What do those numbers on bags and boxes of fertilizer and soil amendments really mean?  And what is the difference between a fertilizer and an amendment?

Whether a material is considered an amendment or a fertilizer depends on its effect on plant growth.  Amendments, such as peat moss or leaf mold, affect plant growth by conditioning the soil.  Fertilizers, on the other hand, influence plant growth by improving the supply of available nutrients in the soil.  Many organic products (for example steer manure), act as both amendments and fertilizers because they fertilize the soil with nutrients and amend it with organic materials which improve soil tilth and water retention.

All commercial fertilizers are labeled with three bold numbers, indicating the percentage of primary plant nutrients that they contain.  The first number is the percentage of nitrogen (N), the second number is the percentage of phosphorus (P - in the form of phosphate, P2O5) and the third number indicates the percentage of potassium (K - in the form of potash K2O).  Nitrogen, phosphorus and potassium (N-P-K) are the primary nutrient elements needed in the greatest amounts by plants.  Therefore, a bag of 15-10-6 fertilizer contains 15% nitrogen, 10% phosphate and 6% potash.

Nitrogen is an important component of proteins, enzymes, chlorophyll and DNA.  It is essential for photosynthesis, plant metabolism, and many growth and development processes.  Symptoms of a nitrogen deficiency in plants may include slowed growth; pale, yellowing leaves (a condition called chlorosis); and the tips and margins of older leaves turning brown and dying.  Because they tend to be heavy “feeders,” many vegetables other than legumes (beans and peas) are prone to this condition.  Nitrogen deficiency can be addressed by amending the soil with manures, which are high in nitrogen.  Of manures, chicken manure generally has the highest nitrogen content, at 3-5-1.5.  Other organic fertilizers with high nitrogen content are blood meal (13-2-3) and fish meal (10-6-0).

Phosphorus is a constituent of proteins, cell membranes, enzyme systems and DNA.  It is essential for energy systems and photosynthesis, and stimulates root and seed formation.  Abnormally dark green, purple, or bronze foliage is a symptom of a phosphorus deficiency, as is browning along leaf margins.  A phosphorus deficiency may also result in poor fruit or seed development.  Phosphorus deficiency is relatively uncommon, but grasses, carrots, lettuce, spinach and apples are plants that seem to be more highly susceptible.  Here, too, chicken manure is a good organic amendment, as it is high in phosphorus, while bone meal (4-23-0) is the organic fertilizer containing the greatest amount of phosphorus.

Potassium is necessary for transport of materials across cell membranes, the opening and closing of leaf pores, the movement of water and sugar up and down stems, the division of cells, and the manufacture of starches and proteins.  It also improves the size and quality of fruits and increases disease resistance.  A potassium deficiency causes leaf tips and margins to curl upward, brown and die, starting with older leaves.  Plants may have weak stems that fall over, or “lodge,” and may have poor fruit or seed development.  Among vegetables and fruit crops, this deficiency is most common in potatoes, broccoli, tomatoes, apples and raspberries.  It is also common in almonds and prunes in Butte County.  As a soil amendment, steer manure (1.5-1-2.5) has slightly more potassium than chicken manure.  Wood ashes (0-2-6) are high in potassium, but are highly alkaline and should be used with caution.  Other organic fertilizers containing potassium are blood meal and cocoa shell meal (3-2-3).  It should be noted however, that cocoa shell meal can be toxic to dogs and other animals and should be avoided by pet owners for this reason.

In addition to nitrogen, phosphorus and potassium, other elements may be listed on fertilizer labels.  It has been established that there are 14 other essential elements necessary for plant growth.  The secondary nutrients (those needed in smaller quantities than N, P and K), are calcium (Ca), magnesium (Mg) and Sulfur (S).  Micronutrients, needed in trace amounts by plants, are boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni) and zinc (Zn).  All of these nutrients (including N-P-K) are taken up by plant roots only when they are dissolved in soil water.  Three final elements, carbon (C), hydrogen (H), and oxygen (O), are supplied through the uptake of air and water by the plant.  Carbon is absorbed through pores in the leaves (stomata) in the form of carbon dioxide (CO2), while plants obtain hydrogen and oxygen from the breakdown of water (H2O) during the process of photosynthesis.  Although plants need much higher concentrations of primary and secondary nutrients, all 17 essential nutrients must be available for healthy plants.

This information is based on Dennis R. Pittenger, California Master Gardener Handbook (Oakland, CA: University of California, Agriculture and Natural Resources Communication Services, 2002).

UC Master Gardeners of Butte County are part of the University of California Cooperative Extension (UCCE) system. To learn more about us and our upcoming events, and for help with gardening in our area, visit our website. If you have a gardening question or problem, email the Hotline at mgbutte@ucanr.edu (preferred) or call (530) 538-7201.