De-coding fertilizers

Fertilizers are food for plants, right? Technically, plants do not eat, so the scientific answer is "no."

Fertilizers are a source of plant nutrients. These can be natural or manufactured. Some soils are fertile and may supply all the nutrients a plant needs, but since this is a rare occurrence in Nevada, gardeners often try to improve crop and ornamental production by adding supplemental nutrients.

The earliest fertilizers were manures, animal and plant remains and wood ash. In addition to these, we now have multitudes of products combining things such as ammonium sulfate, ammonium nitrate, superphosphate, phosphoric acid, potassium sulfate and other ingredients to supply the 16 essential plant nutrients.

Plants obtain carbon, hydrogen and oxygen primarily from water and air. The other 13 nutrients are absorbed from the soil by plants' roots. These nutrients are nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), zinc (Z), iron (Fe), manganese (Mn), copper (Cu), boron (B), molybdenum (Mo) and chlorine (Cl). Nitrogen, phosphorus and potassium (N, P and K) are the primary nutrients. The rest are secondary or micronutrients.

Fertilizers may supply only one nutrient or many nutrients. Multinutrient fertilizers have a numerical designation or analysis on the label stating the percentage of fertilizer by weight. The first number represents nitrogen, the second represents phosphorus, and the third represents potassium. A common fertilizer is 21-0-0 -ammonium sulfate, which has 21 percent nitrogen, and no phosphorus or potassium. Often with a 21-0-0 fertilizer, there will be a fourth number on the label to indicate the percentage of sulfur in the fertilizer. Another readily available product is 16-16-16, sometimes called "sweet 16." Read the labels of fertilizers before purchasing them to see what nutrients they contain.

Fertilizers may be quick-acting, such as 21-0-0, or slow-release, such as sulfur-coated urea. Organic fertilizers, such as fish emulsion and bone meal, are slow-acting because the amount of nutrients in these products is usually in the 1-percent to 5-percent range.

Contemporary soil science indicates that simply adding fertilizers to soil will not optimize plant growth and production. Fertilizers are not cure-alls, and in fact, are often overused to the point of causing groundwater contamination. Instead of relying on fertilizers, build up the organic material in a soil, minimize compaction, plant nitrogen-supplying cover crops in the off-season, and rotate vegetable crops. After building up the soil, use fertilizers to augment a fertile soil, if needed.

For more information on fertilizers or other gardening topics, contact me, 775-887-2252 or skellyj@unce.unr.edu, or your local University of Nevada Cooperative Extension office. Check out many useful horticulture publications at www.unce.unr.edu. "Ask a Master Gardener" by e-mailing mastergardeners@unce.unr.edu.

- JoAnne Skelly is the Carson City / Storey County Extension educator for University of Nevada Cooperative Extension.