Issue: July 6, 2002

Fertilizer

As I read the label on my fertilizer bag, I remembered something that I learned in school many years ago - that plants need more than just nitrogen, phosphorus, and potassium. Those three minerals are all that this bag of fertilizer contains. Where can I get the others?

There are many other nutrients needed by plant besides nitrogen (N), phosphorus (P), and potassium (K). It is just that N, P, and K are the nutrients most often deficient in soils, so these are the nutrients most commonly available in bags of fertilizers. Plants also need large amounts of hydrogen and oxygen that are readily available in water, so we apply these when we water. Plants also need a lot of carbon that is available in carbon dioxide. We provide this nutrient each time we exhale. Except for carbon, hydrogen and oxygen, all other plant nutrients must be extracted by plants from the soil.

In New Mexico, we often see nutrient deficiency problems in some of the nutrients other than N, P, and K. Iron deficiency appears as chlorosis, or yellowing of leaves while the veins in the leaf remain green. Iron chelate compounds are applied to alleviate this problem. Zinc deficiencies are also observed and zinc chelates are also available. A chelate is an organic molecule that loosely binds to the nutrient needed. This nutrient is easily given up to plants by the chelating compound.

Unfortunately, it is also given up easily to the soil. Some nutrients are present in our soil in excess. One of these is calcium that can react to form insoluble compounds with other nutrient elements. Thus, the element may be present in the soil but since it is not soluble, it is not available to the plants.

Nutrient availability is often the cause of nutrient problems in New Mexico. In addition to chemically binding with other nutrients, calcium and other minerals common in our soil cause our soils to be alkaline. In alkaline soils, many minerals do not readily dissolve. Since plants must absorb minerals in dissolved form, they are not available to the plants even though they are in the soil. Some plants are better able to extract minerals from alkaline soils; others are less able. Plants native to alkaline soil regions do better in our soils. Common vegetables and flowers, however, do better in slightly acid soils. For these plants, we must supply the unavailable nutrients in the form of fertilizer, or modify the soil to release those nutrients.

We can modify the soil by adding organic matter. Organic matter consists of material being decomposed to humus and humic acids. The decomposition or organic matter releases carbon dioxide into the soil. When dissolved in water, carbon dioxide forms a weak acid that acidifies the soil. The carbon dioxide with the humic acids release nutrients bound in the soil. We can also apply acidifying agents, usually compounds of sulfur. By acidifying soil, we temporarily make minerals more available to the plants. In time, the soil again becomes alkaline, so we must continue to modify the soil to grow plants needing more acidic soils.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!