Page 152 - AG 7-2011 Revised 2016

FERTILIZER BASICS
By: Gary King, Ph.D.

There are 16 chemical elements necessary for plants. Three of these, carbon from carbon dioxide in
the air, hydrogen from water, and oxygen from air, are used in large amounts for the building blocks of life. The
roots in the soil normally take up the other thirteen elements that are needed. These are grouped as primary
nutrients, secondary nutrients, and micronutrients. Plants use these nutrients to make the cellulose, protein,
enzymes and other components of the leaves, roots, flowers, stems, fruit, etc.; to run numerous metabolic sys-
tems; and to capture the sun’s energy to make sugars during photosynthesis.

The nutrients are supplied in many forms, some organic (e.g., decaying organic plant and animal materi-
als; chicken, horse and cow manures; fishmeal; blood meal) and some manufactured products, such as liquids,
soluble powders, and granulars.

Primary Nutrients
A common granular fertilizer label is shown on the opposite page. As with all modern fertilizers, the label

tells the amounts of the 3 primary nutrients, AKA macronutrients or major nutrients, which are nitrogen (N), phos-
phorus (P), and potassium (K). These numbers are the percent by weight of each in the mix, in this case repre-
sented by the numbers, “16-6-8.” The higher the number, the more concentrated the nutrient. The balance of the
3 nutrients (referred to as the NPK ratio), their chemical formulation, and the concentration of them are designed
for specific purposes.

In a grossly over-simplified rule, products with high nitrogen, relative to P and K, generally produce more
lush leaf growth. Products with higher phosphorus, relative to N and K will generally stimulate flower blooms.
Materials containing higher potassium, relative to the other two, will encourage plant hardiness in thicker leaves,
better rooting, and disease resistance.

The chemical formulations of the NPK will make them more or less rapidly useable to the plants. Plants
most rapidly assimilate nitrogen when it is in the nitrate form, so most quick-acting fertilizers for use in cold
weather will have a calcium nitrate or ammonium nitrate formulation. Next in order of rapidly used forms is the
ammonical form, such as ammonium sulfate, ammonium phosphate, or ammonium nitrate. Even slower releasing
forms of nitrogen are urea, urea formaldehyde, and isobutylidene-diurea (IBDU), which are useful for prolonged
feeding of plants.

Phosphorus can be formulated in a number of common forms, such as finely ground rock phosphate,
phosphoric acid, ammonium phosphate, and triple super phosphate.
Potassium can be supplied in fertilizers as potassium chloride, potassium sulfate, potassium nitrate, and sulfate of
potash- magnesia. Nitrate and sulfate forms are preferred over the chloride form in areas where chloride build-up
in the soil is a problem.

Secondary Nutrients
Calcium, sulfur, and magnesium are considered secondary nutrients. Calcium is supplied by calcium

nitrate in some fertilizer formulations, manure, soil amendments (gypsum, dolomite, hydrated lime, etc.), and even
irrigation water. Calcium supplements may be unnecessary for most growing situations, but especially beneficial
for fruiting plants such as tomato.

Sulfur is beneficial in acidifying the soil, increasing iron availability and uptake, and increasing the hardi-
ness of plants. It is often supplied as sulfates of nitrogen, gypsum, elemental sulfur, iron sulfates, manure, river
and rainwater, and some pesticides.

Magnesium can be supplied by magnesium sulfate (Epsom salts), commonly used for palms, or as potas-
sium-magnesium sulfate and magnesium nitrate, which may be used on citrus.

Micronutrients
Iron, copper, manganese, zinc, boron, molybdenum are micronutrients that are available as inorganic

salts, chelates, and natural organic complexes. Chelated and organic complexes make the micronutrients more
soluble for uptake from the soil and are the easiest for plants to use.

Coated Fertilizers
In addition to using the more slow-release forms of nitrogen, special coating can encapsulate fertilizer

prills to release the nutrients in a more controlled time frame. Some of these coatings may be sulfur, resin, or
polymers. Temperature, moisture, microbial degradation, weathering, or a combination of these factors may af-
fect the release of nutrients from these specialty fertilizers. Consult your UNIVAR representative.

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