Fertilizer

With the nice growing fall/winter growing season nearly here lts take a look at fertilizers. Dubbing it the 'Anatomy of a Fertilizer Bag' lets explore the world of plant nutrients at a more technical level.

To start, plants require 16 (maybe 17 counting nickel) elements called essential to grow properly. They all must be availabe and taken in by the plant in certain quantities. If a plant can not get enough of a certain one it will suffer. Likewise, if a plant gets too much of one it suffers also.

The elements required are needed in various amounts to be adequate. Even though plants need about one one-thousandth as much molybdenum as nitrogen, if it does not get it the plant will react.

Dividing the elements into categories by amount needed we see that Carbon, Hydrogen and Oxygen needs are high but plants get them from the air and water.

The next group required in great quantities are called macro-nutrients. Nitrogen, Phosphorous and Potassium are macro-nutrients.

The next group consists of three elements that are needed is smaller quantities than the macros that are usually not applied as nutrients but as amendments to the soil. Thes are called Buffers and include Sulfur, Calcium and Magnesium. These elements aid in changing the chemistry in the soil so plants can do better at absorbing other nutrients.

The last group includes many elements needed in small quantities. Called micro- nutients they include: Zinc, Iron, Manganese, Copper, Boron Molybdenum and Chlorine.

Usually a fertilizer bag does not contain all the nutrients together. If the bag contains any of the three macro-nutients it must, by law be listed conspicuosly in order starting with nitrogen then phosphorous then potassium. The numbers represent the percentage by weight. Nothing else in the bag needs listing to meet the law.

Since nitrogen ranks number one in need in the desert, lets look at it. Most of the bag purchased for nitrogen is not a nitrogen form for plants. Yes some of it is nitrogen the rest of the material are the other halves of the ions to hold the nitrogen and fillers. A bag saying 21-0-0 insures that 21% is a form of nitrogen used by plants. In reality the nitrogen form called ammonium is attached to sulfate that has sulfur free and at no extra charge for the plant to use.

Not all nitrogen fertilizers are the same. They can vary by amount and form of nitrogen and it may seem confusing at first. Remember that plants can only take in nitrogen in three forms at the root. In descending order nitrate is first followed by ammonium and urea.

Nitrate is most readily taken in because it is repelled by soil particles and leaches quickly into the soil being available to more roots more quickly. Amonium is held tightly by soil particles near the surface. It must then undergo a microbial transformation to become nitrate to then leach to deeper roots.

Though urea can be absorbed directly the more common path is the urea molecule is broken down into ammonium and/or nitrate.

Inexpensive fertilizers only tell the percentages. More expensive fertilizers not only tell percentages but give the sources and forms of the nitrogen, how much of each form and the amount of water soluablity. The information is also listed for the other nutrients as well.

By looking at a couple more expenive bags of fertilizer and with a calculator you can see that the combinations used cover many of the fertilizer bases.

Not as easy to see is the label on a shovel of goat- or a bucket of horse manure. These organic forms though lower in nutrient percentages benefit the soil by feeding the soil organisms. Remember though, at the point of absorbtion the roots take in the nutrients in the same chemnical forms as the bagged fertilizers.

Written by Terry Mikel, College of Agriculture and Life Sciences, the University of Arizona, 602-470-8086.
Material originally appeared in Arizona Republic
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