Berks County Cooperative Extension -

FERTILIZER

Fertilizer defined.

A fertilizer is simply a substance that is added to the soil to increase fertility.

What do the numbers on the bag mean?

On all fertilizer bags, three numbers (fertilizer ratio) are prominently displayed. These numbers disclose the percentage by weight of three essential nutrients: nitrogen (N), phosphate (P₂O₅) and potash (K₂O).

A 25-pound bag of fertilizer labeled 10-10-10 contains 2.5 pounds each of nitrogen (10% of 25 pounds = 2.5 pounds), phosphate and potash. Even the most math-challenged person is likely to wonder what the bag contains aside from 7.5 pounds of nitrogen, phosphate and potash. The balance of the 25 pounds may consist of other elements, additional nutrient sources, conditioners, filler or limestone. The numbers on the bag allow home gardeners to follow specific fertilizer recommendations, such as those contained in a soil test report.

Nitrogen sources.

Nitrogen, a nutrient that encourages vegetative growth and desirable green color in plants, is present in fertilizer in two broad types: slow-release and quick-release. Slow-release nitrogen is identified on a fertilizer label by the initials "WIN" (or "CRN") and is typically expressed as a percentage of total available nitrogen. For example, a bag of 10-10-10 may display a guaranteed analysis of 4% water-insoluble nitrogen. To determine the total amount of nitrogen that is available as slow-release or water-insoluble nitrogen, the percentage of water-insoluble nitrogen must be divided by the total percentage of nitrogen and multiplied by 100. The math: 4% ¸10% x 100 = 40% water insoluble nitrogen; the balance (60%) of available nitrogen is quick-release.

Slow-release nitrogen.

Slow-release (water insoluble or controlled-release) nitrogen is available for a relatively long period of time. While factors such as microbial decomposition, physical processes and/or chemical reactions determine the rate at which the nitrogen is released, slow-release nitrogen may continue to supply a growth response effect for up to ten weeks. Soil moisture is critical to bringing about release. Slow-release nitrogen may either take the form of water insoluble nitrogen (WIN) or coated nitrogen sources, referred to generally as controlled-release nitrogen (CRN). Sulfur or polymer coated urea is an example of controlled release nitrogen. Although slow-release nitrogen fertilizers are more expensive than quick-release varieties, they may be applied less frequently and in greater amounts without the danger of "burning" associated with heavy applications of fast-release nitrogen fertilizers. Slow-release nitrogen is typically identified on a fertilizer label as ureaform, IBDU (isobutylidene urea), sulfur-coated urea or polymer-coated urea.

Quick-release nitrogen.

Quick-release (water-soluble) nitrogen, as the term implies, is, assuming adequate soil moisture, immediately available to plants. While application of this form of nitrogen results in rapid vegetative growth, the effect is short-lived. Depending upon conditions, the nitrogen will be depleted within three to six weeks following application. Frequent light applications are required in order to sustain growth. Care should be exercised to avoid heavy applications of quick-release nitrogen, as soil dehydration or "burning" may occur. Quick-release nitrogen is typically identified on a fertilizer label as ammonium sulfate, ammonium nitrate, ammonium phosphate or urea.

Source: The Penn State Master Gardener Manual
Document created: 15 December 1999; Revised: 3 March 2001