used. Statements made on the fertilizer bags indicate the degree that the material will impact mix pH.  Mix pH management is easier with the understanding that mix pH is modified by a number of known factors other than the amount of lime in the mix.