15-01-2013, 03:07 PM
AGRICULTURAL FERTILIZERS
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AGRICULTURAL FERTILIZERS
Anyone who has grown a garden, maintained a lawn, or kept house plantsknows that it is necessary to
apply a fertilizer to the soil to keep cultivated plants healthy. As they grow, plants extract nutrients they need from
the soil. Unless these nutrients are replenished, plants will eventually cease to grow. In nature, nutrients are
returned to the soil when plants die and decay. However, this does not occur with cultivated plants. Humans
cultivate plants mainly for food, either for themselves or for livestock. When cultivated plants are harvested, the
nutrients that the plants extracted from the soil are taken away. To keep the soil productive, it is necessary to
replace these nutrients artificially. The kinds and amounts of nutrients that plants need have been determined and
can be supplied by applying to the soil substances that contain these nutrients.
A plant contains a great number of chemical compounds. The major compound in all plants is water. The
percent of the plant's weight that is water varies greatly from one kind of plant to another, from less than 20% to
more than 90%. After the water is removed, the bulk of the dry plant material consists of carbohydrate compounds
containing the elements carbon, hydrogen, and oxygen. Using the energy of sunlight in a process called
photosynthesis, plants make carbohydrates in their leaves. The carbon and oxygen in carbohydrates come from
carbon dioxide, which the plant absorbs from the air, and the hydrogen comes from water absorbed both through
the roots and through the leaves. About 90% of the weight of carbohydrates is carbon and oxygen. Therefore, a
plant obtains around 90% of its dry weight from the air.
Phosphorus is also significant in the storage and distribution of energy throughout the plant. Chlorophyll, the
compound that makes plant leaves green and is central in photosynthesis, also contains the element magnesium.
The fluids inside the plant's cells also contain other dissolved minerals which provide the proper environment for
the many chemical reactions that occur in the fluid. Among these minerals are compounds of potassium and
calcium.
Plants must obtain the elements essential for their growth, other than carbon, oxygen, and hydrogen, from
the soil. Thirteen elements essential for plant growth have been identified, and these are listed in the table on the
previous page. These essential elements are called nutrients; those needed in the greatest amount are called
macronutrients whereas those needed in lesser amounts are called micronutrients.
Among the macronutrients are nitrogen,
phosphorus, and potassium. These three elements
are those most rapidly removed from the soil by
plants. Therefore, many commercial plant
fertilizers supply these three essential elements.
The amount of each element is indicated by N-PK
numbers. The analysis information at right
(taken from a package of garden fertilizer) shows
an N-P-K rating of 15-30-15. These numbers
indicate the percent by weight of nitrogen,
diphosphorus pentoxide, and potassium oxide in
the fertilizer. The 15-30-15 rating indicates that
15% by weight of the fertilizer is nitrogen (N). It
also indicates that the weight of phosphorus in the
fertilizer is the same as it would be if the fertilizer
contained 30% diphosphorus pentoxide (P2O5).
The amount of potassium in the fertilizer is the
same as it would be if the fertilizer were 15%
potassium oxide (K2O).
The sources of nitrogen used in fertilizers
are many, including ammonia (NH3), diammonium
phosphate ((NH4)2HPO4), ammonium nitrate
(NH4NO3), ammonium sulfate ((NH4)2SO4), calcium cyanamide (CaCN2), calcium nitrate (Ca(NO3)2), sodium
nitrate (NaNO3), and urea (N2H4CO). Phosphorus is generally supplied as a phosphate, such as diammonium
phosphate ((NH4)2HPO4) or calcium dihydrogen phosphate (Ca(H2PO4)2). Potassium comes from potassium
sulfate (K2SO4) or potassium chloride (KCl), which is also called muriate of potash.